Idle Hunting on 454 MIE

[rondds]

Head, intake manifold, carb and distributor was off in early spring to retrieve a broken exhaust manifold bolt. All was reassembled seeming without incident and after adjusting the timing and idle, the engine is hunting at at idle btwn 700 and 760rpm. The carb gasket is new and there are no visible issues with the intake manifold gaskets. I didn't touch the mixture screws at the front of the carb (original Rochester 4BBL).

Engine runs fine other wise, although I have not run the boat in gear much yet - only from the sling to the slip.

Any ideas?

 

[wileecoyote]

Did you verify that the timing was set properly?

 

[JediJD]

Hunting is pretty classic vacuum leak (or dripping carb). I really like to double check intake and base gaskets with a bottle of propane and a hose...just squirt a little propane around the bases with the engine running. Watch the tach for movement of 50-100 rpm and you will find your vacuum leak. If no vacuum leaks, run your idle mixture screws all the way in...bring them out about four turns, then take them back in a quarter turn at a time until rpms drop 50 rpm or so, then back out a quarter turn, and you will be within spitting distance of pretty close to right.

 

[rondds]

Mark
We did verify the timing initially but I haven't check it again. I will look at that again to be sure.

Jedi
It might be hard to verify the vacuum leak as you describe b/c the engine is up and down by around 50rpm all the time. I have been using a handheld digital tach that works off reflective tape.

 

[wileecoyote]

Maybe it's a bad governor!:smt043:smt043:smt043

Sorry for the bad inside joke! Just couldn't resist!

 

[stuckinforward]

Vacuum, no doubt, thru the shafts on the carb?? seeing you have twins, swap the carbs and will the problem follow?

 

[wileecoyote]

Did you ever fix it Ron??

 

[fc3]

Hunting is pretty classic vacuum leak (or dripping carb). I really like to double check intake and base gaskets with a bottle of propane and a hose...just squirt a little propane around the bases with the engine running. Watch the tach for movement of 50-100 rpm and you will find your vacuum leak. If no vacuum leaks, run your idle mixture screws all the way in...bring them out about four turns, then take them back in a quarter turn at a time until rpms drop 50 rpm or so, then back out a quarter turn, and you will be within spitting distance of pretty close to right.

Yes, on a car this works but holy crap! No propane on a boat!

Anyway, obviously, if you're going idle hunting, you need an idle gun. Can't hunt idles with an elephant gun for Pete's sake!