[jeff beckley]

I have a 0-360 with 90 hrs on rebuild. Ran some what smooth until I installed the carb kit for the ma4-5 that fattens up the top end. Now the engine runs like a dream. But... I now notice a loopy idle until it has completely warmed up.
I have tried to lean the mixture but showed no positive results. The engine smooths out above 1500rpm. No change with a mag check. Checked for a leaky lifter and inspected the valve springs. It idles kind of like a big block with a big cam. Anything else I could check?

 

[SportAvServ]

Carb stuff

It's hard when your not there, but, basics to me would be what ever you changed is more than likely the problem. Loping could be because the idle circut is too fat. When I used to race motorcycles, if we had a problem we would run the bike at the speed that the problem exisited, then shut it off electricaly at that speed and pull the plugs and look at them. Sooty or lean? If you lean the mixture to shut the engine off, it will change the color of the plug, just shut it off with the mags. If the mixture is OK, the next thing I would look for is a leak in the induction, squirt WD40 at intake joints and carb mounting while the engine is idling, if it blocks a hole for a moment you may hear the engine smooth out.
Just a reminder, that metal spinny thing on the front of the engine doen't have a conscience. Other than that pull the carb and try again.

RT

 

[mahlon_r]

Try richening it up at idle and see if that helps. If it gets better as it warms up it sounds like it is lean.
Good Luck and Happy New Year,
Mahlon

 

[AlexPeterson]

snip

I have tried to lean the mixture but showed no positive results.

snip

Just to be clear, this is the idle mixture screw you are talking about? The description fits too rich. Does it sound similar to when one just starts a primed engine? That is generally when they are too rich. Also, you might see a difference with OAT's being cold outside, but usually it will go lean in the winter, not rich. This is because more air can go through the mostly closed throttle (denser air).

 

[jeff beckley]

I think i can remember that after it is running for a few min that i could in fact lean the mixture slowly and get a substantial rise in rpm and a little better idle.
The carb kit I installed was a F mod. That included a new fatter jet for the main. A restrictor plug, and I think a little fatter idle jet. I was having some back fire issues when I pulled back power on approach and an intermittent studder when the rpm was around 2200-2300. The kit solved all my running problems. I did adjust the idle screw when I reinstalled the carb for best idle but only after the engine was completely warmed up.