i've got a 80 toyota 4x4 20r motor the other day i was coming up a hill in 4th gear when it started cutting in and out and backfiring, by the time i got home it would hardly pull it self up the slightest incline in any gear but 1 or 2.

So i just started checkin everything this is what ive done so far

new fuel pump
new fuel filter
new carburetor
new plugs, wire, cap, and button
new coil

so none of that helped ive checked the valves, did a compression test got 110psi on each, timings good, checked for vacuum leaks none found, ive tested the spark its all good. ive put new gas in, i though maybe the clutch was slipping so i put one in yesterday and it still acting up.

it will run just a smooth as can be till u hit 3rd gear and up, as soon as u get it under a load it starts cutting in and out and backfiring thur the carb and bucking and jumping, ive tested and checked bout all i know to anyones help would be great, thanks

 

i would check the fuel lines, i had the original on mine, it would plug up the stock fuel filter in no time, i always put on a really large filter on my trucks like off a 80s ford truck.

 

I wonder if there is a lobe on the camshaft that is worn or a valve spring that is weak causing your problem?

 

check the vacuum advance.

 

i had the same problem in my 4runner and it was the exhaust,turns out the cat was plugged.