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Engine running poorly when cold

Rick RV-4

Well Known Member
This may be a carb issue as I just installed a freshly overhauled carb, but want to get other thoughts. The past few starts my engine will start, then want to quit after running well for about 7-8 seconds. I find I have to run it at quite a high RPM (1300 RPM or so) or it wants to keep dying on me until it has been running for a few minutes. When it is running it is running smoothly, but just wants to die. Once it is warm the problem completely goes away. Post flight it will idle just fine.

I'm not seeing any big difference on EGT/CHT from one cylinder to another. I'm guessing I my have adjusted the carb too lean for the idle setting, but wanted to get other thoughts before I start digging.

O-360, with P-Mags (timing was checked and is good)
Sensenich GA carbon fiber prop (lightweight so not as much inertia)
Plugs are new

Thoughts?

Thanks,

Rick
 
As I casually read this in my A&P cram course, it sounds like an improper idle mixture setting. Especially since you just installed the carb.

After more thought it sounds like your idle mixture setting on the carb is too lean for a cold engine. Since you must rev the engine to 1300 to keep it running cold, the actual idle system isn't a factor at that RPM. Once the engine is warm and can idle fine it is because the hot engine needs less fuel at that specific idle rpm. Probably just needs the idle mixture richened just a bit. But someone else can chime in cuz im not an A&P, yet.
 
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Idle mixture is too lean. Cold engines need quite a bit much re fuel than warm ones.
 
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A light weight prop may need more RPM to idle, because it has less flywheel effect.

It seems likely that you may need to richen up the carb. To do so get the engine up to temp (a good time is when you taxi back to the hangar after a flight). Run the engine at 850 RPM (or a little more if it is not idling smoothly), slowly pull the mixture while you watch the RPM. You should see 50 or so rise before is starts to falter. Less than 50 RPM: turn the screw out to richen. More than 80RPM: turn the screw in to lean.
 
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