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First flight with new (to me) engine - possible detonation event?

Hey Dan, The things I've read as suggestions in this thread are to retard your timing a bit, perhaps down to 23 degrees, and to check for intake leaks.

Another thing to look it is do your mixture and throttle cables move fully to the stops on the carburetor in each direction?

I think technically it is possible to check flow in a carb on the bench, but you probably need something that will move a measured amount of air through it as well, which is probably going to be quite a job to construct.

You might have mentioned this, but are you running 100LL? Is the fuel "fresh"?

And as usual, Kitplanes has some good advice in this article:

https://www.kitplanes.com/carburetor-maintenance/

I’ve tried both 25 and 20 deg timing on both, and encountered thermal runaway in both cases (up to 1.5 dF/sec CHT rise). I had fresh 100LL in the tanks, nothing mixed in that I know of. As for intake leaks, the engine idles smooth at approximately 12” MAP with the throttle closed, and I have SDS intake flanges installed. Both throttle and mixture controls were verified to go to both stops.
 
I’ve been brainstorming how I want to tackle the troubleshooting, and am wondering if there is a way to me to bench test the carburetor flow myself? I haven’t come up with anything in my searches thus far.

Another potentially contributing factor - the other day when I ran it up to 7500’ for the full throttle run I noticed my fuel pressure had dropped to about 1.5 PSI. On the ground with the electric pump off it was running at 3-4 psi. Turning the electric pump on would cause the pressure to jump to 5-6 psi, but there was no change in EGT or fuel flow. My old engine ran pretty steady on fuel pressure, so is this something else to investigate? My gage is mechanical, so obviously nothing electrical to check.

1.5 psi is fine and not that uncommon. All that matters si raw flow and the BP experiment showed that th volume was fine.

No easy way to flow test a carb for fuel delivery. Most testers only measure air flow, not fuel delivery. It may be worthwhile to tear into it and look for issues. Have you pulled and checked the screen in the carb? Have you confirmed that the mixture arm locking screw is tight and not slipping on the shaft? Probably worth the effort to drill out the main jet a bit and see if FF goes up at WOT. Can also borrow a carb with known good flow and test it on your engine and see if FF goes up.
 
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I’ve tried both 25 and 20 deg timing on both, and encountered thermal runaway in both cases (up to 1.5 dF/sec CHT rise). I had fresh 100LL in the tanks, nothing mixed in that I know of. As for intake leaks, the engine idles smooth at approximately 12” MAP with the throttle closed, and I have SDS intake flanges installed. Both throttle and mixture controls were verified to go to both stops.

that pretty well rules out intake leaks. I would recommend a borescope by a pro. You should be looking for "hot spots" that could become glowing and create pre-ignition. That said, fuel delivery makes more sense, as you have detonation signatures on all cylinders (missing deposits on crowns), eventhough only 2 cyl's are showing mild pre-ignition (melted Alum on plug). Severe detonation can lead directly to pre-ignition (where the serious damage occurs) without hot spots.

Do you have mags or EI? Any chance a mistake was made timing these? Extreme advance will absolutely create detonation and would likely cause it on all four cylinders. Then again, so would a lean mixture.
 
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Have you pulled and checked the screen in the carb? Have you confirmed that the mixture arm locking screw is tight and not slipping on the shaft? Probably worth the effort to drill out the main jet a bit and see if FF goes up at WOT.

All things I can check when I get the chance to work on it next. I was hoping to avoid drilling the jet for now since it’s already a fat jet (.1065, on paper should be flowing north of 15 GPH).

that pretty well rules out intake leaks. I would recommend a borescope by a pro. You should be looking for "hot spots" that could become glowing and create pre-ignition. That said, fuel delivery makes more sense, as you have detonation signatures on all cylinders (missing deposits on crowns), eventhough only 2 cyl's are showing mild pre-ignition (melted Alum on plug). Severe detonation can lead directly to pre-ignition without hot spots.

Do you have mags or EI? Any chance a mistake was made timing these? Extreme advance will absolutely create detonation.

