Sputter and then backfire issue

[OEM1984CJ7]

Ok, 84 CJ7 with AMC 258 i6 / 4.2l 4pd. I have had this problem since owned but seems worse now. With the original Carter and ignition setup when I get down to about 1/4 tank on gauge is starts sputtering like it is out of gas. I pump on the pedal and it sputs and sputs and I stop and turn it off or add fuel and it seems to run fine. Done this for a couple of months. I run 4-5 tanks of gas through it with no problems. I then change the ignition system to the TFI with the ford swap and also replace the carter with a MC2100 and runs fine for another 3-4 tanks. Then coming home from work tonight with just under a 1/4 tank is starts sputtering under a load uphill in 4th , then I downshift and it stops then starts in 3rd. I pump the gas and it runs fine again for a minute then sputters again and then backfires out the tailpipe. It does this for about 10 miles home on backroads and I stop and fill the tank to almost full. Starts right up and then starts sputtering again before I pull away from the pump. All the resot of the 5 miles home sputtering and backfiring out the pipe. I get it home and raise the hood and it idles fine but when you idle it up just a tad and hold the speed it starts sputtering again. You can rev it high and it does fine but holding a steady throttle it sputters.

I still have the elec fuel pump in line the PO had so I plug it in and it still does it either way. I have never seen this before and kinda lost.


any help? Is the fuel line clogged/or tank trashy? It seems to run a little better with the manual choke on and not sputter but have not tried to drive it that way yet.


any ideas? sorry so long post.

 

[OEM1984CJ7]

Nobody??

 

[OEM1984CJ7]

So I get in it this morning and drive 15 miles down the road and it runs perfect like nothing is wrong. Its really wierd. I ran it up to operating temp figuring it might be some type of vapor lock but it runs fine. Hmmm

 

[BusaDave9]

You may be right about vapor lock. It sounds like the problem only happens when your engine temp gets up there.
What confuses me is the backfire. That could mean a timing issue. You might want to get out the timing light.

 

[Old Dog]

It might be running out of gas, cj gas gauges can be off sometimes.
It can backfire if it running way lean (no gas).