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Vapor Lock Confirmed and Replicated

I need the forums help with this one. I have recently confirmed that that my RV-9A, O-320 E2A (Carbureted, FP Prop) is suffering from bouts of Vapor Lock. Early in the week after a prolonged taxi and hold I was cleared to depart the main runway. Elevation 2880, air temp 85 degrees F. All checks normal so I departed with the boost pump ON. About 10 feet off the ground the engine coughed and just about died. Luckily with lots of runway I was able to land with no issues. Taxied back to run-up area and engine seemed to be running rough and when power was applied about every 2nd or 3rd time the engine would cough and just about die. Something was wrong so I taxied back to the hanger.

Day 2 84 F I took the cowl off and looked for anything obvious, nothing. With the cowl off I ran the engine, hard, to try and duplicate the problem. No luck. Limited by CHT?s I must have run it for an hour ranging from static max rpm to quick idle to max. Limited by CHT?s I would have to back off after a while to prevent damage.

Day 3 85 F Replaced the cowl and repeated the ground runs. However, this time after the engine was heat soaked from prolonged running it began to cough again. Interesting note that with the boost pump on it seemed to make it worse, not better! From idle to around 2000 rpm it would sputter and want to die almost 100% of the time with the boost pump on, and only 50% of the time with it off. The fuel pressure would hover around 3-4 psi at idle. When the engine is cold it would stay there until full power, however, when hot, once power is applied it would drop to near 0 psi with the boost pump off, around 2 with the boost pump on.

Day 4 83 F Wild Card (Auto Fuel 91) I have been using 91 oct (Ethanol free, check every batch) auto fuel exclusively for the past three years with no issues, even in very hot 90-100 degree days. However after some VAF forum research I thought this might be the culprit. I drained one tank and filled it with 100LL. Went back out and did my ground runs. When operating on autofuel I could get the engine to cough and sputter like the day before. Once purged, the 100LL would not do it! Fuel pressure stayed around 3 psi at all times with no fluctuations. With the boost pump on it would go up to near 5 psi with no fluctuations and/or sputters. This is what I believe to be classic vapor lock. I also think that I probably have some of the last ?winter? blend 91 octane as I fill up a 40 gallon tank, which was done around April. Winter blend has a higher vapor pressure thus more likely to vaporize.
Gascalotor mounted on firewall copilot?s side. Fuel lines fire sleeved, fuel pump NOT shrouded.

Thought about running a blast tube to the gascalator and the fuel pump, however I am not sure there is enough airflow on the ground to make a difference. This is when my vapor lock issues are likely to occur so it probably wouldn?t do much good. Has anyone had this issue and were they able to solve it? I realize that 100LL ?fixed? the problem but I would like a better solution. Anyone have any thoughts?

If you can help it please reserve your autogas vs 100ll arguments. I have access to 91 oct and I have made the decision to use it so I would like helpful suggestions on how to solve this vapor lock issue, outside of exclusive 100ll use.

Brett in Boise
 
It sounds like the problem may be before the electric pump. If the problem gets worse with the pump on it is because the electric pump is pulling a lower pressure. Vapor lock occurs when there is less pressure not on the pressure side.
 
Maybe wrap the fuel lines with heat protective sleeve. It's interesting you still get it with the boost pump on. Any chance you have developed a small leak around the boost pump (before the pump) that is making this show up now?
 
mixture

Any chance it is running too rich?
You did mention the field elevation was almost 2900msl and temps at 85*.
In my plane, I would be leaning some.
I didn't see any mention of the mixture control in your write up.
Good Luck.
 
Been There...

If you can help it please reserve your autogas vs 100ll arguments. I have access to 91 oct and I have made the decision to use it so I would like helpful suggestions on how to solve this vapor lock issue, outside of exclusive 100ll use.

Brett in Boise
Hi Brett.
I had to drain most of my Auto Gas (91 Oct non ethenal) from one wing on conditions that you describe. I have seen the exact symptoms that you describe. Therefore In summer months (i'm in AZ so we see some heat!) I will run on one tank 100LL for departure and climb and the other tank is 91 Oct auto for cruse. There are a few of us doing exactly this type of operations in the summer. I believe that FankH has the solution where the engine driven fuel pump is removed from the system and all fuel pumps are installed away from heat- in the wings. Please do a search with this in mind and you will find much info on altering the fuel system install for exclusive Autogas use. For myself (and others) it is a wing to wing mix in the summer.
regards
 
I dont think there is a leak, I will double check tonight. I also have been running full rich. I will lean it out and see if I can duplicate tonight. Just about everything that can be insulated is insulated, except the gascalator.

