Two things I would check:
1. intake: manifold leaks, carb needle,seat, & float height
2. ignition: points, coil (check for weak spark)

 

I would buy a newadjustable main jet Bendix carb for it and then the carb issue would be eliminated.[/align]

 

You're bike sounds just like mine did when I first started riding it. I didn't have much trouble with backfiring, but definitely sputtering at high speed.

Part of my problem was the loose intake o-rings. Make sure you tightened those sufficiently, and also make sure you didn't tighten them SO much that they got crushed and pulled off the spigots anywhere (I've had both problems in the past).

The other thing that helped me, as I was running open drag pipes, was to put a lot of restriction in the pipe. What kind of pipes/mufflers do you have?

Before all this was straightened out, I felt the bike was running so poorly that it was draining the carburetor at speed, and then running lean, hot, and popping. This will cause a terrible-looking white/grey deposit on your plugs. Take a look at them, too, and see what they tell you...

 

And if none of the above helps pull the points plate and check the advance fly weights, look for broken springs and wear in the pivot points and that they open freely. Slip cam on the shaft and see if they`er operating correctly with out bind and close all the way.

 

ok so my iron head spits pops backfires through pipes around 50 mph and iam only getting around 150 miles out of a set of plugs i can run around town all day under 40 mph 50 55 it starts acting up let off it clears up please help

 

Let's think..Isn't backfiring caused by a: either timing, where power stroke forces combustion out the exhaust too early. And/or b: unburnt fuel ingniting in the exhaust? Losing power can and is related or may be a totally different prob. Just an opinion.

 

When you put intake bands on you have go loosen cylinders and heads to line up intake then tighten back after yoy get the intake on,also a lean condition will make it backfire

 

My very first Harley was a 1972 XLCH that I bought new.

I would do a complete service including but limited to check advance and replace as necessary as if they are not serviced regular the wear out quick. If the bike has been sitting up could be froze up.

Replace points and condenser and make sure you set the point gap correctly and check on both lobes. If they are off more then a a few thousands you will have to tap the lobe cam till they are real close to the same.

Adjust the lifters all this needs to be done before you try to do anything to the carburetor as all this has to be right to be able to adjust the carb. Yes check for manifold leaks but I like to use WD-40 as it is not as dangerous as starting fluid.

May years ago I had a guy bring me a iron head XLH. He said he just put new points in and set the time and the carb needed rebuilt as it would not stay running unless the choke was on. So I did and it did not help the problem any so I asked him does he what me to do something else to fix or would he like for me to find and fix the problem. Well he had the points timing and the spark advance was shoot. So redid all he said he did and it ran like a top.

I have worked on hundreds of iron head sportsters. I do not recommend a Bendix with an adjustable jet from experience.