CFM for 76 Q-Jet

Pastmaster · June 5, 2001

Since you have a 76 truck, can you tell me what the factory CFM rating was for a 350 in 1976? My buddy has one on his truck, that is in need for a rebuild, and I'm helping him find some parts for it, aswell as a rebuild kit. Can you recommend one? Also, have you ever came accross a Tach instrument panel from the 1973-1987 years? I know that earlier trucks had it as ? option as well as the earlier 1980 models.

Thanks

76BBSub · June 7, 2001

Whew...just got in yesterday from being in Europe for two weeks, nice to see that the Forum is still busy!!

As for the carb, on a '76 350 it's 750 CFM. But, there are ALOT of variations, as '76 was somewhat of a transition year. My suburban is heavy enough to be exempt from all the light-duty truck/car stuff. If his truck is also exempt (and it should be) it will have a divorced choke. There should be a small metal box mounted to the intake manifold on the passenger side with a little rod attaching to the choke linkage. If this is what he has it's the non-smog. DO NOT buy a parts carburetor with the integrated choke, which has a round plastic covered housing on the carburetor for choke control, very few parts interchange.

Do you have the numbers, they should be stamped on a pad, vertically, on the driver's side about 2/3 the way towards the back of the carb. Should start either with a "7" or a "17".

Post what you can, pictures would help, and we can figure out a little more about it.

Jeff

June 7, 2001

I thought all Q-jets were 640cfm, with different metering rods and jets for different motors.

76BBSub · June 8, 2001

The vast majority are 750 CFM, with the larger ones, such as my '76 454, being 800 CFM. This is from Doug Roe's quadrajet book, which is pretty much the authority on QJets. I've never flowed one on a bench, though. That flowrate may sound kind of large for some of the engines on which they were used, but the primaries are so small, but the secondaries are quite large, when compared to a square bore carb, such as a typical Holley or Carter.

Jeff

76BBSub · June 8, 2001

Forgot to mention, Friz you are totally right about the different metering rods and jets between the different engines and engine configurations. The secondary jets are actually passages in the body of the carb and are not removable or changable, unless enlarged with a very small drill, about a 1/32" or less.

In addition, the accelerator pump arm, power piston spring, needle, seat, and secondary linkage vary from engine to engine. One big item that some people overlook is the secondary rod hanger. This controls how quickly the secondary rods richen the mixture. There were some 20 or more hangers originally used, supposedly GM only offers one now, and most rebuilders notch and bend the hangers to adjust them. I have been collecting as many of these as I can from junk carbs.

Incidentally, passenger car carbs made from '75 and up use metering rods that are NOT interchangeable with earlier models, although they appear to be identical when sitting side by side.

Jeff

June 8, 2001

Thanks, the Q-jet is something that I never messed with because the Holley was always so much easier to work on.

76BBSub · June 9, 2001

I haven't messed with Holleys, but I have always heard they are easier to tune and work on....I guess I'm a glutton for punishment!

Seriously, though, most of my "toys" have been '70s GM vehicles with QJets, so I have learned the long and hard way...

Jeff

Pastmaster · June 10, 2001

will do 76!

Pastmaster · June 26, 2001

76: I did some checking and it is Non-smog. Has the divorced Choke like you mentioned. He had the carb rebuilt by a specialist. But he can't get the rich/lean perfect. It continously runs rich. What can he do?

76BBSub · June 26, 2001

Depending on the camshaft and some other factors, he may need to check the ignition timing. If it is too far retarded (not enough advance) then the throttle plates will have to be opened excessively to maintain idle speed. If the throttle plates are too far open the primary fuel circuit will bleed into the venturis causing an excessively rich condition. To check for this, start the engine, let it warm up, remove the air cleaner, look into the primary venturis. If you see fuel coming out of the primary fuel feeds, or fuel pooling on the throttle plates inside the bottoms of the venturies this is the problem. Advance the timing, the idle speed will come up, turn out idle speed screw. Check again for fuel bleed.

Do you know any details on the engine as far as mods, condition, etc? That would help too.

Pastmaster · June 27, 2001

no mods, uses about a quart every 100 miles. He uses 20W-50? oil. Runs good still with almost 200k, but I told him to rebuild it. He still claims the carbs screwed up. They tuned it up and timed it.

76BBSub · June 28, 2001

If he's burning a quart every 100 miles, he's not getting jack for compression, and has tons of blow-by. That would have a similar effect to having a cam with a lot of overlap, meaning that the vacuum signal is probably diminished. I suggest that he look into the primaries as I mentioned above and see if they are bleeding into the primary venturis.

Of course, at 200K miles and burning oil at a quart/100miles it could be just about anything....

Jeff

Pastmaster · June 29, 2001

It is severe blow-by. It acts very similar to what early diesels did. Going up hills it blows oil badly. A couple of months ago he had a stuck valve on cylinder number 6. He had that fixed and it used more oil. I think he'll rebuilt it soon. I'm just so suprised how well it runs despite it's ailment. I've driven it a few times, and it still has good power. Thanks