Quadrajet-Idles good, runs poorly at low speed.

[FlatheadSmith]

I have a Quadrajet with part number 7041202. It is on a stock 350 (as far as I know it could possibly have an aftermarket cam, but I am not sure). The engine starts right up and idles good. The idle mixture screws seem to have very little effect on the operation of the engine at idle and the engine surges (or bucks) under low or moderate speed. If the truck is under acceleration everything is fine. The carburetor is very clean with all new gaskets and the float is adjusted to the correct setting.

I had this carburetor rebuilt locally several months ago by someone who seemed to know what he was doing (an older GM trained mechanic). Normally I would have done it myself, but since it had some parts missing I decided to let someone else do it. Before I had it rebuilt the truck ran okay once it was warmed up (the parts that were missing were related to the choke and air valve), but did have a little bit of very light surging at low speed, but now it is not tolerable. The guy who rebuilt the carb basically put it back to stock specs, but does not have an answer to my problem and said it might be related to the distributor. I know that he also removed the spring under the power piston and replaced it with one that he said was supposed to be there. I thought that maybe the one he replaced it with was two strong, but I really don’t know what to do.

[Shark Racer]

A spring that is too strong will have the carb go to the high-load (WOT, hill-pulling) sooner than expected. This would cause a rich condition, assuming that rods/jets/bleeds are all relatively close to stock and the engine is stock.

Surging/bucking implies leanness, so are you sure the correct rods/jets are there? and if they are, are you sure about the air bleed sizes? If it was commercially rebuilt, all bets are off, they very well could have drilled out the main air bleeds and then stock rods/jets will be way too small.

[Cliff Ruggles]

Those carbs are pretty lean in stock form, and rebuilding it may have lowered the fuel level and made it even worse.  I can supply the correct jets and metering rods for what you are doing, and power piston spring. 
Take a vacuum reading at idle speed, which will help tell us how big the cam is.  Also nice to know what heads are on the engine (compression ratio).....Cliff

[FlatheadSmith]

I replaced the #67 jets with #76 as recommended by Mr. Ruggles. I also pulled the idle tubes and they measured .042”, which seemed large when compared to what is in his book. Since the tubes were damaged when I pulled them out, I went ahead put in new tubes that I drilled out to .035”, I figured I could drill them out to original measurement, if there was a problem with the smaller measurement.

The truck runs much better, but there is still a very slight stumble under very light acceleration from about 30 or 40 mph.  I’m thinking that the distributor might need to have lighter springs on the advance as the problem seems to go mostly away when I advance the distributor several degrees (I replaced the weights and springs several months ago as the original ones were in poor condition). I can’t leave the distributor in its advanced state, because the engine is hard to start when it is warm.  I'm also wondering if a slightly heavier spring under the power piston might help things along.

[Cliff Ruggles]

What metering rods are being used with the 76 main jets?......Cliff

[FlatheadSmith]

#44...I think

[Cliff Ruggles]

We would have supplies smaller metering rods.  If you can, free up the APT screw in the baseplate, and replace it with an external screw.  This allows for tuning the part throttle A/F without taking the carb apart.  Those APT screws can be difficult to remove, as the factory loctited them in.  I always heat up the castings before trying to turn them, as they break easily.  No big deal if you can't get it out, just might take a bit more effort to tune the part throttle A/F by changing parts......Cliff