Crab distributor vacuum advance.

[burly]

Fellas, attempting to break in and tune fresh built 59A. Noticed around a 100 rpm fluctuation at idle which is reduced to a 25 rpm fluctuation when I unhook and plug the vacuum advance. Wondering if doing this would effect drivability? 3 & 5/16 x 4, potvin cam, running single 97 until I get a few miles on it.

[ford38v8]

The vacuum brake is best suited to modern fuel by a slight adjustment of screwing in the adjuster, to prevent the wild fluctuation of timing caused by the plate bouncing without any braking action. I’d start by removing the brake piston to check the condition of the leather, which may well be dried out or missing altogether.

[Lawrie]

Also, I wonder if the fit of the piston now leaks vacuum after all the years.
Lawrie

[burly]

Crab distributor was completely rebuilt by Bubba shortly before going out of business.

[aussie merc]

if bubba did it it should be good so i would give it a further 1/3 turn out and recheck .Also modern fuels are nothing like the old ones even from the 2000s let alone the 40s so a vacuum gauge is paramount when tuning

[flatjack9]

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What speed are you idling at and what is your initial advance? The vacuum plunger should not be touching the plate at idle (high vacuum). When you plug the vacuum, the plunger is fully engaged. Seems like the bushings might be worn.

[35fordtn]

Sounds like a worn breaker plate bushing or vacuum leaking past the plunger. Try removing the plunger covering it in a thin layer of wheel bearing grease and see if that helps.

[burly]

Thanks guys!