Well I finally think I've done the compression testing on my 77 Suburban's engine properly. Earlier testing was done improperly (was not even opening the secondary throttle plate). This time I blocked open both plates in the secondary, pulled out all the plugs, and started pulling the plugs etc with the engine as hot as I could. Wet readings done with two teaspoons of 20 weight oil and five seconds cranking to distribute. Cranked four compression strokes for each reading. Each average below is the result of three readings. So here are the results:
Driver side bank (starting with cylinder closest to radiator)
Dry Wet Difference
1 170.3 178.3 (8.0) 2 173.3 183.0 (9.7) 3 174.3 186.3 (12.0) 4 171.3 179.3 (8.0)Passenger side bank (starting with cylinder closest to radiator)
Dry Wet Difference
1 168.0 180.7 (12.7) 2 169.7 185.3 (15.7) 3 170.7 181.3 (11.3) 4 171.3 179.7 (8.3)My 1977 (150k miles) Suburban's power has been steadily declining for the past 8 months. It feels like it's going through 6 inches of mud and it gets about 5 to 6 miles a gallon(used to be 10 to 12). It has a distinct chugging at lower to mid rpms. It has new AC Delco Rapidfire #2 plugs, new dist cap & rotor. Plugs seem to be pretty clean with a nice medium tan but have oil on threads. Found valve cover bolts 1/4 to 1/2 turn loose and tightened them but I don't see how this could get oil on the plug threads unless it drips on during plug removal. I guess I will know next time I pull the plugs.
Today it will get new premium spark plug wires. Defective vacuum advance and transmission vacuum modulator were replaced and helped but did not stop the decline of power. Both vacuum and mechanical advance are correct acording to factory specs although I have not checked the curves. Carb was rebuilt about 50,000 miles ago so that probably needs attention. It has new dual Flowmaster 40 Delta Flows with 2.5 inch all the way. I installed a vacuum guage and these are the readings I get when it is at normal operating temperature:
idling - 16 - 17 inches light acceleration - 10 to 15 inches moderate acceleration - 5 to 10 inches hard acceleration - 0 to 5 inches cruising steady 45 mph - 20 inches decceleration - 20 to 22 inches
A number of people have recommended replacing the carburetor with a Edelbrock Q-Jet (my model would be the 1902, about $460) or rebuilding the Rochester 4MV. Whereas I'd love to get a new Edelbrock and be done with it I have to have a good level of confidence that it is at least the main contributor to the problem and the rest of my engine is in decent shape.
Is there anything else I should check/eliminate before I take the plunge for the new Edelbrock or rebuild the Rochester 4MV. Are my compression and vac readings decent. Thanks to all the people who have given their advice. Any help would be most appreciated as this beast is sucking me dry at the gas pump (Phoenix 195/gallon regular).
Thanks, Bob