I just picked up a holley carb , it is a 750 vacuume secondary, it has a proform main body, I think iam having issues with air bleeds. Since this carb came off another engine totally different than mine, tuning is different than what i need, i need baseline tuning for this carb. i have a fairly built 350. 650 dp ran good with a couple issues just not perfect on street so i picked up this carb. this carb runs like crap. wont even idle without mixture screws 4 turns out. 71pri main jets 83 rear 4.5pv front(engine makes 10 inches in gear at 800rpm) and ran good in dp. i have no experiance with the air bleeds in proform main body and suspect they need to be changed for this setup since carb origanally had 76/86 main jetting. the front bleeds are as follows 75 36 36 75, 4.5 pv 71 main jets
the rear is 43 36 36 43 no pv 83main jetting. also 35 front squirter with red cam pos. 1. if anybody reading this has some experiance with this please post iam about ready to put my doublepumper back on a little rich at idle but goodperformance no adjustable airbleeds in this thing though to clean idle up.

 

I have a little experience with air bleeds...I would touch them last. First of all, the 12 jet split from front to back is huge. I have a 7 jet split on mine (no secondary PV) and I think that is too much. I bet either carb will work on your engine. First thing to do is figure out why you need to turn the mixture screws out 4 turns. I would start with your timing curve. Need some more engine specs, but somewhere around 20* initial advance (at 800rpm idle) and 16* mechanical would be good. You also want vacuum advance hooked up. At idle with the vacuum advance added in, you should be around 50* advance. Next, set your throttle blades so the primary and secondary transfer slots are about .010" exposed. Put the idle mixture screws 1.5 turns out. Set your jets to 71/77. This is a baseline. You may need to open the blades a little to get it to idle. No more than .030" exposure on the transfer slot or it will idle pig rich. Do these things and get back to us. Then, we can chase the air bleeds if we need to.

 

Thanks for the reply, i found the problem after messing with it all day, someone drilled holes in butterflys and butterflys were not closing properly, secondary butterflys were open too far causing a problem also, i jetted 73/83 and think for sure iam gonna step it back 71/80or less. test drove good, plenty of power altough a slight tipin sag though i think i can cover that up with a blue cam. my timing is 18 at idle 38 total, iam idling 750-800 in gear 8-10 inches of vacuume, cam is decent size though. on test drive car pulled real hard 5000 up but could feel secondarys engauge at about 5000rpm, iam gonna have to try different springs, i think this carb is gonna work out better, i just street drive this car, not quite the throttle responce of my double pumpers though.

 

Sounds good. It will be tough to have the vacuum secondaries tip in as good as a double pumper. You may need a big shooter in the front (37 or bigger). Make sure there is very little slop in the pump arm. Also, make sure the pump arm spring isn't compressing instead of moving the arm. I had that happen before. The blue cam should be okay.

What are the cam specs in that 350? I have a pretty decent HR in my 350 too.

 

yea checked it out better have some play, iam going to buy a better base plate. my engine is a 355, forged intermals, i have a scorpion intake portmatched to dart pro 1 aluminum heads.200 intake runners 70cc chambers whith trw .120 dome pistons. cam is a bracketmaster 2 cam luniti makes it its solid cam but seems to be working good for what i have, i forgot specs on cam but i believe it to have .510/.515 lift .300/.310 adv duration, should i run jets where you are at? i dropped shooter to .31 before test drive i have a .35 shooter i am going to install to see what that does. my 650 ran good with these but 2 of them.

 

I ran that cam in 406. It ran strong. I think with the vacuum secondaries, you may need to step up the shooter to a 45 or so. You will also need to use the hollow screw to hold the shooter in place. Set the pump arm so it is just touching the pump lever. Make sure at max pump cam movement that it is not bottoming out the pump arm or you will ruin the accelerator pump.

Are you running vacuum advance? Have you messed with different PV's? I don't like the holley method of "1/2 your idle vacuum is the PV you run". At idle, you may be at 10". As you cruise down the road, you are probably at 15". It takes a lot of pedal movement until you drop from 15" to 4.5". I would try to get the PV just 2" less than your idle vacuum. Then, you will be okay when you are cruising and then give it some pedal.

 

I just ordered a billet base plate, i think iam going to order a 50cc acc pump and a 45 shooter to try, what cam would you run for that setup? iam not running vacuume advance, but that is one thing i think i will try. ive been thinking about crane distrubuter it has preset ignition curves in it and a hookup for vacuume line. it would be nice to beable to crank the advance up on light cruise. i dident even think about power valve i may try higher pv and see how that works. thanks for the time ill let you know how things work.

 

Nothing wrong with the cam you have now. There are other cams that will make more power, but then you need springs, pushrods, etc. Stick with what you have until you can afford to step up to a roller cam. That way you can get similar (or slightly less) duration, but more lift. I would also hold off on the 50cc pump until you try the 45 shooter with hollow screw. A 30cc should be fine, but you never know.

 

no i was talking about accelerator pump, sorry i dident specify that

 

I use the stock pink type in mine. I think the blue one is aggressive enough for you. You will probably need to put it in the #2 hole. If you are cruising along and the throttle is open enough so that it is past where the pump cam does anything...when you step on it you get no pump shot. This is what makes the double pumper a little more friendly in the acceleration department. The pump shot will help from idle to WOT. For the cruising situation, you will need to rely on the PV opening and then maybe the air bleeds. Float level will also affect how quickly the different circuits activate.