Slick mags, I timed them and then had that verified by the field A&P as an idiot check prior to first start. I unfortunately wasn’t able to verify if those deposits on the picture of the plugs were lead or aluminum. That said, based on the data traces for both flights I’m not seeing a pre-ignition signature. Per my Sky Ranch manual, detonation would show as an increase in CHT with a flat EGT peak (present on all four cylinders in the traces at initial takeoff, 06:30-07:15 and 05:00-06:30 time stamps, respectively). Conversely, pre-ignition would cause a noticeable drop in EGT, which I don’t see on the traces prior to where MAP also drops when I closed the throttle.
 
All things I can check when I get the chance to work on it next. I was hoping to avoid drilling the jet for now since it’s already a fat jet (.1065, on paper should be flowing north of 15 GPH).



Slick mags, I timed them and then had that verified by the field A&P as an idiot check prior to first start. I unfortunately wasn’t able to verify if those deposits on the picture of the plugs were lead or aluminum. That said, based on the data traces for both flights I’m not seeing a pre-ignition signature. Per my Sky Ranch manual, detonation would show as an increase in CHT with a flat EGT peak (present on all four cylinders in the traces at initial takeoff, 06:30-07:15 and 05:00-06:30 time stamps, respectively). Conversely, pre-ignition would cause a noticeable drop in EGT, which I don’t see on the traces prior to where MAP also drops when I closed the throttle.

To me the melted alum on the plug is a pre-ignition signature. Not an expert in detonation, so it is possible that detonation severe enough can cause melting aluminum. I just don't have enough experience to say either way. That said, I was always taught that molten metal deposits on the plug was a sign of PI. Whether it be detonation or PI, if the combustion gasses are getting hot enough in the combustion chamber to melt aluminum, I am not convinced that EGTs will be on the low side. Again, no real world experience with PI on an EGT instrumented engine and have no reason to not trust your source. However, it just doesn't make sense to me. Yes, typically more advance = lower EGT, but we are dealing with extreme temps WAY above normal if we are melting metal.

Here si an FAA presentation on the subject

https://www.faasafety.gov/files/notices/2019/Jul/Preignition.pdf

They say PI causes both to spike and detonation causes High CHT and low EGT
 
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Did the impulse coupler snap before you backed up the prop to set the timing?
Also, are the carburetor gaskets sealing well?
 
To me the melted alum on the plug is a pre-ignition signature. Not an expert in detonation, so it is possible that detonation severe enough can cause melting aluminum. I just don't have enough experience to say either way. That said, I was always taught that molten metal deposits on the plug was a sign of PI. Whether it be detonation or PI, if the combustion gasses are getting hot enough in the combustion chamber to melt aluminum, I am not convinced that EGTs will be on the low side. Again, no real world experience with PI on an EGT instrumented engine and have no reason to not trust your source. However, it just doesn't make sense to me. Yes, typically more advance = lower EGT, but we are dealing with extreme temps WAY above normal if we are melting metal.

Here si an FAA presentation on the subject

https://www.faasafety.gov/files/notices/2019/Jul/Preignition.pdf

They say PI causes both to spike and detonation causes High CHT and low EGT

I think at this point the best I can do in this regard is see if I can find any missing aluminum with the borescope. I imagine it would manifest as pitting but I’m not sure. I had been ground running the engine for short intervals several times earlier in the day, which is why I first just assumed it was lead fouling.

Did the impulse coupler snap before you backed up the prop to set the timing?
Also, are the carburetor gaskets sealing well?

Yes, I snapped the impulse coupling then backed it up, then forward again to set the timing. I have no reason to believe the carb to sump isn’t sealing.
 
Engine

I haven't seen any mention of spark plug heat range. Replace the lower plugs with the next colder heat range.
What is the history of the fuel hoses form the firewall to the pump and then to the carb? If any doubts replace. An internally damaged rubber hose could cause reduced fuel flow under pressure that would be below what is seen with a ground test. A flap of rubber inside the hose. that only obstructs fuel flow under pressure.
Remove the fuel flow cube and inspect carefully for damaged or missing blades on the impellor.
 
Since you say you already have an oversize jet, I lean towards something between the tanks and the carb. Did you perform the *airframe* fuel flow test I recommended a few pages back?
 
All things I can check when I get the chance to work on it next. I was hoping to avoid drilling the jet for now since it’s already a fat jet (.1065, on paper should be flowing north of 15 GPH).

