What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Lean Carburator R&D - thinking out loud

Lycosaurus

Lycosaurus

Well Known Member
I've been flying for 25 hours now, and my carb is showing that it is lean for my setup. I have 11.5 gph at full throttle, full rich, -10C, 2000 ft. According to the o-320/160hp manual, I should be flowing at least 13.5 or 14 gph (from memory).

Many on VAF have had success drilling their jets one or two drill sizes larger. Bart at Aerosport Power offered to do the drilling of the jet for me on a lathe, so I sent it off yesterday.

Meanwhile, little wheels are spinning in my head, trying to understand what is different with our RV setups. Here's some of my out-loud thinking :

For one, we have a 'more efficient intake filtering'. While that is true, the greater air flow, should also result in greater fuel being sucked up through the nozzle.

Another thought is that propeller pulses are causing resonances in the airbox, that mess up the fuel metering. Given our highly pitched propellers, there may be something to this. Going to carb heat should resolve that .... and it does a little bit.

Some say that the lean carb is due to our low restriction exhausts. Maybe, but I have dual mufflers on mine, and still have a lean condition.

Yet another idea, and this one I found on the Matronics site, dates back to 1988: maybe the carb float chamber vent is not seeing the proper pressure. Imagine, if you will, that the ambient pressure is decreased around the chamber vent hole due to some strange turbulence, then the amount of fuel that is being sucked out of the jet will decrease. Hmmm.... that sounds kind of right to me. It would seem that this vent can be found in different areas for different versions of our carburators. Could this in fact be the root cause of the lean carb jet issue?

Interested in other people's thoughts / ideas on this.... (Mahlon, any thoughts on this?)

[Please pardon my ramblings .... I am suffering RV flight withdrawal until I get my carb back, and temperatures/weather get back in line.]
 
I know if you have put that much thought into it you have already confirmed the mixture full fwd pushes the arm against the stop on the carb, right? And there is a little bit of cable throw left so in the heat of the battle when everything else is going on a little push fwd does in fact push the mixture arm to the full rich stop?

Just checking.

It has happened before to others but I knew it couldn't happen to you :D
 
asav8tor said:
I know if you have put that much thought into it you have already confirmed the mixture full fwd pushes the arm against the stop on the carb, right? And there is a little bit of cable throw left so in the heat of the battle when everything else is going on a little push fwd does in fact push the mixture arm to the full rich stop?

Just checking.

It has happened before to others but I knew it couldn't happen to you :D
Click to expand...

Yup, mixture hits the stops at the carb, and still room to push further the control cable at the panel. My situation is not unique.

No problem on the checking .... all info needs to be put out there.
 
Exhaust system

Exhaust systems can play a big part in the mixture issue. The systems on RVs are far superior to the ones found on production airplanes. Remember that most of carb setups were "tuned" for some really inefficient exhaust systems.

John Clark
RV8 N18U "Sunshine"
KSBA
 
John Clark said:
Exhaust systems can play a big part in the mixture issue.
Click to expand...

Agreed, however I believe the Mooney also has a richer jet (the richest, as manufactured by the carb company?). I don't think they have anything special for an exhaust, but they certainly have a similar high end speed... as our planes. What happens when they install one of these super flowing exhaust systems that is supposed to dramatically increase horsepower in these planes? Do they have to modify the carb as well to enrich the mixture?

We have a static pressure line for our instruments. You would think that we would need something similar for our carburators to use as a pressure reference. Not as critical, mind you, however given the tight cowling we have, what are the pressures surrounding the cowl. Would be interesting to measure with a manometer in-flight.
 
Alfio,

Here is an article from Kelly Aerospace on Marvel Schebler carburetor (Kelly owns it):
http://www.kellyaerospace.com/articles/Accessory_AMT.pdf

The last section on the last page is entitled "Choosing the Correct Carburetor". It mentioned the complex interactions that affect the fuel/air ratios. So according to the manufacturer the causes are many.
Airframe performance, cowling configurations, baffling, and choice of air-box, all contribute dramatically to fuel/air ratios and to
engine cooling.
Click to expand...

I am no expert on the issue and this is just what I read.
 
