Holley 4779 750dp question

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Mopower73

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Hey guys you ever changed jets on a carb to richin it and nothing hardly changed? My carbs factory jets had 71 80. Now overtime they are at 78 87. I'm thinkin could my acc. Pump be too small or the squirters too small?
 
if you have gone up 7 jets sizes in the front and rear and dont see any change especially in the plugs...something else is wrong....

the accelerator pump is for accelerating from a dead stop or part throttle...not for wide open throttle enrichment....

and do you have power valves in the carb?
 
It doesn't stumble from take off and pulls all the way to 6500rpm. But it has acted the same way even with the 71 80 jets in it from the beginning. Just feel like I've went no where in progress
 
Well I mean I guess I'm not sure sure. I've checked the plugs and they are tan which is good but don't seem all that black as they should be. I mean maybe the car is liking how fat the jets are. When would you know to change the squirters?
 
That's exactly what I was thinkin too just donno where it would be coming from
 
I would upgrade the accelerator pump if there was a momentary hiccup, or hesitation when pushing the pedal down.
 
Okay yeah there are 30cc pumps in front and rear now. You think I should just get one 50cc for the primaries or 2 50cc pumps?
 
I meant to mention there is a slight hesitation but only at a light peddle take off. But it won't hesitate at all when I'm all in it.
 
I have a 4779 in the parts tank now. Factory 73 jets in it, power valve in the primary side only. I am jetting it up to 75 and 85. I am not liking having just the power valve on one side so will probably sell it or trade for a different model. You have to understand that just because you are giving the car more fuel, it doesn't necessarily mean your motor is able to handle it. Timing, compression, spark plugs, ignition, etc ... have very large roles to play in burning all that extra fuel and putting it's power to the wheels. Just dumping more fuel doesn't automatically mean more power.
 
Mopar to ya said:
I have a 4779 in the parts tank now. Factory 73 jets in it, power valve in the primary side only. I am jetting it up to 75 and 85. I am not liking having just the power valve on one side so will probably sell it or trade for a different model. You have to understand that just because you are giving the car more fuel, it doesn't necessarily mean your motor is able to handle it. Timing, compression, spark plugs, ignition, etc ... have very large roles to play in burning all that extra fuel and putting it's power to the wheels. Just dumping more fuel doesn't automatically mean more power.
Click to expand...

Do you understand what a power valve does?

Unless you cruise with the secondaries open, the use of a PV in the secondary block are like tits on a bull.

If the OP jetted up that far and saw no change in performance, he's likely got a fuel delivery issue. That thing should be PIG RICH!
 
He is not taking cruising, he is talking power. You know better than most that he needs to be able to handle the fuel he is giving it or he won't get power. As far as I know, that carb never came factory with 71 and 80 jets. Someone jetted it down in the primary and up in the secondary. Better cruising mpg and more power when stomping on it. Pretty common. The OP jetted it even larger. To get the value of that fuel, the ignition system has to be able to burn it all. If it can't, it's pig rich and no appreciable gain in power.
 
Refresh our memories.......What size engine, compression, cam, trans, converter, rear gear, etc is this carb on?
 
Let me approach this from a different angle. You keep jetting it up because you get a surge at light throttle cruise, but adding jet doesn't do anything. Would the rpm's happen to be in the 1800-2200 range when it happens? Also, as for the stumble, does it occur when leaving a stop light (like as soon as you touch the pedal), or at light cruise speeds? Anytime you take your foot off the gas and try to accelerate again (like taking your foot off the gas to go down a hill, then getting back in it to go up the next hill), does it stumble? We're talking less than 10% throttle here.


What are your idle mixture screws set at? Did you set the idle with a vacuum gauge?

Have you messed with the pump cams or anything to do with the accelerator pump arm adjustment?
 
UOP said:
Let me take a crack at it. You keep jetting it up because you get a surge at light throttle cruise, but adding jet doesn't do anything. Would the rpm's happen to be in the 1800-2200 range when it happens? And when leaving a red light, just lightly pulling away, it stumbles, but if you give it some gas it'll jump and get going. While cruising it's not so much as problem, but only when trying to accelerate from ~ idle speeds. Am I close to describing the issues?
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Sure .... confuse the issue with facts. :)
 
Just because it was jetted up DOES NOT mean that the engine is receiving more fuel.

If you are trying to read color on plugs with pump gas, don't bother. You want black spark plugs? Really? They should be black as night now!

Maybe gary busey can help him out... :)
 
Okay let me tell you guys about my engine

mopar performance distributer with chrome box mechanical advanced, fully advanced at 36 degrees
-340 bored 30 over
-60405 lunati cam 242/252 @50 lift 533/552
-flat top forged pistons with 10:1 compression
-eddy rpm heads
-eddy air gap dual plan intake
-trans is 727 with 3500 stall
-8 3/4 rear with 3.91

the car has the 750dp carb on it as we speak and I bought it new out of the box. But yes at about 10% throttle taking off it sputters and also has a slight miss at a consistent cruise of about 2200rpm or so. Motor pulls 10 inches of vacuum at idle with idle screws 1 turn out on all 4 corners. need anymore info please let me know. There is a lot of knowledge on here so anything you guys say will help me out in some way
 
Timing first, primarily initial timing setting.

What's it got for initial timing? That's the first stop.
 
Ok, so you set your idle mixture to maximum vacuum, if I read that correctly. Just as an experiment, try turning the idle screws out to 1.5 turns and go for a drive to see if that does anything. Does the ~2200 rpm surge get better, and does taking off lightly from a stop seem to improve?

Just as Rob said, what is the initial timing? Also, are the floats set to the correct level?
 
The floats are set correctly I checked that now this is where a problem may be now that you mention initial timing. The springs in my distributer are so light that I can't get a good initial. When I checked that last time it was bouncing around close to 36 degrees because the springs are so light which could be the cause of why this is happening you think? Oh I forgot to mention I do drag race this car but also street it.
 
36 @ idle?......there might be something to that. :) Yeah, I'd say those are some light springs. No good.
 
Yeah I mean that could be why it seems to want more fuel because it burns hot cause the ignition is pretty much fully advanced all the time.
 
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