Binding Butterfly

[dmeloche]

Anybody ever have a binding butterfly on a marvel ma-4spa carb? I've been battling a problem with what now appears to be my carb. I was noticing high forces on my throttle cable after flying for a hour or so. At first I thought i was being stupid and not releasing the knurled knob enough.

About a month ago on downwind the throttle was essentially locked up at a partial power setting. I was able to land safely and decided that the cable was defective. I bought another throttle cable and installed it.

Today the same thing. I was able to land again safely and quickly removed the cowling and disconnected the cable from the butterfly level and sure enough, the butterfly would only move if subjected to high forces and a screaching sound was emitted. Kind of like when you loosen a rusted nut. 15 minutes later the butterfly was moving freely like nothing happened.

As this point a assume that the clearance between the shaft O.D. and bushing/hole I.D. is too tight and when the carb expands due to engine heating the shaft binds up.

This is a rebuilt carb from airmotive which was included in a new engine from aerosport. the whole setup has less than 30 hrs on it. I plan a calling Bart tomorrow but has anyone heard of such a thing before? This is scary situation and it would seem to me that a newly rebuilt carb with paperwork wouldn't have these problems. I plan on putting the carb in the oven tonight @200f when my wife isn't looking and see if i can duplicate it.

Frustrating since the weather here in Philly finally looks good for the next couple of days and I'm grounded again.

It's always something.

Doug Meloche

[danny]

Doug,
i hope you've already fixed the problem. i have seen the same in a "new" carb on a new engine and the end result wasn't pretty. seems the o/haul facility didn't stake the screws holding the butterfly in. consiquently they backed out on the first flight and each went different cylinders. luckily the pilot was still in the pattern , unluckily a brand new engine (tt since o/haul was, oh, about 10 minutes) had 2 new cylinders installed.
coulda been worse.
hope this helps
danny

[dmeloche]

Talked to Bart at Aerosport today and said this the third time he heard of this. Apparently the bushing for the butterfly shaft hasn't been reamed properly. I've mailed it back to him for repair.
So if you've got a Precision carb that has been rebuilt by Precision Airmotive and the throttle force is unconsistent have it checked out.

This manifested itself while i was trying to land. The aircraft wouldn't slow down in the flare and i was floating down the runway. I now know that the engine wasn't going to idle. When i decided to go around the throttle took a very large force to get it to full power. On the next landing i came in high and reduced power to idle further out and slipped it in. 20 minutes after i landed the throttle worked normally. Could have been a lot worse!