Why is it backfiring

My 70 cj5 with a 225 v6 was running fine, I parked it in the garage for two weeks went to start it and all it does is backfire through the carb. I cant even keep it running. It's a repititious backfire that just won't stop. where do I start.

 

timing, plugs, wires, coil, did something make a nice nest in the exhaust some where?

 

Check the timing, timing gear might have given up.

 

I was fairly sure it wasn't a timing problem so I removed the valve covers. Number 4 exhaust push is missing in action. I could just see the end of it in the hole it passes through. I tried to get it with a piece of wire but lost it down the hole. I'll remove the intake manifold this weekend to see whats going on. The valve isn't stuck and looking through the pushrod hole the end of the lifter looks good, so what happend. Has anyone had a simular problem.

 

Over reved it??

Bad lifter, adjuster nut backed off, something kept the exhaust valve open while running and push rod fell out of place, lots of causes.

 

225 has hydraulic lifters so no adjusting nut. The push-rods are solid metal not hollow so the bending part doesn't happen unless something in the valve system forces too much tension on the p-rod.

 

I removed the intake manifold and it is a bent push rod. The lifters are brand new and lifter involved in the problem looks good with no visable damage. The valve isn't stuck, I can take a heavy wooden dowl and hit it with a hammer and it moves just fine. Is it possible that the push rod was just wore out from years of use and just finally gave up. I'm going to replace the whole bunch as long as I'm this far into it.

 

You say the lifters are brand new.. Had the motor been run after the new install?

 

The engine was just rebuilt, and I put it together from the short block to the long block. I put new lifters in it but used the old pushrods. I only had about 15 minutes of running time on it when the push rod bent.

 

I had two pushrods bend on mine on the same cylinder, but that was due to the engine setting for several years and those two valves had rusted shut. It was really my own stupid fault (in a hurry) and didn't pre-spin the engine by hand.