Thread: O-320 Help!
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Old 01-20-2020, 12:20 PM
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avrojockey avrojockey is offline
Join Date: Aug 2019
Location: Appleton, WI
Posts: 482

Originally Posted by 8213C View Post
I am having issues with my EGTs on my O-320 . My mechanic looked it over and concluded that I needed to have the carb replaced. So I did, flew, and this was the result. I could never get #3 cylinder to go under 1500 degrees even in cruise at full rich and, 2400rpm. My temps are, #1 1380, #2 1260 #3 1510, #4 1378 full rich. On takeoff on the ground it wouldn't rev past 2200 rpm, where it use to be 2350 with the old carb. She seemed super gutless in climb out and takeoff roll. In the climb out now, I saw 1605 and 1590 on #3 and #4. I'd have to throttle down to 2200-2300 rpm to get #3 into the 1470s in cruise at full rich; she feels real rough overall and more so on the left mag. I tried different power n mix settings for about 20 minutes but no change, just higher EGTs. I had the mags sent off and one was rebuilt right before all this at the time of the new carb install. Twice now I have been told she runs great, and the mechanic has released it like this. On the 1st test flight attempt, it was backfiring bad on run up with one mag dropping 500, so I didn't even fly it. They then cleaned the brand-new plugs which have barley 50hrs on them and richened it some. I don’t understand why the plugs would be fouled already since I always lean on the ground and always lean out for cruise but anyway no bad mag check or misfiring with mags dropping 100 on both L & R. Now still the EGT issue. They are higher temps then I had before and a greater spread between hottest and coldest. The other thing is my highest temp cylinder before the carb was #4 at about around 1500. Also, when I kill it now, it has a slow drop and no rise at all.

My compression is mid 70s and had no metal in the oil. The engine has about 800hrs SMOH. At this time, I had the 4-cylinder EGT gauge installed and I realized it was too hot and was told to change the old carb out for a new one would solve that issue. I have flown it 55hrs since the annual and now decided to go for it and replace the carb in effort to help those EGTs. I feel like I should have just left it alone at this point.

My mechanic seems to not be sure what to do right now.
Do I just need someone who knows how to tune this thing or any other advice out there?
Why would #2 be so much colder?
Why is just once cylinder so hot?
Why the lower RPM on the ground after being richened up?

I would tend to think this was originally an ignition issue, and has been complicated by incorrect solutions. Poor ignition will slow burning and as a result will increase EGTs because the air/fuel mixture is still burning while exiting the exhaust. You can see this when you drop to one mag on a run-up...EGTs will rise and CHT will drop. The fact that EGTs are different from one cylinder to another is a little misleading and not necessarily bad, EGTs are not to be diagnosed like CHTs.

The fact you had a mag rebuilt and you were obviously having ignition problems at the same time tells me there's something wrong with that mag or that side of the ignition system.

Read this Mike Busch ARTICLE and then do a mag check.

Do a run-up in accordance with Lycoming SI 1132B. Make sure the RPM drops are within said limits, and note the EGTs and CHTS during the mag check. This will give you a good idea how your ignition system is running and possibly narrow down the problem.

If the run-up is good and you feel OK flying, then do an in-flight ignition check after leaning for cruise. This may give you more information regarding the health of you ignition.
Tim Holmes
Appleton, WI
RV-9A N904DC
2022 Dues Paid

Last edited by avrojockey : 01-20-2020 at 12:30 PM.
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