WHole bunch more than the jet size to determine flow. The carb is a somewhat complex set of physics principles. First, the mixture vane needs to be fully open (it is a choking system designed to reduce fuel flow out of the bowl). Fuel level in the bowl needs to be at the correct height (i.e. various float & needle/seat issues). Air bleeds need to be fully open and not restricted, Venturi needs to be intact (some used 2 pieces pressed in and can separate, spoiling the proper pressure reduction necessary to pull fuel through the jet), entry at base of bowl needs to not have debris restricting flow, Upper gasket needs to be properly sealed, etc, etc. Given the feeling that a lean mixture is relatively high probability for causing these issues, it is probably worth the effort to have a pro go through the carb. Obviously a quick ground test to insure that the system feeding the carb is pushing adequate flow first is recommended.
 
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Engine

This is not a new engine. 200 hours on a Pitts. What has changed?? THE SPARK PLUGS!!!!! PLEASE install some new plugs, at least on the bottom, before you spend $600 to 1200 on carb work on a carb that is probably fine.
At least one heat range colder on plugs.
 
This is not a new engine. 200 hours on a Pitts. What has changed?? THE SPARK PLUGS!!!!! PLEASE install some new plugs, at least on the bottom, before you spend $600 to 1200 on carb work on a carb that is probably fine.
At least one heat range colder on plugs.

He is running aviation plugs; Pics in first post. Don't even think they sell those in a hot enough range to cause detonation or PI in a Lyc. OP never mentioned new plugs in any of the posts. Just that he took them out and cleaned them. Maybe I missed something.
 
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This is not a new engine. 200 hours on a Pitts. What has changed?? THE SPARK PLUGS!!!!! PLEASE install some new plugs, at least on the bottom, before you spend $600 to 1200 on carb work on a carb that is probably fine.
At least one heat range colder on plugs.

He is running aviation plugs; Pics in first post. Don't even think they sell those in a hot enough range to cause detonation or PI in a Lyc. OP never mentioned new plugs in any of the posts. Just that he took them out and cleaned them. Maybe I missed something.

The plugs are from my old engine - they are Tempest UREM38S. They were running fine previously so as part of the engine installation I cleaned and rotated them into the new engine. Both flights I have performed with the current engine were with these plugs.

As far as how it ran on the Pitts, there was no engine monitor. It was previously run with UREM38E plugs (Massive electrode equivalent to the fine wires I installed) and a pressure carb.
 
Plugs

Nothing in my post could be interpreted as my saying these were not aviation plugs.
My stock compression 0 360 Pitts with REM40 plugs would VERY rarely shake a bit on a long climb on a hot day. REM 38 plugs cured that.
One popular Continental engine goes from #38 plugs to #32 plugs for a relatively modest increase in compression.
The simplest thing for the OP would be to ream the carb jet. He seems to not want to do that. The next simplest would be the plugs.
 
With 10:1 comp O-320, I'd be looking for 15 GPH at WOT on takeoff/ climb

Absolutely! Years ago, when I bought a standard new certified 0-320 I had redlined CHT on first takeoff. Lycoming suggested I leaned it during climb etc , etc. LAME had no answer. Always had to do stepped climbs & 120 kts to get the temps lower.
Going forward a few years someone asked what my takeoff fuel burn was, it was 10.5 gals/hr whereas it should have been ~13 gals/hr - go figure.
As OKAV8r says your higher compression engine will need more fuel at max power.
This has been stated many times - If the fuel flow &/OR timing are incorrect you will have issues.;)
 
As someone eluded to 0320s notoriously run very lean in rvs. The carburetors were intended for let's say a Cessna with very restrictive take and very restrictive exhaust. The Vans filtered air box and the straight pipes or crossovers have much less restrictions and you need a larger Main jet. In general what I did on my rv4 was drill it out incrementally. If you have a fixed pitch prop you can run the engine up on the ground to let's say 2300 RPM. And start leaning. Slowly. The rpm should increase slightly. If the RPM just starts dropping at a certain point without rising first you're probably too lean and need larger jet.
 