I think there is something wrong with your carburetor if you are seeing real fuel flows that low out of it. That is pretty lean. What are the egt's like? They should be really high, if the fuel flow is that low. Many things affect the fuel delivery out of the carb. 320's tend to be leaner rather then richer. I think most drill their jets because they have a good carb but may need for more fuel because they are battling temps or may be they just want to richen things up a bit from a standard fuel flow. Your flow seems pretty lean to start with, making me suspect the accuracy of the fuel flow reading or that maybe the carb has a problem.
Good Luck,
Mahlon

?The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."
 
mahlon_r said:
I think there is something wrong with your carburetor if you are seeing real fuel flows that low out of it. That is pretty lean. What are the egt's like?
Click to expand...
I see 110 kt climb temperatures of close to 400F, with one cylinder going to 410F (#3?). In flight , at about 75%, I see an average of about 370F (I should download my EMS info so I don't have to work from memory... I'll do that this weekend. This is all at approx -10C to -20C air temps.

I expect that my cylinder temperatures will get hotter in summer, so that is why I am looking at enrichening the carb.

mahlon_r said:
I think most drill their jets because they have a good carb but may need for more fuel because they are battling temps or may be they just want to richen things up a bit from a standard fuel flow. Your flow seems pretty lean to start with, making me suspect the accuracy of the fuel flow reading or that maybe the carb has a problem.
Click to expand...
Fuel flow was checked wrt to one calibrated fillup at the pumps. Came in within 1.5%.

My understanding is that one numbered drill size increase in the jet gives you 1 gph increased fuel flow. Most people seem to be drilling 2 to 3 drill sizes larger. If I have the carb drilled 2 sizes larger, that would bring the fuel flow to 13.5 gph, which would be within Lycoming minimum fuel flow spec.

Aerosport will also be looking over the carb, and see if there is anything wrong with it.

What kind of fuel flows did other builders see that have a carb'd o-320 (before and after having the jets drilled)?
 
Just two data points during initial climb:

Before jet enlargement (at size #41):

OAT FF RPM highest CHT VS
83F 11.2 gph 2200 rpm 420F 1000fpm

After jet enlargement (to #38)

98F 13 gph 2380 rpm 336F 2400 fpm

Please note that I climbed at different rate.
 
I had Bart install a larger diameter jet for me at about 10 hours. Solved the overheating problem on take-off by dropping temps about 30 degrees.

I see about 14 gph on takeoff now.

It turns out that this is very common, and most of the RVs that I'm familiar with end up having the same problem/fix.

I'm suprised Bart doesn't just install a larger jet when he ships an engine destined for an RV.

Vern.
 
FF

Lycosaurus said:
Yet another idea, and this one I found on the Matronics site, dates back to 1988: maybe the carb float chamber vent is not seeing the proper pressure. Imagine, if you will, that the ambient pressure is decreased around the chamber vent hole due to some strange turbulence, then the amount of fuel that is being sucked out of the jet will decrease. Hmmm.... that sounds kind of right to me. It would seem that this vent can be found in different areas for different versions of our carburators. Could this in fact be the root cause of the lean carb jet issue?
]
Click to expand...

FYI: This is how Stromberg adjusted the mixture on the old NAS-3A1 carb used on the Continental 65 engine in the Cub, Luscombe and Aeronca in the 30's and 40's. They had a series of different size holes in two plates, one stationary and one that rotated with the mixture knob that regulated vacuum (manifold pressure) to put a suction on top of the fuel in the float bowl. This was done so that the fuel drawn through the main jet by the venturi could be regulated thus leaning the mixture.

Didn't work very well and later had an AD that called for disconnecting the mixture control and wiring the lever in full rich position.

Trivia for the Young Guys (and Gals!).
 
mannanj said:
FYI: This is how Stromberg adjusted the mixture on the old NAS-3A1 carb used on the Continental 65 engine in the Cub, Luscombe and Aeronca in the 30's and 40's. They had a series of different size holes in two plates, one stationary and one that rotated with the mixture knob that regulated vacuum (manifold pressure) to put a suction on top of the fuel in the float bowl. This was done so that the fuel drawn through the main jet by the venturi could be regulated thus leaning the mixture.

