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Rough running engine at 100?

Thappened a week back and I am out of town so I don?t have all the details...

Rv7a. O-360 engine, bendex mags, 800 hours ttl time. Never an issue.

Jumped in to fly and for the first time ever the engine was very hard to start. It would crank and back fire every once in a while but never really fire. Went through flooding the engine and all those techniques. Finally got it going. On the run up the left mag was very rough. Leaned it out and it cleared up. Good to go. At about 100? the engine started backfiring and sputtering. Pulled it to idle and landed no problem. Taxied off and did the run up again. Left mag number one cylinder was significantly cooler get than the rest. Pull the plugs and the bottom plug was completely loaded with lead. Cleaned it out and put it back together.

Run up again at 1600 rpm and this time all was good. Went to full power and held it there for 30 seconds or so and again, sputter and back fire. Same result with the number one cylinder and the left mag cooler than the rest.

Installed new plugs. New run up.. 1600 rpm all good. Full power for 1 minute all good. Temps all good. Let?s go fly... climb out good, circle the field and all good... off to the practice area.. a couple of backfires or did i imagine that... well upon landing, short final it back fired.. on rollout the engine quit. Hard to start again but got it going. Did a run up and both mags were misfireing now and again the number one cylinder was cooler than the rest. Taxied back to the hangar and put it to bed. I had a beer.

So it appears not to be the plugs as they are new. Seems not to be the mags as both are acting the same. 100ll fuel with no contamination so don?t think it is that.

I have not pulled it apart since. Going to check compression... thinking a sticking valve maybe.

Any thoughts..

Thanks in advance.
 
Could well be a sticking valve but don't assume the plugs are good just because they're new.
 
Back firing

Suggest that you check the HT wires resistance - make sure that they all are within normal limits. I always check the cheapest to replace parts first. Best to you.
 
Do you know if the mags had their 500 hour inspection or otherwise serviced since new?

Larry
 
Are you sure it was a backfire or was it possibly an afterfire (gas exploding in exhaust). That is a pretty important distinction for diagnosing this. If it was an afterfire with stumbling and it is mostly happening while cold, I would start by checking the exhaust valves for sticking via the wobble test. This is recommended at 500 hours for all valves, so time well spent even if not the issue. Backfires at high RPM are pretty uncommon and point to serious issues, so its likely this was an afterfire.

A sticking exhaust valve can prevent compression and keep the cylinder from firing. THe raw air fuel mixture then gets pumped to the exhaust tubes and if the cylinder fires on the next stroke, the raw fuel will ignite, creating the afterfire.

Larry
 
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I will not be home for about a week. I will check all but here are some answers to the above.

I did not check the wire resistance. First on the list.
I bought the aircraft with 600 hrs on it. I will check for the mag and valve inspection.
That being said. Valve check is second on the list.
If that all checks out the mags will be checked.

Thanks for all the advice.
 
I'd go straight to the mags

You say you have Bendix mags. I have Bendix mags on my engine. I've had some issues. I've installed 3 rebuilt mags on my engine in 1500 hours. And installed new point sets also.

Do you know if you have impulse coupling(s) or the "shower of sparks" vibrator?

I would not expect a sticking valve to make the engine "never really fire" on starting like you described.

You also said "Seems not to be the mags as both are acting the same".
Don't get caught up in that trap. I've been there!

Good luck,
Mark
 
Been there, done that

I'd put money on a stuck exhaust valve. I had exactly those symptoms about a year ago. Mine finally stuck open and bent the push rod, so if I were you I wouldn't start it again until you check your valves. It's a lot easier before things break!

Good luck!
 
Hey Jerry

I just started another thread on my experience on a rough engine on take off

Symptoms of a cracked head on take off:
 
Bottom Plug Loading Up With Lead?

The lead has likely been happily building up on the piston heads for some time. If you switched to hotter plugs, it would make sense that the bottom plugs are the ones loading up (gravity at work). Is this the case? If not, something else is causing the cylinders to run hotter and melt the lead.
Are you monitoring/tracking EGTs and CHTs? Are you seeing a spike?
Terry, CFI
FR9A N323TP
 
I finally got time to spend on the engine.

I ran the engine and as before. Once it got hot ran rough. Did a hot compression check and all cylinders were 87 or better. So no stuck valve. But thought of a cylinder running lean. So I pulled the number one cylinder intake tube and it had oil in it. Pulled the plugs and they were black. So my final analysis is the intake valve guide is leaking oil and causing the plug to miss fire. I am going to pull the cylinder and have it IRAN. Hope that is the end.
 
I finally got time to spend on the engine.

I ran the engine and as before. Once it got hot ran rough. Did a hot compression check and all cylinders were 87 or better. So no stuck valve. But thought of a cylinder running lean. So I pulled the number one cylinder intake tube and it had oil in it. Pulled the plugs and they were black. So my final analysis is the intake valve guide is leaking oil and causing the plug to miss fire. I am going to pull the cylinder and have it IRAN. Hope that is the end.

Jerry,
Some oil in the intake tube is normal for all the aircraft engines I've worked on.
There is no seal on the valve stem like car engines.

You say "once it got hot ran rough". Could be a magneto coil breaking down with the heat. I still think it's your mags.

Mark
 
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I finally got time to spend on the engine.

I ran the engine and as before. Once it got hot ran rough. Did a hot compression check and all cylinders were 87 or better. So no stuck valve. But thought of a cylinder running lean. So I pulled the number one cylinder intake tube and it had oil in it. Pulled the plugs and they were black. So my final analysis is the intake valve guide is leaking oil and causing the plug to miss fire. I am going to pull the cylinder and have it IRAN. Hope that is the end.

Unfortunately a leak down test is not definative for a stuck valve. It often starts by sticking intermittantly and will close up before you can do the leak down test. The only definative test is checking the valve guide clearance (i.e. the wobble test). Once it sticks permanently, the leak down will confirm it if it is stuck partially open (not if stuck fully closed though) but by then you usually have bent push rod. If this were the case, and the issues was a permanently stuck (opwn)valve, your roughness would not come and go. Due to this fact and your intermittent condition, valve clearance measurement is required to rule out sticking valves. Also, excess oil in the combustion chamber will speed up the condition that causes exhaust valves to stick.

Not saying you definately have a stuck valve, only that your test doesn't eliminate it. Usually the valve will stick when cold and free up when hot, so it is only a possibility for your symptoms. As mentioned above, ignition is a possibility, as coils often fail only when hot. However, the cylinder fires pretty well on only one mag. You should be able to eliminate ignition by running on each mag individually when it is running rough if you have a good ear for your engine. Also, a bad coil will wipe out the whole ignition and would not present on only one cylinder.

Larry
 
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I finally got time to spend on the engine.

I ran the engine and as before. Once it got hot ran rough. Did a hot compression check and all cylinders were 87 or better. So no stuck valve. But thought of a cylinder running lean. So I pulled the number one cylinder intake tube and it had oil in it. Pulled the plugs and they were black. So my final analysis is the intake valve guide is leaking oil and causing the plug to miss fire. I am going to pull the cylinder and have it IRAN. Hope that is the end.

Plugs with dry black coating are typically from running excessively rich. Excess oil in combustion chamber will also result in black plugs, but they will be wet and shinny, assuming the oil issue is ongoing. You can also usually see oil pooling in the low point of the cylinder near the piston. Also, at higher RPMs, the combustion temp will typically burn the oil off fast enough to prevent misfires.

Also, if you can make it run rough on the ground, you can put a timing light on both the ignition leads to confirm if the plug if firing or not.

Larry
 
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