i had a 650DP on my engine and it never seemed to idle right and ran really rich. i changed the power valve in it and helped some and it would pull 10inches of vacuum and thats it. so i put a 750VS on it and it idles better and i got 15 inches out of it. it doesnt seem to pull as hard up top, its not violent like the 650 was. i understand the DP is a performance carb and runs richer than the 750. i know theres is gonna be a difference but is there a way to tune the 750 for performance? i know there is the 4150 conversion and stuff like that. which would be the best carb? thanks

 

Take a look at the Quick Fuel adjustable secondary housing. I know Jegs has it, not sure about Summit. It allows you to adjust the rate at which the vacuum opens the secondaries by orificing (needle valve) the vacuum signal; rather than varying the force applied by the diaphragm spring, which changes the RPMs at which the secondaries open. Since the intent of the vacuum secondaries is to avoid the bog of too much throttle/too little gas/too soon, being able to adjust the rate of opening is what it's all about, and why it is better than changing the spring force.

With that and the 4150 conversion, which will allow you to tune the mixture of the secondaries, you should be in good shape.

Oh, I assume you have an automatic transmission. Unless your torque converter is pretty loose and the car is used primarily for racing, a VS carb is the way to go.

 

thanks for the reply five7kid,
the convertor is 2500 stall 12inch, it will flash at 2500 but foot brakes at 2000. sometimes it feels like the sencondaries are opening and others it doesnt. is it possible to get the performance out of VS of that on a DP?

 

No..it's not. With the extra 100cfm you may be able to get it close...but I kinda doubt it.

There was probably a flaw with the 650 in the first place...like a piece of dirt in an idle passage or something that could have been cleaned.

 

so you think the 650 would be a better bet? how much difference are we talking about? .1-.2 tenths? compared to drivebility? thanks.

 

Think of it this way: When you floor the DP, all of the carb flow area is available immediately. Assuming the engine is demanding enough flow to justify that (i.e., the carb isn't too big for the engine), power will be maximized. The VS, however, doesn't start opening the secondaries until flow through the primaries can create enough vacuum to open them up, and even then it isn't full open until RPMs are even higher.

The first rule of VS tuning is, "If you can feel the secondaries opening, they're opening too soon." That may sound contrary, but that's the way it works. As the secondaries open, power should come on smoothly. If you feel them open, what actually happened is they opened before you felt it, the engine "bogged" since there wasn't enough flow through the boosters to pull fuel in, and when fuel finally started flowing, you felt them "open".

The first rule of DP w/auto tranny is the stall has to be high enough to get into the cam powerband immediately. The 2nd rule is, don't over-do it on the carb size.

You'll have to make the choice between driveability and max power. I doubt the difference would be .1 sec., though.

 

Vacuum secondary carbs are always smoother and more civilized
than double pumpers.

The double pumper carb will always feel like more of a "ride".
But if you go to the track and compare them after tuning each
for best performance you'll find that the vacuum carb makes just as much power and will run just as fast, often faster.
The secondary vacuum spring will need to be adjusted to suite your car. If your 750vs carb has a metering plate in the sec side
it may need to be enrichened a little. Either swap in a metering block conversion kit and use #80 jets or drill out the metering plate "jetting orifice" about .003/.004" to get the same effect.
The plate is equal to a #75 jet. Primary jetting should be 70 to 73.
A lighter sec spring will want a larger primary accelerator shooter
and cam.
The seat of the pants feel can be very deceptive. Go to the track to tune it.

 

I'll agree with 57kid about the adjustable secondaries housing, my uncle and I ran the same Holley 750VS, his on GM's 350/350 crate motor, mine in my sig. He bought the adjustable housing, while I opted for the springs kit + track tuning. When he installed his it made a noticable difference aka big seat of the pants difference. I ended up with the taller of the lightest spring (yellow one) and while it did help somewhat, the thing that I noticed after switching to the carb in my sig, was that with the new DP, my car ran ALOT more consistent. instead of jumping around from .02-.04 each pass w/ the VS, the DP always been within .02 of itself, hence my success winning, if I lost, it was almost always cause I cut a .550 or worse light, not because the car was off the dail.
Another thing you could possibly look into is the Proform mainbody, that was another option I was going to pursue with my 750VS, till I stuck the carb I have now on, I had 5 races on the 750 and sold it for $125 to a guy that was running a 600VS on a 355. Best decision I made all year (.3 faster w/ that swap alone), too bad I did'nt do it sooner. So I'll stick to my beliefs on DP being better than VS for "racing" purposes, maybe your guys cars at your tracks are tuned better than ours, but I've yet to see a VS on a car that went to a finals event-all the way down to trophy class. But for a more daily driven and maybe once a month to the track, I'd find a way to tolerate/tune the VS for a more economical stand point.

 

everyone thank you for your replys, they are much appreciated. IHI, thats a HUGE carb IMO. wow. with your 750vs, did you feel that your secondaries opened up every time? sometimes it feels like they do and others not. how consistant was your uncle with the adjustable secondaries? did he vary .02-.04 like you did? i do not have a 2ndary metering plate, thats why i was contemplating the conversion. do you just bolt a metering plate on the back, what exactly does the "conversion" entail? thanks again.

 

My uncle's "baby" 68 camero has never and will never see the track-currently he building tubed S-10 for Super Pro though, but I've rode in it enough before he changed to the adjustable secondaries housing to feel the difference. All he has is 3.50 out back for gears and he could turn em over from a stand still, but not a roller. The first 4 way intersection he took me to, he was almost all the way through and said watch this-nailed it and instant sideways/smoke and tires keep a rollin till he let out, never ever did that before. It made it feel as though the carb was a DP, but that's just my opinion. I had the cheap zinc plated Holley750VS with the metering block, the only mod I did was play with the diaphram springs. One of the other fellas will have to jump back in on the adjustable metering block/conversion.

 

Do you want to see pictures?

(I'm not disagreeing on the DP for racing thing, by the way. Wish I had gone that route instead of the VS.)

 

I said that half heartedly, I know that they're out there somewhere, just not the tracks I've raced at.