help with lycomming engine to lean

Guys, My lycomming 0-320 is running a little lean. When under full power on a hot day with two large people on board. It hits 1480 F on three cylinders. I have to back off the power [only about 300 RPM] to bring the temperature down. I've check my lean control and it goes to full OK. I've turned the screw several times counter clockwise to open it up. Nothing seems to work. I'm OK were it's at. But would like the extra power without taking any chances on damaging the engine. Could it be something with the carb jets. I know very little about this engine. But I do know it's certified. But the mechanic I use has never tore into a lycomming carb before. Any suggestions on how to fix this problem.
Oh ya, I forgot to say, I have two fuel pumps, it does the same with one one on as well as both turned on.
Any ideas is greatly appreciated.

Is it possible it has a vacuum leak.

j bird said:

Is it possible it has a vacuum leak.

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good point.

When I had an issue with my carb I sent it to a place that specialized in aircraft carbs. I don't think they do that work anymore though, I might be wrong. They are in Amelia Ohio.

I do know there are some issues with float bowls on some of the Lyc carbs and this could be related to your issue.

It might also be possible your carb was jetted for use at higher altitudes.

I can tell you that the my Twinstarr with the O-320 can get so rich that it will blacken the prop and lead up the plugs if I don't lean it out. My CHT says nice and low even at full honk.

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sounds like an intake leak,if adjusting the carb does not help. I would check the intake couplers,it that is not the problem,then check the jets.

you can check the intake leaks by spraying some wd-40 on them with the engine running. be careful to not get into the prop while spraying it. if there is a leak the engine will speed up when you spray it.

Tim, no mater where I adjust the carb, it never gets rich enough to blacken the prop. I will try to find a lycomming shop.

Animal, I do not have any vacuum hoses that I know of. I will check for an intake leak.

Brent Drake said:

Guys, My lycomming 0-320... When under full power on a hot day with two large people on board. It hits 1480 F on three cylinders. I have to back off the power [only about 300 RPM] to bring the temperature down....
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1480F on the EGT is not unheard of nor is there a redline that I know of for EGTs. What is the forth cyl telling you? What are your CHT's? High CHT's will trash your cylinders long before EGT's become a problem. There is a definite red-line on CHT's and the factory's number, from what I read, is way too high. Try to keep the CHT below 380F with a max of 400 for a short duration such as climbs.

What carb do you have? From your description it sounds as if you don't have a mixture adjustment in the cockpit which seems odd. Most carbs have an enrichening circuit that kicks in at full throttle only. For this reason it is usually advisable to climb WOT (wide open throttle). I don't believe this setting to be set with an enrichening screw.

From the Lycoming Opertor's manual;

For maximum service life, cylinder head temperatures should be maintained below 435 0 F. (224°C.) during high performance cruise operation and below 400°F. for economy cruise powers.

The red line for the cylinder head temperature would be 475 degrees and the red line for most exhaust gas temperature would be 1650 degrees.

I am going to assume that you have a carbureted engine.

On a carburated engine the only adjustments are the idle speed, idle mixture, throttle position and the fuel cutoff lever.

First check to make sure that the throttle is pushed full in. Open the cowling and make sure the carburetor has touched the stop. If it has not, then adjust the clevis to hit the stop and the throttle in the cabin should have about 1/8 inch cushion.

Second, check the mixture cutoff lever to insure with mixture pushed full in that the lever is contacting its stop with 1/8 inch cushion in the cabin. Also make sure that when the mixture is pulled full aft the the lever contacts the other stop.

After you have made theses checks, warm up the engine and with oil temperature at operating temp, 180 degrees or there abouts, with the engine at idle RPM, 650 RPM, slowly pull the mixture control out and note the RPM increase. If the RPM did not increase then the idle mixture is to lean and you need to turn the idle mixture screw out to enrich the mixture. If the RPM increased the the mixture is rich and you need to turn the idle mixture screw in. You are looking for a 25 to 50 RPM increase when you slowly pull the mixture out before it tries to quit.

The idle speed is controlled by a screw on the throttle shaft and most most engines will set to 650 RPM.

Most common cause is the gaskets between the induction pipes and the cylinder heads. They go very brittle and stop acting as a gasket.

Sometimes it can be seen as a little fuel staining around the clamps.

Remove the 4 induction manifolds.

The old gaskets will probably be as brittle as glass. replace them with new ones.

Before re-installing the induction pipes remove the clamps. Lay a piece of sandpaper on a very flat surface and sand the face of the clamp, you will probably see that it has taken up a curved profile with the bolt holes at the high points. Obviously don't go mad, just enought to ensure the gaskets are tight when refitted.

This is so common with Lycomings we now change the induction gaskets about every 6 months or so.

Try this before messing with carbs and jets. Its normally the simple things, especially if it all works OK otherwise.

Ok, Thanks guys. I will double check all the pipes. The engine has not been torn apart since major in 1994. Besides the engine running a little lean, it runs great, no leaks, and uses only one quart between oil changes. I can see where a cracked gasket could cause alot of problems. Thanks again.

Let us know how it works out, I think you will be suprised how brittle those gaskets are compared to new ones.

One other thing to look at is the priming system if fitted. The pipes crack and allow it to suck air. If the priming system is not fitted make sure the blanks are tight and sealed. They are square drive blanks on the lower left corner of the cylinder heads if I remember correctly.

Brent, once you find your problem you might want to install a mixture control on your machine. I didn`t like the stock mixture control on my Twinstarr so I put this on mine. It is easy to control the mixture with this.