I have a 1967 Harley XLH Sportster. I rebuilt the motor, but when I started it, it backfires out the rear cylinder, any suggestions. I know the cams are all lined up the right way. NEED HELP!!!
"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.
I have a similar problem with an 1967 XLCH with a magneto. I do not have a mechanical advance,
The rear cylinder is miss firing. It was not firing at all until I changed the spark plug and then it ran better but still miss firing. Seems to be also be pushing air out of the carburetor. at low idle. No back firing through the exhaust.
It's been a long time since I've messed with a magneto in one of those bike. First, you must make sure the mag is firing with sufficient juice. The magnets have a tendency to go weak over the years. Check your spark and make sure it a nice blue spark. Now, you are supposed to set the points at 0.020" and then use a "buzz box" or an ohm meter to set the initial time using the top dead center mark on the crankshaft. At idle, you will notice a little spit back out of the carb due to the overlap ground into the camshafts. If you rev the engine up slightly, this should go away. If it continues to give by a problem, check the compression. You should have 100 PSI or better. If not, you've got a problem somewhere. Check you valve adjustment and if you didn't have a valve job done on the heads, you might want to check them. Even with the magneto, the system is dual fire meaning that both spark plugs fire at the same time. The mag has a two lobe cam since it turns at half engine speed. Good Luck
Steve
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