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single vs dual vacume distributors

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I have a '68 J code 302. Rather healthy (350+ flywheel HP). It currently has the original dual vacume dist. with a Petronics conversion. I run 34 degrees total advance. The dist. hoses are plumbed properly. I have to use 24 degrees initial timing because the dist. only advances 10 degrees. What is wrong with my distributor? I was looking thru a Mustangs Unlimited catalog and it lists a single adnvance dist. for '65-'73 302. I show the car, and originallity (or the possibility) is important to me. Did any '68 and later 302 actually come from the factory with a single advance dist.?

Summit lists a high performance, blueprinted, adjustable single vacume advance dist.made by Accel for $99.
Has anybody had success with this? Can it pass for stock?
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Discussion Starter · #2 ·
The new part catalogs are very confusing with respect to the dual vac advance. I have a 68 with a 289 which is supposed to have a dual advance set-up. I tried 3 different suppliers and, each time, received a single advance unit - even one vendor's Ford part cross-reference was wrong and I got a single unit again. I finally gave up and ordered a used dual set-up off e-bay.

That being said, my recollection from this was that 302's used the single and 289's used the dual. It is spelled out in the distributor vacuum hose connection diagram in the Yellow/green Ford repair manual (I can't remember the vendor but can check tonight if you still need it). There is also some trouble shooting info in there that tells how to diagnose the dual diaphram units.

I used an Accel distributor on another car (non-ford, <gasp>) and had such terrible luck I swore I'd never use an Accel part again. Their tech line was even worse. My problems went away when I pulled the Accel distributor out. Can't speak for the Ford application, though.


· Registered
1,600 Posts
Are you talking about the factory Autolite dual vacuum port distributors? If so, I believe (and I'll check) that one port is for advance and the other is used for retard in conjunction with the thermstatic switch atop the thermostat housing. This setup was designed with the ever increasing emission requirements imposed by the Feds at the time. I know my boss is setup with a stock appearing vacuum housing, however, I use NO vaccum advance and have 36 total dialed in. Did you have your distributor rebuilt and recurved for performance application??


Discussion Starter · #4 ·
I had a 68 J code with a single vacuum advance.
Remove the breaker plate from the distributor and see what side the centrifugal advance positive stop is on. There are basically 2 types 10-15 deg and 13-18 deg. Since you only get 10 deg advance I think it is safe to say yours is the 10-15 type set on the 10 deg position.
To change to the 15 deg stop you need to remove the lock ring from inside of the dist cam assembly lift it up and rotate the unit until it lines up with the 15 deg slot. This is also a good time to put a set of lighter advance springs in to get the advance to come on a bit quicker. Reassemble and your done

Greg B
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