If you mean it backfired out the carb then I'd say your ignition timing has slipped or the ignition box is failing.
Additionally, it only takes one pop to blowout the power valve in the stock 4180 carb.
This will cause it to blubber and run very rich at cruise.
Thanks for the response...Will focus my research in that direction...
If it's in the ignition, wouldn't the problem exists at idle and slower speeds? High RPM like at a shift point?
Blubbering is a good way to describe what's happening...As I let off the gas, it gets to a point where she'll kick in for power then drop off...Too much gas then it drains to the correct level gives the power gain then due to foot not on the gas drops off again...
Looking at the PV...Is there an easy test for that?
You can have the ignition box tested at most 'Big Box' auto parts stores.
I had made the suggestion before to unscrew it from the fender and place it on top of the engine before driving there in order to make sure it is hot when tested.
They tend to fail when hot and it gives you a chance to examine the potting (plastic sealant) on the underside for signs of melting or boiling.
The ignition timing will need to be checked with a timing light.
This is a basic test and the emissions compliance sticker on the radiator support will have the info. (usually something like 8-10* BTDC at 750rpm with the vacuum advance disconnected and plugged)
I assume the sticker will still be there if you are in Canada... You are reading in Kph?
To see if the powervalve is blown out look down into the primary throttle bore with the engine warmed up and at idle. You should not any raw fuel pouring out of the annular boosters.
If this is the case you will likely have lots of black smoke, fouled plugs and popping on decelleration.
The extra fuel will also cause excessive engine wear by washing any lubricating oil off the cylinder walls, so this should be addressed right away.
This is a start, and my first thoughts on your symptoms.
See if you find the problem. If not then more specific questions and answers will get to the root of it.
It is a simple engine and very reliable.
Could well be, but how does this explain the backfiring and timing being off?
I did not say it was the only problem, but the speed limited to 80 kph is a classic weak coil issue. The back firing could well be a tune up issue and he should do that anyway, anytime you buy a used vehicle from an individual. Cap, rotor, plugs and wires. Then see what issues the truck still has. I doubt very much this is a carb problem. Timing being off could well be the PO screwed up or the timing chain is bad and it jumped timing, which could cause all his problems and the PO screwed him in that case.
I didn't say that it was a carb problem.
I said that if it had backfired through the carb it would likely blow out the powervalve and that would cause its own set of problems.
(rich running, popping on decelleration and cylinder washdown)