O320 rough idle

hcccs · #1 09-04-2018, 01:29 AM

I'm test running my O320 and it runs rough at idle and when I throttle back all the way it stops. I tried to adjust the idle rpm with the screw on the carb and I could barely get it to run at 650 RPM and then the rpm fluctuates between 600-700 rpm and it misfires. As for the adjustment screw I think it is screwed in almost as much as it can be (see photo). Does this seem normal? I can't even decide if it is running too rich or too lean. The magnetos are correctly set, the leads and all weather spark plugs are new. Any ideas?

Dayz · #2 09-04-2018, 04:39 AM

A rag in the carb? You gasket between carb and filter? Don?t ask why I know. I chase that exact symptoms for a few hours on the ground.

Confirm you have nothing in the airways first. Then it should be proper carb setting configuration.

Jvon811 · #3 09-04-2018, 05:40 AM

What sort of prop do you have? I have a wood prop and I can't get mine to idle smooth down that low. It is only happy at 800+ RPM. I think it has to do with the inertia of the prop (or lack of it in my case). My carb is set up to idle at 600 RPM and it will run, but it's not happy. No backfiring though.

hcccs · #4 09-04-2018, 06:06 AM

Quote:

Originally Posted by Jvon811

What sort of prop do you have? I have a wood prop and I can't get mine to idle smooth down that low. It is only happy at 800+ RPM. I think it has to do with the inertia of the prop (or lack of it in my case). My carb is set up to idle at 600 RPM and it will run, but it's not happy. No backfiring though.

Yes, I have a wood prop. My previous plane had a Continental C-90 with wood prop and it idled happily at 500 rpm when cold. I think I have to check magneto timing and spark plug leads.

moosepileit · #5 09-04-2018, 06:22 AM

The blueish plate between the carb, gaskets and sump- is that leakfree? Can you check for induction leaks with egts on each cylinder? The lighter prop is more revealing of any issues at idle.

Which part number carb? Is it new or newly overhauled?

hcccs · #6 09-04-2018, 08:08 AM

Quote:

Originally Posted by moosepileit

The blueish plate between the carb, gaskets and sump- is that leakfree? Can you check for induction leaks with egts on each cylinder? The lighter prop is more revealing of any issues at idle.

Which part number carb? Is it new or newly overhauled?

Marvel-Schebler MA4-SPA No BL-22-14429. I could check the EGT on Skyview.

lr172 · #7 09-04-2018, 01:05 PM

It is likely too lean. If it were too rich, increasing the RPM would likely help it, as opposed to resisting it. Give the idle mixture screw (It has a large screw head with knurling on it and faces aft at the base of the carb - not the idle speed screw highlighted in your picture) a good half turn out (CCW) and see if it improves. Then follow the procedure for properly adjusting your idle mixture. If it were rich enough to resist additional RPM, you should be able to see a black or grey color in your exhaust output.

Larry

hcccs · #8 09-04-2018, 03:16 PM

Quote:

Originally Posted by lr172

It is likely too lean. If it were too rich, increasing the RPM would likely help it, as opposed to resisting it. Give the idle mixture screw (It has a large screw head with knurling on it and faces aft at the base of the carb - not the idle speed screw highlighted in your picture) a good half turn out (CCW) and see if it improves. Then follow the procedure for properly adjusting your idle mixture. If it were rich enough to resist additional RPM, you should be able to see a black or grey color in your exhaust output.

Larry

Larry,
I'll give it a try.

hcccs · #9 09-13-2018, 05:17 AM

Quote:

Originally Posted by lr172

It is likely too lean. If it were too rich, increasing the RPM would likely help it, as opposed to resisting it. Give the idle mixture screw (It has a large screw head with knurling on it and faces aft at the base of the carb - not the idle speed screw highlighted in your picture) a good half turn out (CCW) and see if it improves. Then follow the procedure for properly adjusting your idle mixture. If it were rich enough to resist additional RPM, you should be able to see a black or grey color in your exhaust output.

Larry

If it were too lean would it run well at higher revs. Mine does so I wonder.
I have removed the carb now to check it and upgrade with new floats. When unscrewing the throttle adjusting bolt all the way back the throttle valve closes before the stop on the throttle shaft hits the stop on the carb body. When the valve is closed only the upper primer delivery hole is exposed (see picture) but no air could go past the valve so I can't see how it could possibly idle in this position. How big would the valve opening be at idle?

fixnflyguy · #10 09-13-2018, 06:52 AM

I am running an O-320H2AD/1600 hp with a Sterba wood prop, and idle at 600-650 is rough. I have it set for that only to allow me to throttle way back, but I keep my idle around 800 RPM on ground to be happy. There is little similarity between an O-320 and a C65-C90 which will purr at low idle with a wood prop. There just isn't enough inertia/weight in a wood prop for the low idle on the O-320