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Misfire, Detonation, or Preignition

Tacco

Well Known Member
At least twice in my last 10 flights, the engine misbehaved upon run-up. As I advanced the throttle, it misfired, meaning there was a definite ?bang? (not a back fire) or two on the way to 4000 RPM. If I left the at 4000 RPM for a few minutes, the problem did not reoccur upon a second run-up. All temps were low, both engine and OAT. The gas was about a week out of the pump. I do have a longish taxi from the hanger to the run-up area. Fouled plugs? Again, the engine did not ?stumble?, it was a bang or impulse for lack of any other way to describe it. So, .....Cause? Avoidance? Negative effects?
 
What happens if you check the ignitions soon after engine start? Say 2800/3000 RPM one minute after engine start with choke (enricher) off?

Airplane hangered? High humidity?
 
Don?t know. They?re fine when checked at 4000. Aircraft is hangered humidity is nominal, I?d say 50-60%.
 
Correct. I would expect good ignition check at 4000. Next time out, check as I describe above and report back.

I'll let the cat out of the bag here... I'm betting carb floats.
 
I?ll give it a try. Hope you have another cat in that bag though. The engine has 68 hours on it, the floats are the latest part number, they were weighed in January during the annual.
 
Watch Oil Temp

What happens if you check the ignitions soon after engine start? Say 2800/3000 RPM one minute after engine start with choke (enricher) off?

Airplane hangered? High humidity?

Jim, you mean soon after 120 deg Oil Temp is reached?
 
No, Maybe a minute or two after engine start. My guess is that one of the ignitions will stumble pretty badly at low RPM shortly after engine start. Mine does this about 25% of time. Not a definite pattern. Might be OK for a bunch of flights and then stumble again at low RPM ignition check. Might even do it when engine is warm like taxing back after a landing. I'm not so sure its even ignition related but problem presents when ignitions are cycled at low RPM. 4000 RPM check is always good and checking ignition in cruise flight is always good.
 
So Jim, I lost you there. Why even do it if the results indicate an uncertain cause. Why not just weigh the floats?
 
Keep in mind that Bing carb floats work just fine - until they don't...

You were right. Even though engine is very low time. Had a “sunk” float. Actually, 7.2 grams on one side, 10.5 on the other. So...running excessively rich, fowling plugs. Old style floats, now replaced with newest part number.
 
Keep in mind that Bing carb floats work just fine - until they don't...

You were right. Even though engine is very low time. Had a “sunk” float. Actually, 7.2 grams on one side, 10.5 on the other. So...running excessively rich, fowling plugs. Old style floats, now replaced with newest part number.
 
Ever since I purchased my RV12 I have added 1oz of "Sea Foam" to my 93 octane auto gas per gallon. I use it incase I don't get to fly so the gas doesn't go bad. It states it protects fuel for two years.. I have used it on any vehicle, boat, lawn mower I've ever had, especially the mower which sits unused during winter, it also claims to be an upper cylinder lubricant as well as carbon remover. I took apart my 70 hp outboard motor (2cyl) and it was nearly carbon free, it was 23 years old, just a very light amount on piston heads and I mean very light amount. My 912 ULS starts easily and I never have ignition issues, she runs very smoothly, I have 315 hours on it and of those I have put 130, maybe it could be of help.
 
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