620 runs great, cuts out, runs great.

[ByStickel]

I put my '78 on the road for the first time, yesterday. On the way to the gas station for PBR, and gas, it just stopped pulling on a steady incline. No sputering, no bucking. No throttle response and no warning lights, either. It didn't stall because I was still in gear, and it did remain idling this time, but has stalled during other episodes. After getting it off the road, the throttle was responsive, again. No sign of the problem.

 

It has done it another 4 times since. The only thing all events SEEM to have in common is that there was  greater load on the engine because I was driving uphill or accelerating. It happened going downhill, on the gas, too. I can't make it happen.

 

I swapped out the fuel filter, but since there was no stumbling, I didn't think that was it. It really feels like I lose spark, and each incident seems to last approx 5 seconds. After that, I can go again, normally.

 

Is it possible that low voltage could cause the ignition to reset? Could higher load cause more drain? (My fuel-injected moto had that same problem) I tried checking the battery, but it read 25V (my meter is fried and I can't get an accurate number)

 

Any help would be fantastic. I can't drive it like this.

 

And because everybody likes pics:

 

[abbylind]

Might want to check your fuel lines especially near the carb. If youve never replaced them they are 40 years old. Mine were dried up so bad they fell apart when I loosened the hose clamps. Some of the pieces got into the carb.  YMMV

[abbylind]

Oh also check your fuel pump mine had a bad seal and was spurting fuel out the sides.....

[ByStickel]

There wouldn't be any sputtering as the carb runs out of fuel? This is either full on or completely off.

 

You're both right that the fuel did have small particles of rubber in it...

[bilzbobaggins]

Junk in the bowl blocking the jets up?

[cruznude]

I don't know what to tell you. I put my weber 32/36 and de-smogged everything. After doing it all, I was puttring and couldn't get any acceleration,  A quick 170.00 to a carb specialist here in albuquerque and even though I have a 5 speed, I smoke them on the take off in 2nd gear. I really like your truck. I have a 79.

[datzenmike]

The ignition either works or doesn't. There is no reset. Throw that meter away. Wrong readings are worse than no readings.

 

 

Take it for a drive up a steep hill and get it to quit. When it does, immediately get the clutch in and the engine OFF.... pull over safely. Hopefully you have now preserved the gas contents inside the carburetor at the time of the problem. Lift the air filter and take a look at the sight glass on the front of the carb. Gas should be at the horizontal line or the dot on the glass bull's eye. If no gas is showing then you have a fuel delivery problem.

 

Sounds like a fuel delivery problem.

 

Filter may be good but line plugged from tank or pump not right. Pull the hose off the outlet of the pump and direct into a suitable container. Pull the plug wire off the coil and have someone crank the engine over with the starter. What you should see are a strong pulses of fuel from the outlet a the engine turns.

 

If this checks out as good the carb may not be allowing gas in fast enough to keep up with the high demand when under heavy load like climbing a hill or passing. This could be two things...

 

Your float inside the carb may be set too high.

The brass screen on the banjo inlet on the front of the carb may be blocked with dirt.

 

Make that three... the line from pump to carb is blocked or kinked.

[Elkie]

That would be my guess. I had junk in my bowl caused the truck to idle fine, and as soon as you put it in gear to drive, it died. Drained the bowl, and it fixed it.

[ByStickel]

Great info, thank you.

 

I will try all of that. My helper is put of town, but I'll get started doing what one person can do.

 

Edit/mini update: I replaced all the rubber lines under the hood and checked the screen inside the banjo: perfectly clean. I adjusted the float up, because the fuel level was near the bottom of the sight glass. It's now right at the center. (after re-reading Mike's post, I need to check this again)

 

Then, I drove around and the problem was worse than ever. I did notice that it's particularly bad accelerating out of a corner. This time, it was taking longer to correct itself and one time, it refused to correct until I shut the truck off and started it back up, immediately... this makes think about chunks in the carb being stirred up under higher draw/cornering loads and not releasing from jets until the engine is shut off. Most of the time, when this happens, the truck will idle, but anything past 1/4 throttle made it bog and die. Each time I shut it off and started again, it was back to normal.

 

I think I need to get the carb off and start taking it apart. I already bought a 12mm wrench that I can heat and bend 45 degrees to try to get those bolts...

[ByStickel]

I pulled the carb off and started going through all the jets and passages on top. Everything looked really clean. Then, I flipped it over and removed the plugs that go to what I assume are the main jets. Speaking of 'big chunks being stirred up'...

 

 

The biggest deuce that little rodent could drop, right there inside my carb. No idea how it would get in there.

 

 No issues on the way home. Problem solved.

[datzenmike]

I had a similar problem in the 70s with my 521. Hit a bump and it would run like shit. Stop and shut it off, count slow to ten, restart and everything fine. There was a small casting flake under the primary jet that would get disturbed and sucked up against the jet. .

[banzai510(hainz)]

had this happen to my 510.

If I pulled the choke  it drove better so I knoew my eleltrical was good and carb related. was a tiny stone in the main barrel blocking the gas. idle was fine and pulling the choke made gas go tru the 2nd barrel

 

PS nice 510 and 620 you have

[ByStickel]

Thanks again, everybody, for everything.

 

I'd be effed without this site.