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! ;)

Re: Could it be THIS simple?
 

Correct, which also means as you're getting into your top end, and the vacuum is rising again, a 6.5 PV will close up sooner. That's why I feel pretty confident about suggesting an 8.5 PV. :D

Re: Could it be THIS simple?
 

There's no bog at all. It takes off like a freakin rocket! I don't really feel anything.My 15yr. old son once said it looks like my car slows down after the 1/8 mile....I don't know...it's possible I guess. The 12.85 at only 102MPH is what has me baffled. It should be around 105-107mph....despite the excellent 60' times I'm pulling. I'll try the 8.5 valve and see what happens.

Re: Could it be THIS simple?
 

I ordered the new 8.5pv today along with a set of 74 jets for the front. If the 74's are too rich, then it's still ok 'cause I can use them next year when I finally hook up my nitrous. I'll need to jet up for that anyway.
PKRWUD, the 15hg I have at idle IS when it's in park.

Re: Could it be THIS simple?
 

I just check my vacuum reading again. It's close enough to call 17hg in park and between 14-15hg in gear. A 7.5 valve is what's called for, according to Holley but an 8.5 will give me a slightly earlier opening and later closing.
It won't be until October when I get back to the track but you can bet I'll be back here with the results when I find out! Hopefully, it IS THIS simple...after all this time!

Re: Could it be THIS simple?
 

Why would you need to jet it richer for N20?

Re: Could it be THIS simple?
 

It's recommended that you jet up when you use nitrous as a precaution against a lean fuel/nitrous mixture. It's better to be slightly richer, rather than lean when running nitrous. A lean fuel/nitrous ratio is what melts pistons and all that other good stuff you don't want to happen.

Re: Could it be THIS simple?
 

I've learned 2 things about Nitrous.

#1. Don't GIVE any advice.
#2. Don't TAKE any.

Think about that for a few, you'll "get it"...........

OK, now I'm gonna break Rule #1. Leave the carb alone if you're running a plate system. :rolleyes:

Re: Could it be THIS simple?
 

How are you supposed to regulate fuel/nitrous ratio? I don't want melted pistons or spark plug electrodes.

Re: Could it be THIS simple?
 

Most companies have it set that the recomended jetting is a bit rich.

Re: Could it be THIS simple?
 

Ya have to start thinking of carb and N2O as two different systems, cuz THEY ARE!

If the car is runnining "OK" leave the damn carb alone. Change spark plugs, adjust base timing, and let her rip. N2O plate will take care of itself.

Please refer to the first 2 nitrous rules of this thread.

Re: Could it be THIS simple?
 

OK, now that I don't have so much beer in me :eek: , I can clarify that last statement! LOL


All I meant was the carb should be tuned seperate from the nitrous system. Get the carb setup right first, then hit the spray. If you want to run the carb a little fat, that's fine. A little insurance never hurts. But remember one of my rules: "Fat is safe, but Lean is Fast!" (Maybe I should start righting these down? LOL)

Then all you have to do is fire up the nitrous plate, and if the thing gets PIG fat, lean up the plate a little, but leave the carb alone. Same story for the lean direction. If you have problems on the spray, tune the plate, not the carb.

Dammit! I broke rule #1 again, didn't I? LMAO :D

Re: Could it be THIS simple?
 

This was kind of one of our rules from last night, but I won’t elaborate right now. :D
Ryan

Re: Could it be THIS simple?
 

Brian

Is this why you're going for the 4-corner idle circuit?
You know that you can take the power valve out completely and just square jet the thing and it'll run fine. So long as you run the power valve you have to use a 10-Point spread in the jets. Seems simpler to me to remove the PV.

Denny@JDS Induction Products

Re: Could it be THIS simple?
 

No, that's not why I want 4 corner idle. I think I'm running rich at idle(burning eyes syndrome) and my jetting isn't overly rich either(72/80).
As far as removing the PV, I don't want to do that because I still drive this on the street and was told that wouldn't be a good idea because I would be rich ALL the time.

Re: Could it be THIS simple?
 

Brian
It was just a thought I didn't know you still drove it on the street - yes then it is best to change over to the 4-corner and 2 power valves the engine will run a bunch cleaner. When I convert Holleys to 4-corner I always use 3310 metering plates. I don't have the part # handy but its in their parts book.

That overly rich condition is common for Holleys

The main jet has nothing to do with the idle circuit and only partially controls the intermediate transfer slot just above the the idle hole in the base-plate.

I'm assuming that you have a 4779-2 750
It doesn't have holes drilled in the butterflyes and to get it to idle you have to turn the RPM screw about 3 to 5 turns to get idle speed. If that is correct then with the blades open that far the boosters are trying to dribble causeing the rich idle.

To find out - hold the throttle lever tight against its stop (you might want to have someone help you with this) and unscrew the adjustment until it is completely closed - then turn the screw in 2 turns - Start the car and see if it will idle there out of gear - if not then you are going to have to drill holes in the butterflyes to conpensate for the added air/fuel - Of course this may not be the problem at all! It could be something internal ????

At this point all we can do is make suggestions

Denny@JDS Induction Products

Re: Could it be THIS simple?
 

Burning eye syndrome is usually "Lean" not rich.

Re: Could it be THIS simple?
 

Head Doctor, I can't do that test for you because I already have the carb. off ready to be shipped out for the 4 corner idle to be done. Should I also tell the guy to drill 3/32 holes in the butterflies while he has it or will the 4 corner idle negate the need to do that?
I don't remember, offhand, without looking but I "think" it is a 4779-2 model. I sort of recall it taking quit a few turns to get it to idle speed when I first hooked it up.

Will it hurt to have the butterflies drilled if it doesn't really need it, although, I don't think it would pass that test you just mentioned
I don't think 2 turns would be anywhere near enough to get it to idle clean, which, right now is about 1000rpms. I don't know if that's normal or not though(1000rpms to idle clean)

Re: Could it be THIS simple?
 

Brian I too still have this problem. I am quite sure that I am past the two turns to keep mine idleing, although I dont have mine that high (900-950). So much has changed on mine in the last week or two, it will be some time and some small tuning before I get it.

Ryan