I am considering going the route of a return line from the carb inlet. Andair makes a fuel selector that has a return port built in so what fuel is not used by the engine will be returned. Not sure if this will help as it seems to me this would not actually cycle new fuel but just recirculate hot fuel.

Maybe the return line needs to be vavle operated and ported to the opposite tank, similar to the purge systems recommended by others on this forum.

Brett
 
Purge line

Brett
In one of the recent discussions about fuel purge lines it was suggested to attach the purge line to the carb float drain. This should completely flush the system.
 
may be upstream of your boost pump

From your description, this sounds like it may be vapor locking upstream of your boost pump.
Factors to consider:
1) having winter gas may be a significant contributor.
See if you can duplicate with summer gas.
2) was airplane parked in the sun so the tanks would be warmer?
3) is there much height change between the tank and the boost pump? Asking the boost pump to pull fuel uphill will aggravate the situation.

These three factors could easily transition you from a marginal situation to an actual vapor lock situation. When everything is normal, you never know how close to a vapor lock you might be. A few small changes might explain why it is suddenly happening to you now, and not before.

In summer, you could consider keeping 100LL in one tank, as others suggested, or try a blend of 50/50 or so, and see if that solves it. You have a head start on these solutions for the time being, since you already filled a tank with 100LL. So when it is half gone, top it up with your 91 auto gas and see how that works.
 
I recently had vapor lock running 100LL. The RV6 I am now flying has some goofy fuel line routing which is prone to vapor lock.

The most vapor-lock-proof fuel system is going to be one that has a vapor return line, is all-electric, with minimal fuel line routing firewall forward.

That said a return line via an orfice fitting (around .040" or so) at the carb will go a long way in preventing vapor lock since it is constantly flushing hot fuel/vapor with cool fuel. A shroud and blast tube on the mechanical fuel pump will also help quite a bit.
 
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Brett
When you ran 100LL the bad behavior went away. Would fresh mogas do the same as the suspected winter blend?

Some of the Peterson mogas STC documents warn against the winter fuel in warm conditions- so what you are seeing isn't really a surprise. The easiest solution might be to use up the winter gas in your car, and resupply the aviation fuel tank with the summer blend.

I've got a Hodges Tester over here in Caldwell. You are welcome to try the different fuels and see if the suspected winter fuel does vaporize easier. The Hodges kit lets you reduce pressure on the fuel to the point it begins to vaporize. When it boils, you read the pressure on the gauge. The winter blends begin to vaporize at a higher absolute pressure.
 
Thank you for all of the replies. I am beginning to suspect that it may be the winter blend as well. The next experiement is to get the winter blend out and see if new stuff still has issues.

I am thinking about a purge system just off the carb inlet running back to upstream of the selector valve. Some have suggested the carb drain but wouldnt that kill the engine when the purge system is opened? I want to be able to purge on the ground with the engine running so I think carb float drain is out. Thoughs are to install a solenoid similar to the primer system so that I can purge the stystem and run the excess back to the tank.

Does anyone still sell a vapor pressure tester? I contacted Peterson and they dont sell it anymore as they cant get parts. If I cant find one I may have to travel over to Caldwell to test some of the fuel I have.
 
Vapor Pressure Tester

I built my own tester using a BBQ "flavor injector" syringe and one of these gauges:

http://www.harborfreight.com/fuel-pump-and-vacuum-tester-93547.html

You'll probably have to add a few drops of oil each use to keep the syringe lubricated, but it's a pretty cheap and easy solution.

As far as the engine running worse with the boost pump on, you may be seeing the fuel in the float bowl boiling. If the engine idles better leaned out near a cruise mixture setting, that's what is happening. On a hot restart I often have to lean it out until I get enough cool fuel through the carb.

With regard to winter vs. summer blend, not all summer blends are created equal; take a look at this chart:
http://www.epa.gov/oms/fuels/gasolinefuels/volatility/standards.htm

Winter blends can run as high as RVP 15.
AVGAS is about RVP 7.