Didn't work very well and later had an AD that called for disconnecting the mixture control and wiring the lever in full rich position.

Trivia for the Young Guys (and Gals!).
Click to expand...

Hey! I'm young but I know that! :) Same on the ole Ercoupe 415C.

Anyway, we are noticing that on our C177, after installed Powerflow 4n1 exhaust, much higher initial temps. So exhaust could be a factor. On the other hand, crossovers aren't exactly new, or unique to RV's....
 
Had a long talk with Larry Vetterman yesterday. He is pretty familiar with these phenomena and says the leanness on RV's is caused by the ram air effect on the float bowl vents of most 320 carburetors. He says the leanness kicks in at about 135 knots of airspeed and stays after that. He says the test for this is to climb and dive the aircraft, while maintaining the same power setting. If EGT readings increase and decrease with airspeed, the float bowl vent is being effected buy the ram air change as the airspeed changes. He feels that due to the design of Vans air-box and its ability to deliver some ram air effect to the carburetor; this is what causes the issue. Maybe this is why RV?s with O-320?s seem to be leaner then O-320's in other aircraft. I know in the test cell we can manipulate the air-box, we use on the 320's, and change the fuel flows we get at the same power settings with the same engine, so there seems to be some validity to what he says. Maybe it is also why some seem to exhibit the issue while others don't. In the certified world many of the air-boxes have a grid at the carb inlet portion of the air-box, of maybe 1/4 to 1/2 squares that diffuse the airflow going into the carburetor. This may combat the issue. Larry said they had on guy that rigged a butterfly valve and duct on the back of his air-box and while flying, he would operate the duct open and closed. He could change the fuel flow doing that. So he opened the duct relieving the ram air effect when he didn't want the leanness and closed it when he was leaning.
Anyway thought I would pass this along. You guys might try his test and see what results you get.
Good Luck,
Mahlon
?The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."
 
Mahlon, where does the float chamber vent to the atmosphere? Are you saying the vent hole is inside the airbox, or inside the lower cowl?

I would have thought either location would result in a HIGHER pressure in the float chamber and therefore a higher differential pressure between that location and the venturi resulting in MORE fuel to the engine. Help me out please.

I am assuming that the lower cowl is running at a pressure between static and the upper cowl pressure.

I see high egt at full rated power before the wheels lift off.
 
The float vent in question is located on the inlet ramp of the carburetor air intake, pointed directly into the air flow that is coming into the carb. I guess if you pressurize this area slightly, it affects the float chamber and leans the carb out a bit.
Good Luck,
Mahlon
?The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."
 
A bigger jet.......

Alfio a.k.a. Lycosaurus and all those who have helped me. I have a larger jet on my carb now and I am delighted with the improvement. I need to do more flight testing, and perhaps its now a little too rich, but its a vast improvement.

I have tried to capture what I learned here.

I hope it is helpful to someone.
 
I just received my re-drilled jet/carb back from Bart late last week. Quick assembly (wow, did not remember there were so many parts attached to the carb), and I test flew it on Saturday. Now my max fuel flow I see is 12.8 gph. I did not have a chance to do much testing.

The original jet was .093, and it was drilled to .101". Looks like one drill size smaller than yours, Steve. Then again, the original jet was one drill size smaller than yours as well.

More to come...
 
Little boys gallons....?

Lycosaurus said:
Now my max fuel flow I see is 12.8 gph. I did not have a chance to do much testing.
Click to expand...

Alfio, Did we teach you right in Canada and that is Imperial gallons, or are they just lilly livered little American ones? :)

The reason I ask is that 12.8 is less than the book number for a Lyco at 160hp and we are looking for more. At 59LPH I am now sucking up about 15.5USG, though its probably on the rich side now.
 
Little American ones ... gallons that is.

Steve Sampson said:
Alfio, Did we teach you right in Canada and that is Imperial gallons, or are they just lilly livered little American ones? :)
Click to expand...

Canada ditched the gallons all together, and went with Liters (metric) a while ago. In aviation however, I decided to stick with gallons (US), kts, and pounds. Old school I guess (why am I called Lycosaurus?). Only air temps are in Celsius (go figure....)

