Idle surging?

[maxxmopar]

What would cause the idle to surge? Its a new 318 with a 625 cfm demin carb and a electric fuel pump. You can hear the surging through the exhast and sometimes you can hear it while driving around 2500 rpm.

[pishta]

Surging is usually caused by a lean condition. Jet up, or go to a .5 larger power valve. That would get more gas going at a higher vacuum.

[Dr. Righteous]

FIRST of all check for vacuum leaks. Your jetting is probably ok if your 318 is relatively mild. If you have not already gone over ALL rubber gromits, hoses, carb spacers, gaskets, ects looking for a leak do so. A vacuum leak problem can drive you crazy trying to 'tune' it out by dicking with the carb settings.

[turbododge]

Like the Dr said, look for vacuum leaks first, and also check your plugs to see if they are running lean or uneven between cylinders. Going to a different power valve is not a good idea, as you are running lean under cruise conditions, so you don't want the pv open. Jetting up would help the 2500 rpm situation, but would do nothing at idle. Richening the idle a bit might take care of both, as the idle does contribute at low cruise. Also be sure your float level is correct, as it will affect all of the carb functions.

You might also try unplugging the vacuum advance to see if the surging goes away at cruise. If it does, you may need a different vacuum can, or less mechanical advance.

[Luther]

My vote is that you probably have a vacuum leak since it surges while cruising and idle. Check for leaks first.

If it only surged at idle, I would think a higher vacuum power valve is needed. Another possiblity for surging is the idle adjustment may be turned in too much which would cause the idle fuel circuit to fluctuate.

If the engine only surged while cruising at speed, (say down the highway at 3000 rpm without much load, i.e. all of the mechanical and vacuum advance is in), then you are probably running too much vacuum advance. Do a test by running it with the vacuum advance hose disconnected and plugged to see if the surge is cured.

[65DartMan]

sounds like a vacuum leak but also check the vacuum advance if it has 1, you can pull the hose off and block it to check it.