hello. i just bought a 81 ironhead sportster. it runs and drives with some tlc i put into it. short trips up and down the road...less than a mile or so the bike does fine. if i get into forth gear and stay there for a extended period of time going down the highway. sounds like a backfire and a dramatic loss of power. and sounds like front cylinder is not running right. if i stop and let it idle, it will idle high then drop down and the front cylinder picks back up. or if i shut it down, i can restart it and it will be fine. it had the s&s super carb on it with drag pipes. i do not believe that there is any baffles in it. i already changed the plugs and got the same result. any feedback would be appreciated. thanks mike

 

Not sure about your problem, but I do know that the ironheads have valve train problems. You have to run them lose or the burn up valves. You might have a valve sticking open after the engine warms up and everything expands. You need to get it to act up and check for spark and compression while it's acting up. I'm not a prof mech or anything, just have a family member that has a 79 ironhead that we have done lots of work to over the years.

 

Check the intake seals for leaks, and then you can perform a compression test, and a leakdown test to pinpoint the issues.
It COULD be a valve sticking in the guide, if it was rebuilt, and the valve/stem clearence was established incorrectly.
Scott

 

i have herd that the drag pipes can throw off the pulsing of the carb. is this someting possible that can be contributing or just a myth. there is not any baffles in the pipes yet either. i jetted down the carb because the plugs were pretty sooty. thought maybe up top was getting too much fuel. but havent taken out again to verify. as soon as the front cylinder falls out, if i shut it down and restart it right away, it runs fine for a few minutes and does the same thing again.