deaconco Posted March 25, 2020 Report Posted March 25, 2020 Just purchased this aircraft. Runs well and flys great. On run up with carb heat applied drops 300 RPM, is this excessive drop for this model? Quote
David Lloyd Posted March 25, 2020 Report Posted March 25, 2020 My drop is about 200-250 RPM. On a Cherokee many years ago, had an increase in carb heat drop of 3-400 rpm. Turned out the exhaust pipe split directly under the pickup for the carb heat. Might want to check the system a little more on the ground. Quote
Hank Posted March 25, 2020 Report Posted March 25, 2020 The Owners Manual for my 1970 C says to check carb heat briefly adter startup (RPM should drop). No numbers or guidelines are provided. It does not mention checking the carb heat at runup. What does your Manual say? Quote
David Lloyd Posted March 25, 2020 Report Posted March 25, 2020 My 1975 POH does not say to check at runup either. The checklist placard does. Hmm. Wonder if that placard is original. No numbers provided that I found looking quickly. Did not see anything in service manual. Quote
carusoam Posted March 25, 2020 Report Posted March 25, 2020 Welcome aboard deaconco. I think you may have combined a few steps in one... and got a different result than documented... Carb heat is tested separately to make sure the plumbing is working as expected... Each Mag is tested separately at specific conditions... Then the prop gets tested to make sure rpm is being controlled properly, and circulate some oil... Se if you can find the proper documented run-up procedure... follow that, and then report back... a 300rpm drop sounds like a lot... Do you get 300rpm mag drops at 1700 rpm with no carb heat? Got an engine monitor? PP thoughts only, not a CFI... Best regards, -a- Quote
deaconco Posted March 25, 2020 Author Report Posted March 25, 2020 Mags check fine drop 75 to 100 rpm each at 1700 RPM. POH does not call for a carb heat check at 1700 RPM only at idle and does not specify any amount of drop only that it works at idle. Just wondering because every other carburated aircraft that I have flown always called for a carb heat check at run up RPM. Thanks for all the replys. Quote
carusoam Posted March 25, 2020 Report Posted March 25, 2020 When you get a chance... Give the carb heat system a good once over... There are plenty of small push/pull wires and worn bushings that can cause some motion challenges... All the parts are replaceable if needed... For the most part the pre-flight is checking works/doesn’t work... It doesn’t qualify how well... that is what the annual inspection usually does... So if you haven’t put your eyes on it yet... it is pretty easy to do... Best regards, -a- Quote
PT20J Posted March 26, 2020 Report Posted March 26, 2020 Carb heat heats the air reducing its density and making the mixture richer. The excessive drop might be explained if the mixture was already a bit on the rich side. You didn’t mention your elevation when you ran the test. If you were above sea level, this could explain it. Try leaning the mixture for max rpm (best power mixture) and rerunning the test. Also, try running the test at different rpm to see if the power enrichment function of the carburetor has an effect. 1 Quote
markgrue Posted March 26, 2020 Report Posted March 26, 2020 Short answer is "it depends" It depends on altitude, outside air temp, mixture, and egt. As long as it works I would not worry about it. Mark 1 Quote
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