I need to verify the fuel flow accuracy. I should be getting at least 13.5 gph according to the Lycoming engine manual. I do have a fixed pitch, and don't get full rpm (drag, climb etc) at this point. I suspect you have a constant speed on your setup, which allows you to go 2700 rpm.

My takeoffs are at around 2250 rpm. I don't have any gear leg fairing, pants (spats for you ...) or intersection fairings. Difficult for me to achieve 2600 rpm propeller redline. Will be doing more testing at altitude, and perform the lean-out peak test to see how much richer my mixture is than peak.
 
Volare Conv Kit?

Go to the Aircraft Spruce website and search for "666-660". It's labeled "Volare Conv Kit" and I am pretty sure it is a re-jet kit. The Corona store is supposed to be checking what carb it's for, they just started carrying it, and didn't have any in stock. It was listed in the online catalog only. It may be for the MA 4-5 carburetor.
 
Float SB

Just wanted to add... while I had the carb out at Aerosport, I asked them to look in to the float SB. Mine was affected, so I asked for brass floats. Bart at Aerosport agreed that these brass floats are fine, especially if you intend to use mogas at some point. They can also be adjusted more easily.

Recommend that anyone opening up the carbs check into this service bulletin, and decide whether to comply or not at that point. Heard of some people having their floats fill with fuel (new carb).
 
Alfio,

I was one of those people who had carb filled with fuel after 136 hrs. Bart replaced them with plastic floats. So far I have no problem.
 
Ted, I did not remember that you had a problem with the floats. I had heard of other people on this board that had some issues as well. Did not want to find out the hard way... given that other people have had real-life experience with this, I decided to change them out.

I basically paid $100 for the brass floats, since I believe they will fit my application better. I prefer the devil I know .... [edit: confirmed that I have new Kelly Aerospace floats now. They are not covered by SB]

Issue with the brass floats is you have to have them installed properly (no opportunity to rub on carb bowl), and when the carb is serviced, need to verify for wear. Then again that applies to plastic floats as well, you would think. [edit: confirmed that I have new Kelly Aerospace floats now. They are not covered by SB]
 
Last edited:
I had fuel leaked down my airbox before I did any carb mod. I thought that I over pumped the throttle (accelerator). Took me a little while to realize it was due to the carb bowl overflow. Nothing bad actually happened before I called Bart. I only remember that brass and plastic both had their own problems. Just don't remember what they are.
 
Lycosaurus said:
Just wanted to add... while I had the carb out at Aerosport, I asked them to look in to the float SB.
Click to expand...

Alfio, the carb SB has gone over my head, and I am having difficulty finding it.This is my float, clearly its not brass and I wouold not have thought delrin. Is this what I am meant to have? Can it take mogas do you know? Is there a downside to it?

Thanks
 
Steve Sampson said:
... clearly its not brass and I wouold not have thought delrin. Is this what I am meant to have? Can it take mogas do you know? Is there a downside to it?...
Click to expand...

Steve,

You may want to check the VAF archives. I believe someone referred to the problem floats translucent or black (see a further post on this thread for Brian Chesteen's post). The 'smurf blue' seems to be the latest one according to Brian. Seems older ones soften up even with avgas, as evidenced by other VAF members, and will fill up with fuel .... not good. New plastic floats are supposed to be better, but I just don't know what the compatibility with mogas will be. It may just be fine, but I don't think they would test for it anyway.

The brass floats had some issues whereby they were worn, and in that state, could rub agains the bowl sides I believe. Anyways, it caused them to leak. There seemed to be a limited number of such cases, however a SB was issued. Again, this "because of the long life of these carburetors...", the hinge point was worn, causing issues.

Read the SB below:

http://www.lycoming.textron.com/support/publications/service-bulletins/pdfs/SB582.pdf

Upon my suggestion to replace the floats with brass, Bart confirmed that brass would be my best bet for mogas use (I trust his expertise and judgment). It cost me $100 or so ... small price to pay (0.1 AMU). [edit: confirmed that I have new Kelly Aerospace floats now. They are not covered by SB]

Sorry for diverting this lean/rich jet mixture discussion, however floats that sink (partially) will probably cause a mixture change, and if opening up the carb to rejet, then this SB should be looked at as well. There..... that's my justification :)
 
Last edited:
Alfio,

The SB actually requires to replace BOTH the brass and plastic with a foam float! When my original carb was flooded in 4/07, Bart replaced it with another carb. When I looked at the photo of the float during my carb mod, my floats look exactly the same as Steves. They don't look like foam! I think neither of us are compliant with the SB!
 
tc1234c said:
The SB actually requires to replace BOTH the brass and plastic with a foam float!
Click to expand...