I did some back of the envelope calculations last fall and came to the realization that winter blend auto gas could boil around 92 deg F...I think you've validated that.

Paige
 
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Classic Vapor Lock

Your situation and symptoms are classic vapor lock indications
and end in many unexplained engine failures and crashes that can easily be avoided.
When I say situation and symptoms, I am refering to mogas users who have not made any modifications to their fuel sytem and symptoms exactly as you describe in you near disaster had it happened on a shorter runway.
The question is, are you willing to make some modifications to your fuel system that will immunize your fuel system against vapor lock?
FrankH has an all electric fuel supply system, he can explain it if he wants to.
Mine is electric/mechanical with a constant flow RETURN LINE.
As others have posted, the return line is key to this system, where a constant flow of cool fuel replaces hot fuel and vapor from the mechanical "boiler" fuel pump. You simply T into the exit line on the mechanical fuel pump and route the return line back to a tank. The return line must be orificed to allow an average of about 6 GPH.
Fuel metering and fuel pressure must be measured after this T fitting in the line.
A cool air blast tube directed at the fuel pump helps too but is in no way adequate without a return line.

A bypass line would also help where your electric fuel pump can bypass the mechanical pump in case it vapor locks. Your low pressure pump probably cannot overcome the vapor pressure build up in the mechanical pump, this is why you see no fuel pressure improvement with the electric pump on.
Of course this would not happen with a return line installed.

IMG_2462.JPG
 
"T" off right at the fuel line connection at the carb and run it back to the left tank. Install a valve that you can reach with your left hand.

Open the valve, turn on the electric fuel pump and then touch the return line. You will feel the hot fuel returning to the tank. And you can tell when the cooler fuel passes through.

Then just shut off the valve. You are good to go.
 
I have just about the identical engine/prop set up as you. I too also had an engine out on takeoff. Except it happened a 400Ft with the runway fading away quickly. I pulled the power for a landing on a closed runway and with reduced power the engine restarted. Needless to say I left the power setting alone and landed without incident.

Upon taxing off the runway I did a full power static runup and every time the engine shut down.

Well after a lot of trouble shooting I came to the conclusion it was indeed a vapor lock issue while using high test mogas. I'm firmly convinced that is the problem, for I have been using 100LL since and have never had a repeat of the problem. My personal view, don't try to save a few bucks by going the mogas route, especially with the ethanol. It just isn't worth putting you life in peril, to save a few dollars.

Just getting too old for that kind of excitement on takeoff!
 
With recent accident data coming to light and consequent discussion
on how to reduce these tragedies, I can't help but post one more time on this subject.
Can I use Mogas in my Lycoming?
Yes, BUT It's the fuel systen installation that needs modification
and may be different from RV to RV and works for some and not for others.
I am not selling parts or advice but fuel system installation improvements will truly reduce some of the engine stopage accidents and near accidents.
100LL gives us a nice margin for the most extreme heat and altitude conditions, where as lower octane mogas does not. If you want to use mogas you must install a compatible fuel system. As with an all electric system or a return line system, there is more than one way to do it.

"T" off right at the fuel line connection at the carb and run it back to the left tank. Install a valve that you can reach with your left hand.

Open the valve, turn on the electric fuel pump and then touch the return line. You will feel the hot fuel returning to the tank. And you can tell when the cooler fuel passes through.

Then just shut off the valve. You are good to go.
Yesterday 09:55 PM
 
PaigeHoffart makes a good point. There is “vapor lock” and there is fuel boiling in the carb bowl. I have heard the latter referred to as “pneumatic lock” which is apparently the problem Petersen had in his attempt to certify auto gas in carbureted Mooney’s. When fuel boils in the carb, the float “sinks” a little allowing excess fuel to be ingested, resulting in an overly rich mixture. The symptoms of pneumatic lock are rough running, black exhaust, and possibly raw fuel exiting the exhaust pipes. Factors contributing to pneumatic lock include a hot exhaust pipe running near the carb, a heat soaked engine, and high oil temps. The winter blend auto gas is also a significant factor. You can keep an engine experiencing pneumatic lock running with the mixture control. Vapor lock occurs in the fuel pump and results in low or zero fuel pressure. When you have vapor lock the engine just quits.
I have hundreds and hundreds of hours on auto gas in my -6. I have often experienced pneumatic lock, but it has always been on the ground, usually with auto gas, but sometimes with av gas. Cool fuel flowing into the carb on takeoff roll resolves the problem. I have never had vapor lock.
My -6 is ten years old and has over 1300 hours. Originally, I had a fuel return line from the carb inlet with a valve as others have described. After a few years I removed it. Now I keep 100L in the right tank for takeoff and landing; and auto gas in the left tank. Inbound at five miles I switch to the right tank and turn on the fuel pump. After takeoff and climb out I switch back to auto gas. This is a strategy that I am comfortable with and allows me to use auto gas at least 2/3 of the time.
 
