Need Help With Off-Idle Surge

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4woody

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I've been trying unsucessfully to tune this very slight surge out and am getting frustrated. It is a slight, repeating at a regular interval surge at what I'd call parade speed: Just rolling along with my foot barely resting on the accelerator, ~975-1100 rpm. Put your foot into it a little bit more and it behaves totally as it should.

My combo:
383 w/ 9-1 compression
Cam: Comp #21-670-4 "Nostalgia"
Dur. 280/287, Lift 474/474
727 stock rebuild
TC is TCI ~1800rpm stall
3.0 Rear gears
4000lb car

This combo ran fine without this problem till recently when I replaced my old dizzy (no vac advance) with a new Firecore w/vac advance, and replaced my old Edelbrock 1406 with a Street Demon 625 cfm.

I tried opening the idle mixture screws 1/2 turn (and backing the idle back down to 850) and that didn't help. Gave it a little more initial advance was at 18*, took it up to 20*, but that still didn't help.

What do I try next?
 
I think you are getting too much vacuum advance. When the throttle is cracked the ported vacuum comes in, the advance speeds up the RPM. When you open the throttle more the vacuum drops reducing advance, and that works. If there is screw in the vacuum advance you can adjust to limit the advance under high vacuum. I adjust by applying vacuum with a mighty-vac, and visually look at the vacuum and the distributor plate movement, as you turn the adjustment. You will then know the direction to turn. Try moving it 2" of Hg and try, increase if not enough. Document where it was, so if you want to go back you can.

Another alternative is to place a slightly stronger weak spring to slow down the mechanical advance at low RPM. If you feel the rotor by twisting and releasing, it should return softly and with certainty. If it is sloppy at the end of return, it may a problem of slop in the various parts.
 
Kit-
Did you mean too much vacuum advance, or that it is coming in too soon?
It looks like my vacuum can is adjustable, but the adjustment affects the maximum * it can add, not the vacuum level at which it starts to advance.
 
Disconnect the vacume, run it and see if problem is still there. When disconnected make sure you plug the hose.
 
tekslk said:
Disconnect the vacume, run it and see if problem is still there. When disconnected make sure you plug the hose.
Click to expand...

Even with this, if you still experience the problem it is timing.

My old MPP dizzy did this. Timing comes in WAY too fast on them and at low RPMs you can get surge.

That being said I do not run MPP dizzy's for a variety of reasons and I do not run vac, advance.

YOur car is a very heavy car with a very low stall. Don't go crazy on the advance curve.
 
It is coming in too soon. If you insert the mity-vac or vacuum gauge with T- inline, you can see what is happening to the ported vacuum. There is a release trigger on a mity-vac that will bleed the vacuum for testing. Tekslk, has a good idea, but the mity-vac will give you the numbers you need to tune. His test is great to first identify the problem.

I may me wrong on the ability to ability to adjust, I tune electronically with 3D RPM/MAP so I have complete flexibility.

Too much accelerator pump fuel, or a spill over could also result in surge.
 
Well, I finally had a little car time and made some progress, so here's an update:

I monkeyed around with the advance some without getting the result I wanted, then one day when I had time to kill at my desk I decided to call the Holley/Demon tech line. Good thing I had time to kill, because from "1st in queue" to talking to a live person took almost an hour on hold.

The very nice guy I spoke to asked a few questions and said the issue was probably fuel pressure too high, or float level set wrong. I'd been running the basic Carter #4600 7psi pump with the Edelbrock carb for years with no pressure regulator and no problems. Demon says the 625 Street Demon wants 5-6psi, and apparently they mean it.

So I got the Holley 12-803 adjustable 4.5-9 psi regulator and a cheapie Mr. Gasket gauge and re-did some plumbing and finally got to try it yesterday. The good news is the problem went away!

The small bit of bad is either my adjustable regulator doesn't adjust very well, or my gauge is not very accurate, 'cause I can adjust 'till I'm blue in the face and the change in pressure reading is minimal. In any case I have it reading 5-ish psi (the needle bounces a lot), and the engine is happier. The surge is gone and it still runs fine at higher rpms. (I've only had it up to ~4000, but I drive like an old lady anyway.)

I now have a very slight hesitation when accelerating at a little higher rpm (1500-1800) than my old problem, so I'm not done yet, but it is so mild I really could live with it. But it's a hobby, so I'll have to chase this new issue around too : )

I appreciate everyone's advice- Thanks you guys!
 
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