Holley carb off idle bog/hesitation

**Jeff H** · #1 04-08-2007, 02:42 AM

I finally got the T-block fitting in and put the new fuel lines/T-block on the L78 and got it fired up and running without a leak. But I'm getting a little bog/hesitation just off idle. This is with a rebuilt by me Holley 780(4557) with stock jets(72 and 70 I think) and a 9.5 and 8.5 power valves. I tried advancing the timing a little to see if it would help but didn't seem to. I tried adjusting the accelerator pump bolt but it didn't cure it either. The carb has the stock acc pump cam too. I'm wondering if the secondaries aren't quite closed all the way and air is getting past causing the bog. Open to any thoughts on getting rid of the bog. I'll pull the carb and check the secondaries but what else can I check without opening up the carb?

**firstgenaddict** · #2 04-08-2007, 02:47 AM

What position is the accelerator pump cam in?
if not in the quickest setting... try that..

OR
go with a little larger squirter... if you think that the secondaries are not closed... you could check the set screw that allows them to close under the vacuum diaphragm...

**Jeff H** · #3 04-08-2007, 02:52 AM

[ QUOTE ]
What position is the accelerator pump cam in?
if not in the quickest setting... try that..

[/ QUOTE ]

It's the stock cam and I assume the stock position. I need to wait for it to cool down to pull the carb and check the secondary screw. Should it be closed all the way I'm assuming?

67rscoupe · #4 04-08-2007, 09:43 AM

your to lean.
holley 780,this is vacc secondaries right?
does this happen in the drivway or driving?
try running a vacc gauge into the car and watch it as you drive.whats your reading?you have what power valve? 9.5?could it be blown?whats you vacc at an idle?does it idle clean?

**Rixls6** · #5 04-08-2007, 03:32 PM

I had a 780 vac secondary doing the same thing once.
Mine turned out to be a small vacuum leak at the throttle shaft.
With the carb still on the engine, not running, open the accelerator slightly, then try to wiggle the shaft up and down/ back and forth. If you feel movement, it probably will leak vacuum there.

I put a new throttle plate on mine and the off idle stumble was gone.

VintageMusclecar · #6 04-08-2007, 06:07 PM

Jeff;

Several things could be causing your problem. The very first thing to check is your ignition timing. Once you've verified that's in spec, proceed to the carb.

Your carburetor should be set up as follows:

70 primary jets

76 secondary jets

8.5 power valves front & rear

White "218" pump cam (screw in the #1 hole should be fine)

.025 squirter

To adjust the accellerator pump arm, make sure the choke is completely "off" and the primary throttle shaft is resting on the curb idle speed screw. Tighten the adjusting screw just until there is some slack in the linkage, then loosen it just until the slack is taken up. The pump lever (underneath the float bowl) should start to move immediately as the throttle is opened.

Next, verify you're getting a good pump shot through the squirters by slightly opening the throttle (engine off of course) and watching for 2 steady streams of fuel coming from the squirter.

As far as the secondaries go, the secondary stop screw should be keeping them just barely open.

Lastly, check the primary throttle shaft for excessive play as Rick mentioned above. If it is excessively worn, it can cause a lean condition which can cause a tip-in stumble.

There are also a few other possibilities that lie inside the carburetor as well; if the main body is warped badly enough, it can cause a poor gasket seal at the accellerator pump transfer port (just above the power valve cavity) which can allow fuel to bleed out of the pump passage into the power valve cavity, or there may be a corrosion issue inside the pump cavity on the bottom of the float bowl.

If you can't pinpoint the issue, give me a call if you'd like and I'll try to help you through it.

Eric

**Jeff H** · #7 04-08-2007, 08:45 PM

Thanks for the suggestions. I checked the secondaries and they look fine, the squirter looks to be spraying nicely, the timing is in good shape and I played with float levels and the pump adjustment. I can feel some wiggle in the primary shaft so at this point I'm guessing that's my problem.

**SuperNovaSS** · #8 04-09-2007, 01:17 AM

Was there a hesitation with the old carb? If not, I would rule the timing and advance curve out. Try spraying some carb cleaner on the outside of the throttle shaft, if the idle changes there is a problem there.

Jason

68Biscayne427 · #9 04-10-2007, 06:44 PM

Not to alter the subject here too much but how is your vacuum advance connected? Is it connected so it receives "full manifold vacuum" or is it connected so it receives "ported" vacuum. I recently moved my vacuum advance connection from ported to full manifold vacuum, and this help in my stumblingly problem. This also made a huge difference in the overheating problem that I was having at idle. Any comments or ideas on what is going on here?

Thanks - Rob