Vapor Lock

Having done some testing a few years ago in a C172 with Lycoming Engine with carburetor; and running 92 Octane Auto Fuel in one tank and 100LL in the other tank. Experienced engine failure due to vapor lock at climb power above 7,000' MSL. Never-ever tried that again..!!! It's 100LL or nothing. My life is worth more than a few extra bucks it costs for AVGAS.
What if you forget to switch tanks from auto to avgas someday. Another statistic for the NTSB.
 
Safety

This won't be the most popular post but this is my take on it. Your airplane is talking to you. Listen. It likes 100LL, it does not like auto gas! Were I you I would (A) reserve one tank of 100LL for takeoff, climb out, and landing or (B) give up on auto fuel and run 100LL in both tanks. Any thing less and you are putting your personal safety as well as any passenger you have at risk. Sorry but I have seen one friend crash because of this issue and another put down off the end of the runway. Niether uses auto fuel anymore. With all the alcohol in auto fuel these days there are just to many variables to be sure what your getting.
Good luck,
Ryanq
 
Drained auto gas out of tank yesterday. Still don?t have a vapor tester but I put in a fresh batch of 91 oct. Went out and ran last night, it was 90 degrees and I could not get it to vapor lock! It was the winter blend fuel that was doing it to me, however, who knows how close to the edge I am with the new stuff. As many have posted, until I make some modifications I will now always fly with 100LL in one tank, just not worth the risk even though the summer blend appears to prevent it.

Since I cant find a vapor pressure tester online, at least an economical one, can one of you explain how you made your own vapor pressure tester? I have never tested fuel for vapor pressure so admittedly I don?t know what I am doing. I get that I need a vacuum gauge of some sort then a syringe, then what?
 
Since I cant find a vapor pressure tester online, at least an economical one, can one of you explain how you made your own vapor pressure tester? I have never tested fuel for vapor pressure so admittedly I don?t know what I am doing. I get that I need a vacuum gauge of some sort then a syringe, then what?

Basically my procedure is
1) Start with an empty syringe
2) Pull a few CC's of fuel in the syringe
3) Purge all air from the syringe
4) Connect to the vacuum gauge with a short (couple of inches) hose
5) Pull the syringe until the plunger is almost ready to pop out
6) Read the vacuum gage

It really doesn't matter how much fuel you pull in as long as 1) the volume is small compared to the remainder of the syringe, and 2) you still have some liquid after you pull the vacuum. You are basically lowering the pressure and causing the fuel to boil at room temperature. Since the system is closed, it will come to equilibrium at the vapor pressure, which you then read on the gauge.

The gauge won't give you a direct readout in Reid Vapor Pressure because vapor pressure depends on temperature (RVP is defined at 100 deg F). If you wanted to, you could run the calculations with temperature and barometric pressure, but in my experience, the difference between 100LL and auto gas is pretty obvious just by looking at the gauge reading. I can usually get around 24" of vacuum on 100LL, and 19-20 ish on auto gas. Winter blend gave me a reading of 15" last fall (measured at 50 deg F :eek:).

Another thing to think about when running auto gas is fuel filtration. I've found a good amount of dirt and trash in auto gas.

We have a couple of guys around here that run auto fuel religiously, and quite a few others that tried it and decided to go back to 100LL. Feeding a Cessna auto fuel is pretty easy, but a tightly cowled RV...not so much.

