Thread: disconnect vac adv-popping out exhaust

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I finally got my damn engine running and I was messing with the timing again. With the vacuum advance hooked up it was about 20 degrees advanced. I plugged the vacuum to check the baseline and I could get it to fire up but not keep running it was so erratic.
It would blast wicked loud pops out the exhaust.
The only thing I could figure is that the burn was starting so late that when the exhaust valve opened it would let out a tremendous amount of energy still being releasing.
Does this make sense?

Also, the more I advanced the dist the better it ran and the faster it idled. It would also quiet down a lot when I advanced it up to about 35.
Should I advance it more since it idles better or leave it lower?

you should be able to unplug your vac. adv. set the timing and plug the vac. back up and the motor shouldn't be poping. sounds like your timing mark is not on TDC. if you're setting there with 35 degrees idling then the marks are off. you need to find true TDC.

I agree with Mike. I think you need to check TDC on your #1 cylinder and see if it corresponds with the timing mark on the damper.

I had the same problem.........long story short, when i put on the balancer and installed the timing tape i was off, and had the same experience you are having...right now i have the hei unplugged all the time. no vacuum advance. Use a vacuum guage to set and tune your carb.....i had the popping, might be too lean and too advanced in the timing. sometimes it takesa few cracks at ti to get the engine to run where its happy......takes time. thats the fun part about hotrodding. I have also found that with my holley it has a very small ideal setting...slight movement eithere in or out on the mixtures wreaks havoc....be patient

The firing order is right, it runs great with the vac hooked up.
The popping was only when I didn't have the vac hooked up.
(#1 is driver side front right)

Just so I know what I'm looking for, is it likely that my balancer is telling me I'm more advanced than I really am?
So when it was running good with vac hooked up reading 20 it was probably a lot less than 20? and when I unhooked it and it was popping like hell it was probably close to 0 or maybe even retarded a few degrees?

I'll have to get a piston stop and check this sucker.

Any chance you have a timing cover that doesn't match the harmonic dampener?

Not all SBC's have the timing tab/pointer in the same place so even if you're on the money with the timing light your timing is off.

The guys are on the money recommending you find true TDC and going from there.

You need to run the vac advance on the street as well.

You should be able to check and see if your balancer has slipped by looking at the TDC mark and the keyway centerline. It's usually 10º before the centerline, unless it's a pre-'69 engine and then it's 2º before.

Pelligrini Is probably right if you have a SBC I had that problem once and it near drove me nuts. I finely noticed the damper that was on the engine wasn't the one for the engine. It ran but couldn't time it with light.

it is a big block chev, not sb.
I remember from when I had the heads off that it was lined up properly, but I suppose it could have slipped since then.
I'm not really able to look at the keyway unless I disassemble the front end, I'll just use a piston stop.

But is popping something that retarded timing does?

When I rotated the dist to adv it while it was running it would really smooth out and run great (this was around 35 according to the balancer). Backing it off closer to 20 would make it kind of choppy and a clearly different exhaust note.

Originally posted by tcodi
it is a big block chev, not sb.
I remember from when I had the heads off that it was lined up properly, but I suppose it could have slipped since then.
I'm not really able to look at the keyway unless I disassemble the front end, I'll just use a piston stop.

But is popping something that retarded timing does?

When I rotated the dist to adv it while it was running it would really smooth out and run great (this was around 35 according to the balancer). Backing it off closer to 20 would make it kind of choppy and a clearly different exhaust note.

truth is, you should be able to unplug the vac. adv. and set the timing at 5 degrees without the motor popping back. you might have to idle it up some, but thats all. if you had wires crossed or something like that then it wouldn't run no better adv. or not. checking the true TDC is just a start. if that right then you can move on.

Yes retarding the timing will make it backfire out the exhaust advanceing it will bring it out the carb. Are your wires in the right opening on the cap?? is your distributor in one tooth off?? I would find TDC then check where the rotor is in relation to the number 1 plug wire.

Well.........I personally run my hei with the vacuum advance unplugged, and it sees both street time and track time. Its not a daily driver. now originally my engine had a 6" balancer and the timing tab on the timing cover. Now with the 400 balancer i needed the timing tab that bolts on the side. now with that said. some 400 blocks had the timing tab on the cover so if you change the tab location you need to use timing tape to re adjust where the TDC mark is. this to me sounds like what is happening, and the timing is that far off. well it might not be off its just being read at a wrong location. Tune your car for the best vacuum at idle while in gear. If its not a race motor then you really are not too concerned with the initail or when it all comes in at. but if you want to check it and confirm it is right. set up the motor so its coming up to TDC on the compression stroke of number 1.....put in the piston stop that screws into the number 1 plug hole. rotate clockwise until it touches the stop. mark the balancer at this location with the pointer. then rotate counterclockwise i full revolution until it hits the stop again. make a second mark on the balancer. not measure the distance between both marks and put a new mark in the center of them...that is as close to true TDC you will get with out using a degree wheel. Hope this helps.

I'm not sure what you mean by "are wires in right opening in cap," but the wires are all routed around the cap in the firing order and go to the right plugs.

the thing runs fine, it's just liking very high advance numbers to idle.
I'll just have to check that timing tape and make sure it's in the right spot and go from there.
hopefully I can track down a piston stop today.