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Old 11-10-2019, 01:33 AM   #1
fftoddster
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Default spark knock under load

Crab distributor with pertronix and correct coil. Vacuum brake hooked up to manifold vacuum. Edelbrock slingshot intake with dual 97's. Edelbrock heads... 6 volt pos ground/

Runs good, maybe a bit rich but knocks under load/hard acceleration.

New to all this. What should I be looking for or try?
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Old 11-10-2019, 04:42 AM   #2
Mart
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Default Re: spark knock under load

Knock the timing back a little. Was it ever timed with a light? Do you have a timing mark?
The timing adjuster is on the usa drivers side of the distributor and would need to be loosened and moved upwards to back the timing off a little. (Known as retarding the timing)
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Old 11-10-2019, 10:30 AM   #3
fftoddster
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Default Re: spark knock under load

No. Never timed with a light. No mark for TDC. Thought I had read somewhere to not hook up the vacuum brake but I cannot find that again, so wondering if those with the pertronix hook up vacuum to it or not?

I will try to rotate distributor up a mark or two and see what happens.
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Old 11-10-2019, 06:31 PM   #4
JSeery
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Default Re: spark knock under load

I think you want the vacuum brake hooked up. Bubba claims to see a difference in the stability on a scope. The vacuum brake was originally for help with ignition ping, it slows the rate of advance I believe.
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Old 11-10-2019, 11:39 PM   #5
fftoddster
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Default Re: spark knock under load

On thehamb someone suggested to try adjusting the vacuum brake by turning in ash bolt 1/2 turn at a time.

Thinking of trying that first as idles good. Can’t test drive till ties though.
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Old 11-11-2019, 09:09 AM   #6
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Default Re: spark knock under load

Yes, you do want the vacuum connected to the brake.
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