258 w/ HEI, vacuum advance issue?

258 w/ HEI, vacuum advance issue?


Red Springs, NC
1984 Jeep CJ7
258 Inline 6
T176 4 speed
Dana 300
When I bought my jeep the vacuum advance didn't have a line hooked up on the HEI distributor. I took an extra vacuum line I had and hooked it up. When I revved the motor she was poppin and backfiring and making a big fuss. I disconnected the vacuum line and she runs smooth again. Any idea what would cause that? I have had a few AMC 6's and 8's and have never run into a problem like this before. I did adjust the timing and that didn't change anything as far as the backfiring and popping. :confused:
Where did you have the vac hose comming from? It should be ported vac.
I don't know alot about HEI systems, but from what I understand they came in four, five and seven pin configurations. The four was set up for vacuum advance while the seven wasn't, so if it is a seven pin HEI, then it wouldn't need the vacuum attachment and this could be what is causing the problems you are mentioning. Do you know the configuration?
is it "poping & backfiring" through the carb or the exaust?
what is the timing set at? You set it without the vac. line hooked up right?
Did you keep the timing light hooked up when you reved it up to see what the timing did?
Might be because i ws using the intake vacuum rather than ported. I'll try again. It's backfiring out the exhaust. I'm not sure what the HEI configuration is. I also didn't reve the motor while timing, but yes, the vacuum line ws disconnected when it was being timed. Still is disconneced as a matter of fact. SHe seems to run just fine without it. I'll try the ported vacuum and see what it does. Thanks y'all.
hey I am having the same problem as well, I just bought and installed a 4wheelparts hei and some reason it runs well without the vacuum. I've played with the initial timing from 12 degrees back down to 8 but some reason it seems like when i hook up the vacuum (ported) it pulls it too far advanced and it stumbles and run like :dung:. if i run without the vacuum it gets about 5mpg and this has become more of a daily driver btw and i need to get it back up to 15mpg if not better (hence why i bought the damn thing in the first place)
Your problem is too much timing. Most hei vac cans provide 20-23 degrees of advance with a static of say 10 plus mech adv of say 12 your at 42-45 total whereas 28-32 is max. Hei vac with stock profile cams runs best with manifold vac. Some Small rv type cams like ported a little better. The real gains with vac. Advance is in mileage so you can run without it. So, set timing at stock spec. With adv.Disconnected ,snug dist. Rev motor to 2200 rpm or so and check timing at this rpm. This number minus your static number is your mechanical advance. Now with the engine running apply vac. To the vac. Adv. and check it again at idle. This number minus your static number is the amount of vac. Adv. you have. So add static,mech and vac. And what do you have.
As a side note a dial back timing light is best in this situation.
Adjustable vac advance pots are available at most parts stores as well as hei mech. Adv. weights to tailor your mech adv. as well.

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