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Engine stumbles on power advance

Herc

Herc

Registered User
Big Lake, Alaska
Having some engine issues lately. On a moderately fast power advance my engine will stutter and sometimes backfire, to the point I think its about to quit. This is a high time and tired 290-d2. Compressions mid 60s and mag drop right at the edge of acceptable. I have a new o320 coming from lycon hopefully by late June. Looking to keep this bird in the air till then. Going in for annual next weekend, just looking for some suggestions on parts or what to do to help minimize the time my IA needs to troubleshoot.

Thanks!

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A little more info would be helpful:
Did this condition start recently? Sudden onset? Does it consistently?
engine hot/cold makes it worse?

maybe start by confirming your accelerator pump is functioning correctly.
 
Started months ago intermittent but would go away once the engine warmed up. Most recently it has been constant, even once the engine warm. Oat was -10, oil temp was 180. Thanks I will look into how to check the accelerator pump

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Just a wild guess but it's probably because the compression is low. You're throwing a lot of fuel at the engine with the accelerator pump and without the compression it needs it just can't make good use of all that fuel so it bogs on an overly rich condition. Any unburnt fuel is going out the exhaust causing a backfire.
 
Herc said:
.... mag drop right at the edge of acceptable. Going in for annual next weekend, just looking for some suggestions on parts or what to do to help minimize the time my IA needs to troubleshoot.

Thanks!
Click to expand...
Tell him to reset the mag points to maximum magnet strength of "E"-gap. That will take care of the "mag drop right at the edge of acceptable."
Time wears the point and cam follower which changes the point setting. A small point adjustment will do wonders.
 
SJ said:
Sounds like timing to me.

sj
Click to expand...
More than just timing sj. First the mag must be internally timed correctly, then the mag must be timed properly to the engine. Also it is not unusual to find the mag internal timing should be just a little bit different than the magneto manuals say it should be. Each magneto has it's own best highest magnet strength in relation to "E"-gap. Sometimes this can be a very small spot in the rotation of the magnet. Making the points open just a little before or after this "best" spot and the mag just will not run the engine or will do it poorly.
 
skywagon8a said:
More than just timing sj. First the mag must be internally timed correctly, then the mag must be timed properly to the engine. Also it is not unusual to find the mag internal timing should be just a little bit different than the magneto manuals say it should be. Each magneto has it's own best highest magnet strength in relation to "E"-gap. Sometimes this can be a very small spot in the rotation of the magnet. Making the points open just a little before or after this "best" spot and the mag just will not run the engine or will do it poorly.
Click to expand...

Maybe an over simplification on my part, but doesn't all of that relate to timing?

sj
 
SJ said:
Maybe an over simplification on my part, but doesn't all of that relate to timing?

sj
Click to expand...
It does, however many seem to believe when they talk about timing that it means timing the magneto to the engine crankshaft position alone. When in fact in order to do it correctly it is more than that. That is the impression I get when listening to discussions here. The universal suggestion to correcting mag issues is to "send it out". Most mag issues can be easily corrected on the home bench. Usually without the need for any parts.
 
skywagon8a said:
It does, however many seem to believe when they talk about timing that it means timing the magneto to the engine crankshaft position alone. When in fact in order to do it correctly it is more than that. That is the impression I get when listening to discussions here. The universal suggestion to correcting mag issues is to "send it out". Most mag issues can be easily corrected on the home bench. Usually without the need for any parts.
Click to expand...

Even with some minimal tools, it's pretty easy to overhaul a mag without sending it out, but I agree, when people talk about timing, they often forget the point gap and E gap setting being a very important part of setting up magnetos to operate correctly. Since this was described as an intermittent issue, while you are in there, change the condenser!
 
Slick or bendix mags? Have you ohm’d the spark plugs? I have fixed issues by a plug change due to too high ohms.

Have you tried to accelerate with carb heat applied? That will help determine if it’s running lean...
 
Last edited:
Bendix mags. Accelerator pump was good. Replaced all the intake gaskets and couplings and the issue went away. I'm thinking it was an induction leak. Mag drop is still borderline but I can live with that till my new engine arrives

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I am glad you got it fixed! Now I just need to convince you that I need an O-290 that is leftovers...
 
If you're looking for an O-290-D2 I'll have two of then up for sale shortly. One with 100smoh (20 years ago) and the other thats currently flying

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