How can I increase the fuel mileage of my GMC?

Don't replace anything until you know the condition of your engine. Randomly changing engine parts is economically foolish unless you know exactly what you are changing them for. The two most critical issues are timing and compression of the engine. That is assuming that some PO has not changed the cam and other parts out and screwed the engine up. So I would:

1. Check the compression of each cylinder. Your manual will tell you what that should be.

2. Check the timing.

3. When you are checking the timing also check the vacuum advance of the distributor as well as the mechanical advance and make sure everything is working to specs.

4. Check the vacuum of the intake manifold. This can tell you a lot. For instance if you have vacuum leaks.

5. Check the fuel pump pressure to the carburetor. If it is to high you will overload the needle valve in the carburetor and it will be consistently running rich.

6. Reset the mixtures on the carburetor using your vacuum gauge.

7. Make sure you have an unrestricted exhaust. Sometimes you can have a defective muffler (the baffles collapse) and it caused to much back pressure resulting in the terrible mileage.

After you do these things drive it again and see if there is any improvement. If not I would:

1. Change the cam to one of Mondellos RV grinds

2. Change to direct fuel injection NOT a throttle body unit. The throttle body injection units will give you better starting and driveability but NOT better mileage. Save your money.

3. Install a 3 inch unrestricted exhaust system

As for the Jacobs system, it is a waste of money in my opinion. The 403/455 engine develops its power in the GMC at a very low RPM, to low in fact to get any benefit from a add on system. That's assuming of course that your HEI system is OK.
(Tom Warner 5-29-00)
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