Low RPM O235-C2C-Help

Just had the carb on my O235-c2c rebuilt with a new one piece venturi as required by an AD. Put it back on my champ and only getting about 2400 rpm versus 2750 before. Maybe I installed the throttle cable wrong or something. Before I dig into it I though somebody here might give me an idea where to look first. Appreciate the help.

I don't think the AD is a "required" from the perspective as unconditional or Manditory. If it's th AD I think your refering to,one option would be to continue to inspect the 2 piece configuration for tightness. Many aircraft owners have remained with the 2 piece because they feel the engine runs better. DOn't know if that's your RPM problem but it certainly could be.

Read the AD. There is a remeady for the loss of rpm. You can inspect the two piece every 100 hr. When you say rebuilt, was it flow checked?

Actually - I think there was a AD and then it was rescinded. But at that time I already had taken the carb off so my mechanic figured just as well put the one piece in. When he was in there he found that the float was either shot or one that could no longer be used, and he replaced that. So basically it has a new one piece venturi and a new float. I do not think he flow tested it. Do you think there is a problem with the new venturi or float? Or did I screw up when I put all the cables back on? Carb heat gives a 200 rpm drop, as does mixture, so I think those are right. Runs real smooth, just feels like it has a rev limiter on it. Looking for ideas before I take the #$% *%@# cowl off again.

Checking the cable travel is easy. Look the carb with the throttle in full position and see if you can move it any farther and check to see if its against the stop.

Are you comparing like outside conditions? I rebuilt an O-290-D2 recently. The kid who flew the airplane swore it wasn't developing full power. Figured out he had only flown the plane in the cold of the winter and the hat of the Texas summer made a big difference in performance. On the other hand the one piece venturi changes air flow in some carbs and they need to be flowed and rejetted.

I had trouble with my one piece venturi and found that the item was very rough with casting flash. Smoothing it out with some emery cloth and reinstalling made a world of difference. My friend with a taildragger c-150 had to go back to the two piece after he had rich mixture problems with the one piece. After hearing what I did, he smoothed the one piece and tried it again with no problems. This would all be noticed, I guess, if the carb was flowed.

Thanks for the input - I'm going to relay this info to my mechanic. I have flown it a lot on floats in the summer, so I know what the performance should be. It's on wheels now so it should be even better. Normally as soon as it breaks the ground you need to throttle back or it redlines at about 2750. Now, it level flight, could only get 2400 with the throttle to the stops. Of course I didn't find out it was going to rev up until I was halfway down our 800 foot grass strip and committed to flying. I was definitely puckered up going over the trees just above the tops. I guess it is a good lesson-tie it up to something and test it after you have messed with the carb before you have to make a short/soft field takeoff.

See what kind of rpm increase you get while leaning on the ground.

OK - I took the lower cowl off, the throttle goes to the stop, as does the mixture both ways. The carb heat was hung up in the 1/2 on position (the cable was sliding through the bolt hole) but not sure if this just happened or if it was this way when I test flew it. I fixed that, now the carb heat opens and closes all the way. I tied it up, and it would go to 2375 rpm on the ground, with about a 75rpm increase to 2450 when leaned a little bit. Red line is 2750, so this still seems low. It used to redi line in the air, and I cruised at about 2450. But I don't know if it will red line on the ground when tied as I had never tried this before. I wish I would have tied it up and tested it right when I found the carb heat in the 1/2 on position - before I fixed it - then I might know if that was the problem. Do you know if an airplane will go to max RPM tied on the ground? or do they need to be in the air moving? I'm hesitant to take it in the air again off of our short strip until I'm sure its fixed - then again I also hate to take the carb off and put the old two piece venturi back in if all it was was the carb heat stuck 1/2 on. Anyone else ever tied em up and compared the rpm on the ground to rpm in the air?

When you are on the ground (not moving) with full throttle, this is called the static thrust or rpm. Your airplane TCDS or operational manual should have the minimum static rpm that your engine combined with the propeller that you are using. If this is within the acceptable range it should be correct. If you are still in doubt, no matter what, call your mechanic.

Well the mechanic put the 2 piece venturi back in and it runs great. Damn FAA - all the hassle for something that didn't need fixing. It was more dangerous with their fix than the old part.

superchamp said:

Well the mechanic put the 2 piece venturi back in and it runs great. Damn FAA - all the hassle for something that didn't need fixing. It was more dangerous with their fix than the old part.

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I think I'd blame the people that designed the new venturi and the people that approved it-- Oh I guess you did half of that :roll:

See my post back up the page and get out your emery cloth...... :-?