Hi, we were flying at about 1000 feet with an outside temperature of 12 degrees, in cruise power 2350 rpm x22 inch, when applying hot air to the carburetor the engine began to cough, the air was removed and the engine began to improve, when applying again the air did the same and it could be seen that the collector rose to 28-29 inches while coughing as if stuffy. can someone help us what can it be? we checked everything and there is no sign of problems

thank you !!!

Sounds like a cold air induction leak. Might need to set the shop vac to slightly pressurize and squirt all areas with a bubble solution and start looking.

I'm confused by this, when you apply carb heat in cruise with the mixture full rich you will get a rough running condition, that is normal. When you introduce hot air you are enriching the fuel/air mixture which can and normally does cause rough running. If you are leaned close to peak you can add carb heat without the stumbling.

It is odd that you observed a rise in manifold pressure? Again normally when you introduce hot air the manifold pressure should drop slightly.

As the other poster noted you can also get weird things going on with an induction leak, they can be quite insidious on a big continental.

Need a little more information:

What aircraft?

Location? (super high humidity in AK for these temps currently)

What winter kit are you using?

Is your intake balance/crossover tube insulated?

Checked crossover tube for cracks?

Have you run any red bottle alcohol in your fuel lately? This is counter intuitive but it tis the season.

Exactly how long did you leave the carb heat at full hot?

Is your carb air inlet temp gage functioning? And do you use it to apply carb heat accordingly?

These engines are ice machines and under certain conditions it can take some time to melt all the ice. Some aircraft of the same make seem to ice up more then others although I’ve always felt this was really just a function of the carb heat box air door seal and engine baffle condition.

If you have her in a warm hanger only takes about 20 minutes to pull the top Cowl top plugs and do a compression check. Bad intake valve?

Last thing, is this a known plane and a new symptom or new plane out for her first cold winter run with you at the controls?


Rocket