Idle Increase after Cylinder Replacement?

tx_jayhawk · #1 10-13-2019, 09:08 PM

I recently changed a cylinder due to extremely low compression (was no obvious problems during operation).

After the cylinder replacement, all seems to be well. The only oddity is that the engine now idles 400-500 RPM higher. Nothing on the fuel side was changed...purely just an overhauled cylinder with new piston / rings.

Any thoughts on what could cause the idle increase?

Thanks
Scott

Ralph Inkster · #2 10-13-2019, 09:39 PM

One day my engine wouldn?t idle under 1000 rpm. Found one intake runner totally loosened away from the cylinder.

mahlon_r · #3 10-14-2019, 04:54 AM

if your engine is fuel injected. you likely have an intake leak somewhere. guessing that is due to the replacement cylinder or it's installation. If its carbureted maybe you some how affected the throttle rigging and it is no longer hitting the idle stop at the carb.
Good Luck,
Mahlon

tx_jayhawk · #4 10-14-2019, 08:44 PM

Intersting feedback...thanks. After cylinder replacement the aircraft idles higher than before with the old cylinder. Now that you both mention it, the only other thing I did during cylinder replacement was reposition the intake tube on another cylinder where it looks like it wasn't perfectly seated. It was very minor that I would have never thought even made a difference, but maybe that repositioned intake tube caused my idle increase?

mahlon_r · #5 10-15-2019, 07:04 AM

That could definitely cause it or a problem with the intake pipe or a missing intake port plug on the replacement cylinder, as well.
Good Luck,
Mahlon

airguy · #6 10-15-2019, 08:00 AM

Keep in mind that moving that intake tube on the other cylinder, could CAUSE a problem, or FIX a problem you didn't know you had.

If the other tube was leaking all this time and causing one cylinder to be very lean at idle, and the idle stop was simply adjusted higher to compensate and 3 out of 4 cylinders were carrying the load, you might not ever have noticed it without good EGT/CHT monitoring. Now that you fixed the leak, that cylinder makes good power at idle, and you also replaced a different weak cylinder with a new good one - and bingo, you've got a higher idle RPM due to making more power on the same throttle position.