XP-360 Exhaust Popping

[JordanGrant]

Smart Engine People:
My XP-360 engine has a 'popping' noise coming from the exhaust when at idle power in flight. Is this a normal sound for my configuration? Or could there be something wrong? Here's my data:
45 hours TT on the XP-360
Airflow Performance fuel injection
Lightspeed Plasma II+ Ignition on top, Slick Mags on the bottom
Vetterman Exhaust
Idle Mixture has a 30-50 RPM rise when mixture pulled to cutoff
Slightly (though noticeably) less noise when running on Lightspeed only

[hngrflyr]

Check for leaking exhaust gaskets or an air leak anywhere high in the exhaust system.

Bob Severns

[Brian Denk]

Unburned gas going out the exhaust ports can ignite and cause the popping. Mixture too rich?

[AlexPeterson]

Quite normal for FI engines and Vetterman exhausts. They scavenge reasonably well, and at closed throttle in flight, the combustion in the cylinders is not complete. There is the same amount of fuel going in, but much less air (compare MAP during flight with closed throttle to on the ground).

Interestingly, I noted a distinct reduction in the popping when I converted from Lasar to dual Lightspeed Plasma III's. The new system must burn it more completely at the low MAP's of idle in flight.

[osxuser]

Basically what has been said previously. Make sure there are no intake or exhaust leaks, then you can tweak the idle mixture down a bit 10-25RPM rise is sufficent in FI engines.

[Skid]

I have the the same problem with the the same set-up. I contacted the engine builder, Aerosport Power, and their suggestion was to lean the engine (at the throttle body) slightly. I backed the mixture off 1/2 turn and there was a reduction in the popping but it did not eliminate it completely. I have heard of others with the same problem.

[Tom McCutcheon]

I agree that you should check all of the things already suggested. However,I think this is pretty normal. Expecially with constant speed props. The prop, when put to idle, is trying to turn the engine faster causing the engine to always be in a deceleration mode (if in flight). Most cars do this also but you don't hear it much because of the mufflers.

[JCN247DE]

Skid,

I just posted up with the exact "popping" question as this thread poses.

My engine is an Aerosport power O-360 as well. You mentioned that you leaned the idle mixture on the carb by "backing off 1/2 turn." Do you mean you screwed the mixture screw in 1/2 turn to lean? As in tightened 1/2 turn to lean?

Thanks,

Jeff

[dan]

Originally Posted by osxuser

...you can tweak the idle mixture down a bit 10-25RPM rise is sufficent in FI engines.

Best advice I've heard so far...and it is exactly why the popping occurs. You're running idle mixture at 2000 or so RPM (throttle to idle while the plane is still flying).

Three things...

1) I run a leaner idle mixture than most folks. Stephen's 10-25 RPM rise is exactly what I see (I see 20 RPM). No more. I don't want any more of a rise than that.

2) When you pull your throttle back to idle, do it slowly. That will help.

3) My mixture knob almost never hits the full rich stop. I mean once in a blue moon. This goes against what we were taught back when we learned to fly. Full rich mixture? In normal circumstances, why would you ever flood the engine like that? (I'm speaking strictly about fuel injected engines.) I lean for the conditions in all phases of flight.