620 Mechanic In Colorado

[BrianBColo]

I have a '78 620 and having fuel system issues.  Looking for a good mechanic in northern Colorado who understands these trucks.   Thanks.

[datzenmike]

We have a guy somewhere in Co.... https://ratsun.net/profile/4544-yenpit/

 

Meanwhile what does it do or not do??? May only need a fuel filter.

[BrianBColo]

I have replaced the carb, fuel pump (mechanical), fuel filter, put in a backflow valve, and replaced all of the fuel lines and clamps.  I also removed the tank, cleaned it out and replaced the sender.  If I prime the carb, it starts, but when I shut it off, it won't start again.  And the glass fuel filter empties out, and it never really fills to the top.  When it stops, the drains back into the tank.    Is there a vacuum issue?

[datzenmike]

In a carburetor, once the gas is in the fuel bowl it can't magically jump up hill past the needle valve and return to the tank. It must be draining into the intake and why it's hard to restart.

 

Next time you shut it off, pinch the rubber hose closed with vice grips. Bet the gas still magically disappears.

[BrianBColo]

I have replaced the carb, fuel pump (mechanical), fuel filter, put in a backflow valve, and replaced all of the fuel lines and clamps.  I also removed the tank, cleaned it out and replaced the sender.  If I prime the carb, it starts, but when I shut it off, it won't start again.  And the glass fuel filter empties out, and it never really fills to the top.  When it stops, the drains back into the tank.    Is there a vacuum issue?

 

Pinch which rubber hose?

[BrianBColo]

Pinch which rubber hose?

[datzenmike]

Miss read. Fuel filter emptying doesn't matter. The carburetor will have enough gas in the float chamber to start and drive half way round the block.

 

Filter siphoning back to tank is likely a loose hose clamp letting air in allowing the furl to flow sown hill. Check all hose clamps at the pump inlet and both sides of the filter.

 

 

[Stoffregen Motorsports]

Are you pumping the throttle pedal before you try a cold start?

[Stinky]

I bet it is your carb....as already mentioned.

 

I really, really doubt that your fuel line has anything to do w/it.

 

As mentioned, pinch your rubber fuel line, that will stop a back siphon (which can't be there because of the check valve).

 

I bet your PU sill start if you hold your foot on the floor, while cranking it (it is flooded, and that fixes flooded as gives it air for the excess gas).

 

My guess, is that you have a crack, or a pinhole allowing the gas to drain.

 

Those things aren't rockets they are really simple to work on.

 

 

[BrianBColo]

Yes, it starts when I floor it and idles and runs when warmed up.  I shut it off and won't start.  The carb is new, but could be leaking.  I'll clamp the hose and see what happens.  Fuel pump could also be bad (even though new) and thinking of replacing with new electric.  Any recs on a fuel pump for a '78?  

[BrianBColo]

And I'll recheck all hose clamps.  More to follow and thanks everyone.

[datzenmike]

There should be lots of gas in the carburetor for restarting. So tightening the clamps may stop gas from siphoning back into the tank on the tank side of the fuel pump but this will have no effect on hot restarts. Gas cannot siphon back through the fuel pump. There are two one way valves inside. There is a one way valve at the fuel tank but it's there to prevent gas running out in the event of a roll over.

 

Next time you have it running take a look at the front window on the carburetor. Where is the gas level??? It should be at the half way point on the small round glass window. This is important as it is designed to work properly at this half point.

 

Look down inside. Do you see any dripping gas???

 

Shut it off and look. Anything?

[Stoffregen Motorsports]

Floor it on a hot start and see if that gets it to fire.