Forum member Tom (aka, "Runningwild") was having some problems with his Vette, and he was getting some bad advice about the Q-Jet carb on his '74 L82:
https://www.corvetteforum.com/forums...placement.html

The "mechanic" was telling him that the base of the carb was rusted out, and that "rust had built up in the base of carb bowl." The carb, obviously, needed to be "replaced."

So Tom sent the carb out to me for a checkout. From visual appearance, the carb appeared to be a '74 carb, correct for his car. I think the appearance frightened the "mechanic":









Upon closer inspection, the carb is actually a 1978 454 HD Truck carb, but it can be set up to run on his Vette since he has no other carb available at this time. These HD truck carbs are based off the 1974 passenger car carbs, so it visually looks like a "correct" Vette carb.

But, obviously, the "base of the carb" is not rusted out, and the airhorn has never been removed off the carb, so the mechanic had no way of knowing if "rust had built up in the base of carb bowl." So we raised the BS flag on the "mechanic's" diagnosis and decided to take a look at saving this carb to get Tom's Vette running nicely.

A quick teardown revealed some issues. Notice the blackening in the throttle bores from exhaust gas reversion through the intake system. This is a sign of badly set timing (retarded), which is contributing significantly to the car's poor performance. A well-built carb can never compensate for badly-set timing, so Tom is going to do some tuning once he gets the carb back:






Next little item was the accelerator pump. This is a very common problem on older carbs exposed to modern pump gas: The pump diaphragm swells up, seizes in the pump bore, and gets shear off the pump shaft. Obviously, this accel pump is completely inoperable:



Once the carb was disassembled, run through the hot tank, de-rusted, and assembled, it came out like this:





I never did find any rust in the bowl in the carb - the carb was remarkably clean on the inside and free of any corrosion or rust. This is certainly not a carb that should have been discarded and replaced. It just needed a little attention.

The carb then went on the test engine for setup and verification. It fired up instantly, and ran well. Here is the carb running on the engine:


Running cold with the choke closed and cracked open by the pulloff. The cold fast idle speed had been set really high, which was probably done to "force" the engine to idle with the retarded timing spec. I lowered the fast idle from 2000 rpm down to about 1350:


Once warmed up, I ran it with the choke wide open, and set up the hot idle speed to about 850:


This produced a perfect hot idle mixture right on the numbers at 14.7:1


So don't let a dirty carb discourage your tuning efforts, and don't listen to incompetent "mechanics." Always set up timing correctly before tackling the carb, and then give the carb a little tuning and attention to make the car run as it should.

Lars

 

Saved another one, guy probably had no clue about the Qjet & didnt wanna say so.

 

 

Thanks Lars. My first vette and it will run great with your help and this forum

 

I like reading these posts you put out, they are very informative to those of us that still use Q-jets. What do you use to remove the rust from the linkages...they look very nice after your treatment.

 

Great work yet again......

Jebby

 

Yep great write up great job.

 

Really great work. I am also interested in the method you used to remove the rust.

 

It wasn't 'rusty', if you read Lars' post. The only rust on the carb was on the steel parts: some choke system parts and some linkage rods. The choke pulloff got replaced and the linkage rods were cleaned with rust remover or wire-brushed. Same with the steel screws, if you want them cleaned, also.

 

The choke linkage is what he maybe referring to. It looks pretty rusty

 

Yes, that's what I was referring to, the choke linkage parts and throttle linkage.....it just looks different to me than a buff job on a wire wheel would look.

 

One quick question Lars since this carb was from a big block truck was there a need to change any jets or metering rods or are the same jets and rods used in small blocks also. It looks like this carb has never been opened since the retaining spring is still on the metering rods. It seams like the first thing Bubba throws away as an extra part on every Q Jet I have ever opened up.

Phil