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RPM decrease during warmup

Acenels

Active Member
I’m experiencing what I would call interesting RPM behavior on warmup, and I’m kind of curious if anyone else has seen this or similar. After about 3 minutes from engine start and holding at 1000 rpm, if I pull the throttle to idle it starts off settling at its normal (slightly high) 900 RPM, then slowly fades to about 550 rpm, idles a touch rough there for about 30 seconds, then climbs back up to about 900 and stays there (all while on the idle stop).

O-360, about 700 hours snew. Dual PMAGS. I’ve tried leaning out (little increase, about 50 rpm), flipping of the alternator, verified both mags are working independently (and that they are running on ship power at that RPM), and added the electric fuel pump. This has been 40-50 degree weather.

No other issues beyond this. And it runs like a champ after warmup. I’m just scratching my head on it. My fuel PSI has been reading at 8 during warmup, and gets flutter when the engine is running in the lower RPMs, but not sure if that is just from the added vibration. It tends to sit at about 5 in flight, and it is the original mechanical gauge, so I do suspect it is potentially due for a replacement. (Plane used to fly around at about 3, so I suspect it is reading a touch high...but tbd)
I haven’t seen much about mechanical gauges having issues, but it is about 15 years old.

Is all this somewhat normal engine behavior?

Thoughts? I don’t have any immediate concerns on the engine, but the curiosity bug has me on it for sure. And I’m not very smart on engines to begin with, so I have no baseline if this is a normal thing.
 
Assuming you have checked the security of the throttle cable especially at the carby end erratic idling is usually caused be varying fuel/air mixture especially in carbureted engines. For an engine speed to vary either fuel or air has to change. Several things can cause this. Butterfly valve shaft worn (allows air in to effect ratio). incorrect idle setting, I've even seen dirty fuel create erratic idling. When you retard the throttle there may be excess fuel momentarily causing the rough running, when that's consumed the correct air fuel ratio returns as does the normal idle RPM. Vaccum leaks also cause erratic idling. Hope you get to the bottom of it:)
 
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I had similar symptoms with my 0-320. Sent the carb out for overhaul. Tech said the only thing he found was the throttle/butterfly shaft was a ?little? worn and it might be drawing air by the shaft. Replaced shaft with new and no problems since. So....perhaps it was the shaft, perhaps the cleaning/overhaul. I like to think it was the shaft. 😗
 
Carb ice may not be the problem but 40-50*F is definitely not too warm for ice unless the humidity is quite low. High humidity can push the optimum temp for ice up into the 70's.

Yeah, good call. I?m apparently rusty on carb-ice conditions?I?ll give it a longer test with carb heat on just to completely rule it out.
 
Thanks, Sam, for confirming I?m not a muppet!

Acenels, remember if carb heat is being effective there is every chance that the rough running will get worse before it gets better (the ice is melting and weakening the mixture further).

On the other hand, you might not necessarily be able to replicate the carb icing if conditions have changed!

Good luck!
 
Also, you won't get much carb heat on initial start-up. The exhaust pipes need to get hot before carb heat works.

I also notice that you state that 900 RPM is your normal idle with the throttle at the idle stop. Is that correct? If so, your idle is set way too high. Should be around 500-600 when the engine is fully warmed up. You should have to open the throttle somewhat to get an idle of 900, especially when cold just after a start.

An engine that "idles" at 900 RPM will extend your landing flare and rollout by quite a bit.
 
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Also, you won't get much carb heat on initial start-up. The exhaust pipes need to get hot before carb heat works.

I also notice that you state that 900 RPM is your normal idle with the throttle at the idle stop. Is that correct? If so, your idle is set way too high. Should be around 500-600 when the engine is fully warmed up. You should have to open the throttle somewhat to get an idle of 900, especially when cold just after a start.

An engine that "idles" at 900 RPM will extend your landing flare and rollout by quite a bit.

Correct on all, my friend?it is high and I?m going to adjust it down, as a known issue. My target is to get it to 650.

The engine runs smooth up until the RPM decrease, and I?ve seen this behavior in various conditions, albeit only for about the last 6 weeks. I?d be surprised if that is it, but I?d be happy to be surprised and have the answer.
 
I would be looking closer at the fuel pressure. 8 psi is about the limit on what the needle/seat can control. If the pressure overcomes the resistance of the needle/seat, the bowl overfills and the overly rich condition could cause the reduced RPM. The bouncing needle could be oscillating pressures that exceed 8 psi. I would watch to correlate the bouncing needle to the RPM drop issue.

These type of pumps can and do fail in an overpressure situation and had a very similar situation on a 350 chevy engine (same pump style, even same manufacturer - AC), though it would totally overrun the needle/seat and ultimately kill the engine after running rough at a low RPM. I didn't expect that type of pump failure and kept going back the needle/seat until I figured it out.

Any chance you use gas with ethanol? Thats what did this to the one I referenced - old, pre-ethanol era pump exposed to E10 gas. This is very common in the marine world in some states that make ethanol free gas hard to come by.

Larry
 
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