Holley carb issues

[pkonopasky]

So I have a pretty healthy Chevy 383 in my prerunner (pretty aggressive cam, afr heads, full forged rotating assembly, fully roller setup) and I bought the Holley 770 truck avenger offroad carb as on Holleys website after putting in my motor info that's the carb it told me to get. I took the truck out to test it out and I was having horrible bog issues. At anywhere from 3-5k Rpms as soon as I would open it up it would fall flat on its face. Occasionally I would be able to get it over its bog and open up good but it was very hit or miss, which is no good when you are going through a whoop section trying to stay on top of it or going through a corner trying to keep the back end out. After giving them a call one of Holleys tech guys told me the 770 offroad carb is only for rock crawlers with massively built big blocks, even though their website told me to get it after putting in my motor specs. He said for me to just ship it in to them and he will send me a carb that will work better for my application. The new carb came in the mail yesterday and it was the Holley 770cfm vacuum secondary street carb. Now won't that have issues in the rough stuff? I'm not very experienced in the carb world so any insight would be greatly appreciated.

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[scottm]

There are many factors involved in this situation. That second carb will work, but the avenger carb could be made to work well too. First of all, verify you have proper fuel pressure with a gauge. You need 5 psi at full throttle. Do you have a good fuel filter and a good 4 to 6" tall x 14" diameter air cleaner? Tiny low-profile air cleaners kill power and throttle response. For offroad in general, fuel sloshing around in the bowls is an issue. You need the Holley spring loaded rubber-tipped float needles, part number 510-6-513 in the Jegs catalog. Set the fuel level a little on the low side - I put it about 1/8" below the threads in the sight port. Also extend the metal bowl vent tubes with short peices of fuel hose to raise them up a couple inches. This reduces bogging from fuel overflowing the vent tubes.

Once the basic stuff is in order, you can start tuning and diagnosing any problems. You may need to change jetting, power valves, accelerator pump nozzles, etc, but its more likely you just have a fuel pressure problem.

[pkonopasky]

There are many factors involved in this situation. That second carb will work, but the avenger carb could be made to work well too. First of all, verify you have proper fuel pressure with a gauge. You need 5 psi at full throttle. Do you have a good fuel filter and a good 4 to 6" tall x 14" diameter air cleaner? Tiny low-profile air cleaners kill power and throttle response. For offroad in general, fuel sloshing around in the bowls is an issue. You need the Holley spring loaded rubber-tipped float needles, part number 510-6-513 in the Jegs catalog. Set the fuel level a little on the low side - I put it about 1/8" below the threads in the sight port. Also extend the metal bowl vent tubes with short peices of fuel hose to raise them up a couple inches. This reduces bogging from fuel overflowing the vent tubes.

Once the basic stuff is in order, you can start tuning and diagnosing any problems. You may need to change jetting, power valves, accelerator pump nozzles, etc, but its more likely you just have a fuel pressure problem.

Wow thank you, very thorough! I talked to the holley tech guys and they agreed that they thought it was a fuel problem, possibly that i am running the red holley pump and not the blue or black one, or could be a clogged filter.... How should I check the fuel pressure, kind of hard in the truck! Haha I have 2 rather large UMP filters with new filters so I don't think the air cleaners are the problem.

Also what kind of puzzles me is that if I hold it open through the bog sometimes I could get it to power over it. At 6000rpm it feels great, just from 3-5 it's a crap shoot if it'll go or not. Would at 6000rpm the most fuel would be consumed?

[pkonopasky]

I can also post a video of the truck tomorrow where it is clearly having the issue

[orbot]

no vacuum leakes???

set the fuel level and try it. is it tuned right?

[pkonopasky]

no vacuum leakes???

set the fuel level and try it. is it tuned right?

Nope no vacuum leaks as far as I know, I set the fuel level right to the bottom of the sight port before taking it out, and I just took the carb out of the box and bolted it on, didn't even know where to begin tune wise... I do need to check the fuel pressure at full throttle next time I take out the truck. On Sunday I went out testing and threw on a 670 truck avenger just to see if it would still have the same issues, and it did.

[PDANK Racing]

You can check the fuel pressure without taking it out, just have to run it and hook up a fuel gauge. Earl's makes a fitting that goes between two connectors and has a port for the gauge. My engine guys says 7 psi is good. Is the truck blowing black smoke? That will indicate a rich mix. The off road needles and seat are very important, but won't affect driving on flat ground. When was the bog happening, all the time?
Vacuum leaks are easily detected with by spraying brake clean by the intake gaskets, carb flange and vacuum lines. I would ditch the vacuum secondary for a mechanical, like a 1450 series 750 cfm.

FWIW
Jetting: I prefer to use percy's adjust a jets instead of the regular ones, no mess, and quick to adjust.
Search around the net and you can find some good articles on tuning carbs.

[orbot]

Nope no vacuum leaks as far as I know, I set the fuel level right to the bottom of the sight port before taking it out, and I just took the carb out of the box and bolted it on, didn't even know where to begin tune wise... I do need to check the fuel pressure at full throttle next time I take out the truck. On Sunday I went out testing and threw on a 670 truck avenger just to see if it would still have the same issues, and it did.

2 diferent carbs and same issues..... is the timing ok?? what distributor are you using? are the spark plug wires good?

[partybarge_pilot]

OK, which accel pump cams are you using and if you have 2 UMP's, whats the plemun on top of the carb look like?

[pkonopasky]

You can check the fuel pressure without taking it out, just have to run it and hook up a fuel gauge. Earl's makes a fitting that goes between two connectors and has a port for the gauge. My engine guys says 7 psi is good. Is the truck blowing black smoke? That will indicate a rich mix. The off road needles and seat are very important, but won't affect driving on flat ground. When was the bog happening, all the time?
Vacuum leaks are easily detected with by spraying brake clean by the intake gaskets, carb flange and vacuum lines. I would ditch the vacuum secondary for a mechanical, like a 1450 series 750 cfm.

FWIW
Jetting: I prefer to use percy's adjust a jets instead of the regular ones, no mess, and quick to adjust.
Search around the net and you can find some good articles on tuning carbs.

Heres a video from under the truck... you can see the mufflers and I dont see any black smoke.

And Ill check the fuel pressure asap... my buddy has a 750cfm mechanical secondary I could try that next time I'm out.