HEI Distributor Question (Quick, I promise)

[ArkansasJeeper]

I just replaced my old Prestolite with a new (!) HEI. In-cap coil, 15 degree (?) advance can, basically right off a 1980 Jimmy.

The problem- the engine times out great, the centrifugal advance follows the curve beautifully, and without vacuum advance hooked up it runs like a charm.

The advance, though, causes the engine to lose power and backfire horribly under throttle. Is it just way too aggressive? I'm hooked up to one of the lower ports. It pulls under open throttle, no pull at idle.

I don't have a manifold vacuum port (one suggestion I read). Is it that hard to swap out an advance can? Is it even necessary to have a vacuum advance if the truck runs strong?

Thanks in advance for any suggestions.

 

[redrooster]

You need timing advance as the RPM increases or you would get out of time in a hurry.
Where are you getting vacuum from now? Where was your old vacuum line connected? What is your base timing at idle set at now? Did you set the base timing with the vacuum hose disconnected and the engine idling at 1000 RPM?
I'm not an expert, but a 15 degree advance sounds right throughout the full range up to redline. I think your problem lies in the vacuum source. Sounds like too much vacuum too soon. My guess is that you have vacuum from the carb instead of the manifold.

 

[ArkansasJeeper]

Hrm... I actually do have the vacuum from the carb. That's how the old Prestolite was set up. Do I have a manifold vacuum port somewhere? If so, where is it? My exhaust manifold and intake manifold have both been hacked on a bit.

I timed the engine to 9 BTDC at 1600 RPM with the vacuum advance disconnected and capped.

 

[slo-wrench44]

let me know what you find out. what are you running for a motor in it? i-6?