poor performance, holley/qjet?

[ruby76]

I just bought a '76 stingray, the engine is not original, but I think might be a "clone" of the L82. It has a Edelbrock Performer intake and a 4160 double pump Holley Carb with vacuum secondaries. I've never had a vacuum secondary car before but it seems like it just doesn't have a good pull under accelerating. I used to have a stock 350 in a 1968 stepside chevy truck with a quadrajet with mechanical secondaries and it would run laps around the vette. Kinda dissapointed :cry . I readjusted the fuel level as when I first got it the secondaries didn't have any fuel in the bowl. It runs alittle better but not to my satisfaction. Someone mentioned that there was a kit to change the vacuum secondaries to mechanical? Wonder if that would help. Timing is at 12* with vacuum advance disconnected and plugged. I also don't know what has been done with the internals of the engine.

It came with a new Qjet im thinking about putting on, it also is vacuum secondaries. I wish I would have kept my old Qjet with mechanicals. Its only problem was you had to pump it about 15 times if you didn't run it for a week or so.

Can either of these carbs be turned in mechanicals? I've heard that vacuum secondaries are good for "heavy" cars and smaller/lighter cars are good to run mechanicals in, I would consider a 76 Vette to weigh less than my old 1968 C10.

Okay Im rambling now, Any ideas?

 

[The73vetteman]

Im no expert on carbs but I do know that the base '76 was SLOW. That's probably why your '68 truck seemed faster. Of course your performance will depend on the spec of the engine installed, but think low compression, single catalyst, emissions gear ... you get the picture?

 

[ruby76]

Yeah, I know the 76 was the "beginning of the end" as far as emissions and stuff. I guess I forgot to mention:

The 68 truck had a stock 1976 350 from a chevelle station wagon, except I put a 1969 iron 4 barrel intake and a qjet

The vette has a 76 350 from a truck(from the numbers anyway), all of the emssions stuff is gone

both have the *882 head casting, which means both have 76cc chambers

The truck did have headers, the vette has rams horns exhaust manifolds

Both have true dual exhaust, no catalytic converters. So I would think that any 350 that has any kind of aftermarket mods would be quicker - or feel quicker - than the one that I rebuilt and knew was all stock.

 

[titan81]

feel lucky your vette didnt have a stock L81 in it lol.
These yr vettes are soooo detuned, smog controlled, emissioned, and low compression.
I bet your truck had 10:1 or higher, where as yours has 8.5:1

 

[Culprit]

ruby76,

Q-jets with vaccum secondaries can run great if they're set up properly. I'm definitely not the guy to give advice on that, but I can offer this - have you checked to see if the vaccum secondaries are even opening? If the spring tension is not adjusted correctly, they won't open and you'll be driving around on the primaries alone.

Some of the smarter guys around here can jump in with the details.

Semper Fidelis,
Culprit

 

[ruby76]

I know both engins had 8.5:1, with the 76cc heads.

I can hear the "howl" at higher RPMs when flooring it, so I think that is a good sign the secondaries are at least opening a little bit.

Is the spring tension something on the Holley something I can adjust, or is it one of those you have to buy separate springs to tune?

Thanks

 

[tomtom72]

Hi Ruby76, There is a thread in the C3 tech section about holleys, and it has a link to MorTech. It might help ya out to go there. I'm not a mechanic! I had a 72 LT-1 & those motors used 780cfm vac-secondaries single pump Holleys. I remember that holley used to sell a kit with different tension secondary diaphram springs. Also there is a mechanical adjustment on the secondaries. I DO NOT remember all the details, been too long. I do remember it was a bit of trial & error for me, but I eventually got it right. Probably would have been easier if I had some experience as a mechanic or if I went to someone who knew holleys. I remember I bought a book from holley on how to tune & fix their stuff. I could not have done without the book it was a good teaching aid for a beginner. Sorry this is not really the answer, but I hope it points ya in the right direction. Good luck. Tom. I found the kink: http://www.mortec.com/carbtip1html

 

[vette-dude]

I just bought a '76 stingray, the engine is not original, but I think might be a "clone" of the L82. It has a Edelbrock Performer intake and a 4160 double pump Holley Carb with vacuum secondaries.

Are you sure this Holley is a double pumper?? I don't remember ever seeing a vacuum operated secondary with a fuel pumper attached to the bowl? I might be wrong but if this is single pumper you can buy a vacuum spring assortment to get the secondaries to open up sooner. It takes a little experimentation. You may also have the wrong power valve in the primary plate. There is a book out that tells you how to tune the holleys but I gave mine away. Also on the primary pump cam there are two mounting positions to give your squirters an earlier shot on acceleration. The squirters may also need to be changed to match your engine combo. It takes alot of fooling around with different areas of the carb to make a Holley run right but once you got it they are pretty much bullet proof. Good luck.

Randy

 

[JohnZ]

Correct - no such thing as a double-pumper with vacuum secondaries. Holley sells a kit that replaces the secondary diaphragm cover (so you don't have to remove the housing from the carb to remove the cover) and includes an assortment of different springs to tune the opening rate of the secondaries. Disconnecting the vacuum advance is a big mistake, usually done by a "Bubba" previous owner who thinks that will improve performance - it won't, but it will definitely sacrifice idle stability, idle cooling, throttle response, and cruise fuel economy.

 

[SG4206]

Ruby76-
Consider the carb not your weak link, and look at timing first. You can advance the timing untill detonation is heard ; then retard it 1-2 degrees. This will optimize power for your engine load combination and give you a more responsive throttle.
The vac. sec. carb only delivers what the engine requires under load.
It is much better than a mech. sec. carb. that goes lean and creates a bog at high load situations.
Next, re-lash the hyd. lifters and rocker arms at a pre-load of 1/4 turn past zero lash. Now , with this new base line , you can weaken the diaphram spring in the Holley untill a bog is created; then back up to one stronger spring.
This little bit of tuning can make major gains in a lazy small block.
Food for thought ; this works for me !

 

[ruby76]

thanks guys, I have to admit I don't know too much about Holley's I'll follow some of the advice here. Also, when I said the vacuum advance was disconnected, that was only when I set the timing - thought that is what I was supposed to do?

So I guess just because the Holley has fuel lines going to both front and rear it isn't a double pumper?

I also noticed it diesels when I turn it off it is running on anything less than 91 octane - not to sure what that means. I think it could be a sympton of a couple different things. At first I thought it was compression related, but with 76CC 882 heads, I think Im pretty much at 8.5:1 or even as low as 7.5:1. Im not sure what timing could be as I have no clue what the cam specs are. I did 12* - so you think I could still go more?

I probably have too many questions to ask on a message board but I really appreciate everyones responses.

 

[SG4206]

The term double pumper refers to the acellorator pumps on the bottom of the float bowls on all mech. sec. Holleys.

Primary butterflies are open too far. Adjust sec. butterflies open a little further , too allow closing the primaries down and controling the run on after killing the ignition. You need to purchase a Holley and a Q-jet book if you are going to pursue working on your toys.

The fact remains, that all the information in world may not fix your problem, you have to experiment with the info and taylor it to your circumstances.