Carb issues...

Engine ran perfectly before i removed the carb, but a couple drops were found on the FAB top after sitting over night seeping from the base of the bowl. I just had my carb (MA4-5) bowl replaced (shop that yellowtagged it) because it was seeping and a Kelly aerospace float installed. I just reinstalled the carb and did a test run (air screw 1 3/4 out gives 10-20 rpm at idle cutoff), idle circuit runs perfectly but as soon as I advance the throttle past about 900 rpm it almost instantly wants to kill the engine.
Only thing I've changed besides removing the carb was removing and reinstalling #4 intake pipe with new gaskets etc to replace oil return line fitting.
I used new gaskets for the carb and all temps and pressures were normal with 56 OAT.
Ideas?

I would try backing out the idle mixture adjustment a bit more for about a 50 rpm rise, to lean an idle mixture will cause off idle stumble.

Thanks Walt, I can try that but I did go another 1/4 turn out with the same results. Idle was perfect but anything off the idle circuit above 900 ish RPM the engine fell flat.. I double checed the idle screw position and it was correct, originally it was 1 7/8 out when it previously ran fine. I tried advancing the throttle slow/fast half to full throttle with the same result every time. I think it is something internal in the carb. An A&P friend said he would help me pull it apart tomorrow so I will probably go that route.

Mixture control?

If you only changed the carb bowl, perhaps you didn't get the mixture control properly inserted in the angled tube in the bowl. Easy to get it off to the side.

Not sure if that would cause your symptom, however.

Thanks Pete, I'll check that when I take it apart tomorrow.

I didn't replace the bowl/float, the engine shop that provided the yellow tag for the carb serviced it for me. I'm a little reluctant to send it back a 3rd time with my airworthiness inspection in 2 1/2 weeks.

float level

hard to tell without being there but it sounds like the float level is low. Either that or a massive air leak. could be accel pump problem too.

Chris M

Accel pump is pumping, boost pump on and pumping the throttle sends gas running into the airbox.
A&P who volunteered to help me tear it apart tomorrow said it is definitely a lean condition, although I'm thinking its lack of fuel not an excess of air. He did ask if I stuffed any rags/etc into the engine that I might not have removed, surely I'm not that careless and dumb.......although its an easy fix and I have been short on sleep...
I'll be sure to check the float level tomorrow.

Maybe go back and check both ends of that #4 intake tube.

I've thought about the #4 intake tube, but besides being certain I put everything together correctly it started and idled perfectly. Not pretty good but in the 5 or 6 runs it started within a couple blades with no pumping and would idle perfectly smooth to about 550 Rpm with a Catto prop. I believe intake leaks would be more prevalent at idle correct?
Because was it runs so well at idle and the carb was recently taken apart I'm suspecting something wrong with the main needle/jet.

crabandy said:

... (shop that yellowtagged it) because it was seeping and a Kelly aerospace float installed. ...

Click to expand...

Why would they do that ... Doesn't meet the MS SB

Today my A&P friend helped me check my carb out, we found nothing. Float level 13/64 above the gasket, mixture metering valve positioned correctly in the mixture metering sleeve, air metering pin jet adjusted correctly, accelerator pump functioned correctly and all ports clean and clear.
We reassembled and installed on the engine, it ran the same. It idled well, but was still lean we progressively moved the air screw out until it almost fell out (6turns) without any signs of a rich mixture. We were able to get it to run at 1700-1800 rpm by rapidly pumping the throttle 6 or 7 times, it ran solid once it achieved 1700-1800rpm but adding carb heat would increase RPM 100 indicating it was still lean.
I'm mystified how it ran perfect before, than has the lean issue after swapping bowls with no other changes. I talked with the guy who swapped the bowls and he used the internals of the old bowl and swapped them to the new bowl.

Good news is I now know what all (mostly all) the various ports/screws/doohickeys on the MA4-5 are supposed to do, bad news is I still have a messed up carb. I ordered another carb today for several different reasons but would still like to solve this puzzle.

I received my old carb bowl today, while comparing the old (ran good but seeped from steel ball ports at the bowls base) and the new bowl (no leaks but ran terrible) I think I found the problem.
The old bowl has two ports next to the mixture metering sleeve, 1 on each side.


The new bowl only has 1 port next to the metering sleeve.


I'm going to do some more checking but I bet a six pack of good beer this is the cause of my carb problems.