You are correct, Ted. SB says to replace both types of floats. This where-in lies the beauty of experimental aircraft, in that you can use judgment and assess your own risk.

The issue with the brass float was wear over the extended life of the carburetors. This wear should have been caught at overhaul time, and not have been placed back into service. I believe the affected units were limited in numbers (I recall reading this somewhere but lost track of the reference). In hindsight, the brass floats were much better (low failure rate) than what was used to replace them. With the benefit of that hindsite, and Bart's opinion, I am back with brass floats. Don't know how the new plastic floats will behave over time .... so, better the devil you know. [edit: confirmed that I have new Kelly Aerospace floats now. They are not covered by SB]


For people reading this out there ... I am not a carb expert (by any stretch of the imagination). Contact your engine builder .... or maybe Mahlon can chime in with his opinion.



Edit:
Brian C. on VAF thread indicates the the new ones are supposed to be smurf blue​

More good info on that thread...​

Incidently, I checked with Aerosport as to what brass float was installed: Kelly Aerospace PN CF666-915​
 
Last edited:
Lycosaurus said:
I believe someone referred to the problem floats as 'smurf blue' which looks like yours, I guess. Seems they soften up even with avgas, as evidenced by other VAF members, and will fill up with fuel .... not good. New plastic floats are supposed to be better, but I just don't know what the compatibility with mogas will be. It may just be fine, but I don't think they would test for it anyway.
Click to expand...

The Smurf Blue float is the latest design and what is recommended by the SB/Volare.

The hollow translucent white plastic ones and the solid foam ones (I think these are black) that are not Smurf Blue are the problem children along with the certain brand of brass ones.

The problem with the brass one is that unless you know for sure what manufacturer made the thing, you have no idea if you should replace it or not.
 
Last edited:
Thanks for chiming in, Brian, and correcting me on the smurf blue floats. I have corrected my previous posts accordingly so my posts don't induce others in error before they get to read yours.

My new float is Kelly Aerspace... for which the SB should not apply.

[edit] ... and here's the SB from Kelly Aerospace, if you happen to have one of their brass floats

http://www.kellyaerospace.com/service_pdf/SL027_A_brass_float.pdf
 
Last edited:
Lycosaurus said:
My new float is Kelly Aerspace... for which the SB should not apply.
Click to expand...
thumbs_up.jpg
 
Lycosaurus said:
Thanks for chiming in, Brian, and correcting me on the smurf blue floats. I have corrected my previous posts accordingly so my posts don't induce others in error before they get to read yours.

My new float is Kelly Aerspace... for which the SB should not apply.

[edit] ... and here's the SB from Kelly Aerospace, if you happen to have one of their brass floats

http://www.kellyaerospace[/INDENT].com/service_pdf/SL027_A_brass_float.pdf
Click to expand...

Alfio,
Your web link didn't work for me. Is it just me, or is there an error in the link?
Charlie Kuss
 
Link fixed to previous post

chaskuss said:
Alfio,
Your web link didn't work for me. Is it just me, or is there an error in the link?
Charlie Kuss
Click to expand...

Fixed the link ... as mentioned, I had an indent in there. Sorry about that ....
 
Last edited:
Mooney Mod Richer Jet Kit Available From AS&S

Spruce has it as part number 08-06512, available online only.
 
Lycosaurus said:
.... The original jet was .093, and it was drilled to .101" ...
Click to expand...

Update: I finally did the 75% power at 8500 ft. last week, and found I could lean 150 degF from full rich until first EGT peak. I'll need to repeat that test a couple of more times, however things are looking good. Looks like Bart hit the nail on the head.
 
You must log in or register to reply here.
Back
Top