Hesitating Holley

[YENKO DEUCE REGISTRY]

If my Holley stumbles just off idle, how do you tell if it requires a pump cam change or a different shooter? I have already checked/adjusted the following:

-secondary throttle plates fully closed at idle,
-vacuum checked, 18-20" at 1,000 rpm,
-set primary mixture screws to achieve the 20",
-set idle rpm to 1,000,
-acc. pump arm gap set at .0010",
-rejetted the primary side from 70's to 68's,
-something I've forgotten in my frustration [img]/ubbthreads/images/icons/mad.gif[/img]

[69L89396]

From my experance, you have to make sure there is NO play in the acc. pump arm. Just make sure that the pump arm moves as soon as the throttle arm moves. I have tried the acc. pump arm gap set at .0010" and have not had any luck. If you get it too tight, you won't get a full pump but that usally doesn't effect anything. Most of the time it is just a little too much slack.
If you don't find any slack, check the plunger under the nozzle. It might be letting the fuel drain back to the pump a little. If this is the case, when the pump starts to move, it just pumps air.
What is happening is the vaccum is dropping when you first open the throttle and the pump shot covers up the dwell time until the vaccum picks back up. I hope I explained this right.
Good luck

[YENKO DEUCE REGISTRY]

Your explanation is correct, the acc. pump is a mechanical delivery of fuel until the vacuum signaled fuel shows up. I'll reset the gap at 0 and see what I have. My hunch is that the duration of the fuel from the shooter is too short, thereby requiring a smaller diameter shooter to 'lengthen' the jet stream.

In the book I have on Holley's, it says: 'This function - mechanical injection - is most important when the carburetor is somewhat higher in capacity than ideal for the application, or with a double-pumper type carburetor using mechanical secondaries.' I know that my stock 780cfm Holley is much too large for the LT1, but I don't understand why that makes the acc. pump circuit more critical. Any ideas?

The most frustrating part is that the carb ran excellent before I sent it out over the winter to get 'concoursed' [img]/ubbthreads/images/icons/confused.gif[/img] [img]/ubbthreads/images/icons/mad.gif[/img] [img]/ubbthreads/images/icons/confused.gif[/img] [img]/ubbthreads/images/icons/frown.gif[/img]

[69L89396]

One other thing I have done before to help with this sort of problem. I had to up the idle a tad and it helped. You might need a cam that will give more shot when the throttle first opens. I don't think the problem is with the carb size. LOL

[COPO PETE]

Hey Marlin.....I'll try and take a look what squirter and what colour cam is in my Nova carb! Stay "tuned"!
Peter

[restorer]

69L89396,your right about upping the idle a tad, your starting to uncover the transfer slot sooner. If nothing else works it's an easy fix. Wes

[NOCOPO]

Marlin, Just my 2 cents on hesitating holleys.(1) Power valve, make sure its not blown out. even the smallest backfire can ruin one. (2) The old fuel gyser out of the vent tube. If the float level is just the slightest bit high, when you jump on it, the fuel sloshes up and out down into the primaries giving you a hesitation. just my $.02 [img]/ubbthreads/images/icons/grin.gif[/img]

[YENKO DEUCE REGISTRY]

Thanks for the tips!! I purchased a pump cam kit and found a graph for their parameters, putting the orange one in Friday night. I found that the Pink one that was currently in there didn't activate the acc. pump until the throttle linkage moved almost a 1/4 in" [img]/ubbthreads/images/icons/shocked.gif[/img] It runs much better now, but still needs some more tweaking. I will have to try some of the others to find the one that works best.

I believe that this 780 Holley is too big for the LT1, especially with stock manifolds and smog system. We know for a fact that they run quicker in the 1/4 mile with the secondaries disconnected, (as do the '66 L79 Novas). GM put 780's on the solid lifter big blocks, and our theory goes that they just tossed one on the LT1 knowing that most would have headers installed anyway.

Pete, what did your dyno guy use on your 427/425 test from MCR? I thought he started with a 750 shop carb, and then went to ??

[COPO PETE]

Marlin. I am running the clear (no colour) cam with a 31 squirter. 72 primary and 82 back jets. And sorry, don't agree with you on the 780 being to big. You need it to run 106 mph through the traps! My car revs like a motor bike with the 780 on. On the 427, we stated with a 650. Way to small. Then we tried two different 780's on it. Oh ya, are you running a vent baffle on the metering block...that can help stop the stumble.
Peter

[YENKO DEUCE REGISTRY]

Thanks for the info Pete, will try the clear one next.
As for the 780, Neal and I accidentally disconnected his secondaries and he ran 102mph!! I plan on keeping my secondaries connected, putting a plugged power valve in there and bumping the jets up - - after I get the stumble ironed out.

When will we see the silver bullet again??