Bucking bronco

[militarypotts]

I do not have an electronic ignition. I did purchase a Pertronix ignitor kit, but never installed it because the truck runs so well.
Any suggestion Nailhead, on what to do, should it be a fuel starving situation?
24v inline fuel pump??? Bad diaphram in pump?
I only use Mobil or Shell gas in the tank and the tank is in perfect shape.
Could the sending unit be bad? This all sounds unreasonable because the truck idles so well.

[Von]

Somebody put marbles in your gas tank? .. don't know that that actually works, always heard that though. Does sound like a fuel problem though.

A hole in your pickup tube allowing air in below a certain level? ... $20 worth should have kept you above that level for a while though.
Weak pump or stuck float...? although I've never heard of a float sticking in a down position.. you could be the lucky first.

[brute4c]

Wish i knew for sure...years ago a member had a problem like this after he changed his points...seems the leaf spring of the points was not installed right...didnt put tension on the points to close them...rough running happened in high gear as rpms rose...motor was under enough load for it to show up...
I realixze you didnt touch the points but could the leaf spring in there have loosened up or broken?
Just my wag...

[militarypotts]

You know Jon, I will check that out. My rough running takes place as I'm cruising along in 4th gear at a steady speed of about 35-37 mph. I'm a little confused as to why it wouldn't happen as your rpm's increase as you reach your shift points in the first 3 gears though.
My thoughts are, if it was a fuel pump/supply issue, why would the truck run so smoothly at idle? I really don't suspect a bad coil issue either. My electronics knowledge is limited to fairly basic stuff, but a loose/broken points spring should be easy to detect I would think. Although, if the spring was loose or broken, wouldn't this effect the firing order through all gears?
Now that I think about it, I believe I heard one or two very small backfires while this was happening last Friday evening. The backfires came while the truck was bucking and loosing throttle power. Any clue there?

[Don Cavey]

I don't remember if you have checked the coil in the past. I can tell you that my "No Restart When Hot" was one of those bloody Chinese chrome coils. I installed a vintage, used coil and that problem went away. Have you been inside of the ignitor? Wires OK, etc.? A condenser could also cause issues.

But bad points could do exactly what you describe. And, you are using very little fuel when idling so if it is a carburetor or pump or clogged filter thing, that could cause it too.

Do you have a clear fuel filter? I put one on my truck temporarily so that I could monitor the fuel. I have a new steel one but have been too lazy to change it. (I could say, "If it ain't broke, don't fix it" but I know it is laziness....)

[Von]

You know Jon, I will check that out. My rough running takes place as I'm cruising along in 4th gear at a steady speed of about 35-37 mph. I'm a little confused as to why it wouldn't happen as your rpm's increase as you reach your shift points in the first 3 gears though.
My thoughts are, if it was a fuel pump/supply issue, why would the truck run so smoothly at idle?

Had a similar discussion on an Isuzu FB group the other night. A member had the opposite problem, His truck ran great at speed but was dumping fuel down the throat and dying if it idled for a little bit. This was after he put a new carb on. I haven't heard back from him yet, but the top two theories was either his fuel pump was putting out too much pressure for that carb or the float was adjusted a little too high. Figuring that if the valve was staying open just a little too long it might not be a problem while he was running at speed, but when it sat idling and not using as much fuel then it would fill the bowl up and dump.

Maybe I just had carb problems on my mind when I read your post, but it sounded like a starvation problem... but just a slight starvation... more like a diet..
Like you're getting enough fuel to idle all day long and run through the gears but after running at speed for a bit you run through the available fuel faster than what's being supplied.
That's why I was thinking weak pump or a float problem. The fact that you can pull over when it starts (letting the bowl refill) and then get back on the road for a bit reinforces it a bit.

[militarypotts]

All of these answers seem to make sense, but I'll start with the ignitor. All seems weird to me though, that it was running perfect up until last Friday evening. I know that sounds dumb, because issues can happen at any time, but their wasn't a sign of this at all, in any way. I guess one thing I should do is get the truck back out on the road and see if it does it again. Trouble is, when things like this happen, I get a little gun shy about taking her back out.

[Don Cavey]

...Trouble is, when things like this happen, I get a little gun shy about taking her back out.

Boy, do I understand this...

[kyken]

Me too...Hope you have an easy solution!

Keep us posted!

[militarypotts]

Will do kyken. Unfortunately, I haven't had the time or energy to start looking at it. Maybe this week I'll start with the points spring. I really think I need to bite the bullet and take her out on the road again to see if it starts happening again though.