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Dead Parrot · 02-09-2017, 11:23 AM

Easy test for vacuum canister. Check timing with canister disconnected(make sure this is how you are setting base timing.) Then connect the canister to a manifold vacuum source(verified with a vacuum gauge). Timing should advance 10-20 degrees(different canisters have different advance amounts). If it does, canister working, if not, time for a new one.

Check the mechanical advance for proper operation. Fairly common for them to quit working after a few decades. If you have the worse case scenario that both the vacuum advance and mechanical advance are not working, will have a big hit on mileage.

Connect a tach to the tach port on your HEI. What are your RPMs at highway speed? Compare to the RPM calculated using your tire size and gear ratio. Too large a difference might indicate a transmission slippage problem.

02-09-2017, 11:23 AM	#13
Dead Parrot Registered User Join Date: Aug 2012 Location: Oklahoma City, OK Posts: 2,484	Re: 78 K20 Gas Milage Suggestions Easy test for vacuum canister. Check timing with canister disconnected(make sure this is how you are setting base timing.) Then connect the canister to a manifold vacuum source(verified with a vacuum gauge). Timing should advance 10-20 degrees(different canisters have different advance amounts). If it does, canister working, if not, time for a new one. Check the mechanical advance for proper operation. Fairly common for them to quit working after a few decades. If you have the worse case scenario that both the vacuum advance and mechanical advance are not working, will have a big hit on mileage. Connect a tach to the tach port on your HEI. What are your RPMs at highway speed? Compare to the RPM calculated using your tire size and gear ratio. Too large a difference might indicate a transmission slippage problem.