Ok, I will bow out of this and leave it to others. You guys keep saying he is not flowing enough fuel and he should be drilling out the jet, but his first post says that he flowed 12 GPH at 2200 RPM/25". A simple look at a Lyc chart shows this to be well above the minimum required fuel. AND he is detonating at that power; also uncommon. He also doesn't have a stock jet. He has already up-sized to the jet designed for 340 CI. Others say the 38 plugs are the problem, but there are lots of guys here running those same plugs with all sorts of aggressive CR's. Others are saying high CRs need more fuel. Simply not true; The amount of air ingested is the same (dictated by displacement, not CR). CR just determines how much the ingested air is compressed. Higher CRs DO create more detonation potential and therefore a need to deal with that. While more fuel is one way to decrease detonation risk in high CR engines, most do this with reduced timing and NOT excessive fuel and the OP has already dropped down to 20* of advance.

Happy to answer PMs, but checking out of this one.
 
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There are a lot of opinions on fuel flow for takeoff. I have always used the 9% of HP rule. The OP’s 320 is probably putting out over 160 HP. Here is what Savvy says.
For a normally aspirated fuel injected engine designed to run on 100-octane fuel (8.5-to-1 compression ratio), takeoff power fuel flow in GPH should be roughly 9% of the engine’s maximum rated horsepower. (For example, an IO-520 rated at 285 horsepower should flow about 25 to 26 GPH.)
 
Fuel Pump

The fuel pump for the Bendix pressure carb on the Pitts is a medium pressure pump. The Marvel carb requires a low pressure pump. I don't know if this is the problem but there is a reason why fuel pumps of three different pressure ratings are available for these engines. The high pressure pump is for the Bendix fuel injection.
As far as fuel flow people seem unable to understand the significant difference between 2700 r/m at approx 29" with a constant speed prop and the 2200 r/m the OP is getting at start of takeoff with fixed pitch prop!!!!
I have a near identical setup but with gravity flow. My cowl is much tighter than RV. No carb mods. CHT 400 degrees at 130 knot climb. I do have a cowl flap and a plenum. Timing 22 degrees for break in then 25 degrees.
 
So that everyone is clear on what fuel flow I experienced, here it is from the data:

Ground static runup, full throttle: 28.3" MAP, 2209 RPM, 11.1 GPH

In flight, approximately 1,000MSL on climbout, full throttle: 27.4" MAP, 2685 RPM, 12.5 GPH. It should be noted that fuel flow reached 12.5 GPH at approximately 2550 RPM and 27.9" MAP (still full throttle). This appears to have been its peak. Throttle was retarded after fuel flow stabilized to control the thermal runaway.

In flight, approx 7100' PA, full throttle, full rich: 21.5" MAP, 2730 RPM, 11.5 GPH.


As stated above, the carburetor had previously been modified with a new factory jet sized for a stroker engine, prior to first run I assumed this would lend itself to being sufficient for the higher compression ratio.

The engine driven fuel pump is not outputting what I would expect a medium pressure pump to output, as stated in an earlier post. On the ground its outputting around 4 PSI, which is what I would expect the proper low pressure pump to output, unless I'm very confused on this or have a sick pump (doubtful, given that turning the boost pump on doesn't cause an increase in FF or decrease in EGT).

This weekend I intend to run a flow test of the airframe fuel supply system, as well as checking the carb inlet screen. I have a gascolator that is installed just prior to the engine driven pump that I serviced as part of the engine installation; no debris was found on that screen. Of couse, the elephant in the room is did I buy myself at top overhaul with those two minutes of hot CHTs; I'm working to get a second set of eyes on the borescope to determine that.
 
So that everyone is clear on what fuel flow I experienced, here it is from the data:


In flight, approximately 1,000MSL on climbout, full throttle: 27.4" MAP, 2685 RPM, 12.5 GPH. It should be noted that fuel flow reached 12.5 GPH at approximately 2550 RPM and 27.9" MAP (still full throttle). This appears to have been its peak. Throttle was retarded after fuel flow stabilized to control the thermal runaway.

From your original link to the Savvy data at https://apps.savvyaviation.com/flights/6630447/55228b29-cb24-4fee-98c7-f00b726832bc
Right at the run away point there is a droop in fuel flow at the 7:02 mark. RPM is flat at 2200, but the fuel flow declines, and then peaks back up again. At the dip the flow is down to 10.6 gph. This assumes we can trust the flow numbers. On my installation my red cube shows higher numbers than actual flow when my boost pump is on. REGARDLESS... how do you explain the dip in fuel flow at that point?
 
