I got a 750 dp holley on my 383. The motors built, and is well over 400 hp. Ive got 3.08 gears in the rear, and i can spin the tires all the way into 3rd gear with straight throttle. The problem is mid to upper power feels like a ford taraus. Ill drive at 30 or 40 and try to take off, and a v-6 could probably take me. The engine roars loud and strong, but it just accelarates slowly. I looked into the problem and found out my secondaries on my holly dp are NOT opening at all. So I went and bought a new Vacuum diaphram to fix it, and that wasnt the problem. I did the paper clip test and the secondarys aren't opening. SO i switched it to Mechanical secondaries. Ive adjusted it for about 2 hours, and now with my secondarys actually working, im only yeilding worse results than when the secondaries stayed shut. My timing is spot on, so thats not it. Any ideas?? The only thing i can get my car to do with the mechanical secondaries, is either feel the exact same, or make my car bog for a split second up top. Ive heard about people adjusting their pump shot, idk if that would help or not. But its got tons of power off the line, and looses so much from 2nd into 3rd gear.

 

.... on what?

What is it, ACTUALLY, at 3000 RPM with the vac advance disconnected? And I don't mean just, "what does the mark say"... what is it REALLY? Have you physically verified that the mark actually concides with TDC, like it usually doesn't?

 

my timing is off the balancer that came with the motor freshly rebuilt. ive adjusted the timing from 22 degrees upto 38 degrees and everything inbetween with the vac disconnected. their actually isnt any noticed difference in power from 28 degrees to 38 degrees. Currently, its 36 degrees with base and mechanical. I have the vac advance locked out. i dont use it. If it helps, ive ran up to 38 degrees mechanical, and have a 10.5 compression, and havent experienced any pinging yet.

 

double pumpers dont have vacuum diaphrams. they have double pumps (2)
and you did not just switch your vacuum carb over to mechanical secondaries by whatever mod you did to it.

 

So... does the stationary side of the mark agree with it? Do you know FOR CERTAIN that when the mark says "0", the motor is ACTUALLY at TDC?

Sounds to me like your timing is WAY retarded. At 10½:1 compression, you should have been able to easily make it ping. No matter what the mark reads.

Have you even tried advancing it until it pings, and then backing it off until it just quits, and seeing how it runs? That may or may not be the "best" "right" "optimum" setting, but it shoudl give you some kind of a clue where you ACTUALLY are, independent of an unreliable mark.

 

Lol, yes they do, and yes i did

 

Ill give it a try when i get my car back tommarow

 

thanks for your help sofaking. Any other ideas besides timing??

 

Carb. jetting, fuel pressure, fuel volume, float level

 

Only, put the carb back like it belongs; since there's no way to make a mech sec out of a vac sec carb. Except the "2 easy steps" method: 1. sell vac sec unit, 2. buy mech sec one. I can't imagine what you did but whatever it was it wasn't that, which means it probably wasn't an improvement.

And, make sure your fuel delivery is adequate. If the car runs through 1st & 2nd OK and then starts to fall flat on its face halfway through 3rd, or some pattern like that, it's likely because the fuel bowls are running dry.

 

alright, ill put it back how it was, with the Vacuum secondaries. But any ideas on how to get them to kick in? I already replaced the spring with a lighter one, and a new diaphram, and that didnt do anything to make the secondaries engage at all. With the car off, i can engage the throttle all the way and twist the secondary linkage tube that runs through the base of the carburator, and get the secondaries to open. And, if i DONT engage the throttle, and just try to twist open the secondaries, they wont open. SO that gives me the impression that nothing is wrong with them mechanically. But they still wont open when im driving.

 

The way they open, is from venturi vacuum in the primaries; basically, the same suction that pulls the fuel into the engine. Flow in the primaries creates that signal. When it reaches a sufficient value, meaning when the engine is nearing the flow limits of the primaries, the vacuum generated by the flow through the venturi begins opening the secondaries.

When you get your other problem sorted out, to where the engine demands more air, the secondary issue will go away. They will open just like they're supposed to.

How does it run with the timing advanced just to the point where it almost pings?

 

Not enough vacume to open the secondarys? could that be caused by the timing being VERY retarded?

 

well, i dont think my car would be running or idleing to well if the timing was wayyy off. But I have pushed it upto 4,000 rpms with WOT and the Secondarys havent opened even a milimeter. So im a little confused why the vacuum wouldnt be strong enough, but its always a possibility. Ive got a 490 lift, and 280/280 degree cam if that helps anyone. I think im going to go to the dyno tommarow. Hopefully i can see whats up with it there. Ill let you all know how it goes if i end up making it. Im still open to any ideas or comments you car gurus have tho. Thanks for everyones help so far. Its given me alot to look at.

