Q-Jet Rebuild

[vetteboy86]

Thanks again for all the help. I have posted several times regarding this carb, and each time I get great replies. Well my question is how hard is it to rebuild the carb? I am totally unhappy with how the car runs. It seems like it is one thing after another. The carb is the original. I dont have very much experience with this type of thing, however I am somewhat handy when it comes to cars, and do have a wider variety of tools. Are their any special tools, or instructions I will need? About how much should I pay for a good rebuild kit?

One last thing, is it worth the money buying a new holley? I am seriously thinking about it, however if i can rebuild this one and get good results and save a couple hundred dollars that is what I will do.

Thanks in advance for the replies.

Craig

 

[78SilvAnniv]

We just had my 27 year old carb rebuilt by a professional in Texas and are very happy with it. Cost was extremely reasonable, including shipping.
If you're interested, send me a PM and I'll let you know how to contact him.

If you wish to rebuild it yourself, do a search of grumpyvette's posts, I recall he mentioned a great book that detailed a carb rebuild.

My decision to keep my original carb was purely sentimental. It has already been with the 78 for well over 200k miles and I couldn't see separating them.
Heidi

 

[MMM]

Keep the Q-Jet and forget the Holley. However, prior to attempting the rebuild, buy a copy of the book "Rochester Carburetors" by Doug Roe. Read the section on Q-jets to get an understanding of how they work and the basis of their design.
Always replace your float during a rebuild unless you have previously replaced it and know it is less than two years old. The plastic floats are the achilles heals of Q-Jets. One other area to carefully examine is "slop" between the bores for the primary throttle shaft in the base plate and the primary throttle
shaft. This area is prone to wear, especially on the left side of the carb. Air will be drawn in around the shaft and create an inconsistant idle. If you have a lot of wear in this area I recommend you send it to a reputable rebuilder to have bushings inserted. You can purchase a kit yourself to install the bushings, but if not done correctly, you will end up with a lot more problems than you started with.
I may be prejudiced, but in my 30 years of carb work, I have never seen a Holley replacement carb perform better than a properly operating Q-Jet.
In fact, I have made a bunch of Chevy owners happy by rebuilding and reinstalling their original equipment Q-Jets after they had installed a Holley replacement and were very disappointed with the results.
Mike

 

[TWINRAY]

I think doing it yourself is pretty simple but I've done it a few times before. Some things are simple for some, difficult for others and vice versa. Stay with the Q-jet. You will get better mpg and when the big secondaires come in, you'll get all the performance you'll need. If you do it yourself, go slow and take digital pictures. Watch when you take the ball check screw out inside the carb because there is a little ball bearing inside. I think they give you a new one in the kit which probably costs about $25. And as said previously, buy a new float for about $10 or less. If uncertain, I'd get Heidi's recommendation and see how the price sits with you. Good luck.

 

[tom102]

Qjet Rebuild

Napa carries the Echlin rebuild kits for the carburetor, you will need the nimber of the carb to insure getting the right kit. The number is located on the left side of the carb (drivers side) just above the secondary linkage and to the left of the rochester logo. The floats are a seperate item and do not come in the kit but are a seperate item and are also available from NAPA the kits cost about $26.00 and the floats about $7.00 if memory serves me correct. The rebuild itself isnt very hard to do, one thing though if your carb has an adjustable power piston stop in it, do not alter the adjustment on it as this is a very critical part of the operation. Hope this helps some.

 

[vetteboy86]

I have been told by many people that in aspects of power the Q-jet is the way to go, however adjustments are easier on the holley. I really would like to keep the Q-jet because this car has the shaker scoop, and I want to keep it as original as possible, and still be as mean as can be under the hood. There is nothing more I would rather do then tackle this myself. Although Heidi's recommendation is hard to pass up. I will stay with the Q-jet and either build it myself, or find a good shop to do it.

Once again thanks for all the replies. I really appreciate them.
By the way are their any Trans Am owners out there?