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So that everyone is clear on what fuel flow I experienced, here it is from the data:

Ground static runup, full throttle: 28.3" MAP, 2209 RPM, 11.1 GPH

In flight, approximately 1,000MSL on climbout, full throttle: 27.4" MAP, 2685 RPM, 12.5 GPH. It should be noted that fuel flow reached 12.5 GPH at approximately 2550 RPM and 27.9" MAP (still full throttle).

according to Lyc chart for 320 B/D series. 2700/27" should flow a bit over 13.5 GPH. The flow for 2500/27" is a bit over 12.5 GPH. The fact that you make that at 2500 but if fails to increase with the RPM increase points less at the carb and more to a restriction upstream or a pump that cannot keep up. Regardless of jet size, fuel flow should incrementally increase with airflow (A/F goes up with RPM) - pretty much what a carb does (match fuel flow with airflow). If there is a restriction, the carb is unable to deliver flow beyond what the restriction allows, which is exactly what the mixture vane does. Still think you need to be sure the shaft screw is tight.
 
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From your original link to the Savvy data at https://apps.savvyaviation.com/flights/6630447/55228b29-cb24-4fee-98c7-f00b726832bc
Right at the run away point there is a droop in fuel flow at the 7:02 mark. RPM is flat at 2200, but the fuel flow declines, and then peaks back up again. At the dip the flow is down to 10.6 gph. This assumes we can trust the flow numbers. On my installation my red cube shows higher numbers than actual flow when my boost pump is on. REGARDLESS... how do you explain the dip in fuel flow at that point?

The only possible explanation would be the carburetor leaning slightly as the takeoff roll progressed. I’m informed that the model carburetor I have has a tendency to pressurize the bowl as ram increases, causing a minor leaning effect. On the flight linked, I performed a static takeoff. Brake release is observed at 6:44 by the slight dip in RPM (I just let RPM stabilize, not fuel flow). At the takeoff roll progresses fuel flow reaches approximately the peak I observed in the static RPM tests then slowly dips down as airspeed increases. The return to peak is likely a function of airspeed as the airplane pitched based on takeoff trim. The fuel flow falls after that corresponding with the throttle being retarded for CHT.

I don’t have an EFIS that records flight data so I can’t validate this hypothesis with numbers; I can only state my observations.
 
Let me ask something really stupid (?!)...
What model RV is this? And build year?
Do you have sloshed fuel tanks?
 
On the flight linked, I performed a static takeoff. Brake release is observed at 6:44 by the slight dip in RPM (I just let RPM stabilize, not fuel flow).

I'm completely confused by this. "I just let the RPM stabilize, not the fuel flow" During the take-off roll??? Fuel flow should be stable with stable RPM. If it isn't.... that isn't right.

And as soon as you start rolling, RPM should *increase*, not decrease. No "slight dip" should be seen.

I just wonder if you have crud in the tank that is shifting... and partially blocking the pickup. Or the pickup tube is loose. See Van's old SB.
 
Let me ask something really stupid (?!)...
What model RV is this? And build year?
Do you have sloshed fuel tanks?

I'm completely confused by this. "I just let the RPM stabilize, not the fuel flow" During the take-off roll??? Fuel flow should be stable with stable RPM. If it isn't.... that isn't right.

And as soon as you start rolling, RPM should *increase*, not decrease. No "slight dip" should be seen.

I just wonder if you have crud in the tank that is shifting... and partially blocking the pickup. Or the pickup tube is loose. See Van's old SB.

-4, 1982 tail kit, first flight 1988. The tanks are partially sloshed - the joints were painted with the orange slosh compound. When I bought the plane at the end of 2020 I pulled both tanks for leaking access panels. At the time I inspected the slosh and found it to be in good shape, no peeling or lifting. I complied with the SB to safety wire the pickup tube flare nut on reassembly. I visually inspect the inside of the tanks regularly and haven’t seen anything, nor have I seen any debris when I sump the tanks.