Hope that helps,
Paige
 
Vapor Lock

I have been using 91 AKI (no ethanol) in my RV9A for the past three years and I've never had any problems. My home airport in Idaho is at 4700' where the normal summer time temps in the morning are in the high 50s or low 60s. Since it is difficult to find mogas on cross country trips I then have to switch to 100LL.
When my 9 was new I was having issues with elevated engine temps and so I installed 2 louvers on the bottom of cowl hoping that would allow for a greater
volume of air through my cowling. I would guess that a side benefit of this installation is also a cooler fuel pump and fuel lines. I have recently acquired a datalogger and some thermocouples and plan to be taking some temperature readings at various points inside my cowl.

Tom
 
Hi Brett, just read your post on suspected "vapor lock".
For some reason people always think your symptoms mean there is vapor in the lines causing their low fuel pressure problem when this is mostly not the case.
You were right when you said "the boost pump seemed to make it worse."
The problem is not lack of fuel but too much fuel.

On hot days especially when a lot of time is spent in taxiing or holding there is a lot of heat built up in the cowl and the pump and carby get very hot and do not have a lot of fuel flow to cool them.
When the fuel inside the carby reaches boiling point (easier with mogas) the specific gravity of the boiling fuel is too low to support the float and it sinks to the bottom of the bowl!
This allows fuel to fill the bowl and flow out through the carby bowl vent which on an aircraft carby is vented to the throat not atmosphere or the tank as on cars.
If you want to see this in action, take a block of hardwood that barely floats in water (as a carby float does in fuel), place it in an open container of water on the hotplate and watch what it does when the water boils.

Oh, and the fuel pressure drops, as it would when the float needle valve is wide open! This drop in pressure is what makes people think it is lack of fuel.

When you lose power you are trained to make sure fuel is on, apply carb heat, mixture is rich and boost pump on.

Every action will compound the problem! You need to restrict the fuel with the fuel selector, not an easy task. (Though I know of one gliding club that did this at times on there Decathlon tow plane. It was on Mogas!)
The mixture control has little effect as the fuel flows through the vent not the mainjet.

If someone is watching you take off when this happens they will see you trail black smoke if the engine manages to keep running. A sure sign of over richness.

A lot of earlier cars had this problem in Australia where the heat can be extreme. The temporary fix was to wrap a wet towel around the carby to cool it. VWs and early GMH Holdens were prone to this and it fact it was the retired Chief Engineer of GMH Australia who made me aware of this after an accident to a T18 that I later bought. Holden fitted an insulating block between the carb and manifold to try to rectify the problem.

This form of failure is not limited to low wing aircraft, I have seen a C172 have the same problem trying to take off on a 110 degree day and another C172 using Mogas that could not climb over 3,000 feet after sitting on the ground all day in over 100 degree heat. As he tried to climb higher the engine ran rougher. Later in the flight when the fuel cooled he was able to climb higher.

PS. The Avgas did not do it to you because it boils at a higher temperature. Mogas with Ethanol would be worse I would think.

This is an issue more people need to be made aware of.

Hope this helps you Brett, safe Flying, Brian
 
Good info!

This has been a very informative thread.
The last post clearly points out yet another possible problem in the fuel system
and again highlights problems associated with different installations.
As the previous poster points out, a carburetor installation has an additional worry besides the most offending component in the fuel system, namely the fuel pump. I am sure Brian's analysis is quite right, except I would argue that the hot fuel was first and foremost generated in the fuel pump and further heated in the carb bowl, sandwiched between hot exhaust pipes.
Even though the OP has confirmed the absence of vapor lock after now using summer gas I hope he is considering a fuel system modification.
The idea of testing fuel does have a place for those of us who use mogas but does not solve the problem of a marginal fuel system installation.
Using 100LL in one tank for take off and landings is an excellent way to work out a solid new fuel system while not exposing yourself to unnecessary
risk.
 
I am going to install blast tubes to my fuel pump and gascalator. I realize that this will not solve the problem but this is a quick step that I can take to at least aleviate some of the problem. I also realize that on the ground there will be little if any cooling pressure as the prop just isnt moving much air.

My question is can I use the vacuum pump dishcarge port as an additional cooling duct? I have heard of this before but I am not sure if it is actually cool air or hot air coming out.

Once again, I realize this will not fix the problem. Until I modify my fuel system I will be flying with 100LL in one tank for takeoff/landing. Just looking for thoughts.
 
RV-12

I understand that most RV-12's are flying with auto gas. Yet I have never heard any reports of vapor lock issues with the -12. Is there something about the -12 fuel system that makes it less likely to suffer vapor lock problems?
 
