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Help me troubleshoot rough running..

Tram

Well Known Member
Ok, bear with me, this may be a long post..

2 flights ago, during a cold start (on a chilly day) the plane fired right up and then quickly **** back down. This is abnormal for our 6. Normally always fires right up. Anyway, fired it right back up, started fine, searched for idle for a bit but stayed running.. Good runup, no issues. Went flying.

Today, the same deal. Fired up, shut back down, fired back up and zero issues. Good runup, went flying. No issues.

Fly for about 30 minutes. Land. Plane sits for about 45 minutes, crawl back in, fire it up (starts fine), decently short taxi out, normal run up and depart.

While climbing out, above ~2250 rpm the engine starts running rougher than I like, pull the power back to around 2100rpm and all is fine. Turn downwind and land.

Taxi in and do a quick runup. Oil temps climb a littler higher than normal, about 240*ish, and the plane runs s rough at 2200 rpm and above.

Taxi in, pull the cowl, look it over. Everything looks normal, but there was a funny smell. Semi-electrical. Semi-heater on for the first time in winter. Semi-burnt electric motor type smell. Not a normal smell.

Let it rest, call our mech and he suggests mixture setting as the first place to start looking. Mixture was slightly off. We set it for about a 50rpm rise when killed from idle.

Do a couple quick run ups to about 2250rpm static and then idle shut offs messing with the mixture.

Taxi’d back out for a runup just to see how it’s acting. Run up is good until a few seonds into high power and it begins running rough again. Rpms drop to 2000-2100ish. Back to the hangar. Smell is kinda there, but nothing like the first time.

Call A&P and he suggests maybe it’s fouled plugs and to attempt to burn it off.

Taxi out, attempt to burn off plugs with high power/leaning on the ground and am unable to get steady enough running to really accomplish anything. EGT’s got to around 1250. No improvement.

Pull the power back and just for grins, I checked the mags. Nothing. No drop on either mag. Ah ha! I think. Found it.

Taxi back to the hangar and before shutting down, power back to ~1800rpm and check mags. Got about a 75rpm drop on both. Decent check.

I’m stumped.

Unmetered Air? Bad Venturi or float in the carb? Mags toasted?

Engine details:

Recent (<100 hours) rebuild O-320.
Bendix mags.
Running non-ethanol mogas.
 
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I see your running mogas. I had a Cessna 150 years ago that would start sticking exhaust valves and loosing power. May check compression. I heard marvel mystery oil does wonders in the gas and oil as a good lube/fix
 
Sounds like low fuel flow? 250 on oil temp? my red line take action is 230. are all EGTs equal when it starts to run rough?
 
I see your running mogas. I had a Cessna 150 years ago that would start sticking exhaust valves and loosing power. May check compression. I heard marvel mystery oil does wonders in the gas and oil as a good lube/fix

Compression check is going to be the first thing this weekend.


I would definitely look at axhaust valves as well as the venture in the carb.

Thanks.

Sounds like low fuel flow? 250 on oil temp? my red line take action is 230. are all EGTs equal when it starts to run rough?

Our EGT?s are at best, sketchy. Which should soon change with a Sky View install, but right now, they all seemed to be the same.

Also, I?m glad you caught the 250. I fat fingered that. Tempts got to just shy of 240, which is still warm for our bird.
 
My -7A did the same when starting in cold PA weather last week. Started, quit, started, quit. I had to leave the boost pump on to get it to stay running the 3rd time. Once warmed up, all was fine and the flight back to GA was uneventful.

Another cold day in the south and it did about the same thing. I used the boost pump again to keep it running upon start and let the oil warm to 100, as usual, before take-off.

Your other symptoms sound like you are not getting enough fuel for the denser winter air. Also, consider changing to a multi-viscosity oil for easier cold starts.
 
Winter blend MOGAS on a warm day, or with a hot engine bay, can cause the symptoms you are seeing. (Or could be a fuel line getting hot from the exhaust pipe, or whatever, causing the gas to vaporize.)
If that is the problem, mixing about 15% to 20% 100LL with the MOGAS should fix it, once the gas in the lines and carb is used up.
I saw this problem once and fixed it by moving the fuel line away from the exhaust and shielding it.
YMMV
Good luck
 
You may have more than one problem here. If it were me, I'd track down that burning electrical smell first because that should be the easiest. Just offhand, the only things I can think of which would cause that would be 1. Alternator; 2; Starter; 3 Magneto Coils; 4: Oil pan heater (didn't you say you had one?). 5 Master or Starter relays; 6 Boost pump; 7 Wiring.

The only one of those items which would be related to the rough running would be the magneto coils, right? You can check those pretty easily.
 
Toward the end of your post you say you got no drop in rpm with a mag check, then later on a normal check? I would disconect the p leads and run it. That would eliminate the switch and associated wiring. You wont be able to check the mags individually, but it might just solve the problem. Just be carefull because the mags will be hot all the while the p leads are off.
 
Engine

I would think oil at over 220 would be a problem and is the bad smell at 240 also high prob of vapor lock if your oil is that high just think how hot the engine is and fuel pump are the fuel has got to be boiling,I think you need to get your oil temps under control as soon as possible that kind of temp this time of year I don't see how you made it thur the summer.
Bob
 
I missed the 240 oil temps!! Something is not right. Either you have massive friction - something very tight. Or the timing is way off.

Or no air or no oil is passing through the oil cooler.
 
Sticking Intake valve guides

Had a similar problem, my intake guides start to shrink because the press fit of the guide to the head was excessive. This tolerance changes as the cylinders expands and contracts on low time rebuilds. Listen for air to exhaust thru the carburetor with the mags off.
 
Does this sound like morning sickness/ stuck exhaust valve?

The more I read about it, the more I think it does.

We're going to throw some 100LL into one of the wings and run that through it, see if it may help smooth it out..
 
Rough Running

Could be shorting out under the distributor cap on the magsneto,would cause a burning smell and shorting and tracking at higher rpms, I would not fly it and possibly turn of the only good mag you have, I recently saw this on a continental engine, same problem,you could see the burning in the high voltage cover, just a thought !
 
I would not fly it..

Yeh, she's not going anywhere until all this is sorted out.

It's sitting over an hour from me at the moment, so I haven't really been able to do anything with it.

Fortunately, it broke at my mechanics home base, so he's going to be looking at it today.
 
I would think oil at over 220 would be a problem and is the bad smell at 240 also high prob of vapor lock if your oil is that high just think how hot the engine is and fuel pump are the fuel has got to be boiling,I think you need to get your oil temps under control as soon as possible that kind of temp this time of year I don't see how you made it thur the summer.
Bob

Very good points. We didn't have a single issue during the summer and the temp spike was very brief. It hasn't occurred since during any run up since the issue.
 
Well, too many hoops to jump through to post an image, but it's new cylinder time....
 
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