Craig

 

[MMM]

Don't own a TA, but literally put one togther about 15 years ago. Was approached by a father (who wanted to relive his childhood) about upgrading his then 15 year old son's '77 for his sixteenth birthday. Did the following
- replaced the 400 block with a .030 over 455 HO block
- installed SD rods and balanced the rotating assembly
- installed windage tray and High Volume oil pump.
- installed a Comp Cam H 280 with Rhodes lifters
- Roller rockers and Poly-locs on Ram Air III heads
- The RA III heads were pocket ported and cleaned up
- Edelbrock Performer intake with a 780crm Holley (3310, and their choice not mine!)
- MSD 6 ignition
- Beefed Turbo 400 TX
- 12 posi rear, 3:55 ratio
- Battery relocated to the trunk
- Hurst ratchet style shifter
- Maintained the stock hood scoop but modified it with an electrically operated door with option to leave open, close or use throttle to activate.
- Kept all of original options, i.e., Air Conditioning and Cruise
- Used the RA III exhaust manifolds (headers are a PITA), with a custom 2.75 inch dual exhaust system.
- Cat converter was not re-installed, i think it must have been stolen. I looked really hard for it but could not find it. Same happened to the EGR too. But the kicker is the sucker still passed the snifter test that was in use at that time.
The car was wicked. Way too much power and torque. Would easily burn the tires off. Still needed a lot of suspension work to be able to transmit the power to the pavement. Really liked the highest octane gas that was available at the time (93 octane) but loved AvGas. Got 12 MPG. They drove it for two years and sold it to a collector. I got stiffed and was paid nothing for my work. The only kudo I got out of the whole deal was in the ad they posted when they sold the car, "All work Professionally Done!"
Mike

 

[Stallion]

I wouldn't mind just rebuilding and modifying a QJet. Something to do on the side, and the power benefits (so I hear) are amazing. Definitely something worth considering. I should start looking for one at a junk yard, but you have to be careful, because QJets were not created equal. Different years, different models.

 

[MMM]

Yes, all Q-Jets are not created equal and all were built for a specific application. Not only the primary jets, metering rods and secondary rods changed, but idle emulsifying tubes, idle air by-pass circuits, entire choke systems (pull offs, divorced or integral choke assemblies) were changed from model to model. Not only were they changed for applications within an automotive group, but changed per automotive group.
Even down to the main body casting. On most B-O-P applications the fuel line entered the carb perpendicular to the throttle shaft, where as on Chev and Cad, they entered parallel to the throttle shaft. Then in '75 came a major redesign, model designation changed from 4MV to M4MV. The actual best for performance (IMHO) are the '68/'69 models because they still had a somewhat "richer" idle circuit. The earliest models ('65 through '67) had some issues with leaking fuel wells and the original "balanced" fuel valve design. These can all be modified or fixed, just more work. The lean idle circuits of the '70 and later will bite you with an off-idle stumble if a more radical cam is used (that creates less vacuum) and the idle tubes can be difficult to modify.
Most radical factory version of the 4MV came on the '70 Olds 442, W-30 cars equipped with a manual tx. The 324 degree duration cam installed in these engines presented very low vacuum to the carb so the power piston/ primary metering rod system did not respond well. So on these carbs GM did not use primary metering rods, plugged off the vacuum inlet to the power piston well and installed a unique set of small main jets with a fixed orifice.
Mike

 

[Stallion]

The million dollar question is, how can you tell if you are getting the better designed QJet for what we want?

 

[MMM]

This is from memory recall, so forgive me if I err a little.
For the 4MV carbs a 7 digit model number was used.
I'll use carb # 7042202 as an example
First three digits - 704 - I think this is a designator for various versions of the carb. The earliest Q-Jet models used 702, this went to 703 in the late 60's and to 704 by the early 70's, 705 and on after 1975.
Fourth digit - 2 - in all cases, the 4th digit is the model year car the carb was built for. In this case, 1972.
Fifth digit - 2 - This too is a designator for the use of the carb, specifically, I believe to indcate emissions calibration. On most "49 state" carbs the 5th digit was a "2". For cars calibrated for California emission standards this digit was a "9".
Sixth digit - 0 - indicates what GM make the carb was designed for;
Chevy used 0 or 1 or 2
Cad used 3
Buick - 4
Olds -5
Pontiac -6
Seventh digit - 2 -specific application as to engine cid, performance level,
and transmission type. Digit could be anywhere from 0 to 9. In Chevrolet applications the higher digits were usually used for truck applications. In the specific example the 2 meant base 350, automatic trans. I think I remember the carbs with the last three digits being 200 and 201 being big block, automatic and manual (but I forget the order).
The M4MV's started in '75 (though the 4MV continued to be used in specific applications after '75) and I think all carbs after '75 had a 8 digit model number with the first digit being a "1" and it was the additional digit.
So for a '75 Corvette with L 48 and AT, I think the carb # would be:
17055202. The indications for the numbers just move by 1 spot, i.e, year now being the 5th digit instead of the 4th.
So for a '68 Chevy carb the first 5 digits would be; 70382** or possibly in some applications 70282**.
Hope this is not too confusing. Oh yes, the second set of numbers stamped on the carb indicate the build date (Julian dating) and I think the shift on which it was built.
Mike