On my RV6 I have an open ended carb heat muff surrounding ten inches of the two exhaust crossover pipes. This has the added benefit of shielding the carb. from radiated heat. This muff produces a heat rise in excess of 50 degrees C which was a requirement in Australia pre experimental category.
 
Interested in RV 12 fuel system

I believe the RV-12 has dual electric pumps only.
Someone could confirm that before further speculation please.
 
Engine driven pump question ?

I have followed all the MoGas discussion and have two electric pumps available and a return to tank line. My engine came with a new mechanical pump. I would like to use it with an electric boost ( like a Cherokee 140 ). Is the mechanical pump designed such as to hold fuel pressure if the diaphragm or pump check valve failed and the electric boost was required ?? If not, I will need an independent check valve between the carb and the mechanical pump.
 
I'm resurrecting this thread because Brett's vapor lock issue happened to me today. Wouldn't you know it, I'm in Boise, too! I'm in a -7A, fixed pitch O-360 with carb...same setup, it seems.

Funny thing is, me and a buddy flew to Payette airport to get Auto-gas (91 no-ethonol)...it's the only airport around that has it. Not sure if many know about it. May I ask if this is where you got your gas? If they don't sell much of it, then it may be the winter blend everyone is talking about. I usually drive down to the Shell Station on Hwy 44 to get their 91 octane no-ethanol gas...but got lazy today. Not sure if either is winter gas, or neither is.

Upon landing and taxi back to runway for takeoff, I applied full throttle and the engine quickly died. I'm thankful it happened before I even hit 5 mph. Taxied off the runway, and taxiing back I turned on the boost pump and it died again. Thought this may be coincedental, though. I haven't done any further testing.

Quick question about running 100LL in one tank...are those who are doing this only doing this during warmer months? I never have this vapor lock issue unless it's hot out. Today's OAT was 84 degrees (F).

Also, Brett, have you T'd into the fuel line for a vapor-return-line? If so, how'd you do this? Did you run the return line through the tunnel?

Thanks!
 
Glad you decided to post your experience, I for one am very interested in what others have learned using mogas.
I have been using mogas E10 since Februry of this year out of one tank mostly for cruise. I keep precise records of how much I use as well as records of all my flights. My EI MVP-50 engine monitor records all engine parameters and EG View software displays all of it in colored graphs along with trends and GAMI reports on all flights.
To answer your question, I have not been able to detect any difference or noticed a trend when going from winter gas to summer gas, in fact I have no idea when the change occurred.
I do know one thing after using mogas for the last 75hrs, My engine runs better, smoother and leaner on mogas and I do not see a speed penalty nor a significant temperature difference.
I have also done some hot weather mogas testing this summer doing take off and landings in 101F. Taking off at sea level on mogas after heat soaking the engine compartment exhibited no adverse effects on engine operation,
in fact the engine ran smooth and strong all the way up to 14000 feet.(end of test)
The electric fuel pump was turned off at 1000 feet just to be sure the engine pump could handle it.
I did get a bit carried away hoping to be able to do the same thing on the engine driven pump only. For this test I turned the electric boost pump off just before landing and planned on heat soaking the engine compartment in preparation for another take off test. However, without the boost pump on, the engine
began to stumble half way up the taxiway and proved to me that the boost pump is necessary to keep positive pressure on the supply line to the mechanical pump.
Once the flow of cool fuel increases at high power settings the boost pump is no longer needed.
The heat soaking was done on purpose to simulate extreme circumstances.
My engine runs cool in flight and on the ground and oil temps are hardly ever over 200F.
In case you missed it, I do have a constant flow fuel return line, one mechanical pump and one boost pump.
 
Don't mean to be a stick in the mud but can someone explain to me why you would run mogas versus aviation fuel? It's an airplane, it's an airplane engine designed from the 30's-40's. Is a dollar or two per gallon that important to save versus the ongoing dangers associated with meddling with tried and true aviation technology?
I'm keen to learn what I'm missing. The way I see it, I'm getting around 25 mpg at 145-155 mph. That's still pretty good economy to my way of thinking.
 
can someone explain to me why you would run mogas versus aviation fuel? It's an airplane, it's an airplane engine designed from the 30's-40's

If you have an engine with modern day technology (eventually modern technology will take over the aircraft engine business) like a Rotax then you of course use mogas because they are built like car engines and do not like lead. Low Lead AVgas will go away in spite of everyone's best efforts. Ethanol will not in spite of everyone's best efforts. Unfortunately, ethanol causes vapor lock as well (as me how I know). The real concerns are why is the piston freight business allowed to prevent unleaded AVgas and why is the subsidized ethanol lobby allowed to poison mogas?
 
