Help in Diagnosing Strange Engine Operation

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Direct C51

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To get to the point, I have a Corvair powered Sonex with about 10 hours on it. I have spent a good portion of those 10 hours diagnosing a rough running condition and am hoping I can get the collective input of the crowd here and maybe we can stumble upon something I haven't thought of.

Engine: 3.0L Corvair with all WW/SPA conversion parts, built exactly as the conversion manual suggests. WW Electronic/Points ignition. MA3-SPA carb.

Problem: Mixture full rich, DA above 3000 or so. Advancing the throttle from 2700 RPM - wide open throttle, I get rough engine operation. Reducing throttle through that same range, I get rough engine operation. 2700RPM and below, the engine runs smooth. WOT the engine runs smooth. below 3000 DA the problem is less severe to almost not noticeable. Mixture leaned aggressively, the engine runs smooth.

Troubleshooting: I have noted the #2 EGT is not responding normally. It is roughly 150 degrees F below the average of the other 5 cylinders when the mixture is full rich. The #2 EGT decreases another 100 - 150 degrees when advancing the throttle from 2700RPM to WOT, then increases at WOT. The same is noted when reducing throttle from WOT through that range, the #2 EGT decreases 150 degrees or so, then increases once below 2700 RPM. All other EGTs respond normally. When leaned aggressively, the #2 EGT will increase significantly and settle right in the average of the other 5 EGTs. Advancing or reducing throttle through the range of 2700 RPM - WOT again result in drastic decrease of #2 EGT followed by an increase once out of that range.

It seems my #2 cylinder is running very rich, or otherwise not combusting correctly. Compression check is normal at 76/80. Spark plug looks slightly richer than the others but not so much as to be a concern, and certainly not fouled. Has anyone seen these symptoms before? I'm not sure if this is a mixture issue or ignition issue. I plan to further test on the points ignition (so far I have only tested on electronic ignition, and possibly change the spark plug and wire.
 

TFF

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I would check the cap;probably change it. Plug and wire, too. No leaking gaskets? Put a timing light on it and see if you get the same advance as the other cylinders; maybe some shaft wobble in the distr. Measure cam lift. Rocker arms ok? Bad valve spring? Valve not sticking? Auto fuel or 100LL?
 

Pops

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What happens when you use carb heat ?
 

Direct C51

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I've tested multiple times with carb heat 1/2 and full. It doesn't have much of an effect. If anything it makes it slightly worse.

I'm running 100LL. A lot of those things like plug, wires, valve spring, etc should also show symptoms at 2700RPM and below and WOT right? That is what is confusing me. This only happens at 2700 RPM to just under WOT. Maybe 70% - 95% throttle.
 

TFF

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Carb heat richens the mixture so if it is already rich it would be worse. The puzzling part is good compression. Are you checking hot or cold, the compression? How is the intake manifold made; log style or individual tubes? Is this a middle cylinder?
 

Direct C51

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Differential compression checked about 30 minutes after a flight. Intake is the standard WW/SPA system. Carb near bottom of engine connects to a long Y style intake that attaches to the heads. Log style, not individual. #2 is the aft cylinder.

I guess I shouldn't say the problem is only in that range of 2700 - WOT as the #2 EGT is always 100-150 degrees lower than the rest. It is only in that range that the EGT decreases further causing the engine to run rough.
 

Direct C51

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Differential compression checked about 30 minutes after a flight. Intake is the standard WW/SPA system. Carb near bottom of engine connects to a long Y style intake that attaches to the heads. Log style, not individual. #2 is the aft cylinder.

I guess I shouldn't say the problem is only in that range of 2700 - WOT as the #2 EGT is always 100-150 degrees lower than the rest. It is only in that range that the EGT decreases further causing the engine to run rough.
 

Toobuilder

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So it sounds like #2 is the bad actor. Some of the Lyc 320 carbed engines have pretty poor cylinder mixture disribution too, but thats a known characteristic of those with that sump. If your induction system is built like all the other Corvair guys and you are the only one with the issue then it would make sense that you can focus on #2 first. I'd agree with the ignition system: plugs, cap, wires. Also look for good sealing of all gaskets, hoses, etc, and check valves. Lift, broken springs, dropped guide, etc.
 

Pops

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So we know with the darker #2 plug plus using carb heat and lower EGT temps that #2 is running rich for sure. Next I would look at the ignition system. If you are using the auto distributor, I would be looking at the hi-voltage side as in a cracked cap, plug wires and plug. Sometimes a crack in the cap can be very hard to see and a new plug can be bad. If you don't have a new plug on hand switch plugs and see if the trouble moves with the plug.
Have had new aircraft plugs that were bad. Agree with Toolbuilder, high rpm roughness , could also be a sticky valve in the guide, broken valve spring.
 

Pops

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So we know with the darker #2 plug plus using carb heat and lower EGT temps that #2 is running rich for sure. Next I would look at the ignition system. If you are using the auto distributor, I would be looking at the hi-voltage side as in a cracked cap, plug wires and plug. Sometimes a crack in the cap can be very hard to see and a new plug can be bad. If you don't have a new plug on hand switch plugs and see if the trouble moves with the plug.
Have had new aircraft plugs that were bad. Agree with Toolbuilder, high rpm roughness , could also be a sticky valve in the guide, broken valve spring.
 

Winginit

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My thought is it's not a problem of being too rich, but an existing problem that is exacerbated and becomes more noticible when additional fuel mixture is added. I'm thinking incomplete combustion because of an ignition defect and the adding of a richer mixture worsens the problem. If it were a problem of being too rich, other spark plugs would show some signs too. With the design of the intake system, if too much fuel was the problem, I would expect both rear cylinders to act similarly. My best guess is a thorough inspection and/or replacement of ignition components. You might also try switching the EGT probes to be sure you don't have a bad one which is causing you to look at the wrong cylinder as a problem. I think I would verify that first.
 

proppastie

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I vote for a bad EGT probe on 2, switch them around to see if or not. Then if it is a bad probe you have bad mid range mixture.
 

don january

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What type of elbow do you have on your intake manifold connecting to the head ? is it the weld on or machined /drilled/ tapped? I was thinking in the area of welding slag hidden on the inside disrupting air flow.
 
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