idle surge problems..need suggestions

[pazcarguy]

I have a 66 coupe with a healthy 289 (11.5~1, 351 heads,540 soild lift cam, torker intake and 650 mighty demon carb. I have been running this on Sundays and have approximately 400~500 miles on it after restoration. The problem I have is the idle will surge (from 950 to 1500 rpm) after I jump on it and let it return to idle (when sitting still). I think it is the fuel pressure regulator or fuel pump. I have a Holley red pump and regulator setup. I set the pressure to 6 psi with engine running and I noticed when the idle surges the regulator is reading 4 1/2 to 5 psi. Short of pumping fuel into a container and timing the amount pumped to determine rate, how do you properly suggest I determine fuel pressure output of pump? It should be possible to deadhead it prior to the regulator with another gauge to determine pressure output, and if it is above what I am running I could assume it is not the problem. Has anyone had this type of problem and do regulators fluctuate this much due to weak spring or spring fatigue? I would rather spend $25 on a regulator than $100 on a pump to resolve problem. Any suggestions? Brian

 

[D.Hearne]

You're chasing up the wrong tree. Idle will not be affected by fuel pressure with a carb. The pressure is there, simply to overcome the resistence the float gives against the needle/seat. When the float drops or is pushed down by too much pressure, it lets fuel into the bowl. The fuel in the bowl is pulled into the jets and other circuits by the pressure differential generated in the throttle bores (or in simpler terms: vacuum does this function) About the only time the pressure would affect it, is if you have too much pressure, which even at 6psi I doubt is the case, even if it was, too much fuel would then flood your engine, the excess would pour up and out of the vent tubes then down into the engine.Too little pressure and you'll have a fuel starvation problem, that would also show itself at higher rpms.Your idle problem lies somewhere else, either in the carb or ignition. By the way that Holley pump flows about 3 times the volume you need. You could have done as well with a $25 Purolator electric low pressure pump from the local parts house.

 

[pazcarguy]

idle surge

The carb I am using is a 650 mighty demon (no choke horns) fed from a Holley red pump and regulator set to 6 psi running. This carb has 4 corner adjustment capability for A/F ratio along with "Idle-ease" that permits air to flow to all 4 butterflies through a common bleed hole underneath the aircleaner stud for cars with racical cams and low manifold vacuum. The A/F mixture seats are set approximately 1 turn from fully closed where idle increases slightly due to a lean condition , then backed off slightly approximately a 1/4 turn more for 1 1/4 turns total each. Off idle hesitation is not there as the engine is clean and crisp upon throttle engagement and high speed runs produce a slightly rich condition on plugs and tailpipes Nothing sooty, just darker tan (I would rather favor this end than being too lean) Ignition is through an MSD-6A with Taylor Wires and new properly gapped properly. The car does not have a vacuum leak through the use of a gauge and "ether" looking for idle rise. The ignition is sound according to wire resistance, visual inspection of cap and no arcing or misfires throughout rpm range. Any suggestions?

 

[D.Hearne]

Try setting the idle mixture just using the primary mixture screws,and see what that does. I don't think your combo is radical enough to need a 4 corner idle, but closing the secondary idle may eliminate that as a source. You didn't say what distributor you're using either. Does the Mighty Demon have vacuum or mechanical secondaries ?

 

[pazcarguy]

idle surge

The Demon carb I am using has mechanical secondaries. I am using a Mallory unilite billet (came with car) that has a triggered pickup (no points) and either a Blaster 2 or 3 coil (can't remember). This setup should provide reliable spark through 20 degrees of crank rotation, though I should not rule it out as a culprit. Brian

 

[D.Hearne]

Do you have a fuel fliter in the fuel line ? If so check that, if you have more than one, check em all. If you have no filter, then remove the carb and take it apart nad look for trash.