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Setting base timing is the same on the '87-97 EFI trucks as it is on the earlier carb'd motors, turning the distributor clockwise advances the base timing, and this must be done with the spout removed on the EFI motors. Advancing the timing from the factory setting of 10deg BTDC will produce more TQ and HP, but it will also make a motor more detonation prone. You should not just set it where somebody else has, every motor is different and it's sensitivity to detonation will be different, so adjust it to whatever the motor will tolerate. How do you know how much? Start with 2 degrees additional advance, and then take it for a drive and try to load the motor and listen for pinging or hesitation. At hwy speeds in OD under part throttle(just before the tranny downshifts for an auto) in hot weather will be highest load. If it's OK with 12degrees, bump it couple more and test again. Once you make it ping you want to back it off to the previous setting.
Ping is what we used to call "Valve clatter". To gain more advantage from advancing the timing, you can upgrade to a higher octane fuel. Bump the timing till it pings, then try more octane and see what the motor likes. Your mileage will likely improve too, offsetting the cost of the higher octane fuel.
Yeah.. pinging sounds like a rattle or clatter, some people say it sounds like shaking marbles in a can. It won't be really loud so you'll have to shut off all noise generating devices and listen carefully.
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