Hey folks, need a pro's suggestions, so I came here. I have narrowed my car's ailments down to 2 things. Too much carb and not enough vacuum. I have a 750 Holley vacuum secondary on it right now. I KNOW this is too big. Knew it when I put it on there. But I wanted to compare it to the edelbrock. Holley is waaay better IMO. Anyway, when you nail the throttle it stumbles. Same spot, every time no matter where the timing is. Give it smoothly, and it runs strong. Just dumps too much in there under hard acceleration. What I want to know is which holley should I go to? Should I try jetting the 750 down? What if I added a spacer and allowed the gas to drop more............would it be enough? It wants to, it just can't quite take the gas. Also, I have heard that a vac. secondary will usually have a little stumble........is this true? Anyway, I'll outline my setup so you will have a little more to go on. 302 40 over flat top aluminum pistons 448 intake/472 exhaust 268 duration cam heads ported/polished/shaved CR supposedly around 10.25:1 Holley street dominator intake Holley 750 vac secondary carb Crane cams electronic ignition convrsion kit inside dist. Flame thrower coil crank has been turned 10 Long tube headers dual 2 1/4 inch exhaust with virtually no restrictions (just enough to keep enough backpressure to be safe.) Anything else y'all need to know? I would really appreciate any help! I'm not looking for a quarter mile killer, but my bottem end is non existent at the moment. It won't come off of the line. I can ease out, get the RPMs up and then nail it and I have no problems. Just want a carb that will allow me to stomp the gas and actually go and not have it stumble and embarrass me. Thanks y'all! Preston
Where does it stumble? Instantly upon application of the throttle or does the car roll out a little ways then start to stumble?
Preston, Todd may be asking the questions for the same reason I would...if you stall immediately on hard acceleration I would say your starving it...the secondaries can't react fast enough...if it's on a bit of a roll out and bogs I'd say your overcarbed. From the looks of your setup I'd say it's starving. I don't particularly care for a vacuum secondary carb on a performance motor.
If it is lean off the line as Rick suggests, you can experiment with weaker secondary springs. Certainly not as good as a double pumper but you should experience some improvement nonetheless.
when the tree comes down and you hit the throttle......... If it instantly bogs (noticed I said "bog" not "stumble"...2 different things), look at the squirter. You can change it to a larger one. I think the 750 VS (well most of them anyway) come with a 32, IIRC. You can go up to 35...37...even 40 if you need to. I would say 37 is about the biggest you'll need. Now, on the accelerator pump lever, there is a spring with a screw through it. It should have JUST a little bit of play in it (shouldn't be tight against the lever)...I like mine around .005" clearance between the lever & adjuster. I've seen them brand new with a 1/4" of clearance. Darn right they'll bog like that! If it rolls out a few feet, then starts to stumble, you'll most likely be looking at the secondary diapragm spring. Most likely, you'll want to put a stiffer spring in it, basically what happens is Holley sends them with a pretty light spring in there and when the secondaries open up, there is no secondary accelerator pump so the diaphragm slowly opens it....to avoid a stumble. Depending on which carb you have, you might have to take the diaphragm housing off of the carb to get the cover off. If you plan on keeping it (and I would.....EVERY 302 I've seen "likes" a 750 over a 600) it might be a good idea to upgrade to a quick change cover, available from Scummit, Jegs, and most any GOOD speed shop. One other thing that might be a future upgrade, again depending on which carb you have, is a secondary metering block with an idle circuit. I did this mod on my old 514-powered Ford truck, and the difference was night & day. Kind of like going from dial up to cable LOL. But, that's a little more difficult than just a bolt-on. But, the very first thing to do is eliminate any and all vacuum leaks. You simply can't tune a carb that isn't metering ALL of the air that the engine needs. Trust me, I've been down that road and it ain't exactly fun! These guys on the boards are pretty sharp with Holley carbs...so long as they get a good description of the problem.
Disregard my post above and listen to what Mavman has said. His explanation is well reasoned and makes much more sense than mine.
Okay, took it out and ran it hard. If you nail the throttle it seems like it picks up then falls on its face. (the front end is just beginning to lift when it stalls) In neutral it will rev up just a tad then cut out. Accelerate smoothly and it picks up good. Drove it and I'm still getting backfires through the carb, which tells me I'm running the timing too slow (I think.........) Got on it pretty good and it was backfiring through the carb like crazy and just melting the 275/60 rear tire. I know the car has power, I'm just not getting it yet...... Another thing, the car will NOT stall when you nail the throttle in neutral if you have it partially choked........does this help? I really appreciate all of the advice!!! Preston
Sounds like your vac.advance is leaking or not sufficent enough.....You may need to install an external vac.pump.....
Well, no one around here knows how to work on carbs, or wants to take the time with one, so I'll just keep the 750 till I find someone. Anyway, which would be better? 600 or 650? Edelbrock or Holley? Vacuum secondaries, spreadbore, etc? I want the best performance I can get from this. Thanks! Preston
At your altitude of elevation I would take the 650 over the 600 and a spread-bore over vac. secondary.I myself like a Holley carb over an Edelbrock.I think they are easier to work on and quick to make adjustments to. Seriously,I would keep the 750 and rejet it to your specific needs.....
Try this: There is a screw in the rod that attatches the vacuum secondary diaphragm to the throttle shaft; take the screw out completely and disconnect the rod. Then go for a test drive. If it doesn't act up, the vacuum secondary is opening too soon (spring not stiff enough). If it still acts up, I'd be looking for vacuum leaks.
sale your 750 go to a 600 550 with a 6000 rpm 302 motor look in the holley book in the back their is a chart (page 260) that tell you more about what to run its a very good article i went from a 750 to a 600 you will be amazed at the performance increase the holley chart solved my problem and the chevy boy (400sb) i sold the 750 to --------good luck--------
