The Stovebolt Site Forums The Shop Area Engine Shop Vacuum Advance-Distributor-228 c.i. GMC

I am new to Stovebolt Forums and this is my first posting. I have a 1946 GMC Canopy Express with a 228 cubic inch 6 cylinder engine. I would like to know how the distributor is suppose to react when you rev the motor at idle. Should I see the distributor turn when the motor is revved ? The engine was running poorly and back firing through the carb. I changed the points, plugs, and condenser without any success. The distributor wires and cap are in good condition,I started advancing the timing and saw a significant improvement in the performance. I then changed the vacuum advance, but I do not see any movement in the vacuum advance rod which attaches to the distributor plate or the distributor when the engine is revved. The distributor does not move when the engine is being revved. I have vacuum to the vacuum advance unit. I have seen postings on the Forum about similar problems, but wanted to know exactly how the distributor and vacuum advance rod should be reacting when the engine is being revved. Thank you. Sron48.

If the vacuum advance is working and correctly installed on the distributor it should move the distributor when the engine is revved up.
It should subsequently return as power is lowered and return to a stationary position at idle. You could check the vacuum advance with a vacuum pump to see if it is holding.
Fred

Hey sron48, welcome. Take a minute and drop by in the welcome centre. Would love to see some pics...

Thank you Fred. I thought this was the case but wanted to be certain. Will get to work on it tomorrow and let you know. Sron48.

Fred, I have vacuum from the base of the carburator to the vacuum advance unit. I can see the vacuum advance arm moving inside the advance housing. The distributor turns when I rev the engine. Here's my dilemma; there's a screw and nut collar which attaches to the vacuum advance and goes around the outer distributor housing. If I don't tighten this collar, then engine comes out of timing when driving. If I do tighten this collar, the distributor does not move when I rev the engine and the vacuum advance arm does not move. What am I doing wrong ? Thx.
Sron48.

Also, does the distributor rotate clockwise or counter clockwise when advancing? When using the vacuum pump the advance should hold the vacuum and not leak out, correct!

Looking at one of my Delco catalogs, the vacuum advance for your distributor looks exactly like the vacuum advance used on the 235/261 chevy motors. So if I have that correct, the advance is made up of two parts encompassing an octane selector plate/vacuum advance and the clamp. You state that you recently changed out the advance. Take a close look at the base of the distributor. As you know from when you installed it, the distributor is held in place by both a bolt and a screw clamp arrangement. The vacuum advance is bolted to the block. If you look close, you should find a hash mark on the distributor pad on the block and a graduated scale with 0 in the middle and the scale on either side on the vacuum advance. This is the octane selector plate. It was a hold over from days when you never knew the quality of the gas you were getting and allowed you to make some adjustments for bad gas or changes in altitude when traveling with out any thing other then a wrench. Nowadays, you should just line up the 0 on the hash mark (making sure the motor is at TDC) and make your timing adjustments using the screw clamp from that point. This really should be your starting point for TDC. Then look at the clamp. It should be fully seated on the advance. If its not, the clamp is in bind with the advance. Loosen the screw and push the clamp down to seat it on the advance. Tighten the screw and twist the distributor. It should twist and come back.

As for the vacuum, that advance uses ported vacuum so you should have little to no vacuum at idle (throttle blade pretty much closed). What that means is that the vacuum port is on the atmosphere side of, or in other words above, the throttle blade. This is why you don't have to disconnect the vacuum line to the advance when you are setting the timing on a 235. You only really get vacuum when the throttle blade starts to open a little and exposes the vacuum port in the carb throttle bore. When the blade opens up past a certain point, the vacuum effect begins to diminish until there is no vacuum (wide open throttle). If you have set the vacuum advance on full manifold vacuum, where you have plumbed your vacuum advance line to a source under the throttle blade, what the advance sees is vacuum right from the get go, then you do have to disconnect the line to set the timing.

Good description of the vacuum advance operation by Mike. Yes, the vacuum should hold when actuated by the pump.
Fred

I'm headed for the AACA Grand National outside of Pittsburgh. I will work on the truck once I return and let you know what happens. Thank you both for your advice. Sron48.

Well, it's been over a year since I first posted my questions about the vacuum advance problem on my 1946 GMC Canopy Express with a 228 cubic inch engine. I want to thank everyone who contributed towards helping me solve the problem/s. As it turned out there were several problems which contributed to the poor engine performance: 1) There was a defective vacuum advance unit. 2) The screw clamp holding the distributor to the block was inverted causing the distributor to bind when trying to advance and last but not least, 3) one of the engine intake rings was missing, ( where the intake manifold bolts to the head) causing a loss of vacuum. I was only getting 10" of vacuum at idle when it should have been 18-22". Thanks once again for all the advice.

Absolutely! Thanks for letting us know the outcome and allowing us to assist you.