motor surges

[chevy 37]

Man oh man. Just when I get thing running great another problem pops up. My truck will surge, tack will move back and forth when I'm traveling up to 40 MPH. Tack will move around 200-300rpm's. After 40 theres no surging at all. Checked for vacuum leaks, carb seems to be OK as far as float,choke,etc. Timing is right on at 35 degrees total. Any suggestions?

[chevy 37]

It seems to just to happen when under load.

[76GMC1500]

It's too lean. The 72-75 Qjets in cars were notorious for this. The heavy duty trucks, over 6000lbs GVWR, were generally ok because they didn't have to deal with as stringent emissions standards.

[sparky2263]

Try swapping the vacuum advance from ported to manifold or vice versa and see if it goes away.

I'm with the above on the lean mixture also. Good possibility.

[chevy 37]

Carb. is a 600 cfm Edelbrock. Denny, will check on the torn diaphram. Sparky already tried that, no difference.

[WRENCHD]

didn't catch what kind of engine you are running, but if you have a sbc and it has a egr valve then maybe you could check out the valve to make sure that it is seating all the way, and check to see if maybe the spring is not too loose and it is coming off seat to early giving you this problem. if it is there then take off the vac line and try again to see if the prob goes away good luck

[chevy 37]

Found that the vacuum hose from carburetor had a crack in it. Fixed the problem real nice.

[chevy 37]

Yep out of this world man. Just jiving with my engine and the bitching thing just pulled a groovy thought out my old geezer head and by golly miss molly I fixed that with like man I was hanging 10

[R Pope]

Glad it was cheap to fix. For reference, surge is caused by lean mixtures, or too much advance.