Distributor

[Donny]

Got a little problem with distributor can't really figure it out
478 magnum 500 Holly , bought A HEI distributor off eBay
When I hook the vacuum advance up it is fine at idle as soon as I give it a little throttle starts backfiring and kills the motor , runs OK without it hooked up
Anybody out there ever have this problem ?

[TJ's GMC]

Sounds to me like there may be something wrong with the pump or maybe the advance springs are weak. What's the initial set at? I would run it with a light and see what is going on by reading the crank pulley. Lot can be read that way.

[Donny]

Timing is set at 2.5° , it doesn't really matter where I set the timing , I advanced it
To 5° And then to 7° Still did the same thing backfires then dies I have a capped off now
Took it out last weekend once in a while when I'm in a hard pull it backfires through the carburetor ,

[TJ's GMC]

Back firing through the carb could be other issues to, could be running to lean, intake valve is hanging open, or improper valve adjustment. These are set at .012 intake and .018 exhaust. If you haven't checked them I would for sure. On my 305 I found several of them to tight do to ignorant shade tree mechanics who think they know it all. There may be some vacuum leaks. With the engine off, look down the throat of the carb and pump the gas, does it give a nice shot of fuel down the manifold? If so, your accelerator pump is fine. If not, there is at least part of your issues.

[ilvracn]

weak valve springs? worn guides? just adding 2 posibilities

[ilvracn]

2 plug wires switched around?

[Donny]

Thanks guys
Yes I believe you're right I haven't adjusted the valves yet ,they did it when they rebuilt the motor but it was not up to temperature only got about 30 miles on it was going to do that this weekend ,

They are brand-new Magnum heads
That were still sealed in the box , New valves new springs and rotators

Plug wires are good check those about umpteen million times runs too good to have one crossed
I will check the carb out


This distributor problem with the vacuum Advance issue still has me baffled
Nobody seems to know

[turbobill]

I have run into problems with the stator wiring on two HEI distributors over the years. When the stator plate was moved by the vacuum advance, it cause intermittent or open circuits in the stator signal to the module.

In one case, the engine would quit when the vacuum advance was hooked up. In the other, it would run very erratic.

[tommr]

Donny Google "Rotor Phasing HAMB" There is a thread over there regarding phasing and vacuum advance. Tom

[Donny]

Thanks Tom thanks Bill
Definitely sounds like this could be the problems