CarburetorsCarb discussion and questions. Upgrading a Third Gen carburetor, swapping TBI to carburetor, or TPI to carburetor? Need LG4 or H.O. info? Post it here.
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Need some input from you guys. I have a Holley 750 CFM HP #0-82751. Its on a 355 SBC with a Comp XS274S-10 cam and an Edelbrock Super Victor on Ported Vortec heads. It has about 7 inches of vacuum at idle. I swapped out the 6.5 power valve for a 4.5. I know I need to do some accelerator pump cam work and maybe some squirter changes. My biggest question is on jetting and power valves for the secondaries. Right now it has 75 and 80 jets front and rear. But, just a plug for the secondary power valve. Do I want/ need a P/V for the secondaries? If I run a P/V what should I change the jets to for starters? What are the advantages to a P/V on the secondaries compared to a plug? Oh, elevation is about 3300 ft. if it makes any difference.
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no power valve needed on secondary side, unless you're getting really up there on the jets like a 97-99 then you can add one to take away some jetting.
__________________ my car thats never done
"I'd rather be judged by 12 than carried by 6"
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Car has an off idle stumble. I was guessing not a big enough shot from the acc. pump. I have access to a W/B O2 sensor. Will use that maybe this coming weekend. Running a TKO 600 with a 9" Ford with 3.89 gears.
my off idle stumble was solved by going from 31 to 35 squirters on the rear when going wot from a stop, you might need to up your primary squirter if it's causing this for you. Stumble could also be from too lean an idle circuit but a wideband is the easiest way to tune it.
Well I tuned the idle circuit with a vacuum gauge. Got it to 7" Vacuum. But the screws are only out 1/2 turn to get this. Any more or less and the vacuum will drop. Not sure what size squirters it has. The wide band should give me a bunch more info.
I am in the process of tuning a 750DP on my '85, it takes off great if floor it. However, if I slowly open the throttle while cruising it has an awful flat spot at 1500 rpms or so. The car has about 3.5" of vacuum when idling in gear, I'm using a 2.5 power valve. I'm not sure what to try next to get rid of the flat spot. Different pump cam perhaps? I'm currently using the pink cams.
Thats the first thing that popped into my head too. I have 6" and my cam is pretty much borderline streetable. I have to run a vac. canister so i can use my brakes. Fill us in on the cam...
Back on topic...
i think you gave yourself the best advice. Run that wideband and see what is acctully is going on. I have a LM-1 installed in the tailpipe of my no-cat car, and it is a great tunning tool. I make sure I am always at 14.7 at idle and 12.5 on the fast pedal. I can literally retune the car on my lunch break. Different atmosferic conditions really effect the carb.
( With all that said, I am still a super noobie at tunning carbs. Only reason mine runs well is cuz I had it tunned at the dyno shop.)
i don't run a PV at all. they are junk and blow out all the time. i run 81 in the front and 81 in the rear. with a 1" spacer(two stage nitrous plate). sounds like you need 50cc pumps. i have 28 and 31 for squirters with a 50cc pump. before i had 37 and 37 for squirters witha 30cc pump.
holly crap what cam are you running to get 3.5" of vacuum?
I don't have the cam card in front of me, but it has about 244 degrees of duration at 0.050" and ~0.515" of lift. The ignition timing is set to 18 degrees of inital advance.
It a drag car, I too to add a vacuum reserve canister in order to use the power brakes, and they are still marginal.
Stumble is on moderate to hard acceleration. Tried the timing thing but car wants to "diesel" when shut off. Took timing back out. five7kid, why is the PV too low? You know what Holley says. Its a bit too high for them. Changed the pink cam for a white one. Better, but still there. Things seem to be going in the right direction though.
five7kid, why is the PV too low? You know what Holley says. Its a bit too high for them.
The number on the PV indicates what the manifold vacuum has to get down to before the PV opens and allows extra fuel into the circuit. If it needs more fuel and the PV isn't opening yet because manifold vacuum hasn't dropped far enough, you need a PV with a higher number.
Right, but it needs to be lower then idle vacuum otherwise it tries to add fuel at idle right? What PV would you reccomend? You can see there is not much of a shot from the primary squirters when you hit the throttle. Thats why I was leaning towards acc. pump cams and or squirters.
Ok, Mine might be out of proportion, I run a big block with a 750 hp.. might sound small but it pulls the best with the 750, I think all hp's bog off idle when new, the brown cam and a 50cc pump on secondaries did wonders, the front with 30cc pump, pink cam in the #2 hole with 35 squirters..
try this, when at idle, only rev it when the front 2 barrels open, probably no bog, then rev it till both open, i bet as soon as the secondaries crack open it bogs.. I done have 3 of these hps on different motors and they all act the same, Why holley does it that way?? i think they build all carbs on 1 test motor? what do i know??
Well I put in a white cam on #2 hole with a #35 squirter. Better but not perfect. I bought a cam assortment today. I also have a #37 squirter if needed. Will play with it when I can. Thanks for the input so far guys.
with a 30cc pump i had 37 squirters front and rear. with the 50cc pumps i have 28 in the front and 31 in the rear. just to give you an idea of what a difference it makes.
Hey, how do you know if you need a high flow power valve or a standard one? My motor makes around 460 fly HP, around 350 at the wheels. I idle at around 6" of vac. and I have an out of the box 750 Holly DP. I hear these have 6.5s so my PV must keep opening. I want to get a 4.5 but am not sure if i should get the high flow one. What do you guys recommend?
i run the 4.5" non high flow one, unless you're drilling out the pvcr passages behind it I doubt you'd need it unless you have to run higher primary jets as well on your carb due to lack of A/F ratio at WOT.
__________________ my car thats never done
"I'd rather be judged by 12 than carried by 6"
First Place Camaro Modified Thirdgenfest '05, Second Place Camaro Modified Thirdgenfest '06, Best Modified Camaro Thirdgenfest '07 , Second Place Camaro Modified Thirdgenfest '08
MFBA March 2008 Featured Calendar Vehicle
, Second Place Camaro Modified Thirdgenfest '08