 

How on earth do you know that your secondaries aren't opening while you're driving?
It's rather perilous to be peering at your carb as you're WOT'ing going down the road eh?
If you're not leaning over your car as it's accelerating past 40mph, you have no idea what your secondaries are doing.
What sofakingdom was saying, is that they open from vacuum, ie, when your engine has a load on it. Revving it in neutral in the driveway isn't enough to get them to open. It's next to impossible to "watch" vacuum secondaries open - unless you're on a dyno.

Did you try the timing test? Can't hurt.

If in doubt, bring it to the dyno and find out. That's a good way to figure out what's going on.

 

Think a little before you post so fast SONIX. Did you know that you can actually determine exactly how much your secondaries are opening by using a simple paper clip attatched to the secondary linkage. (and yes, most car guys use this trick) WHILE DRIVING, As your secondary linkage pushes upward into the carburator, the paper clip cant fit in the carb with the linkage, so it slides down the linkage. When you pull over, you can tell how far your secondaries engaged by how much the paper clip moved. And yes, Ive been testing the secondaries while driving around, NOT sitting in park or neutral.

 

I'm glad you edited your post, but I still get emailed based on what you originally wrote.
Just to be perfectly clear, I was being sincere when I said a dyno run was a good idea. You DO get a lot of information that WOULD be helpful to someone who knows what to do with it. However with all the tuning knowledge you seem to posses I think you have all the information needed to fix your problem as it is.
Have a nice day.

 

yeah i read your post fast and i thought it said "i doubt bringing it to a dyno would find it out" And i guess i posted kinda fast myself because after reading it a second time, i realized you werent trying to be a smart ***. So i edited it. My apologies tho

 

A dyno guy might fix your problems but it wont be free. the guys on here are very smart and know small block chevys very well and the help is free. I would do some of the suggestions that have been given. get your ignition setup perfect so you can eliminate it as a problem 12-20 degrees initial 32-38 total. ballpark is this hows its setup? Are you sure? And your car may idle just fine with the timing off as you have suggested its fine because it idles good. Vacuum will be better with more initial timing. If you are positive the timing is correct move on to the next possible problem. cam was it degreed? Who put it in? This could be a problem for sure. Hows the carb? condition? you need to put the tools down and read, read, read. The more you understand about the systems at hand the better the outcome will be. The motor is either not needing the secondarys open or the carb is not working correctly for whatever unknown reason to me. Your timing curve could create low vacuum if incorrect. the secondarys may not open because they are not needed, due to the timing being off creating a engine that is struggling. Dont guess, we are being sincere about helping you. Verify the mark on the balancer corresponds to #1 cylinder at tdc. when the mark is at the zero on your indicator the piston should be all the way to the top. then move on to something else....

 

rjt76, thanks your help too. Im still waiting to get my camaro back this afternoon before i can try the timing idea. I got a shop putting in 3.73s right now for me. Hopefully they will have it done before it starts raining today. Yeah your right about my timing setup, 20 initial, 15 mech, 35 total. i would have bumped the timing even more since i didnt hear any pings, but i got nitrous on there, so i figured id be safe with 35. Ive only sprayed once because of its problems in the upper RPMS, but the wet shot reacted very well and effective when i used it. Ive got a chance to goto the dyno tonight, and possibly tommarow. I would like to do some tuning and tweaking using everyones ideas before i just run right up there. i was really hoping that the 750 dp was just a bit to much for 3.08 gears, and that 3.73s would help fix that. But thats probably not the case. ill post up my findings regardless. Im pretty sure its down to an improper timing problem, or a carburator problem. Ill get 2 dyno pulls for 65 bux, so it wont be to pricey.

 

alright, i upped the timing 10 more degrees, and it ran the same as before, but the engine got louder up top, but wasnt producing anymore power. I got my foot in it 3/4 of the way, but didnt wanna give it anymore because it sounded like anymore throttle and the motor would gernade itself. I checked the linkage arm for the secondaries and it hadnt moved. So im kinda thinking i am going to buy another carb. I got my 3.73s put in today, and the dyno is backed up for a week b/c of everyone getting their stuff out for spring.

 

alright, i brought my car to a guy with a life that revolves around nothing but carburators. After his inspection, he said the carb had debris that came from the car sitting for long periods of time. That debris was clogging up a passage that controlled the secondarys. He offered to rebuild it for 60 bux plus the cost of the rebuild kit. I decided to let him, and i also bought a fresh 750 holley racing carb with quad corner idle adjustments an mechanical secondaries, that hasnt had any fuel through it yet. Ill get it on and test it out monday.

 

Debris in the vacuum passage is a possibility.

Sometimes there's no substitute for putting an eyeball on something... I don't recall seeing anything in your post that would have pointed us out here in that direction though. Telling the WHOLE story can often be real helpful.

You should still advance your timing until it pings, then back it off until it quits, and see how it runs. Since you already know your mark is meaningless, it's senseless to worry about what "number" it reads. Do the timing thing, find where it runs the best.