Could it be THIS simple?
 

As most of you know, my car has run a best of 12.85@102mph and that MPH is a little low.

I discovered that my 750dp, that I bought on Ebay has a blocked secondary PV. Doesn't rear jetting need to be increased 8 sizes to compensate?
If I'm running 72/80 jets now, shouldn't I have 88's in the rear?
If all this is true, would that be a major reason why my MPH is low?
That would be GREAT if a simple jet change would increase my MPH and E.T!......but I'm not getting my hopes up though...LOL

Re: Could it be THIS simple?
 

Possibly! 80 is stock secondary jet size, so might be worth a try. holley even says on there site that if you do remove the power valve you have to jet up 6-10 sizes.

Re: Could it be THIS simple?
 

Becareful you don't overjet. You'll make it fat and lazy. Your MPH will go the wrong way.

Re: Could it be THIS simple?
 

I have a day of T&T on saturday. I'll just keep jetting up until MPH starts falling off.
This is when those Holley quick change fuel bowls would be nice so jet changes can be make with the bowls still on the carb.

Re: Could it be THIS simple?
 

I would think your plugs would tell the story if you were running lean.

Re: Could it be THIS simple?
 

I think they are trying to tell me that. The porcelain is snow white after each run.....not even a hint of getting tan.

Re: Could it be THIS simple?
 

I'm also going to check my holley book and see what year my carb. is because I'm being told that some holleys didn't come with a secondary PV and that's why the rear jetting is at 80, otherwise it would be 70-72. I don't know how true that is or isn't though..

Re: Could it be THIS simple?
 

The Holley #4779 series of 750 DP's only had one PV, and were jetted accordingly (most were 70/80). If your plugs are bright white, try increasing them to 82's, but don't go any higher than 84's.

Re: Could it be THIS simple?
 

I tried 82 in the rear and lost about 1/2 MPH but I'm not sure if that was due to raising temperatures or what.
WHat about going from 72 to 74's in the front? If jetting the rear gains little or nothing, does that mean increasing the primary will yield the same results?

Re: Could it be THIS simple?
 

You can try 74's in the front. What size PV do you have? What is your vacuum at idle?

Re: Could it be THIS simple?
 

6.5 PV and 15hg of vacuum at idle(neutral).

Re: Could it be THIS simple?
 

82gt-
You have to do it with the car in gear and at an idle.....Unless you have changed out your auto to a stick, which I dont think you did. :D

http://www.holley.com/data/TechServi...wer_valves.pdf


Ryan

Re: Could it be THIS simple?
 

I was just asking him what his vacuum was at idle.

Brian-
Try an 8.5 PV.

Honestly, I think your car would respond much better with a 3310 on it, but that's more money.

Re: Could it be THIS simple?
 

Ok :( I opened my mouth too soon. I just thought I could add some decent knowledge in here before somebody who knew what they were talking about did. I am going back to my corner now.....
Ryan

Re: Could it be THIS simple?
 

Dude, not even! I was just covering Brian's back. I really did want to know his vacuum at idle. I want to know yours, too. 8" can't be right. What is it in Park, at idle? What is your elevation?

Just to satisfy my curiosity, let's get hypothetical for a second. Let's say you were cruising down a straight level street at around 35 mph, when a little kid suddenly jumped out from behind the back of a car parked ahead and to your right. You jump on the brakes, stopping just short of hitting him. You don't lock up the tires, but if you had braked any harder, you would have.

Would the engine stall?

Re: Could it be THIS simple?
 

I will post all that up tomorrow or at least by Tuesday afternoon. I wasn’t running the vacuum gauge, my buddy that had one was. So I need to call him and see what it was running prior to putting it in gear. I will go buy one and look-see for myself.
Thanks, I know we will get it figured out.
Ryan

Re: Could it be THIS simple?
 

Back to the original question. If there was a power valve in the secondaries, then the carb would be jetted squared up (or nearly so). With no PV in the secondaries the spread was jetted correctly to begin with. To go another 8 jets on the secondary would be overkill to say the least. Reading plugs isn't always as easy as people would believe and is not the best way to check the A/F. Take the car to your local dyno shop and have it checked with a wideband O2. On the PV, I've always gone by the rule of thumb of dividing the curb idle vacuum level by two and run that PV (15"-hg /2 = 7.5 PV). Like others have said, if you can put the car under light load when you check the curb idle vacuum level even better.

Re: Could it be THIS simple?
 

Chris, I used to have a 3310 on it and I get better throttle response and 60' times with the double pumper (1.67 60' times)
So, with the 6.5 valve, do you think I'm leaning out on the top end, which is what many people suspect is happening?
The 8.5 valve, if I'm correct, will allow fuel enrichment sooner and close later....right?
Did you want my idle vacuum in Park or in gear?

Re: Could it be THIS simple?
 

You've got the right idea.

I'd like to know your vacuum at idle, in Park. It would be really nice to know what your vacuum is when you feel you're losing your top end power, too. I'm guessing it's between 7" & 8".

Either way, an 8.5 PV will suit your set up better than a 6.5 PV will. May not solve all your problems, but it's a step in the right direction.

Throttle response has nothing to do with vacuum or mechanical secondaries, it's in how the accelerator pump is set up, the idle mixture, and the throttle plates. The reason your car should do better with a 3310 is vehicle weight and an auto tranny. That combo is difficult to tune mechanical secondaries for, whereas vacuum secondaries are tuned by engine demand. You just fine tune them with jets and the secondary spring.

Are you experiencing a bog, or it just feels like the carb runs out of juice way too soon? If it's the later, a 3310, properly set up, should solve it. In the mean time, an 8.5 PV will help.

Re: Could it be THIS simple?
 

Don't forget that most people think of the power valve "rating" BACKWARDS!!! If you have a 6.5 PV, it's open BELOW 6.5 hg.

Above that, it's as tight as an ugly 4th grader!

And back to the original question: NO, it can't be that simple. It never is! ;)