78 c20 quadrajet problems

[AlexC20]

Hey everyone, new to the forum. Ive had my 78 c20 for about a month now. It has a year correct chevy 350, 346250 Intake manifold, and 17058213 Q-jet. Long story short- Truck wasnt running well after doing some work to it. Rebuilt the carb, didn't work. Put another carb that I rebuilt on and it still wont work. The vacuum advance port has constant manifold vacuum, with a vacuum gauge hooked up the needle bounces back and forth between 13"-15" of vacuum. The only was to get a stabilized reading at 20" is if I just barely cup my hands around the primary choke butterfly. The mixture screws have no effect what so ever and the idle wont go any lower then 1200-1000 rpms. This sounds like a obvious vacuum leak but I'm not figuring out where. We eliminated everything on the outside of the carb :brake booster and check valve, all vacuum lines and they're respective spots to be hooked up to, and intake manifold. I've tried different insulator gaskets hoping it would work but it didn't. Why would the carb only pull the correct vacuum with my hand over the choke blade and why do the mixture screws have no effect? I've tried looking for leaks on the outside of carb as well and cannot find any. Also one last detail. When putting your finger over the vent tube I can feel just a small amount of vacuum, this was also confirmed by my father and uncle.

 

[Charlie]

 

[dvdswan]

Welcome to the site. Vacuum on the vent tube? Is that the breather hose from the valve cover? If so, you may have an internal intake manifold leak would be my guess. (underside of the intake manifold).

 

[1987 GMC Jimmy]

Vacuum leaks can be tricky sometimes. I’d plug off every single port on the carb and manifold and see if I get the same vacuum reading. If you’ve already done that, that’s a good first step. However, visual inspections only yield results in the more overt cases. Another vacuum leak that’s a possibility is the throttle shaft area. On high mileage units, the shaft will bore into the soft metal and cause a vacuum leak. The only way to fix it is with a bushing. Also, what’s your initial timing set to? If two carbs aren’t working, then I would throw a warped carb diagnosis out the window. It could be a more serious intake manifold issue as suggested above. Not a big deal though because aluminum manifolds are relatively cheap, and some people can’t give the cast iron ones away.

 

[AlexC20]

It's not the breather tube, it's the vent tube off of the carb

 

[AlexC20]

My initial timing is set to 8 degrees BTDC. My dad has worked for chevy for 35 years and is still stumped. I'm not looking to put any performance into it I'm just trying to enjoy the truck, mines got a cast iron manifold on it. Even though theres no leaks on the outside of the manifold, it could be leaking in the oil galley?

 

[PrairieDrifter]

Yep it’s very possible your intake is leaking somwhere you can’t see. It’s sucking unmeasured air somwhere before the intake ports on the heads. So there’s only so many things you can check and replace

 

[AlexC20]

Alright thank you everyone! I will go ahead and see about the manifold again and probably just replace the gaskets, if that doesn't work I'll go from there

 

[Quadrajet Power]

Did you spray carb cleaner around base to search for vacuum leaks?
Primary shaft may need bushings to prevent leaks.
Did you keep correct original jets and primary rods in that carb? These heavy truck carbs were calibrated much differently than most other Quadrajets.

 

[AlexC20]

Yes everything matched all the way down to the metering rods and jets. I was given another 17058213 from my grandfather who had the exact same truck that I do and when I tore it apart to rebuild it, all the internal parts matched the one that originally came on the truck. I have sprayed the carb around the baseplate to no avail, we haven't been able to find any leaks externally

 

[AlexC20]

I just got done not but 2 weeks ago replacing the known vacuum leak from the manifold gaskets. I did plugs,wires,cap,rotor and coil the day I bought it. After looking at it again yesterday we did a compression test and all cylinders have good compression. I've plugged every port and eliminated every other vacuum source to see if anything would change and nothing did. It's been reduced to the carb and manifold.

 

[Submariner592]

I also have a 1978 C20 350 eng. with 400TH. Had the same problems with my Q-Jet and what I thought were manifold leaks internal. I finally just got a spreadbore Holley new and installed it. It corrected all the problems. Many leaks from the Q-Jet were the problem. Plus you can get away from the divorced choke and go electric. Good luck.

 

[Snoots]

I'll put my money on badly worn throttle body bushings.

 

[ali_c20]

Borrow an edelbrock carb and put it on to eliminate the manifold as source of error. If it's the q-jet rebuild it with bushings, epoxy to prevent leaks, jets etc.
I bought this book, it's really helpfull when you rebuild your carb.