Jay M

I know that it is not the origional carb, but I suspect that it is too small. I have a 427/390 that has only 230 hp at the wheels. The HP droped off drastically after 4000 rpms.

Why would the HP drop at such a low RPM? My best guess is that there is some restriction. Exhaust is stock and it may have a bigger than stock cam.

Thanks,
~Jay

73 LS-4

The small quads were 750 cfm and were used in the lower performance big block vettes for many years, at least 68-74. If tuned right is more then enough for a mild big block. If you can get the numbers off the carb someone here may be able to tell you what it was actually used on, if nothing else you may be able to get the numbers for the proper rods, seats, hangers, etc. that your supposed to have

:cheers:
Pat Kunz

SteveG75

A properly tuned Q-Jet should meet your needs easily (I know of at least one 12 sec 455 GTo running a Q-Jet).

Possible fuel pump problem (not enough pressure to feed demand at higher rpms) or air flow issue. What kind of aircleaner/filter are you running?

The Dude

Oh, if you have Doug Roe's book, "Rochester Carburetors," it shows a nifty way to tell your Q-Jet's CFM by simply measuring the diameter of your primary boosters.

That having been said, I don't have the book here. Sorry. The upside is that you have basically two possibilities--HUGE and FARGIN' HUGE.

Larry B.

It is 750cfm... Later ones are 780cfm. but ... you need to check to make sure the throttle is opening all the way when you operate it from the pedal.. There have been a few threads about the pedal rod bending so the carb never opens all the way. :jester

Tom73

The q-jets used on Vettes were 750 cfm. The differences were internal, different jets/metering rods/etc. Just take it to a good q-jet tuner and get a performance tune/rebuild. It is capable of providing more gas then your engine is ever able to use.

tom...

clem zahrobsky

check for full throttle as i have seen most cable operated carbs not getting full throttle. GM set some up this way as i had to shorten the cable more than a 1/4" to get full open throttle. post your carb number. :chevy

79er

Ok correct me if I’m wrong, but I thought that my 79 had a 795 cfm Q-jet stock? Also is there a rare casting Q-jet out there that’s off a GTO judge I think that flows 815 cfm stock and can go to 850 cfm when polished and ported?

Paul L

Yes, the 1979 was 795cfm. But really that is not much different from a 750. In fact it is probably too much carb for the engine.


[Modified by paul79, 12:24 AM 1/25/2002]

The Dude

OK, I looked it up FWIW. And it's the primary venturi diameter and not the primary booster diameter as I said earlier. Anyhow, Roe writes that 750 CFM Q-Jets used a 1 3/32"-diameter primary venturi, while the 800s use a 1 7/32"-diameter primary venturi.


[Modified by The Dude, 11:19 PM 1/24/2002]

Jay M

Thanks,
The number stamped on the side is 7028217 EG and there is a sticker that says manufactured by Holley.

I had my wife push the pedal to the floor, and throttle opened up all the way. But the flaps on top of the secondaries didn't open up at all, I had to push them opened to see the throttle. When the motor is running they don't open all the way when I floor it. All though I'm a little uncomfortable running it WOT with my head a foot away from the engine!

I guess the next step is to check the fuel pump, then find a guy who still knows how to twesk a carb.

The place that dynoed it for me didn't have a clue about how to tune a carb! In fact one of the guys was puzzled by a sudden burst of richness when he began the dyno run. I looked at him like a dog and said "Isn't it the pump?" another guy there rolled his eyes and said yes.

thanks,
~Jay

clem zahrobsky

7028217 is a 1968 396 truck or car with std trans. you do not have to have the engine running to check WOT. just have some one push the pedal to the floor and you open the air flap by hand and see if you have WOT.

Jay M

A truck carb? Is this the right one for a Corvette?

The throttle does open up all the way when the pedal is to the floor.

Thanks,
~Jay

SteveG75

The secondary air valve is spring loaded and only opens on demand. With no load on the engine, there is no demand, therefore it will not open. I reccomend getting Lar's Q-jet tuning paper from http://www.corvettefaq.com and the Doug Roe book (good pics). The above helped me enormously.

Last resort, send the carb to Lars. You won't be dissapointed.

Jay M

I've looked at that paper before. It's way over my head! I may just send it to him.

~Jay

Smokehouse69

Yes! Send him the carb along with your engine specs and he'll set it up for you very reasonably.

79er

paul the really good thing about a q_jet is that it uses a air valve secondary so you can not have to big of a carb. It on flows as much as the engine can use.

Tom73

Right, that is what makes them nice. Also easy on Chevy to put a 750 cfm on all from the smog choaked 75 to the 454. Only uses what is needed. Some engines will probably not draw over 350-400 cfm. But if you open it up with cam and headers it may draw 600-700. Very very few street engines will need more then 750. As you said, it flows as much as the engine can use.

tom...

79er

If I remember correctly my engine pulled 665 cfm at WOT? I got the bigger 800 cfm casing carb so there was not as big of change between the primaries and secondarys.

JD/76-ZZ4

When I purchased the Edel Performer 795 CFM, I found out it was way too much (rich) for my ZZ4...I sent it to Jet Performance with all the specs and ALTITUDE and they rejet it/swapped rods along with changing the secondary rods....The only thing I had to tweek was the idle since it was still running way too rich...Seems like I finally got it right....