Carb heat issues in a Super cub 0320 150

HI,

I have been chasing a carb heat issue and carburetor (maybe) issue for a while. New Carburetor by the way. I installed a new carb and the Atlee hot rod exhaust. Ever since then I am getting engine roughness with the carb heat on. If you give it a burst of power at low rpm it stumbles pretty bad. Sometimes with out the carb heat on it will stumble at low rpms. Plus just doesn't sound really smooth sometimes. I have been chasing this with idle mixture with no luck. Bought the carb from avstar. Marvel Schebler just like what was on it. No luck getting them to call me back. Just wondering if anyone out there has had this problem and figured it out.

Thanks in advance for any help with this issue,

Thanks Scott

Check your intake gaskets.

Charlie Longley said:

Check your intake gaskets.

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They are new as well. But I will recheck them for sure. thanks

scottsville said:

They are new as well. But I will recheck them for sure. thanks

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A full engine monitor would be very helpful. You could then compare data from when you had the previous carb/exhaust installed and see any differences. Making multiple changes at once always complicates troubleshooting. Do you have a carb temp gauge? Is the RPM drop on runup where is stumbles most? Is the airflow from the exhaust somehow restricted?

But lets start at the beginning. What prompted you to change out the carb?

What did you set the idle mixture to? What is your idle rpm, when you are adjusting the idle mixture?

A leaking primer can cause issues, have you eliminated this? Are your spark plugs black with soot?

you should list your location under your name, so we know if you are dealing with cold or hot air intake.

Need some more info. Are the symptoms constant or just happen every now and then, or just at start up? What is your RPM drop when pulling carb heat at 2400 RPM? Do the symptoms go away if you lean the engine out. What is the EGT change from full rich to first cylinder going LOP. What temps/altitude was the test done at. One thing to check is make sure the screws holding the float bowl on are tight. Look for fuel/fuel stains on the sides of the bowl, shake the air filter see if bowl feels loose.
DENNY

nanook said:

What did you set the idle mixture to? What is your idle rpm, when you are adjusting the idle mixture?

A leaking primer can cause issues, have you eliminated this? Are your spark plugs black with soot?

you should list your location under your name, so we know if you are dealing with cold or hot air intake.

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I am in Texas. It started by having a loss in power. And I am taking it to Alaska this spring. So I decided to replace the exhaust, carb, mags and light weight alternator. Also replace intake gaskets etc... just to have it all new and figured it was the exhaust or carb causing the power loss. Wanted to change them one at a time to find out really what the issue was. But it didn't get to happen that way. It was changed all at one time and it did fix the power issue. The exhaust system was really bad. So I am sure the hot rod exhaust fixed the power issue. It just came out of annual and no leaks on primer line and spark plugs are good and about 80 hours on them. It seems when it is cold/1st starting it up. Really no issue. When you are flying when the issue really comes out. If you pull carb heat while flying at low rpm it drops the rpm more than normal and if you try and give it throttle it will stumble pretty bad then pick up. Sometimes even with no carb heat it will stumble a little at low rpms. Not near as bad or consistent tho. On the idle mixture, we set it for slight rise when you pull mixture. But that is really inconsistent as well. You can get it set to rise and then the next day it will not rise when you shut it down. I have even taken it up and leaned it out and tried it with carb heat on with low rpms and no change. It only happens at low rpms. At higher rpms the carb heat doesn't change it much and the throttle response is there.

FYI. It has about 500 hours on a penn yan overhaul. Narrow deck 150 I have had it on a spectrum and it shows it runs very smooth.

Thanks for all your help. Scott

If you have slick mags check the internal timing. Do you have a 4 cylinder get/cht?.
DENNY

DENNY said:

If you have slick mags check the internal timing. Do you have a 4 cylinder get/cht?.
DENNY

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I have Bendix mags. No engine monitor of any kind. Thanks Scott

A friend had similar issues I think it ended up being the venturi was out of round if I remember right.

Sounds like it's running too lean? Mixture cable set wrong?

Glenn

This sounds similar to the issues caused by some of the “one piece” carburetor Venturi swaps that started back in the early 90s; getting rid of the two piece style that could work loose.
Some would run noticeably rough and also rich.
The factory tried to fix the problem with a fuel nozzle change to an atomizer nozzle with mixed results.

Many people decided to avoid the problems by keeping the old two piece Venturi and inspect every annual.

You may have gotten a bad mix with the new carb you installed.