I have a 1958 Comanche 250, N5109P. We had the carburetor rebuilt. On the ground with a field elevation of 1080 feet, with full rich the carb heat drop is around 600 to 800 rpm, which appears to be excessive. When we lean it out a bit (about an one inch on the mixture control) the drop is more consistent with what we THINK it should be - about 200 RPM drop. Is this normal for this elevation? If not any suggestions - we have sent the carb back twice and they insist there is nothing wrong with it.

Chris,

It has been a long time since I've flown carb - don't' recall a drop that big. Is there anything in the service manual that supports your thinking? Service manual is under the Tech Tab on this website's Home page. Anything on the Lyc website?

BTW, this is how I got more knowledgeable on my Twin, one problem or check written at a time.

Chris,

A few things to check, collapsed SCat ducting from carburetor heat shroud to carburetor air box, or cracked exhaust manifold under carburetor heat shroud.

Clarence

Thank you, just curious what a "normal" drop should be. I don't see it listed in the service book.

Ours is about 2-300 during a 2000 rpm run up. Sounds like idle mixture may be a bit rich?

Andrew
61 250

I am at sea level and during run up after applying carb heat the drop is barely noticeable. I do my run ups at 1800 RPM. I do have a carb heat temp gauge and it will peg high after applying carb heat during run up.

I have a 1959 model.

I'm at 1,000 feet MSL and my '59 250 is about 150-200 RPM drop