Engine spits from carburetor and pops through the exhaust is this timing or carburetor?

check and see if intake valve seat is loose as motor gets hotter condition worsens

By the term "backfire",are you referring to the engine spitting back through the carburetor or popping through the exhaust system? You also didn't tell me what type of carburetor the bike is equipped with. A lot of the owners of early Sportsters didn't like the original carbs and replaced them with aftermarket carbs. Since the carburetors use different starting circuit designs, it makes a difference in how the engine is started. It really makes no difference and I've heard many bikes that do either or both of these things when starting. What does make a difference is how easily it startes and how it runs AFTER it starts. If it's simply a spit or a pop, this is not unusual. Does the bike run the way it's supposed to? If not, that's what we need to look at.

Good Luck
Steve

Aftermarket pipes tend to lower back pressure, this will cause the fuel/air mix to lean out and result in the "popping" you hear---Backfiring, is extremely rare these days with electronic ignitions and the high reliability of engines. Real backfiring means the engine is not properly timed and the fuel/air mix is getting a spark and ignition at the wrong time (when the intake valve is open). Real backfiring is actually the bike "spitting up" in a sense, backwards through the carburetor or intake.You can check for exhaust leaks up around the cylinder heads by putting on some gloves and covering the exhaust tips momentarily. I have seen exhaust leaks that will make the popping worse by allowing air to be sucked in during deceleration...Hope this helps.

"Backfiring" is a pretty broad term depending on where you live. Some people refer to backfiring as popping from the exhaust while others refer to it as the engine spitting through the carburetor.

I'll assume that you're talking about popping out the exhaust pipe. If it does this after you rev the engine and it backfires on the way back down to idle, this is typical of an exhaust system sucking air. The mixture is extremely rich under these circumstances and will not ignite. But, if your exhaust system is sucking air, it combines with the fuel air mixture already there and bring it to an explosive mixture thus the backfire.

You can check you valves but I've always found that if the valves on an Ironhead are too tight, it's extremely difficult to get it started due to the low compression on that cylinder.

Check you ignition timing and points setting. Also, what kind of condition is your mechanical advance in the distributor in? I'm assuming this is an XLH model and not the magneto equipped XLCH.

What type of carburetor is on the engine? Hopefully not the original Tillotson that it came with. If it's an S&S, the low jet should be a 28 and the main jet a 66 to 70.