RPM Rise with Carb Heat On?

[uwochris]

Hey guys,

The other day as I was entering the pattern to land, I put the carb heat ON, and noticed the RPM increased (maybe 50 RPM or so, perhaps a little less). No engine roughness occured, and the conditions were not very favourable for carb ice (cold/dry day).

What could be the cause of this? The only thing I can think of is an improperly set mixture- ie) carb heat ON causes the mixture to be slightly more rich, so perhaps I had the mixture too lean?? I usually do fly with the mixture lean, but never passed engine roughness.

Thanks in advance,

Chris.

 

[minitour]

Maybe the air intake was clogged/blocked and when you went to the alternate source (carb heat) the air was able to flow through the carbeurator "normally" (although hotter) giving you the boost in RPMs...

I noticed that on my instrument 'ride and when we got back we checked it out and sure enough, the filter was completely soaked which restricted the air flow...

I saw quite a bit of jump though (around 3-400 RPM) when I kicked in the carb heat...so with only 50 that may not be it...

just a semi-educated guess.

-mini

 

[avbug]

If you were operating close to stochiometric (peak) on your mixture, application of carburetor heat increases the induction air temperature, decreases induction air density, and increases the mixture toward rich. If operating lean, moving your mixture toward peak will show as an increase in RPM.
,
You'll see the same thing by leaning on a fixed pitch installation, watching your RPM peak (static), and then continuing to lean. You'll see RPM decrease. Enrichening the mixture causes the RPM to rise again. Same with adding carb heat when the mixture is leaned too far.

You may have had some carb ice forming, and cleared it.