Questions and Answers

GMC Sierra ¾ ton Knocking & Backfires Q. I have a 1977 GMC Sierra ¾ ton 4x4 with a 350 and Rochester carburetor. I am 18 and just learning about mechanics. My problem is that the engine idle is rough, there is a knocking noise and it backfires through the carburetor. The timing is at about 6° BTDC. I checked the compression, all were around 150 psi. There is still good spark in all cylinders.

Spark plugs and wires have been replaced. The HEI module was tested and is still good. The mechanical advance still moves. I not sure what else to check. The timing is off about two degrees but I wouldn't think that would cause it to stall out and backfire that severely only at 1,000 rpm. What's wrong? If it is the vacuum advance how can I tell? Or possibly the EGR system? Please help

Keegan

A. Anything that is in doubt, disconnect it! Pull the hose off the EGR and plug it. Unplug the vacuum advance and loosen the distributor. Drive it and gradually move the distributor both ways to see if you can find a sweet spot where it runs good. Coil could be weak also.