Lycoming O-360-A4P Carburetor Issue

[Steve Pierce]

Been long distance troubleshooting a Super Cub with a 180 hp Lycoming via the TigerCub former Penn Yan STC. This issue started the summer before last, when the throttle was advanced from low power/ idle the engine would stumble. After checking primer system etc. the carburetor (10-3878) was sent to Marvel Schebler and rebuilt. This winter the engine started doing the same thing. Everything was checked and double checked. Carburetor sent back to Marvel and no change. Talked to Lycoming and nothing was suggested that hadn't already been tried. Was talking to Ken at LyCon about another issue and brought this up. Without me even finishing the story he tells me I probably have a 10-3878 carburetor and I need a 10-4164. Seems when Steve Tubbs was building engines he ran into this issue and carb could be set up to run fine in summer but then have this issue in winter or vice versa. Carburetor is suppose to show up today so it will be interesting to see the outcome. Ken also has a flow sheet on the set up of this carburetor so he can tweak it to flow correctly on his flow bench. Posted this to help anyone else that might have this issue and to see how many others have run into this problem.

 

[Dave Calkins]

Steve, I dont have the carb part number on the tip of my tongue like Ken, but I can concur that more than one of these engines have been delivered with a leaner carburetor than is optimum.

I recall an engine requiring a top overhaul due to this issue. As I recall, the cylinders were payed for by the overhauler that provided the carburetor in the beginning. That was in about 2001.

 

[DENNY]

If I was to try to dial in a carb I would want a 4 cylinder EGT/CHT and fuel flow to start with MP gage would be nice also. Sounds like it was too rich, I believe you can change out the economizer jet in the MA4-5. Check the nozzle size in case someone has drilled it. You should be able to run at least 100-150 rich of peak. If you cylinder head temps are spread out above 100 degrees (with correct baffling) Try a pepperbox nozzle. If it has slick mags try anther set or better yet bendix. I know you know this I am just rambling because you asked.
DENNY

 

[joecub]

Steve, we had the same issue on a government cub, mechanic changed carbs 5 times, yep 5, different times...still wouldn't accelerate correctly, turned out the Brackett air filter was plugged.... Logs showed it had been on there 5 years.....hmmmmm. Maybe time for different mechanic....