1968 L79 Carb
Thread Starter
Instructor
Joined: Jun 2011
Posts: 102
Likes: 0
From: Phoenix Arizona
1968 L79 Carb
Just bought a 68 L79 car, it has a Rochester Q jet carb, totally not familiar with these. Any opinions on going to a Holley, car does not seem to pull well when you stand on it.
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
Probably nothing to do with the carb. The Q-Jet is a great match for that engine....and if it is running it is fine. First thing to do is check your total timing. You should be at 36 degrees. Disregard the factory spec on the emissions sticker under the hood. Find out where the timing is.....hook your vacuum advance to a port that gives full manifold vacuum at idle.
Start from there. A properly tuned L-79 with a Q-jet is an easy 14 second car.....
Jebby
Start from there. A properly tuned L-79 with a Q-jet is an easy 14 second car.....
Jebby
Last edited by Jebbysan; Sep 17, 2021 at 11:16 AM.
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
If you do find it is severely retarded.....you may wanted to get an octane booster before doing the 36 degree setting. The 36 degree setting is at or around 3000 rpm.
E-Mail our member Lars for his excellent tech paper on timing at V8fastcars@msn.com
Jebby
Tech Contributor
Joined: Aug 1999
Posts: 14,384
Likes: 6,411
From: At my Bar drinking and wrenching in Lafayette Colorado
In addition to the correct and accurate info provided by Jebby, you might want to send me, or post, a couple of photos of the carb that is on your engine. The Q-Jet for the '68 L79 is a unique carb, and very rare - real ones are selling for over $1200, or about the same as a 427 big block carb. Very few of these are still intact and unmolested on these cars. In the majority of cases, someone has removed the correct L79 Q-Jet and installed some POS commercially "remanufactured" carb that is a complete mismatch for the engine. If this is the case, you may have a combination of problems between the incorrect timing curve setup and a badly hacked carb.
...But start with the timing setup. E-mail me if you need my papers on how to set it up.
Lars
V8FastCars@msn.com
...But start with the timing setup. E-mail me if you need my papers on how to set it up.
Lars
V8FastCars@msn.com
Safety Car
Joined: May 2008
Posts: 3,756
Likes: 725
From: Lafayette Louisiana
An original 68 L79 Q jet would have 7028219 stamped on the rear LH vertical face if a 4sp and a "HT" suffix on the engine stamp pad..
Sounds like a great find! please post photos of your new 68 Corvette!
One other thought for you that I learned the hard way, if you have the original intake manifold, there is a cross over passage that exhaust flows through in the front and right under the carb that will heat up the carb for I think cold weather starts. I believe this caused warranty problems for GM and will eat out the base of the carb. So, I plugged these holes and installed a SS baffle to block off. In my case, the exhaust heating also caused rough running once warmed up.
Enjoy!
Sounds like a great find! please post photos of your new 68 Corvette!
One other thought for you that I learned the hard way, if you have the original intake manifold, there is a cross over passage that exhaust flows through in the front and right under the carb that will heat up the carb for I think cold weather starts. I believe this caused warranty problems for GM and will eat out the base of the carb. So, I plugged these holes and installed a SS baffle to block off. In my case, the exhaust heating also caused rough running once warmed up.
Enjoy!
Last edited by 20mercury; Sep 17, 2021 at 03:02 PM.
Tech Contributor
Joined: Aug 1999
Posts: 14,384
Likes: 6,411
From: At my Bar drinking and wrenching in Lafayette Colorado
From the photos received via e-mail, the carb is a nice 1982 factory Service Replacement carb for a 1969 L79. It should run very well if correctly set up and properly rebuilt. Start with a correct timing setup and then move on to the carb if the timing does not cure the issues. Make sure you are using the correct 3-piece carb base gasket with the steel plate if you have the stock '68 or '69 intake manifold.
Lars
Lars
Last edited by lars; Sep 17, 2021 at 07:14 PM.
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