Holley 750 DP off idle stumble

DeeVeeEight

Fast Pedalphile
Joined
Nov 3, 2008
Messages
2,284
Location
Southern New Jersey, USA
My 'Vette's been sitting too long so I start it up when I can to keep the cobwebs out of it. I have an off idle stumble issue once the car is warmed up. I am wondering if it is too little timing (probably not) or maybe a funky accelerator pump from sitting with cruddy gas for so long. It's got a load of fresh gas in it now. Any ideas?
 
I am far from experienced with this but you may be onto something with your assumption about the accelerator pump especially if it ran fine when you were driving it more often. I've been reading Lars' paper on "How To Tune A Holley". He mentioned stumbles quite a bit. One possible culprit is that an off idle stumble is a characteristic of a single plane manifold but further down he mentioned the accelerator pump circuit and said "if the car stumbles . . . on initial throttle opening, try a larger shooter" (discharge nozzle). Maybe yours is clogged a bit?

DC
 
Well, it's definitely a single plane manifold. I'll have to do some more troubleshooting once I get her back on the road and that may be a while...

Thanks for your reply!
 
Have had the same problem + and idle adjustment problem as well. Took me a very long time to figure it all out. So you may be able to profit from my experience

This is what I did :

With a single plain she's going to need more initial than usual. It depends on the cam as well. You may need to go all the way up to locked advance. You will see idle stabilize and AFR's correct. Adjust it accordingly. Start with 18° and adjust mechanical advance to suit. Connect the vacuum advance to ported and have it add some 10-12°. You will find out you can turn the idle scews in a lot more this way. Get it approx good.
Pull the carb off, drain the fuel and clean it.
Reset the primaries and secondaries with proper transfer slot exposure.
Put carb back on, get it to idle right. If necessary turn out the curb idle screw but remember the amount of turns you turn it out so you can return it to it's Original position. Adjust the secondaries with the carb on the engine while running (you need a very short small screw driver for this - I modified one to do this). This may require some balancing : pulling open the secondaries some more until you have the primaries in the original position.
The thing is that I couldn't set secondaries on the bench, as there is some slack int the trottle opening mechanism on the secondaries (no spring etc.). So the vacuum of the engine pulls the plates agains the stop closing off the opening and reducing the amount of air the secondaries allow in. You will need that extra air from the secondaries big time.

If you do not have an AFR gauge. Use your nose. A bad idle mixture adjustment(afr ratio) will stink up your garage big time)

Then you experiment with the cam on the accel arm.

PS : I have a DP carb. May be a little different on a vac secondary. If you have to go to locked you might need a spacer as I've found my vacuum can hitting my fuel log line. Don't let it idle too low, use 900-950 as a guideline on a single plane.
I have also had some comments on speedtalk that the current fuel with alcohol in it, needs a different setting than what the old carbs were designed for...
 
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In addition to all the things Belgian said -

where is your butterfly opening cam for the rear blades set? You have 3 positions with which to choose. Presuming it idles right, and then pulls right after the stumble - I'd start there

I'd also check which cam you have for the accelator pump circuit - the factory setting is the slowest opening, and perhaps you need to change that to get a bit more fuel at tip-in
 
I have testdriven my car today. No hint of a stumble anymore. I had to put in a little larger cam on the accel pump.

However : as I first didn't want to believe some folks over at speedtalk that told me that most idle and stumble problems were ignition related, I must admit that they were right.
To note is that it's not an easy thing to change. Because if you change some setting like the initial timing, you need to recurve the mechanical advance as well. Not easy on a factory HEI.
 
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