I got my first bike ever and it's an oldie (but a goodie). The engine is a 1972 XLCh Iron Head. I'm having backfiring in town and on the highway. Plus its choking itself out and falling on its face when I'm going down the highway. When it chokes it's a sputtering sound and I loose speed. Sometimes it's a mile or two before I can get it back up to speed. If I go down a hill it runs better. But when I'm going 55mph and it sputters I can give it full throttle and it still just sputters and looses speed. Before to long it snaps out of it and the bike will run good for for a little while without sputtering but it just does it again eventually. I replaced the manifold o-rings and clamps but that didn't take care of it.
Before I take the manifold back off and re-do the o-rings and clamps I was wondering if bad Spark Plugs or Timing would cause this?

 

Tilletsons !!!

 

Was the bike in use when you bought it?
Or was it in storage for a while?


Check your battery and connections, a good time to clean them!
Make sure the connection at the frame doesn't have rust or corrosion.
Make sure all cables are nice and tight and clean at both ends.

If that's not it check your carb jets.

An old trick to check for sucking air at the intake is; while motor is running at idle, give a (VERY LITTLE SHOT) of starter fluid aimed at the intake.
If you heard a Rev pickup, then you got an intake air leak.

Did you have any luck getting a service manual yet?

chappy

 

'72s came with Bendix carbs.

 

Chappy covered some stuff, and here's some more:

Check/clean fuel filter. Set points & timing (you can get close enough to run on the timing by setting the points backing plate in the middle of the slots). Adjust valves (pushrods). Pull carb off, strip & clean, blow jets & other air/fuel passages (no wire!), reassemble making sure to set float bowl level.

Manifold anti-leak trick: When you are installing those dang o-rings, roll ‘em up onto the head spigots, but make sure they don’t twist when you slide them into the gap. Then before you put the clamps on, take a wrap or two with some electrical tape around the o-ring. Use some good tape, not the el-cheapo crap from the bargain bin. I like either Scotch 33 or Scotch 88. 33 is thinner and more flexible.
Some folks don’t like the tape routine. It does goo things up as the tape gets warm. And that is one reason it helps seal things. The next time you pull the manifold off, it easily cleans up with some good ol’ gasoline.

 

I found an original 1972 AMF Shop Manual for the bike on ebay and I'll be ordering the newer Clymer Service Manual soon. The battery is brand new and the cables are good to go. Is there any special trick for keeping the o-rings in place when installing the manifold and clamps? I'm wondering if I put them on wrong?
Otherwise... there is a small rod on the side of the carb that travels up and down when I give it throttle. This small rod has a rubber boot around the bottom of it. Gas is leaking from that boot when I give 'er throttle. I wouldn't think that leak would cause it to backfire and cut-out though.
I'm a newbie but I just want to ride this damn thing again. So far the only good ride I had was the three hour ride home when I bought it. Every since then the sputtering and backfiring has gotten worse.

 

Still looking for help with problem...

 

I had the same problem with my 71 iron head. When in forth gear and hitting the throttle hard it would loose power as if it was governed (sp). If I backed out of the throttle it would come out of it. During slow acceleration in forth I would not experience this problem. I found the problem was the S&S carb was not getting enough gas due to a plugged pet ****. I cleaned it out and have no problems since. Mine did not back fire though so don't know what to tell you. I had alway been told the back firing could be caused by bad valves. Does your bike start easy ie..less than five kicks? If not, check your push rods. Set them when the bike is cold. Have no up and down play on the rods when set and allow them to spin freely between your fingers with no binding.

 

Sounds like it`s flooding to me, are the plugs black, does it smoke black out the exhaust. If it`s flooding it will back fire and cough.
Are you running champian spark plugs, i used too have a 72 sporty that wouldn`t run on champians. Try NGK`s or another brand plug. Maybe your floot level is set way too high or the needle and seat are bad which will make it run real rich. Are you sure the choke is all the way open?

 

I had the same problem with my 1981 Ironhead.
Replaced the intake manifold, and also replaced
the pointsby a CompuFire ignition.
Runs like a devil now !!

Greetz Peter