Vac.Advance ?'s

[Flat Black]

Can any of you guys please tell me how to test a vac. advance on a dizzy to see if it's working or not?I threw the timing light on it w/the vac.line still on the dizzy and ran up the r.p.m.'s and the timing mark doesn't move.I hooked up a handheld vac. pump w/gauge to the vac. advance and pumped, but it won't hold any vac. and nothing in the dizzy moves :-|


Oh,by the way it's a 76 cj7 w/a 258

My main problem is, When I get past half throttle it falls on it's face and pops through the carb.That's why I suspect the vac. advance isn't working.Any help or suggestions would be greatly appreciated!Thanks....Mike

[Flat Black]

Nobody? :smile:

[fuzz401]

if the hand pump did not do any thing i would say it is bad

[Flat Black]

Thanks Fuzz,No the hand pump didn't do anything,It just sucked in and released,like you didn't have the pump hooked up to anything.And nothing in the dizzy moved.Is it O.K to use a used one,or should I just go ahead and try to find a new one?Is a 258 and a v-8 the same?(Vacuum advance that is). How hard is it to change?Thanks for the help Fuzz I knew you would chime in sooner or later....You're the man.Thanks.....Mike

[fuzz401]

I dont know if they are the same or not but not to hard to change try a used one first if it holds it is ok a quick test to find out

[Toy Maker]

If the ignition system is original, it is a Prestolite distributor. This is identified by a white (used to be) vacume advance that is somewhat the shape of a figure 8 on its side, with the vacume port on one side. Replacement is somewhat involved, better have a manual handy. If it has been changed to something else, it is probably easy.
Also check the mechanical advance, it should have advanced the timing when you increased the rpm with or without the vacume advance. Lack of mechanical advance would possibly cause the symptoms you describe, vacume advance would not.

[Flat Black]

Thanks guys,I did test the used one I have and it works(off of a prestolite,out of a 360). Any ideas on how to test the mechanical advance?Greatly appreciated.....Mike :sa:

[Toy Maker]

First, remove distributor cap, turn the rotor by hand. It should move one direction with slight spring pressure resistance, then spring back when released. Check the advance weights to see that they are not frozen and that the springs are attached and not broken. If everything checks out, then reassemble the cap, attach timing light, start engine. With vacume disconnected, watch the timing mark. From idle speed, as you raise the RPM the pointer should smoothly move toward more advance from off idle and continue to advance as RPM increases. Check that the advance continues through 3k RPM with smooth change. Should advance a minimum of 12 deg, probably will be more. About 20 deg. preferable.

[Flat Black]

Thanks,I'm going to have -at-it tomarrow.What can I clean the mechanical and vac. advances with?Contact cleaner or?Also Are these supposed to be lubed? What with?Sorry soo many questions I usually don't mess with distributors too much,I don't have much luck with them.Thanks in advance :t: .....Mike

[Toy Maker]

If you look down the top of the shaft under the rotor, there should be a felt wick. Regular maintenance is to put a few drops of light oil (Marvel Mystery Oil works well) on the felt. When the weights are slightly sticking, BP Blaster or such can be used to free them up. If completely frozen, may require disassembily and cleaning. Use wire brushes or fine sand paper(400+ grit) with liquid lubricant such as parts washer fluid, mineral spirits, very light oil etc. Be carefull to only remove rust and old debris so as to not change assembily tolerances. Parts should slide togeather freely without effort, but without slop or play. If you have facilities, do not bead blast mating parts, pickup asm., or vacume advance unit when disassembled, non mating parts ok to blast A/R if you choose, but not required. After rust removal, clean parts with solvent, allow to dry/ blow off with compressed air if avaliable, lightly lubricate before reassembily.

[Flat Black]

Thanks Toy Maker,I'm going to get on it as soon as it stops raining....IF it stops raining.Note to self....Having a Jeep w/out a garage-not good.lol.
Expect a post-back.....Mike

[Flat Black]

Solved it,G.M. H.E.I. :mrgreen: .....Mike