Not that any of that rules out something causing a blockage, but the slosh has not been a previous source of concern for me.
 
Troubleshooting update: Today I checked the carb inlet screen and found nothing. I also tested the fuel delivery while the hose was off and found the boost pump delivered 22.7 and 23 GPH from the left and right tanks, respectively. I again verified the mixture and throttle controls had full range of movement. The mixture control shaft had no play when at the full rich stop.
 
I’m informed that the model carburetor I have has a tendency to pressurize the bowl as ram increases, causing a minor leaning effect.

Dan or others, do you have anymore info. on this? Possible carb mods to counter act the bowel pressurization from ram air? Does the Vent Bushing in the Mooney modification address this on 10-3878 Carbs? Maybe this is a mod that would work on the MA4SPA carb…
 
Reinstalling my injection servo today, I had another thought. Early kits didn't provide you a good way to affix the outer jacket of the throttle and mixture cables to the engine. *** I have seen where people attached the control cable outer jacket to the ENGINE MOUNT instead of the ENGINE ***

So?

Well then the engine can move relative to the mount. It could possibly shift when you power up and pull the mixture off of the full rich stop.

It might explain the fuel flow valley we saw.

Also look for any place where vibration / wiggling can open up a gap and allow the fuel pump to suck air. I suspect the red cube will happily read bubbles as flow, giving a misleading idea of how much actual fuel is passing by.
 
At this point I’m going to pull the carb and clean/inspect. I went ahead and ordered the kit for the float SB since I know for a fact mine has an old brass float. Beyond that and cleaning it out real well, anything else to check for?

On that note, what’s the best way to clean out all the little vent passages on these style carbs?
 
At this point I’m going to pull the carb and clean/inspect. I went ahead and ordered the kit for the float SB since I know for a fact mine has an old brass float. Beyond that and cleaning it out real well, anything else to check for?
carbs?

Look for blockage of the main jet before cleaning. Confirm the main jet size.
 
I opened up the carb today and found no evidence of blockages. The main jet is .1015” diameter.

While I have it apart I’m going to go ahead and replace the needle, seat, and float (I had an old brass float still). What’s a good way to remove and install the float seat without the proprietary tool MSA sells?

An observation I also had while I was inspecting it: is it possible for the mixture control valve to be misaligned with the casting based on how it’s connected to the mixture control level? In other words, could it be mistakenly oriented in such a way that full rich on the lever isn’t actually full rich at the valve?
 
Is the main nozzle from your carb indeed the 47-862 with .1015 hole? Can you show a picture of it with the side holes? Wondering if it has more side holes than the stock 3678-32 nozzle which has .1065” hole?

I talked to MS and they did not think the O340 nozzle would help my mixture issues. If it has a .1015 dia. hole, then I guess it wouldn’t…
 
Reinstalling my injection servo today, I had another thought. Early kits didn't provide you a good way to affix the outer jacket of the throttle and mixture cables to the engine. *** I have seen where people attached the control cable outer jacket to the ENGINE MOUNT instead of the ENGINE ***

So?

Well then the engine can move relative to the mount. It could possibly shift when you power up and pull the mixture off of the full rich stop.

It might explain the fuel flow valley we saw.

Also look for any place where vibration / wiggling can open up a gap and allow the fuel pump to suck air. I suspect the red cube will happily read bubbles as flow, giving a misleading idea of how much actual fuel is passing by.

This is actually a good thing to check. I have seen set-ups where people do not even clamp the outer housing at all, and just rely on the compressive stiffness of the housing from the other end to allow the inner cable to move.

So, just check that you have the mixture and throttle cable housings correctly anchored to the engine.

digesting all the comments here, it does seem like you are running a bit lean. One person mentioned air bubbles getting into the fuel lines, and this has been an issue that is hard to troubleshoot for others in the past. A slightly loose fitting or a cracked fuel line can do it. Even a crack in a tubing flare. The standard Vans fuel selector valves are known to leak air in through the valve shaft when they get old. Do you have a gascolator with a glass bowl. Question for others: would you expect to be able to see bubbles in the bowl of the gascolator if they were present?