Get vacuum advance. That will help bring your idle vacuum up 2" or more. Then, you can step up to an 8.5 or 10.5 PV. Your engine sounds like a good combo. You ever run it at the strip?

 

vacuume advance keeps plugs better from what ive heard too, a lot of guys are starting to go to it in circle track, cleaner, more efficiant and more power , no lose situation from what i understand. iam learning every day i guess, too bad its so much money. i havent got it to the track yet, 40 miles away and iam waiting on a 2 core radiator(aluminum) my car runs hot after about 10 miles. car feels like a mid to low 12 second car though, faster than any z28 ihave had. i hope to see what it does before winter though, i drive it to work everyday, some stupid *** in a new mustang pulled up next to me, thought he had something fast v8auto car, i smoked his *** kind of embarissing for him he had a good looking girl with him that was laffing her *** off buy the time he rolled up next to me at the next light. what do you have? sounds like you really know your way around a sbc again thanks for the help all is greatly appreaciated

 

I have a 1990 IROC. Original L98. The only thing ever replaced in the short block was the timing chain. Big hydraulic roller. Ported AFR 190's. Air Gap and a 750 Mighty Demon carb. Engine runs a lot better than I would have ever guessed. I get a little nervous sometimes shifting at 6800RPM.

The vacuum advance should also help the engine run a little cooler too. Not sure why that is, but it did for me. For a good, cheap distributor, I would recommend the GM Performance HEI. I think it is about $185 and it comes with a gear that is compatible with stock roller cam gears.

 

sounds good, amazing how a pretty stock lower end probably balanced for 6k or less will spin, my buddy had one like that the thing would rev like a motorcycle with upgraded valvetrain. what times you running, not to get off subject i just installed a 2 core all aluminum radiator, cruised it for 30min still got over 205 is this common, seems to get hotter cruising. not near as bad as with single row. still have single big electric fan, still have stock 305 waterpump???just bought billet qft baseplate and 50cc pump if needed. figured i would get radiator in while i was waiting on parts

 

My cooling system is the stock radiator and stock dual fans. I have a 160 thermostat and a high flow water pump. Make sure the air dam under the radiator is still good. I usually hover around 170-180 when cruising.

I ran some 11.89's this past Friday in good air (2500' DA). At sea level on a cool day, the car went 11.66 at 117mph. That 11.89 corrects to a 11.57 at sea level. The new Air Gap with Mighty Demon carb seems to make more average power than the old miniram setup. Next, I am going to see how much my single exhaust is hurting me.

 

That car is fast, what hp you think 500+ good gob on that build. I think i may be missing airdam, I guess iam gonna have to see what one looks like i dont remember ever seing one on my 86 or 83, my 83 had a 283ci engine probably not over 300hp was a strong engine though mild build but had original rad in it and never got hot.
Is the air dam black about 4 inches wide and runs the length of lower bumper, if so its missing

 

Yeah, the airdam is back below the radiator. It is black and about 4" high or so. They are missing on just about every thirdgen!!

Thanks for the compliment. I can't claim anything as far as the build. I bought it with the miniram setup and all the other goodies. The only changes I have made are to bump the compression (shim gaskets), ported air gap manifold, Mighty Demon carb and HEI and also some 4.10 gears in the rear. When TRAXION owned it, it went 11.66 at Cecil. That is a sea level track with good air. I would love to drive down there sometime.

 

Do you have 4-corner idle adjustment? If so, you need 75 idle air bleeds on the secondaries as well.

12 jet sizes split primary to secondary is what Proform recommends. Seems a bit radical, but it worked for me.

I'm no fan of vacuum secondaries, but if that's what you want to do, then at least get a Quick Fuel adjustable vacuum secondary housing. It controls the opening of the secondaries the proper way, by orificing the vacuum signal. Spring stiffness is what you use to control how far the secondaries open, using it to control how quickly they open is just wrong.

Parting shot: Using a vacuum secondary carb for streetability over a double pumper is ill-informed. A 650 DP with choke would be a much better choice than a 750 VS w/o choke. The type of secondaries is irrelevant to streetability.

 

Do you have a part# or link on vacuume housing from qft? just bought a baseplate from them, should be here tomarrow. I quess i dont quite understand that vacuume secondary diaphram new to me. My carb has no 4 corner adjustment. Where sould air bleeds be? is my jetting close; 75/36/36/75 front 43/36/36/43 any help is greatly appreaciated.

 

Hey just installed air dam and low and behold 190 now thanks

 

On summitracing.com, QFT-63-1. Similar Proform part, PRO-67235.

Check Holley's website. They have a description page that shows it well. Of course, they don't get into the adjustable housing, because they don't offer one.

I assume you mean are your air bleeds close. If you don't have adjustable secondary idle mixture screws, then that sounds right.

 

I know know holleys vacuume diaphrams work you were saying qft is different, it regulates vacuume I have not seen anything like that I guess, I have seen quick change diaphrams before not one from qft though. Thanks for the part# iam going to check that out.