Why worry - - - -just drive it until the new one comes in. The only problem a non-working vacuum advance will cause is the loss of a couple of miles per gallon gas mileage. Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
An NOS one will have a removable cap with a spring under it so you can experiment all you want shimming the spring to advance or delay the vacuum advance. I have never mucked around with that but there are specs and I think the trucks were different than the cars. Advance was supposed to start at 4 in hg IIRC.
The fifties GMC medium and heavy trucks I worked on didn't have a vacuum advance at all. Think of a GMC dump truck grossing 32,000 powered by a 270. The throttle only had two useful positions; wide open or idle.
1951 3800 1-ton "Earning its keep from the get-go" In the DITY Gallery 1962 261 (w/cam, Fenton headers, 2 carbs, MSD ign.), SM420 & Brown-Lipe 6231A 3spd aux. trans, stock axles & brakes. Owned since 1971.