One other piece of the puzzle that it doesn't seem like anyone has commented on is why, if you are running too lean, would you be getting such badly fouled plugs? The plug temperature range seems right. In your very first description, it seems like the fouling happened pretty fast, or perhaps was fouled from your previous ground running.
 
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Is the main nozzle from your carb indeed the 47-862 with .1015 hole? Can you show a picture of it with the side holes? Wondering if it has more side holes than the stock 3678-32 nozzle which has .1065” hole?

I talked to MS and they did not think the O340 nozzle would help my mixture issues. If it has a .1015 dia. hole, then I guess it wouldn’t…

See my post from when I first installed that nozzle; it has a picture.

https://vansairforce.net/community/showpost.php?p=1518984&postcount=34
 
This is actually a good thing to check. I have seen set-ups where people do not even clamp the outer housing at all, and just rely on the compressive stiffness of the housing from the other end to allow the inner cable to move.

So, just check that you have the mixture and throttle cable housings correctly anchored to the engine.

digesting all the comments here, it does seem like you are running a bit lean. One person mentioned air bubbles getting into the fuel lines, and this has been an issue that is hard to troubleshoot for others in the past. A slightly loose fitting or a cracked fuel line can do it. Even a crack in a tubing flare. The standard Vans fuel selector valves are known to leak air in through the valve shaft when they get old. Do you have a gascolator with a glass bowl. Question for others: would you expect to be able to see bubbles in the bowl of the gascolator if they were present?


One other piece of the puzzle that it doesn't seem like anyone has commented on is why, if you are running too lean, would you be getting such badly fouled plugs? The plug temperature range seems right. In your very first description, it seems like the fouling happened pretty fast, or perhaps was fouled from your previous ground running.

My engine controls are all mounted to the engine; they run through clamps that are attached to a plate that fits under the rear two carb studs and also has a bracing bar running up to a sump bolt.

Regarding a potential air leak, that would be quite the rabbit hole. I obviously had the FWF fuel lines apart for replacement, I serviced my metal bowl gascolator (placement required removal of the entire assembly), and I also took apart my fuel selector to lube it (it’s an old brass three way valve). Wouldn’t a leak manifest when the system is pressurized with the boost pump?

As for the plug fouling, the engine had been running on the ground extensively earlier in the day (chasing a minor oil leak, ended up being the vernatherm gasket); I’m 99% sure that’s the explanation.
 
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47-773Nozzle.jpg
This is my Nozzle 47-773 with .1065" hole. It flows 13.5 gph full rich. I think I might add the small holes like your 47-862 nozzle to help atomization for better fuel distribution.

If your nozzle has a .1015 dia. hole, I think that is probably the explanation for your low fuel flow. Mine at 13.5 gph seems lean but I need to do the testing as Larry suggested. Just need some decent weather!

My fuel pressure fluctuates like yours also. Maybe that has to do with the float bowel pressure from the higher ram air...?
 
Regarding a potential air leak, that would be quite the rabbit hole. I obviously had the FWF fuel lines apart for replacement, I serviced my metal bowl gascolator (placement required removal of the entire assembly), and I also took apart my fuel selector to lube it (it’s an old brass three way valve). Wouldn’t a leak manifest when the system is pressurized with the boost pump?

Nothing upstream of the boost pump is pressurized... so, in order, there could be a cracked flare in the tank pickup, on the tank outlet, on the tank selector valve, on the valve itself, the valve shaft, or even on the boost pump inlet. Any of those spots could suck air. I still say, from the Savvy graph, that there is an unexplained droop in the fuel flow just prior to the event. Which makes me suspect things that could move, vibrate, or shift in flight. Fuel pickup? Maybe sucking air somewhere and at first the fuel in the bowl hides the issue? I think this is a transient lean problem which means it could run rich and sooty 90% of the time...
 
I disassembled the carb and cleaned it out as best I could; I did not discover any blockages. I reassembled it with a new float, needle, and seat. Float height was set to 7/32”.

I did a static run up just now and the engine stabilized at 10.8 GPH @ 28” MAP and 2191 RPM. OAT was 76*F.

It still seems a little low, but I’m not sure. I’m hesitant to try flying it and just hope the fuel flow increases with RPM this time given the prior issues.
 
