[proge]

I have a 56 chev 350 274 xe cam,vortec heads,flat top speedpro pistons,edlebrock carb,hedman headers,3.73 posi rear axle and stock new hei dist. I set the base timing at 12 degrees and the total timing at 3000 rpm is 38 degrees.I was trying to improve drivability on the street so I hooked up vac advance which had not been hooked up.Th total advance with vac advance is more than 50 dgrees. I took it out to make a run.I wound it up to about 2500 rpm started to take off but it backfired and died.I took off vac line took off same way ran ok.I know there is a problem with vac advance. anybody got any ideas? thanks proge

[SSedan64]

It sounds like there's just too much advance with the Vac can hooked up. Vortec heads usually don't need more than 34 Total (Initial + Mechanical) for best performance. Try backing your mechanical advance down 4 or 5 degrees. How do you have the vacuum advance hooked up? Ported (above butterflies)or full manifold vacuum (below BF's)?

[454C10]

seem that you need a new vacuum advance canister. crane sells a nice one that is adjustable. get it from summit racing.

you have the right idea, just too much timing added in. adjust the vacuum advance to add in about 15 degrees more. so at idle it will be at 27 degrees (12 + 15).

expect a few more mpgs with more timing.

[Jmark]

While I'll say that 50 degrees is too much for a street engine, it should not cause it to backfire and die.

I would take a shot at a defective pickup inside the dist. When the advance pulls, the pickup rotates and sometimes the wires break inside from the millions of pulls they've had. When they move and open up inside the wire, ignition either gets sporadic or quits all together, at least until the advance returns to zero.

The easiest test is: It runs fine without the advance hooked but craps out and dies once the advance IS hooked up and pulls the pickup.

Say lots and lots of this exact problem in Pontiac's back in the day and still see a few in chevies.

[F-BIRD'88]

Distributor rotor/cap phasing is effected by vacuum advance and manufacturing tolerances.
If the rotor phasing is /or moves too far off the post in the cap, the motor will misfire. (spark scatter)
The little magnetic pick up wires are also suspect.

[bigloser5]

Possibly a dumb question, but are you sure that the outer ring of the dampener hasn't moved? I had one where the outer ring spun, I would time it based on the markings on the dampener, it would idle o.k., and advance fine mechanically, but when I thought it was fully advanced at 34 degrees it was actually much higher. So when the added vacuum advance kicked in, it was way too advanced and ran like crap. Just a thought.