Citabria 7GCAA Engine running rough at full throttle

B

bbailey

New member
I need some suggestions as to why my engine is running rough when applying full power for takeoff.
I have smooth mag checks with about 75-100 rpm drop each. One mechanic says I am getting carb ice, another says not. I have flown several carb engined planes in my years and have never experienced ice. I now have gotten rough running engine for about a minute after going full power for the last 20 plus take offs. The engine runs rough and after about a minute is smooths out and runs fine. I have tried leaning for takeoff and also running carb heat for about a minute before takeoff. Sometimes it seems one or the other may help but nothing consistent. Anyone have any suggestions?
 
Roughness at high rpm could be an intake leak.

If that were the case, I would not know why it would smooth out after a minute.
 
I used to get this complaint quite often when I ran a flight school. Engine running rough 7GCBC
Pilots were not engaging the primer properly - in and locked. Sometimes missed the lock part and the engine would run rough. Only a thought.
 
Primer is a good check, also check to see if your carb has all the latest service bulletins complied with. I've replaced about a third of the metal floats i've run into in the last 5 years because of sinking or sticking issues. New blue composite floats work great.
 
Thanks for the input. I still haven't solved my problem. Weather and work has not let me fly but once this week. I did try taking off with only leaning and still got a little roughness. The primer is in and locked. I had a mechanic suggest using TCP fuel additive. Does anyone have any experience with the additive. I am using 100LL gas. If weather clears I plan on pulling cowl off and checking intake this weekend.
 
Don't use a fuel additive. Lycoming engines DON'T need anything besides 100LL to run well if everything else is working right. O-320-A2B? Is this a new development, or has it been doing it the whole time you've had the plane? How many hours on engine?
 
The engine has done it since I bought it in October. The engine was warm when it was test flown. The engine has 102 since major overhaul. It is a O-320-A2D. I looked through the yellow tags and noticed that the carb was sent back to the overhaul facility two and a half years after overhaul. The engine had about 75 hours smoh when I bought it. I called the overhaul shop and he told me they sent it back to have a larger jet put in because it was running lean. I then called the carb overhaul facility and they checked the paperwork on the overhaul but said it did not list what size or part # they installed. The overhaul shop said he tried to get the previous owner to take it back off because of roughness but said the owner said it was not bad enough to worry with. Right now I am thinking it may be the carb, but I can't get it to not run rough by leaning. I am still researching. I guess there is a fine line between to rich and to lean for takeoff.
 
Is it a narrow deck or a wide deck lycoming (wide deck S/N ends with "A")? What is the P/N of the carb? (10-xxxx). These are critical questions when diagnosing carb problems on these engines. Early O-320's had a different sump than later ones. Rarely is there a 'legal' richer jet for a carb. There are a couple instances where it's possible, but you might have a missmatched carb/engine.
 
The engine has sn-L-49235-27A and the carb is part # 10-3678-32 sn-A-43-21275
I flew Monday and took off leaning mixture a little and engine had only minor roughness. I believe it is too rich but have to get a few more takeoffs to be sure. I am not sure what happened with the jet in the carb. I have a 8130 for carb when engine was overhauled (2008) and another in 2011 with no entry in engine logbook for 2011 tag.
 
Ok, engine and carb match. My '70 GCBC with that same setup ran cool, even with 90+ degree F Socal days. There is nothing about the 3678-32 carb that should run lean. My next suspicion would be a sinking float. They started going composite a few years ago. Whenever you have the cowling off next, see what is stamped on the dataplate of the carb. Should be either MF (Metal Float) or FF (Foam Float).

I've replaced about 7 metal floats in the last 5 years that were in various stages of sinking/sticking, and I don't even work on airplanes all that much. If you do have to pull the carb off again, I'd put a stock jet back in it for now. If they drilled that jet, it could be it's not even sealing well against the needle.

Sorry about all the questions, this is a pretty cumbersome process over the internet.
 
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