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Max RPM Ground Runup Question

blueflyer

blueflyer

Well Known Member
So I just installed an overhauled carb on my plane. I was doing a max rpm ground runup. When I did the max rpm ground runup with the new carb, I got to 2250 rpm and it started to stumble a little bit and rpm dropped to about 1900. I then leaned the mixture (throttle still full wide open) and the rpm came back up to 2250. Is it normal to not reach max ground rpm in a full rich mixture condition on the ground? Do you typically need to lean the mixture to achieve max rpm on the ground?
 
It depends on the DA of your airport.
Most carbed planes I have flown had an individual taste for ground leaning that I had to figure out. A newly overhauled carb will likely change those tastes and you will need to adjust your habits.
 
Good point. I’m at a sea level airport. It’s been so long since I did a max rpm ground run, I can’t recall if leaning for max rpm is normal or not.
 
Possible an air leak in the carb gasket or induction leak? I have a carb O-360 with constant speed prop and I can static run to near full power going above 2600 RPM before I have to back off as I'm skidding across the ground as the brakes can't hold it. Engine runs completely smooth full rich from idle to full power. This is at about 80 feet above sea level.

Also check the throw on the mixture cable. Meaning make sure the mixture level on the carb has full range of motion to the stops and the cable isn't trying to push it further. Mine was adjusted wrong pushing. If adjusted wrong cable all the way in can put significant pressure on that lever at full rich and for me seemed to ending up leaning it a little. Backed it off so it there was just a tiny bit of pressure and lean condition went away. Noticed because of higher than normal EGT on take-off.
 
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next time capture the fuel flow indication when that happens. that would help determine if the jetting is too rich. i would say that is not normal, but haven't had a carb in forever.
 
Have someone check your exhaust for black smoke when you try to get static RPM. Best to tie the airplane down. Fixed pitch? You should see 2200-2450. Constant speed should be able to make 2700 or thereabouts, and either way, it should run smooth. I wouldn't attempt a takeoff until this is resolved. It does sound as if you are too rich. Leaning for takeoff at high density altitude is normal. Needing to lean at sea level is not.
Ed
 
How was it running before the new overhauled carb installation?

I was chasing an issue and the suggestion was to replace the carb which I did but that didn't turn out to be the problem. The problem finally figured out was the electronic ignition advance was not behaving correctly. Meaning the variable timing was not working correctly resulting in poor running under higher power settings. Fixed that and now it runs buttery smooth through the entire range of power. I'm happy I have a "new" overhauled carb and I think contributed to the smoothness but at the same time the original problem was the electronic ignition.
 
blueflyer said:
So I just installed an overhauled carb on my plane. I was doing a max rpm ground runup. When I did the max rpm ground runup with the new carb, I got to 2250 rpm and it started to stumble a little bit and rpm dropped to about 1900. I then leaned the mixture (throttle still full wide open) and the rpm came back up to 2250. Is it normal to not reach max ground rpm in a full rich mixture condition on the ground? Do you typically need to lean the mixture to achieve max rpm on the ground?
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Anytime I am taxiing or idling, I am more lean than middle or rich. And with our humidity, I use carb hear for up to a minute before takeoff. Seems to keep the plugs and carb cleaner...full rich for takeoff then lean as I climb...A&P told me to do that.
 
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