Thanks, Ernst. I did miss the part about you having a return line. Are you also carbed? I'll look through the thread again to see if I can find out how you did this.

Don't mean to be a stick in the mud but can someone explain to me why you would run mogas versus aviation fuel?

Nigel, the first 50 hours on my RV were with 100LL. One thing I noticed was that I would get lead deposits on my spark plugs which caused the ignition to occasionally miss. I've never had that happen with auto fuel. Other than a restart within 15 minutes of shutdown, I don't experience heat soak induced vapor lock. My concern is that maybe the gas I purchased was a winter blend.

I've had pretty good success running auto fuel, and would like to stay with it.
 
Nigel,
You can get over 40 MPG in a Prius and you won't even have to risk anything as dangerous as flying.
Seriously, the lead is an undesirable component in avgas and is chiefly an octane booster. Lead has nothing but ill effects on an engine except for the octane boost. Running mogas will keep your engine and plugs clean and you can proclaim to be environmentally friendlier than your lead polluting bretheren.
I share Peterk's sentiments and I am convinced 100LL will go away some time soon. I for one, along with many others who use mogas will be confident in my fuel system and operating procedures when 100LL is no longer available. The price difference is merely icing on the cake.
Peterk, I do disagree with you about ethanol causing vapor lock. Ethanol does dissolve certain rubber parts as well as fiberglass tanks but works very well in RVs with very little fuel system modifications.
A poorly designed fuel delivery system, unsuitable for lower octane fuel is the prime problem not ethanol, in fact ethanol is the octane booster in mogas.
Just for the record, I do not like ethanol poisoning our mogas but here in California it won't go away anytime soon. Just build a fuel system designed
to accommodate what is available to you. All electric fuel pumps or a simple constant flow fuel return line will go a long way in eliminating vapor lock issues.
IO-540, 8.5:1 compression ratio, AFP fuel injection, Light Speed and one mag.
 
Nigel,
Just build a fuel system designed
to accommodate what is available to you. All electric fuel pumps or a simple constant flow fuel return line will go a long way in eliminating vapor lock issues.

I totally agree. I'm building my 7 with two electric fuel pumps (one primary and one backup) and no mechanical pump. I have a fuel rail through which the pump will flow 30GPH and all unused fuel is returned to the tank from which I'm drawing it from....so I'll always have cool fuel flowing to the engine. I'm also running electronic fuel injectors that are mounted directly into the intake tubes. The injectors are mounted about 2" below where the intake pipe attaches to the bottom of the head. I think this is the perfect setup for avoiding vapor lock. The entire system is from Robert Paisley and his EFII system (flyefii.com) I have yet to test the setup but hope to be doing so by December but I think it's ideally designed to avoid vapor lock problems. I also plan on burning mogas.
 
The auto industry eliminated vapor lock by pressurizing fuel delivery from the tank to the engine (with fuel injection systems). Fuel at 30 psi with not vaporize with the temps we see. That's why guys with carb systems are having problems. It was so in automobiles years ago also .

I wish someone would build a mechanical pump with materials compatible with ethanol. The materials certainly are available. Don Rivera has such material in his fuel controllers, why not in that simple fuel pump?

The are Lycoming engines being used with pure enthanol but the engine suppliers will not share the information they have on the pumps - I've tried to get the secret. I strongly suspect one supplier (Tempest) is using such materials in their pumps but they won't confirm it one way or another. I do know they had a test program going a while back to have a pump certified for mogas with ethanol but it was shelved, probably out of liability issues.

Two electric pumps will work well especially with AFP's system. I just don't like hanging it all on the electric system like with the Subby effort when there is a perfectly good mechanical pump to go along with the electric pump.
 