First flight today after cleaning out the carburetor, replacing the float, seat, and needle, and opening the jet to .1065".

https://apps.savvyaviation.com/beta/shared/flight/6761178/94297881-f4ef-48b1-81af-c211f1844dbd

It was a bit cooler today (68*F) and the engine ran warmer than I had hoped, but at least no thermal runaway events like previously. On takeoff I quickly accelerated to 2500 RPM with a flatter pitch attitude and fuel flow peaked at 13.1 GPH before falling off as I climbed. CHTs all stabilized at a tad over 400* in the climb to 7500' PA (I was full rich the entire time). Once at altitude I let the aircraft accelerate and cool off (still full rich) before doing some GAMI sweeps. Mixture distribution at full throttle was disappointing, with a 1.8 GPH average spread. CHTs were hanging out around 380-390 the whole time and would just top 400 as I passed through best power doing the GAMI sweeps. Full rich to peak EGT showed low 200* spread on #1/2 and about 150* on #3/4.

Based on the EGT spread at altitude I'm inclined to say it's running "rich enough" but I'm not sure. Temperatures were still a bit warm given the low OAT (40* F at 7500') and I'd love to get the CHTs below 400 in the climb. More concerning however was oil temperature. At altitude it stabilized at almost 230* F. In limits, certainly, but with absolutely no margin for the impending summer temperatures.

At this point, I feel the aircraft is safe to test fly but still needs optimization. While I had the carburetor off and was waiting for parts I replaced all the old baffle seals with new silicon and sealed up every bit I could find that wasn't going through a fin. I think my baffles look good but I'm at the point I probably need a more experienced eye to see if I can make them any better. Of course, more fuel may also be in order, particularly if I ever want to advance the timing from 20*. For oil temperature I'll verify there are no blockages causing the cooler to be bypassed, but otherwise I'm open to suggestions.
 
First flight today after cleaning out the carburetor, replacing the float, seat, and needle, and opening the jet to .1065".

https://apps.savvyaviation.com/beta/shared/flight/6761178/94297881-f4ef-48b1-81af-c211f1844dbd

It was a bit cooler today (68*F) and the engine ran warmer than I had hoped, but at least no thermal runaway events like previously. On takeoff I quickly accelerated to 2500 RPM with a flatter pitch attitude and fuel flow peaked at 13.1 GPH before falling off as I climbed. CHTs all stabilized at a tad over 400* in the climb to 7500' PA (I was full rich the entire time). Once at altitude I let the aircraft accelerate and cool off (still full rich) before doing some GAMI sweeps. Mixture distribution at full throttle was disappointing, with a 1.8 GPH average spread. CHTs were hanging out around 380-390 the whole time and would just top 400 as I passed through best power doing the GAMI sweeps. Full rich to peak EGT showed low 200* spread on #1/2 and about 150* on #3/4.

Based on the EGT spread at altitude I'm inclined to say it's running "rich enough" but I'm not sure. Temperatures were still a bit warm given the low OAT (40* F at 7500') and I'd love to get the CHTs below 400 in the climb. More concerning however was oil temperature. At altitude it stabilized at almost 230* F. In limits, certainly, but with absolutely no margin for the impending summer temperatures.

At this point, I feel the aircraft is safe to test fly but still needs optimization. While I had the carburetor off and was waiting for parts I replaced all the old baffle seals with new silicon and sealed up every bit I could find that wasn't going through a fin. I think my baffles look good but I'm at the point I probably need a more experienced eye to see if I can make them any better. Of course, more fuel may also be in order, particularly if I ever want to advance the timing from 20*. For oil temperature I'll verify there are no blockages causing the cooler to be bypassed, but otherwise I'm open to suggestions.

Unfortunately a full rich 150-200 ROP at 7.5K doesn't necessarily translate to a full rich 150-200 ROP at sea level. 13.1 sounds about right for 29"/2500, but you could look at the Lyc table to confirm that. If you are getting near 400* CHT in cruise at 150* ROP, I speculate that you have some issues with your baffling.

Larry
 
Here’s some pictures of my current baffle seals. When I put the cowling on and run a borescope inside it appears they all lay flat against the top just fine. I’m not sure under pressure, of course.

The bottom cowling is all sealed up tight with lots of RTV as well as with glass-silicone wraps around the fins.
 

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