I just got the carter YF carb back from the rebuilders and still it stumbles and backfires at half throttle and above. I dont know whats next I really getting POed this jeep has been all redone and it still refuses to run right. The engine runs like a top when idleing an starts great but when it reaches a point it stops accelerating and falls on its face. It reminds me of a 1970s chevy truck with the vacuum advance hose left off the carb runs great to a certain point then stumbles and backfires Im at a loss on this one carbs have never been my strong point in working on engines.

Any chance of swapping with a friends working Carter WO to see the differance?
The Carter YFs were used on F head motors.
Matt

Some Lheads used the YFs, like the ones from the DJs.

It could be a Vacuum leak, worn distributor, or like you said, it isn't advancing. Have you checked the timing?

If it is idling well, then you have no vacuum leak. I'm in the camp with the distributor problem. Gummed-up flyweights or worn posts they turn on, or springs that have lost their tension.

RH,
  You are describing the exact same issue I was having.
  You have to pull the oil pump and move its gear a tooth.

Which I must add, is a real pain in the rump on a fully assembled Willys.  I ended up pulling the exhaust pipe off the manifold.  This gave me a little more room, but it was still tight. 

I will add that once I was able to time the engine correctly, it ran like a fresh rebuild.

Tim

I was turned all the way to the right also.

It's been a while, but if memory serves, I turned it counter clock wise...peering down the distributor hole

or

Clock wise if you are looking up through the oil pump hole.

This is a helical gear on the oil pump.  The trick is to move it one tooth, (which I think represents about a 30 degree change) but the helical style gear will move when you re-insert it and it make contact with the gear on the camshaft.
Same thing when you pull it out.

I had a tendency to think I moved it one tooth, but it kept on ending up in the same place.  !%@$ helical gears!!!

Tim

Randy,
You are correct. You have the original distributor set up.  RH and I do not.  Ours are likely replacement distributors for later Willys.  The mounting hole in your mounting bracket is a mounting and adjusting slot for me.
 
Here is a post at the 2A page while I was fighting this battle.  It may be helpful to you and Ranchhopper.  Pay particular attention to the responses from Sean.  He is the guru on L-134 engine timing.

http://www.thecj2apage.com/forums/per-faq-l134-ignition-timing-by-sean_topic2631...

Tim

I'm not familiar with the Disty on a CJ5. 
This was the best photo I could find of my current distributor. ( For some reason, I neglected to take close up photos of this side of my engine. )

What do you think?  CJ5 Distributor?

Tim