I based my decision on the mechanical pump on the experience of others who have accumulated thousands of hours on the same pump.
To the best of my knowledge, the ethanol formation team has used the same mechanical fuel pump as everyone else.
The Brazilians who burn ethanol in their Lycomings use the same pump as we do.
A local pilot with nearly 1000 hours on mogas E10 in his RV has had no problems.
I am not breaking new ground here and I am aware that none of the manufacturers of the mechanical fuel pump will approve of the use of mogas with ethanol.
Besides, switching to avgas between landing and take off has the airplane sitting in avgas and the exposure to ethanol is very short in the grand scheme of things.
 
Winter versus Summer fuel

Funny thing is, me and a buddy flew to Payette airport to get Auto-gas (91 no-ethonol)...it's the only airport around that has it. Not sure if many know about it. May I ask if this is where you got your gas? If they don't sell much of it, then it may be the winter blend everyone is talking about. I usually drive down to the Shell Station on Hwy 44 to get their 91 octane no-ethanol gas...but got lazy today. Not sure if either is winter gas, or neither is.

Sonny

Is it winter gas? Seems like a logical question to ask. At least in some locales, Sept 15 is the date winter blend can be retailed. If you google the topic, you'll get some hits on that date.

As a local, I've made a few phone calls. A Payette airport commision member reports the jobber does an excellent job sourcing high octane no-ethanol fuel. The octane posted is a minimum, he seemed to recall actually getting fuel as high as 94. Most recently, the fuel was trucked from Montana, which may have a more aggressive schedule for switching to winter blends (my speculation based on Montana's colder climate).

It sounds like trucking fuel into the area is customary. I presume because the Boise terminal has lower octane coming up the pipeline from SLC and that fuel needs the octane bump from the ethanol to make premium, thus bringing in the high octane no ethanol via truck.

The jobber has confirmed the fuel came from Montana and seems to be willing to dig for some more details. I expect to hear more.

Thanks for passing along your experience. It serves as a warning.

I don't know if the Hodges Tester is still available for purchase. If they aren't, they can easily be made (search is your friend). The Hodges tester is a worthwhile investment, especially during the spring and fall when one isn't quite sure what is being sold.

I've got a Hodges here in Caldwell if you want to check your fuel- just PM me.
 
Can anyone point me to a build log or pics of a fuel/vapor return line from the carb inlet? I just can't picture how this would work.
 
Junior MacGyver Vapor Pressure Test Rig

I wanted to test the vapor pressure of some mogas this weekend, so I took some hints from PaigeH and cobbled one together.

Mighty Vac that I use to bleed car brakes and a cheap vacuum gage from HF.

20130428_090853.jpg


Found that the mogas at the airport might still be winter blend as it boils at about 15.5 inhg vacuum at 19C. 100LL would NOT boil at 27 inhg at the same temp. Might mix in some 100LL until the brew changes. Looking at some fuel system mods.....

Pretty cool to test the science on we hear about ........
 
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I wanted to test the vapor pressure of some mogas this weekend, so I took some hints from PaigeH and cobbled one together.

Mighty Vac that I use to bleed car brakes and a cheap vacuum gage from HF.

20130428_090853.jpg


Found that the mogas at the airport might still be winter blend as it boils at about 15.5 inhg vacuum at 19C. 100LL would NOT boil at 27 inhg at the same temp. Might mix in some 100LL until the brew changes. Looking at some fuel system mods.....

Pretty cool to test the science on we hear about ........

Hey Pete, that's some interesting data you've got there. I just did my BFR yesterday evening...It was 92 degrees (F), and I had a mix of 6 gallons of Avgas with 15 gallons of 91 octane auto-fuel (no ethanol). I also recently partially wrapped my exhaust pipes to help lower the under-cowl temps to prevent heat soaking the fuel. I'm not sure how much it helped because last night after an hour flight I was still detecting some vapor lock. Not enough to kill the engine, but it would stumble a bit.

Would you happen to know what the vapor pressure is on 91 octane summer blend?
 
The RV-6 I fly will vapor lock with nothing but 100LL in it. The original builder put some shields on the pipes and they don't help much.
 
Vapor Lock

I have an old IH Scout that used to vapor lock on me all of the time until an old timer told me to put some wooden clothes pins on the metallic fuel line. IT WORKS! Never VP again. If you have metal fuel lines, it is a very easy thing to try. The pins apparently dissipate the heat.:)
 
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