Strange problem with Aerovee

[radfordc]

A guy near here called me last night to discuss his engine problem. He has a tri-gear Sonex with Aerovee. He has had engine problems in the past that required him to overhaul/rebuild the engine. He set the compression to 7:1 and is currently using 91 octane mogas. Because he is very low time in the Sonex he had a test pilot fly the plane after the engine work. The engine started and ran well initially. For the first 20 minutes the engine performed well, with CHTs in the low 300s and oil temp in the 180-190 range. After 20 minutes the oil temp suddenly rose to 200 and the engine started to run rough (apparently the CHTs didn't go up much). When the plane landed the engine died. The owner reported that the engine was very hot, and even over an hour later was too hot to touch.

I'm at a loss to tell him what the problem is? Usually when and engine has been running well and then goes rough I suspect detonation, but that should result in high CHTs, not high oil temp. I suggested he change to 100LL and also shut off the electronic ignition system in flight and see if the problem continues. Any other good ideas?

[daleandee]

I suggested he change to 100LL and also shut off the electronic ignition system in flight and see if the problem continues. Any other good ideas?

My best recommendation would be to install a 3.0 Corvair in it ... ;)

Besides that I think your advice was spot on. I had an issue with the Aerovee that I had that was very similar. It was a very hot day (OAT 100ºF) and I was running 93 octane. Shortly after take-off the EGT's were in the 1500ºF range and climbing. Of course CHTs were high also but the problem was the engine leaning out due to so much heat under the cowling.

Perhaps his CHTs aren't reading correctly (the head fin mounted ones are "generous" as you yourself have documented). What were his EGT readings? Does he have the pants and fairings installed? What was the reason for the recent rebuild? If he is a low time pilot I would guess the engine was low time before the first rebuild. Perhaps he assembled the engine and the timing is not correct. When I first got my Aerovee the initial builder had the timing off on the cam gear by THREE teeth and it ran. It wouldn't idle and top RPM was 2500 but it ran. Don't know what the result would be if it was only one tooth off. Looking through the oil fill hole (where the distributor used to go) with a good light and turning the prop will allow you to see the gears and the timing marks. Just a thought.

What you have given was good advice. 100LL, a very cool and smooth morning flight, over the airfield. I'd also suggest covering the gascolator and adding a blast tube and possibly wrapping the exhaust. As you know the Aerovee works very hard and runs very warm. If he can keep it cooler under the cowling I believe his concern will go away. More break-in time will help also.

Dunno if this helps ...

Dale Williams
N319WF @ 6J2
Myunn - "daughter of Cleanex"
120 HP - 3.0 Corvair
Tail Wheel - Center Stick
64.0 hours / Status - Flying
http://kitplanes2.com/blog/2013/12/ownerbuilder-2/
http://www.youtube.com/channel/UC49h6Qijc17_Ebfz0CbRFtg

[vigilant104]

The part about normal CHTs with abnormally high oil temps is interesting. Could he have an oil flow problem (pump, pressure adjust/regulator valve, etc)? If the oil isn't circulating well through the oil cooler and the day is hot, the OT will climb. Did he mention anything about the oil pressure?
As to why it would run rough and quit on the ground--that sounds unlikely to be related to the "oil flow" theory I've proposed above.

[radfordc]

Perhaps his CHTs aren't reading correctly (the head fin mounted ones are "generous" as you yourself have documented). What were his EGT readings? Does he have the pants and fairings installed? What was the reason for the recent rebuild? If he is a low time pilot I would guess the engine was low time before the first rebuild. Perhaps he assembled the engine and the timing is not correct. When I first got my Aerovee the initial builder had the timing off on the cam gear by THREE teeth and it ran. It wouldn't idle and top RPM was 2500 but it ran. Don't know what the result would be if it was only one tooth off. Looking through the oil fill hole (where the distributor used to go) with a good light and turning the prop will allow you to see the gears and the timing marks. Just a thought.

What you have given was good advice. 100LL, a very cool and smooth morning flight, over the airfield. I'd also suggest covering the gascolator and adding a blast tube and possibly wrapping the exhaust. As you know the Aerovee works very hard and runs very warm. If he can keep it cooler under the cowling I believe his concern will go away. More break-in time will help also.

