1978 620 KC Driving Issues

[flabbychucs]

Was wondering if I could get some advice on a problem I'm having. I Googled but I'm just wondering if maybe it's something funny with these trucks that I'm not familiar with.

 

It starts perfectly and will idle infinitely. Up to a point it drives fine and then it shuts off. If you lay off the gas or go to neutral it'll keep running. Shut it off and let it rest for any amount of time and you might get another 10 miles or you may get 50 feet. I replaced the fuel pump, gas tank, did a tune up, changed the fuel filter, and changed all the vacuum lines. When I changed the vacuum lines the problem went away for probably 50 miles. Timing has been checked. Any advice is appreciated and thanks so much in advance.

[banzai510(hainz)]

Is this a points ignition?

 

If so make sure you have a stock point coil with the ballast resistor. Feel if the coil gets hot is a sign a wrong lower ohm coil is installed. This will fry the poitns to where it arches out.

since I re read your post a 78 should be the EI type ignition 1st Gen

 

also ck the dist shaft wear.

 

Could be carb is just worn out. if still the stock carb,

 

make sure carb squirts gas when you cycle the linkage. this makes sure the accel pump on carb is working.

 

also make sure the choke is opening up after its warmed up. 77 should be electric choke. if wire fell off then choke will stay on . Not good,

 

open up the fuel filter to see if crude in there but since you changed the tank it should be good.

 

this is a EZ fix.

 

unfortuantly the later L motor had alot of emissions

[distributorguy]

The big question: is there anything you worked on recently that may be related?

 

Being that it ALWAYS runs fine at low rpm, I'm guessing come kind of fuel pressure loss or restriction.  It could be as simple as a small leak in a fuel line that sucks air but doesn't leak fuel.  I've seen cracked lines at the top of the tank that do this, as well as a loose clamp on the tank end (not engine end) of a rubber fuel line.  It could also be junk in your replacement tank plugging the filter, or even low float level in the carb.  Or a mechanical fuel pump failing (don't assume new parts are good).  Or a poor ground on an electrical fuel pump.  Or bad vacuum leaks all over the place that lean out your fuel mixture to a point of failure (very bad.)  My guess is that you used the wrong diameter vacuum lines and they leak?  Or maybe one is sucking air from a place it shouldn't.  You have to have several things wrong to get to this point of failure.  

 

If this were ignition, you'd likely have poor running, unless you are the 1/1000 that has a partial condenser failure.  A bad coil will shut the truck off until it cools. Bad point gap will run poorly.  

[banzai510(hainz)]

OK Im giving my touble shoot skills away. Ill find out How EZ this is. but hard for others.

 

ck the engine timming and valve lash

ck distributor timming.

 

if all ok ck for tightness on carb and intake manifold

 

swap distributor as its EZ to swapp

then swap carb.

 

Yes I have spares of both as you should also if in the game a long time.

 

By then if you dont have it fixed ,your fucked.

 

 

I had the above also happen. bad condensor(went to Pertronix )

cracked intake hose on fuel pump.

[distributorguy]

Be careful with Pertronix.  They suck 1V from the coil, meaning roughly 1000V lost from the HT circuit.  They also interpret magnetic noise from the plug wires as their own magnetic trigger wheel, causing random misfires that push the mixture continuously rich and you lose a net 4-7 hp at the rear wheels.  Proven over and over on the chassis dyno.  

 

There are SO many factors missing in Banzai510's post that are easily overlooked by someone with lesser automotive skills...  What if the fuel pump volume is low for example?  It should pump no less than 1/2 gallon per minute.  "By then if you dont have it fixed ,your f'd."   ?????