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-   -   Misfire, Detonation, or Preignition (https://vansairforce.net/community/showthread.php?t=182567)

Tacco 05-17-2020 01:16 PM

Misfire, Detonation, or Preignition
 
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?

Piper J3 05-17-2020 01:32 PM

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?

Tacco 05-17-2020 01:44 PM

Don?t know. They?re fine when checked at 4000. Aircraft is hangered humidity is nominal, I?d say 50-60%.

Piper J3 05-17-2020 02:58 PM

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.

Tacco 05-17-2020 06:47 PM

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.

Piper J3 05-18-2020 07:07 AM

Keep in mind that Bing carb floats work just fine - until they don't...

DaveWelch 05-19-2020 10:22 PM

Watch Oil Temp
 
Quote:

Originally Posted by Piper J3 (Post 1431106)
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?

Piper J3 05-20-2020 05:04 AM

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.

Tacco 05-20-2020 03:52 PM

So Jim, I lost you there. Why even do it if the results indicate an uncertain cause. Why not just weigh the floats?

Piper J3 05-20-2020 06:47 PM

Kind of like a doctor where you look at symptoms first before performing surgery. But yes, weighing the floats is always prudent. Use a good scale with x.xx gram resolution. eBay $6.95 w/ free shipping...https://www.ebay.com/itm/Portable-20...84286f8a4f9740


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