Rebuilding a QJet for high performance - what year?

[Stallion]

I'm looking to rebuild and modify a QJet to deliver top performance for my Vette. I was looking at ebay and wondering what year should I go for?

Also, is a QJet from a Camaro, Corvette, Chevelle, etc. any different from the other ones of that year? See what I mean?

Ideas/suggestions?

 

[JohnZ]

I doubt if you'll see any increase in performance from using a different Q-Jet than you have now; they're all the same in terms of airflow capacity (700-800CFM), which is more than adequate for your engine. They only differ in calibrations for jets, metering rods, etc. Carburetors by themselves don't make power - they just meter air and fuel as the engine requires; the right combination of heads, valves, cams, compression, and manifolding are what makes horsepower.

 

[Stallion]

Ah if only I had the money for new heads, valves, cams, compression, and manifolding. I'm working on limited resources here.

 

[MaineShark]

Aren't some of the Q-Jets limited in terms of how far the air valve will actually open, and such?

And, even if no more peak power can be obtained, I think they can still be "tuned" for sharper throttle response, etc.

Stallion: If you'd like a Q-Jet to play with, I have one that you can have for the cost of shipping. It's out of a truck, so I don't know exactly what would be involved in making it perform, but it's yours if you want it.

Joe

 

[MMM]

I have found the '67 through '69 versions to be the best to play with. Why? The idle circuits in the carbs are not as lean as later versions. Less off-idle hesitation or stumble if you have a cam that does not build much vacuum. It is hard to modify the idle jetting in Q-Jets unless you have access to some very long, very small diameter drills or broaches. Earlier versions are ok ('65,'66), but are very hard to find and you will need to epoxy the main jet well plugs.
In any case, almost any used Q-Jet will need bushings installed in the throttle plates.
Mike

 

[Stallion]

Aren't some of the Q-Jets limited in terms of how far the air valve will actually open, and such?

And, even if no more peak power can be obtained, I think they can still be "tuned" for sharper throttle response, etc.

Stallion: If you'd like a Q-Jet to play with, I have one that you can have for the cost of shipping. It's out of a truck, so I don't know exactly what would be involved in making it perform, but it's yours if you want it.

Joe

MaineShark, that would be great!! Do you know the year of the QJet? You wouldn't happen to see the "serial" numbers on it, would you?

Thanks for the offer! I think I'll take you up on it.

 

[KANE]

I'm looking to rebuild and modify a QJet to deliver top performance for my Vette. I was looking at ebay and wondering what year should I go for?

Also, is a QJet from a Camaro, Corvette, Chevelle, etc. any different from the other ones of that year? See what I mean?

Ideas/suggestions?

Just my opinion, for the money and long term enjoyment.... I'd ditch carburation the way Detroit did in '87.

Don't worry, there are a variety of ways to go EFI. Holley, Edelbrock, GMPP, ACCEL, and many others offer kits and solutions from $500 to $5000.

I am not a fan of carburation if you live in emissons testing states. It is getting harder and harder to pass older cars. Many times, it is the operator of the test equipment that may hold your success or failure in his hands. If that person doesn't know what they are doing, looking for, or what the values of certain gasses are... you could be in for a shock. FAILURE!!!

I feel that 80s and 90's pre-OBD cars are rediculously simple to build, tune, and maintain. A timing light, voltmeter, and jumper wire are the best tools to tune and keep a TPI motor in check. The best part is most of the parts are availible at any store for little money these days. Obscure emissions components, carb rebuild kits, and etc. are becoming harder to come by. So, be prepared for increases in prices as the years go by!

In any case, a well tuned engine will perform better than a sooty emissions emitter.

 

[KANE]

Stallion: If you'd like a Q-Jet to play with, I have one that you can have for the cost of shipping. It's out of a truck, so I don't know exactly what would be involved in making it perform, but it's yours if you want it. Joe

Oh- and truck carbs may give you a different set of metering rods. Depending on your engine.... these may help you out with the throttle response. If you try a search for metering rods for Q-jets, you may be shocked by the prices!!!

 

[MaineShark]

MaineShark, that would be great!! Do you know the year of the QJet? You wouldn't happen to see the "serial" numbers on it, would you?

Thanks for the offer! I think I'll take you up on it.

I'm not sure about the specifics - it was given to me by a neighbor, so I don't know precisely what it came from, other than it was a truck of some sort. I'll see if I can find any markings, early next week.

Joe

 

[Stallion]

Sounds good, Joe!

DarkShark78, can't you just "gut" out the whole carb, metering rods and all, and replace it with whatever you want? I thought QJets were pretty much the same as far as non-modifiable parts goes (besides the primary and secondaries, obviously). No?

 

[black_81_vette]

Hello guys,I just did this not but very long ago.As far as e-bay carbs, gotta watch those ive gotten em cheap but 75% were junk. The real reason most of em get ruined is over tightning the mounting bolts,thus warping the air horn. The main body and such will be fine in most cases, and yes worn throttle shafts can be trouble too.If you want a good performing carb get one from a caddy 425 472 or 500 cid.Q-jets will only flow as much air as the engine needs,yes air door secondary rate is adjustable so are the air the metering rods and on and on.I used a carb off a 77 425 caddy changed the power pistion spring(big cam) and ran 72 primary jets with smaller diameter secondary rods on a H secondary hanger.The last thing I did was to set the air doors to open quicker since ive got a four speed and a gear out back.After all was said and done it pulls like a rocket and the best part is it stays tuned!!! Cant get that out of a holley and good gas milage either.If you decide to go for it let me know if you need some tunning help.

 

[Stallion]

But would this QJet off a 425, 472, or 500 Caddy be too much carb for my 350?

 

[KANE]

Sounds good, Joe!

DarkShark78, can't you just "gut" out the whole carb, metering rods and all, and replace it with whatever you want? I thought QJets were pretty much the same as far as non-modifiable parts goes (besides the primary and secondaries, obviously). No?

Noooooo..... you can sawp the secondary metering rods. They are shaped differently and meter fuel differently based upon application. You'll at least have some flexability.

There are several books that deal with mods to Qjets. If anything, having a few Qjets laying around will pay dividends if you decide to take them under the drill or dremel!!

Oh- and STAY AWAY FROM THE 81(?)-87 Q-JETS..... they have the CCC electro-mechanical carb set up. They are not what you are looking for.

 

[KANE]

Hello guys,I just did this not but very long ago.As far as e-bay carbs, gotta watch those ive gotten em cheap but 75% were junk. The real reason most of em get ruined is over tightning the mounting bolts,thus warping the air horn. The main body and such will be fine in most cases, and yes worn throttle shafts can be trouble too.If you want a good performing carb get one from a caddy 425 472 or 500 cid.Q-jets will only flow as much air as the engine needs,yes air door secondary rate is adjustable so are the air the metering rods and on and on.I used a carb off a 77 425 caddy changed the power pistion spring(big cam) and ran 72 primary jets with smaller diameter secondary rods on a H secondary hanger.The last thing I did was to set the air doors to open quicker since ive got a four speed and a gear out back.After all was said and done it pulls like a rocket and the best part is it stays tuned!!! Cant get that out of a holley and good gas milage either.If you decide to go for it let me know if you need some tunning help.

OUTSTANDING POST!!! Sounds like you have the formula that I never found!!!

Stallion, black_81_vette basically did what I was suggesting to- and he got it to work!!!!

(......I'm going to save your "recipe"- . If I ever delve back into the Q-jets, I'll use your formula. Sounds like a winner!!)