SPARK KNOCK QUESTION - Off-Road Forums & Discussion Groups
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post #1 of (permalink) Old 10-17-2003, 12:44 PM
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Simple uestion here. Do I need to asvance or retard my timing to take care of this. It does it when I go up hills or try and goose it while at low rpms.
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post #2 of (permalink) Old 10-17-2003, 12:55 PM
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Spark Nock is usually premature detonation, so I'd imagine retarding your timing would be the way to go.
Dan 84 CJ-7 is offline  
post #3 of (permalink) Old 10-17-2003, 01:09 PM
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yup retard is the way to stop knock. however retarding timing will also seriously reduce your power. may be better off simply running 93 octane fuel or octane booster. maybe both
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post #4 of (permalink) Old 10-18-2003, 10:52 AM
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Over advancing creates too much heat, then the excessive heat starts the detonation. The "ping" itself isn't caused directly by the ignition system, but indirectly.

I see you have a GM HEI. Many carb's ported spart vacuum are too strong for the HEI.
Use a timing light, set initial to about 8 degrees - no vacuum line.
Measure timing at 1500 - should not exceed about 5 more degrees than what you had at idle.
Measure timing at 2500 - should not exceed about 18.

Reconnect vacuum line - should not change timing at idle.
Measure again at 1500 - should only be 2 or 3 degrees more than it was without the vacuum.
Measure again at 2500 - should not exceed 35.

What that'll do is make sure the timing isn't advancing too fast. Too much too soon will cause ping.

The HEI vacuum advance can is designed for a weak/slow vacuum signal, so it's very sensitive - often comes in too fast with a different carb than it was designed to work with.
The cure: Install an adjustable vacuum advance cannister - about $20 for a Crane.
Set the new one on the least sensitive position - I think that's with the screw all the way out counterclockwise.
And, very important, set the tiny limiter cam that comes with it so it limits the total travel of the advance to about 1/3 of the travel - else it'll advance too much and be worse!

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