WebMonkey
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a.i.r. tubes.
New engine quirks
- Thread starter JohnV
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MikeB
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I can tell you that zero degrees advance was a band-aid to reduce emissions, just like using "ported" vacuum for spark advance. Coupling those band-aids with a super lean fuel mixture typically meant stumbling on acceleration, detonation (from the lean mixture), and poor fuel economy.
As someone suggested above, do some research on spark timing, and learn the meaning and significance of these terms:
Initial advance
Total advance
Vacuum advance, along with manifold vacuum vs. ported vacuum
As someone suggested above, do some research on spark timing, and learn the meaning and significance of these terms:
Initial advance
Total advance
Vacuum advance, along with manifold vacuum vs. ported vacuum
75gmck25
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Set the idle down to about 600 rpm and disconnect and plug the vacuum line to distributor. This ensures you have no mechanical or vacuum advance, only the base timing.
- Set base timing to 8-12 degrees BTDC. For a stock engine it should easily tolerate 12 degrees and may be fine with up to 16.
- Use a setback timing light to verify that as you raise the rpm the timing should advance an additional 18-20 degrees by about 3000 rpm (a total advance of up to 32-34 degrees). Even better if it maxes by 2000-2500 rpm.
- Let it go back to idle (mechanical will still be at 12) and then pull vacuum on the vacuum advance can. This should also advance the timing about 18-20 degrees if its a stock vacuum advance. This timing advance is what gives you better gas mileage at cruise. It does not make the truck accelerate any faster. Your total advance is now about 52 degrees BTDC (12 base + 20 mechanical + 20 vacuum = 52)
Once you get the timing set, follow the instructions for your carburetor to set the idle mixture. Then set the curb idle at about 800-850 RPM in Park, dropping to about 600 RPM in Drive. I recommend using manifold vacuum to the distributor advance to get a smoother idle, but ported vacuum will also work.
Bruce
- Set base timing to 8-12 degrees BTDC. For a stock engine it should easily tolerate 12 degrees and may be fine with up to 16.
- Use a setback timing light to verify that as you raise the rpm the timing should advance an additional 18-20 degrees by about 3000 rpm (a total advance of up to 32-34 degrees). Even better if it maxes by 2000-2500 rpm.
- Let it go back to idle (mechanical will still be at 12) and then pull vacuum on the vacuum advance can. This should also advance the timing about 18-20 degrees if its a stock vacuum advance. This timing advance is what gives you better gas mileage at cruise. It does not make the truck accelerate any faster. Your total advance is now about 52 degrees BTDC (12 base + 20 mechanical + 20 vacuum = 52)
Once you get the timing set, follow the instructions for your carburetor to set the idle mixture. Then set the curb idle at about 800-850 RPM in Park, dropping to about 600 RPM in Drive. I recommend using manifold vacuum to the distributor advance to get a smoother idle, but ported vacuum will also work.
Bruce
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The sticker under hood specifies zero degrees because the primitive computer that came on your truck when it was new would advance the timing. There was a terminal you grounded or unplugged (can’t remember what the process was) then set the timing to zero. Unground or plug in the connector, restart the truck, all was good.
Since the original engine is gone ( I assume you installed a conventional distributor and carb?) you need to set timing the old school way. Follow the instructions that were mentioned earlier. Don’t be surprised if you have to tweak things a little to make the engine happy.
Since the original engine is gone ( I assume you installed a conventional distributor and carb?) you need to set timing the old school way. Follow the instructions that were mentioned earlier. Don’t be surprised if you have to tweak things a little to make the engine happy.
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