[Kato's 8]

Hello,
I have a new lycoming with supplied carb and was wondering about making adjustments to the carb idle and mixture. This was installed on the lycoming with supplied basic instruction and after first engine start the idle just seems real rough. That rpm is about 500 to 600 and I'm wondering if I should adjust it up to 800 or not. If so, should any adj be made to mixture first? I should also add that upon engine shut down, the throttle back and then pull mix to full lean, the engine takes its time shutting down, possibly only 2 or 3 seconds but it just seems like a long delay. I also don't have that slight increase in rpm just before as is ideal. Can someone comment on this and provide some guidance. I could also use some info on how to make the adjustments. Like if engine should be running and then turn set scre located xx 1/4 turn.. Something like that. I don't have a maint book for the carb so I'm what others have done as well. Thanks!

 

[krw5927]

If the idle RPM is around 500-600 with the throttle all the way against the idle stop, I think that is okay. Yes, it's rough on the ground, so you will typically idle with just a little bit of throttle in. But I like the ability to pull it back as far as possible on final, especially with a fixed pitch prop. In flight you won't ever see 500-600 rpm, even with the throttle pulled all the way out.

As far as mixture, make sure that when you pull the mixture knob all the way out, that the mixture lever on the carb is actually hitting the lean stop. If so, then this is all you can do. A 2-second stop isn't bad, especially if you have a heavy prop.

To adjust idle mixture, the way I've done it is to first have the engine warm. Then at approx 800 rpm, slowly pull the mixture to idle cutoff. Note whether the RPM rises at all, just before the engine shuts down. Ideally there will be a slight (50 rpm) rise just before cutoff. If there is no rise, adjust the idle mixture screw out (counter-clockwise) 1/2 turn, re-start the engine, and repeat the test.

 

[Ron RV8]

I can't comment on mixture adjustments except to say, please don't do it with the engine running... yikes

For the two lycomings I have owned, the shut down is cleaner if the RPM is higher when you pull the mixture to idle cut-off. I used about 1200 rpm for the O-360 I had on the Mooney, and about 8-900 seems to work well for the O-375 on the RV... Much less wet dog shake, probably because it uses up the remaining fuel quicker...

My idle speed on the RV is about 550 and the O-375 seems happy at that with the Catto 3 blade...

YMMV

 

[crabandy]

I'm a novice too, but before you adjust the idle mixture screw the engine must be at normal operating temps. Hard to do while limiting ground runs on a new engine. I would get it close and as long as it idles smooth above 600-650 I would fine tune it later. You could back out the airscrew 1/4 turn at a time until you get a 25-50 RPM rise at idle cutoff as a start.

Dual mags or Electronic ignition?
Metal or wood prop, fixed or constant speed?

I believe the inertia of the metal props help to lower the "smooth" idle speed, Electronic ignition can also help lower the idle speed. Rough idle somewhere around 500-600 sounds normal to me.
If you have a fixed pitch prop you want it to idle as low as possible to reduce float on landing. If it idles rough at 500-600 sitting on the ramp but doesn't die it should be fine. Remeber when you're flying and pull the throttle to idle the air is going to drive the prop a little and the idle RPM will be higher.

 

[az_gila]

If the idle RPM is around 500-600 with the throttle all the way against the idle stop, I think that is okay. Yes, it's rough on the ground, so you will typically idle with just a little bit of throttle in. But I like the ability to pull it back as far as possible on final, especially with a fixed pitch prop. In flight you won't ever see 500-600 rpm, even with the throttle pulled all the way out.

As far as mixture, make sure that when you pull the mixture knob all the way out, that the mixture lever on the carb is actually hitting the lean stop. If so, then this is all you can do. A 2-second stop isn't bad, especially if you have a heavy prop.

To adjust idle mixture, the way I've done it is to first have the engine warm. Then at approx 800 rpm, slowly pull the mixture to idle cutoff. Note whether the RPM rises at all, just before the engine shuts down. Ideally there will be a slight (50 rpm) rise just before cutoff. If there is no rise, adjust the idle mixture screw out (counter-clockwise) 1/2 turn, re-start the engine, and repeat the test.

My aircraft manual says to do the 50 rpm rise idle mixture check from the idle speed. It also says not to actually let the engine quit.

The idle speed for your particular engine model should be in the Lycoming Operator Manual. The first idle speed number above is for a O-360-E2G and the second for a O-360-A4K.

 

[krw5927]

My aircraft manual says to do the 50 rpm rise idle mixture check from the idle speed. It also says not to actually let the engine quit.

The idle speed for your particular engine model should be in the Lycoming Operator Manual. The first idle speed number above is for a O-360-E2G and the second for a O-360-A4K.

If the OP's engine doesn't idle smoothly at 500-650 rpm as stated, how could he tell if there was a RPM rise before cutoff?

Also the AA5 aircraft manual suggests to adjust idle mixture with the engine running and repeat the test?

 

[az_gila]

If the OP's engine doesn't idle smoothly at 500-650 rpm as stated, how could he tell if there was a RPM rise before cutoff?

Also the AA5 aircraft manual suggests to adjust idle mixture with the engine running and repeat the test?

The two (idle speed idle mixture setting) are related, adjust as necessary. I would adjust the idle mixture to see if I could get a smoother idle.

And yes, adjust while running is the factory manual recommendation.

If you don't adjust while running, at least do the run up to 2000 rpm bit before checking any adjustments.

 

[Kato's 8]

Thanks for the input guys! I think I ned to try backing the screw out a 1/4 or 1/2 to start with and then check on the next short engine run to see if anything improved. I can also try leaning out to stop with rpm at 100rpm possibly to ee if it shuts down any smoother. I like that idea! And then after engine break in, That is when i do the fine tuning with engine running it looks like. Thanks again! Now i just need to find out which screw it the idle mixture. Hmmm.

 

[crabandy]

Big screw with a spring under it on the backside (toward tail), reach in-between the exhaust near where the carb meets the sump and you'll see it. You can generally reach it with the cowling on.

 

[Kato's 8]

Thanks again. I actually found a video on EAA that showed the mixture and idle screw locations. The idle screw is just where you said and is knurled for turning by hand and sloted for screw driver too. Nice!