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351W will not idle and backfires.

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Last of the Machicans:
I'm getting closer to finding the problem.  The vac lines on this car are hooked up unlike any vac diagram I can find, so it is slowing my newbie butt down a bit.  :D 

There is a vac line from the "Power brake" port on my Holley carb that is hooked directly to a 3-prong loop attached to the firewall (the actual power brake can is hooked to vac supply on the intake manifold). The two other prongs on the loop have ancient-looking vac hoses that are routed down below the firewall. 

I removed the hose and plugged the "Power brake" port on the carb (which should remove the loop and those two old vac tubes from the vac system) and now with the car at idle I still have no vacuum, but if I bring it up around 2k-3k rpm (tach doesnt work yet or I'd be more certain) I get 10 -15 inches of vac. Before I plugged it I had no vacuum at any rpm.

Sooo, it seems I have a vac leak in one or both of those two tubes attached to the firewall loop.  The vac diagrams I have show only a 2-prong loop (mine is three) with only one extra tube that goes to the transmission.  I am assuming one of my two tubes goes to my FMX, which I will replace in the morning.  The other tube, I am unsure of, but I am thinking that it goes to the climate controls inside the car (will trace it in the a.m.)? The heater controls seem to have been rather hastily removed by the previous owner, which I am guessing explains some of the vac leak from one of those tubes.

The power brake can won't have another vac tube leaving it, aside from the line directly to the intake manifold, correct?  My diagrams do not show brake vac.

If my oil filler cap has breather holes, can I just cap my PCV valve at the valve cover and the carb just to eliminate one more vac hose?

Wow... that was long winded, sorry!  :o

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