backfiring, need help!

[91FrankinStang]

Just finished the carb conversion on my 91 GT.. Got initial timing set at 10*..Set idle & fuel mixture, float level & fuel pressure is at 5psi.. I'll probably bring it up to 6psi though..

Anyway, when I have the vacuum advance plugged on the distributor, the motor runs great.. If I connect the vacuum advance to the timed port on the carb, when i give it gas, it bogs & backfires.. I can very slowly bring the revs upto 1500rpm, but thats it.. then it starts to bog..If I give it gas quickly, it backfires throught the exhaust..

I dont get it... Could it be a bad vac advance mechanism? bad distributor?? I am using a used distributor off of an '85 GT with the steel gear for roller cam..

I've been told this dizzy has mechanical advance built in, so I should be ok to run it w/out the vacuum advance.. just set the total timing to 32*, and it should be ok..Any truth to this?

But I would much rather correct the problem, than "band-aid" it..

This ever happen to anyone? I was thinking about just getting a reman dizzy.. They are $50.. drop it in and see what happens..

Oher specs on the car are:
Stock 91 long block..
Performer 289 INtake
Holley 600 carb
MSD 6A & Blaster 2 Coil, 9mm wires, new plugs
Holley Blue pump, mallory bypass regulator..-6an braided hose throughout.
Stock '85 dizzy from a Manual tranny GT w/steel gear.

Any info appreciated..

Joe

 

[Ourobos]

YES.. Run it without the vacuum. Start at 32 if you want, and if it were me, I'd try to add some time until I got pinging, then back it off.

 

[91FrankinStang]

but why is it happening?? is it b/c there is no ecm controlling it & between the mech adv built in & the vac adv its too much?? or is the vac adv can faulty??

 

[bt15]

Don't use the timed advance port. Use the other one. See if that helps. If you're still having problems after that, there are a couple of other factors that may be involved. For example, if you have a vacuum leak and you're setting your initial with a leak, you'll not be able to set correctly. If you don't have a vacuum leak, check your mechanical advance setting, then your vacuum advance limit to figure out what your total is. You should be around 35-45 total. This of course is if you want to use vacuum advance. The big benefit of vacuum advance is fuel economy is significantly improved in street cars with correctly operating vacuum advance, but then again, that's up to you of course.

 

[91FrankinStang]

I checked my mechanical advacne today.. The weights in the distributor are 13L & 18L, its on the 18 side..

So that means I have 36deg of Mechanical advance, plus the 10* initial, 46 total which is kinda high..

I tried to flip the weight so I would get a lower mech advance and give me more room to play, but it seems like you need a special puller to remove it from the shaft..

So I left it as is, and lowered my initial timing to 8*.. So now I've got 44* total.. I could go down to 6*, but left it at 8*..

Reconnected vac adv to timed port, same thing.. I'll try the manifold vacuum port tomorrow.. But i dont think it'll help.. I think the car wont even start then if it is bogging down with the timed port..

I made sure everything was nice and tight.. INtake bolts, carb bolts, header bolts.. So I dont know from where I would be getting a vaccum leak..

I'd like to have it working properly, but the motor feels pretty good without it.. I guess I will ultimatly have to just wait till i get it back on the road to see how much of an affect it has on the driveability & mpg..

If i had too much total timing, could that cause the problems when I hook up the vac advance?? Or would it mainly be due to a vaccum leak? Or maybe the advance canister itself is no good..

 

[91FrankinStang]

ttt