fuel delivery - fuel pump issue...

[pheengurs]

hey all, I've got an oddball issue with my 64, and thought I'd share it to see if anybody's had a similar problem...

The motor is a 78 C10 - 355 (350 bored .030) - 4bolt mains, with mild cam, lightly ported, cast 2.02 heads, edelbrock airgap intake, edelbrock carb, double roller timing chain, flattop pistons... a solid, good running motor...

Anyway, here's my problem: The level in the fuel filter is just fine, until the motor warms up, (enough to cycle the cooling fans a couple of times), then the level in the fuel filter starts getting lower and lower until there's barely a trickle of fuel going into it... then the motor starts running funny, missing and popping and backfiring when you hold the accelerator past 1000 rpms...

when staring at the fuel filter, as the motor cools down, you can see it slowly fill up again, and then it eventually empties... I don't know if that's normal for an old setup that doesn't have a return line

I also noticed that when the motor is warmed up, then you shut it off, there is bubbling happening in the filter. Not alot (like boiling fuel or something), but some bubbling... Now I know for a fact my gas cap does not fully seal, but the design simply doesn't allow for more than a slight pressure fit. There are no threads or like what you see on a more modern gas cap, so perhaps that's why the gas drains, as the pressure releases from the gas cap ???

I've checked all the rubber fuel lines, and they are still nice and soft, no dryness or cracking at all. There is a hard line from the sending unit on the tank that goes through the floor. From there a 12inch rubber hose connects to a 5/16 steel line that runs along the frame to near the exhaust downpipe on the passenger side. From there it's rubber to a steel U line that sets up the angle properly for the rubber line to attach to the fuel pump. From the pump, there is a 3/8 steel line running upwards to the fuel filter. From the steel line, there's a short rubber hose, then the filter, then another 12 inch rubber hose to the carb. There are no leaks anywhere.

On both the old mechanical pump and now after borrowing a friends spare mechanical fuel pump, once the fuel inlet line is attached, fuel will dribble out the fuel pump via the outlet that is to feed the carb... When the main fuel line is removed fuel flows rather freely out of it, so there's no blockage.

I don't really understand what the problem is. How could the same problem happen with both pumps? How could fuel flow just fine until the motor is fully warmed up?

Right now the motor is timed at 8 degrees advance. I held the timing light on the balancer, and revved the motor a bit, and noticed the timing retards as I rev it... IS THAT NORMAL??

Also, since coming back from maylong, my vac gauge shows a steady 11-12 at idle, instead of a steady 15-16 as it was before, and should be reading. (I confirmed this reading by running a vac tester off the manifold vacuum port, which I'm not using and is currently blocked off) AND, when I unplug the manifold vacuum port off the intake, the idle speed increases by about 100rpm or so, but the vac gauge still reads 11-12...

(do you guys use your manifold vacuum port off the intake for anything? According to the edelbrock carb manual, everything is supposed to be run off the carb...)

I re-torqued the carb nuts, the intake and exhaust manifold bolts & exhaust down-pipe bolts, thinking this might be the cause of low vac, but it changed nothing... My engine rebuilding manual say low vacuum gauge readings are most likely a leaking carb to intake gasket...

I DID, make my own carb gasket out of gasket paper. Perhaps that is a factor?

So, I'm stumped. And unless you guys come up with something, I'm gonna throw money at it. I'll buy an electric fuel pump, and I'll buy a new carb gasket, and I'm also going to buy some 3/8 fuel line, and replace the old stuff. I dunno what else to do. This is a very weird problem...

also here's a pic of the truck (a work, of course, in progress...)