His plane is a tri-gear and also has oversize tires so I'm sure it's "draggy". He claims that he can only get 3200 rpm max (Sensenich prop) but isn't certain about the accuracy of his tach. He has worked with the Aerocarb trying to get it right and claims that the engine runs great initially until it gets hot. You're right about the gascolator maybe being part of the problem. I should mention that to him. I guess it's possible the timing could be off too, but that doesn't fit with the engine running good at first.

He doesn't have an EGT instrument so can't tell about that.

Thanks for the input.

[kmacht]

Since this is happening just after re-building the engine I would check the valve adjusters to make sure they are set correctly as well as check the torque on the head bolts. As the engine gets hot it could be the valve timing getting too far off if they weren't set correctly when cold. The 10 degree increase in oil temperature doesn't seem like that big of a deal to me. I would be more concerned with the rough running engine. If it is just happening when getting hot then he should be able to duplicate it on the ground. If it were me I wouldn't go fly it until it was sorted out.

Keith

[wlarson861]

Sounds like the "Aerocarb Burps". I removed my gascolator and went with a straight fuel line and 40 micron filter. The burps dont usually start right away but after the engine compartment is heat soaked. Mine never burped on the ground with the cowl off but would burp on hot days while doing run up or shortly after.

[radfordc]

I agree that it sounds like a hot fuel issue. I talked to Fred and advised him to wrap all his fuel lines with insulation and see if the problem goes away. Also, to use 100LL instead of mogas.

[floridasonex]

"Of course CHTs were high also but the problem was the engine leaning out due to so much heat under the cowling.
Dale Williams"


Wouldn't having too much heat under the cowl mean less dense air and cause it to be RICHER? I've installed a
cockpit controllable fresh air intake (not sealed to carb) that places fresh air right in front of the air filter. I also
have a sensor in the aeroinjector right below the slide. Opening the fresh air intake lowers the temp in the carb
by about 30 degrees inflight. When I do this it's putting more dense air into the carb and the EGT rises (leans). I
then have to richen the mixture slightly.

RT

[daleandee]

Wouldn't having too much heat under the cowl mean less dense air and cause it to be RICHER?

RT

No. The problem for me was too much heat causing the fuel (93 Mogas) in the gascolator and perhaps the fuel lines and even the Aerocarb to boil causing bubbles and burps. Once I had insulated the fuel lines, covered and insulated the gascolator, and wrapped the exhaust the heat wasn't causing this to happen anymore. Older cars with the fuel lines running too close to the exhaust could, when hot, get what was referred to as "vapor lock" but modern fuel injection and higher fuel line pressures have eliminated that.

Myunn (Corvair powered Sonex air frame) uses a float carb (Marvel Schebler MA3-SPA) and has had no issues with heat (the Corvair runs very cool), burps, stumbles, or hesitation of any kind. I have fire sleeve on the fuel lines and put a blast tube and cover on the gascolator.

Your thinking as far as the air into the carb is correct and may indeed be a contributor to the problem. With the Marvel Schebler I have direct ram air and carb heat for use when needed. I had an Aerocarb and flew it 175 hours but I now personally do not consider them to be airworthy. Just my humble opinion ... please no flames!

Dale Williams
N319WF @ 6J2
Myunn - "daughter of Cleanex"
120 HP - 3.0 Corvair
Tail Wheel - Center Stick
64.0 hours / Status - Flying
http://kitplanes2.com/blog/2013/12/ownerbuilder-2/
http://www.youtube.com/channel/UC49h6Qijc17_Ebfz0CbRFtg

[floridasonex]

"Your thinking as far as the air into the carb is correct and may indeed be a contributor to the problem. With the Marvel Schebler I have direct ram air and carb heat for use when needed. I had an Aerocarb and flew it 175 hours but I now personally do not consider them to be airworthy. Just my humble opinion ... please no flames!

Dale Williams"

No flames. I see where you're coming from. I'm not happy with the Aeroinjector either ..

RT