Carb Genius needed

[XXBULLETSXX]

Ok, I've got my motor done and back in the car with my old carb, Holley 600, and it runs like crap. I'm trying to find the best carb for my setup. Here's what's been done to the motor:
Compression is now about 9.5 to 1
World Windsor Jr heads (ported, polished, flowbenched)
Weiand Dual plane intake
Comp Cams Extreme Energy 274 cam
here's the specs on the cam:
http://www.compcams.com/Technical/Search/CamDetails.asp?PartNumber=31-246-3


HELP! My car runs like crap and I know it's the carb.

 

[67Dylan]

What does 'runs like crap' mean? More description on what it does or doesn't do would help. 600 DP? How long did the carb sit? When was the last time it was rebuilt?

 

[XXBULLETSXX]

I think the power valve is busted because I backfired three or four times. The engine acts likes its starving for air or fuel, can't tell which. I pulled the spark plugs and they were really black, hence I think the powervalve is broke.
But when I run the car down the road it's like it has no power, and when it's at low RPM and you give it more peddle it want's to backfire. But if I slowly press down on the peddle it will run pretty good.

I took the carb off at the end of March and put it back on Saturday. It was rebuilt only about 2-3 months before I took it off in March. It's a Holley 600 CFM and the engine was dyno'ed with it @ 349HP and 320TQ

Does that help?

 

[67Dylan]

Why not replace the powervalve? Usually when the car backfires, thru the carb, the powervalve goes bye-bye. When you do that, do me a favor and clean out the carb as well... it's been sitting for a while so some dirt might have found it's way in there. Also, it sounds like a little carb tuning might be in order... good luck.

 

[Max Power]

Almost sounds like multiple issues.

The leaning out sounds like it might be a vacuum leak or an accelerator pump, the richness sounds like a blown power valve, etc.

It's odd that a carb that was perfect before went bad on the shelf after only two months. I would look at other variables. You could just install a new power valve, they are cheap, just to be sure. Many Holleys have power valve backfire protection, so not knowing what model you have leaves that question unanswered.

I think you have other things going on, maybe a vacuum leak, maybe valve adjustment or ignition timing, etc. are making the problem worse.

As far as a carb recomendation, knowing more than just what your engine specs are would be helpful. Street vs. strip? Tranny? Rearend ratio? Stall converter? All are factors in choosing a carb.

 

[69gmachine]

I make absolutely no claim as to being even a carb apprentice... I struggled with two different ones before I broke down and ordered a new one that didn't need anything but bolting down. But I can tell you from my bad experiences that one other thing Holley's are infamous for is a sticking needle and seat, especially if they sit for a while. If it's stuck open, fuel will overflow into the venturis and you can watch it drip into the carb, and your plugs will foul in no time. Sometimes all you have to do is hit the carb lightly with a rubber mallet to free a stuck needle, but you may need to replace the float assembly and/or the needle and seats to prevent it from re-occurring. If throwing money at it isn't your preference, give it a thorough cleaning, re-gasket it, replace the power valve if it's older and didn't come with power valve protection, and while its apart, install a power valve protection checkball so it won't blow out again.

 

[D.Hearne]

First, what gives you the idea that you now have a 9.5 to 1 comp ratio with World heads on a 289? Did you mill them? What size chambers are they? Second, I doubt the powervalve blew out. I've played with Holleys for 30 years and have yet to have a backfire finish one. More likely it got dried out and busted when you reused the carb. I also agree about the needle/seat, I tap on the adjuster screw with a big screw driver handle to free it. Got fuel filters? How did you set the floats? What fuel pump and pressure you running? The 600 will work, but may limit your top end power. You might want to upgrade to a 600 or 650 DP Holley or if you have an automatic, a 670 street avenger. If you don't like the shiny look ( like me) Holley's list 80783 is also a good choice.

 

[XXBULLETSXX]

69MACHINE, I think you may have something there with the needle being stuck. I'll def. look into that!

Max I'll look into the Vacume leek. Another thing I didn't think of.

D.Hearne, I know the compression has been change because the shop my brother works for (which built the motor) set the compression to 9 something. I can't remember though if it was 9.5 or 9.8, so I went with the lower number. And yes I think they set the compression by milling the heads down, but I'm no engine builder so they may have done something else?
I set the floats with the engine running and to where the gas would just barely trickle out the hole. I read somewhere that was the way to do it, is that right?
I do have a screen in the carb as well as an inline fuel filter. Is that good enough?
Not sure what the fuel pressure is at. It's a mech. fuel pump so I just assumed the pressure would be right. Someone recently told me that Holley's like a lower pressure like 4-5lbs. Not sure if that's correct though.

Thoughts?

Edit* Today I found out that the previous owner or someone swapped out the jets and they are considerably larger .014 larger! So I'm going to start by putting the stock size jets back in which are .064, check the needles and fuel pressure. I'll let you all know how it comes out. Let me know if anything else comes to mind that I should check.
Thanks!