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From the members of the GMC Sunshine Statesmen           
           

 

 

 

Here we will list issues and possible solutions related to mechanical aspects of the GMC.  

Vacuum Gauge -  Measures engine intake manifold vacuum in inches of mercury.

While driving: Tells you roughly how much of a load you are putting on the engine. Lower vacuum = more load = more fuel.  Try to keep the gauge above about 11 for best economy. At about 10 and below the vacuum advance on the distributor retards the ignition timing so the engine doesn't knock and ping, however this also increases fuel consumption. At about 6 and below the carb power valve is open to provide a richer fuel mixture and even more fuel is consumed.

Diagnose Engine Problems: A properly running warmed-up engine should have around 18 inches or more of vacuum at idle in park, a bit lower in gear, and a bit lower yet with the A/C on and in gear. The reading should be steady. A constant jumping at idle would usually indicate one or more cylinders not 'doing their share' due to bad ignition or leaking valves. Constant low readings are usually due to improper ignition timing or a restricted exhaust.

                                                                                         

Carburetor Float -  Controls the fuel level in the carburetor bowl by opening and closing the inlet needle. Works much the same as the float in your toilet tank.

The float in the Quadrajet carburetor is made from Nitrophyl, commonly known as a 'foam float' (very common), or brass (not so common on automotive applications). Nitrophyl (foam) is very buoyant and works fine most of the time. One disadvantage is that the 'foam' slowly absorbs fuel and gets heavier. This lets the float sink lower and makes the fuel level in the carb bowl rise above where it should be which leads to the engine running rich, slow hot starts, stalling when braking abruptly, poor extended hot idle, excessive fuel consumption, etc. It is my perception that modern fuels are harsher on the 'foam' float due to the ethanol and other additives in modern gasoline.  Since Mercury Marine engines use brass floats in their QJets, presumably due to the higher alcohol levels in marine gas,  I decided to give a brass float a try in my GMC, especially since all of the gas I buy has 10% ethanol in it. I used a GLM 77220 which fits many Mercury Marine engines with QJet carb 1969-1993. This is the "old style" QJet float and will fit the 1973 and 1974 GMC's with stock carb. So far everything works fine and it has no problem handling the 6 psi from the Carter P4070 electric fuel pump. The brass float should last a lot longer than the old 'foam' float and be more consistent -- it won't slowly get heavier. As always, JWID (just what I did) and YMMV (your mileage may vary).