The Stovebolt Site Forums The Shop Area Engine Shop 235 adjustments

235 backfiring through carb and jerking and missing, especially under load, what do I need to do? Is it timing?

This very thing happened recently to a bolter. I believe his turned out to be in the distributor....a good place to start. The thread is not very old so maybe a few topics down you will find it.

Timing can do it, but so can a lean condition. Lean (insufficient fuel) can also cause a miss. A lean mixture burns slower which means combustion can still be underway in a cylinder when the exhaust valve opens. This ignites the charge in the intake manifold intended for the next cylinder in the firing order - and you get a backfire through the carb. So check your distributor/timing as suggested, but also look for something that is causing a lean mixture (clogged jet, low float level, etc)

I am a start with the basics kind of guy. Pull the plugs and make sure not fouled or cracked. Just for giggles, do a compression test. See if anything is all of the sudden way out of wack. Next, I would bring the motor up to top top dead center or where ever the spec is for no. 1 to fire on your motor. Trace the no. 1wire to the distributor cap tower and put a chalk mark under the no. 1 wire tower. Remove the cap. The rotor should be pointing at the chalk mark or pretty darn close. Check the cap, make sure it’s not all gummed up or filthy or just plain worn out. Check the point gap. How old is the coil? Vacuum advance is next, make up a little adapter and rubber hose arrangement so you can sort out whether the diaphragm is bad. If it is, you have both a bad vacuum advance and a vacuum leak. You might also pull the valve cover and just make sure no bent push rods, broken push rod cups, of flat cam lobes (no lift on one or more valves.

Do a valve adjustment, then a compression test.
Jerry

Ian, I know the engine was relatively “new” to your truck. Did it run fine for a while? Or has it acted like this since installed?

It ran fine for a while and then past few days it got cruddy, I played with timing but that didnt a lot, replaced coil so that's fine. I thought my next thing is to check for a vacuum leak and I dunno what else

Have you checked your fuel tank, line, fuel pump, fuel filter and carburetor for sediment? My truck behaved similarly, I even had to tow it home once, as it would not pull even a modest hill for missing, bucking and backfiring. Even a tiny amount of rust, or sediment in the system can cause symptoms you describe.

this is what i think happened. it explains the sudden onset of symptoms.

When I changed the engine I didn't put a fuel filter, back cause my fuel pump has a screen in it, I'll put a filter in a see it that helps

If it’s already bucking and acting up like that, might need to figure out where the blockage or restriction is and flush it before the filter will help.

Engine off, looking down carb, do you have a good strong squirt of fuel, if you open the throttle?