I have a 78 Z28 350 Edelbrock performer manifold, Holley 600

When it idles (warm) it has 15 in. of vacuum
When you accelerate it dogs (or hesitates) for about 1 second then vrooom it goes

With the gauge on it it drops to 5 psiv, when you crack the throttle

what should I do?

Could it be the timing?
What should the timing be set at?

Thanks

 

you need to check the vacuum idling in gear. then whatever your reading is half that number and get a power valve that size. for instance if you read 7 hg idling in gear you should have a 3.5 or 4 hg power valve. holley 600's come equipped with a 6.5 power valve and that is too high in alot of applications. that hesitation could possibly be from an overly rich mixture. beware that holleys are notorius for blowing power valves. any slight hickup or back fire can blow it. usually in racing apps the power valve is capped and the secondaries are over jetted to compensate. Sick

 

this is from idle, ANY opening of the throttle, it does this

IF you would put your foot on the floor it would start to die then backfire thru the carb

 

We need to know more about your situation.

When you say "crack the throttle" is this in park or while driving?

Does it do it all the time and does it backfire every time?

Does your car have a manual or auto trans?

Are you running a fuel filter?

Are your floats adjusted properly?

What is your current timing?

Anything else you can add?

RM

 

It is a newly rebuilt motor
new holley 600cfm
new edelbrock performer manifold
New HEI Distributor
At idle in park hood open engine running and warm, you open the throttle lever, it hesitates (almost dies) then comes to life and revs up, every single time
the vacuum gauge reads 15 psiv, till you open the throttle, it drops to 5 hesitates, then revs up
IF YOU GO FROM IDLE TO WIDE OPEN THROTTLE, IT WILL COUGH, BACKFIRE AND DIE

When driving, you let off the gas, to idle, throttle up, die cough, backfire, it is very hard to drive

It does this every single crack of the throttle, every stop sign
You could make it backfire every time IF you want to, just step on the throttle hard and bang
auto transmission
yes on the filter, (I don't think it's the fuel pump because the bowel would be full when you start
floats are new with the carb
timing 6 deg
It has to be something simple because once it revs up it works fine?

 

first of all you DO NOT check vacuum in park. you check vacuum idling IN GEAR. is it backfiring through the exhaust or through the carb? 6 degrees is a little retarded but, not enough to make it backfire. Sick

 

backfiring thru the carb

 

When tuning a Holley, the stumble that You feel is because of the accelerator pump, What's happening is that You're getting a huge burst of air going into the engine when You crack the throttle. If You have a bunch of air going in and the acc pump isn't squirting enough fuel to compensate for all that air, it'll bog or stumble. Now, look at the acc pump cam. It has positioning holes that allow You to move the cam to a more or less agressive position. The Carb has 3 fuel stages. Idle circuit, Accelerator pump circiut and finally the jets. The accelerator pump is the transition between idle and full throttle. If You need more fuel in that transition, You have to make the acc pump more agressive through the pump cam, maybe even using a different pump cam or larger accelerator pump to accomplish this. Just make sure it has plenty of fuel in this process.

 

if it is truly backfiring through the carb. you may have a bigger problem than an accelerator pump. the biggest culprit i have found in carb backfires and popping ( besides being out of time ) is a flat cam lobe. hopefully that is not the case. need to pull the valve covers and manually turn the engine over watching for valve lift on each of the cylinders. if you have one that has very little lift that is your problem. Sick

 

Last nignt I flipped the fuel pump cam over, and It worked like a charm, I now have to crank down the fuel it's squirting in
timing by the book is 6 deg I will leave it there till somone can convince me otherwise

Thank you all