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Ironhead Sportster Motorcycle Talk (1957-1985) For all those that wanna talk about Ironhead Sportster Motorcycles

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  #1  
Old 1 Week Ago
steve lewis steve lewis is offline
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72 ch, bendix carb, starts great, idles fine , As soon as I accelerate, motor stumbles and back fires though the exhaust. At speed , 40mph, smooth, If I accelerate, stumble and back fire. I have read every post in can read, New battery, new points , new plugs, new wires, new coil, old condenser, new fuel filter, I restored the bike 5 years ago, and have not ridden it much. Never did anything to the carb, so should I rebuild the carb, I am thinking dirt or grim. Also tested intake for leaks with propane. Tight. I have the FM from 72, Any suggestions, or guidance, or any help what so ever would be great. steve
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The Doctor71 The Doctor71 is offline
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It wouldn't surprise me if you had multiple things going on.... but where to start ? My troubleshooting 101 philosophy starts with..... (in a word)

Ignition. I'd suggest read & follow the pointers in this thread..... http://xlforum.net/forums/showpost.p...05&postcount=9

Get your ignition on the money first.... then follow the evidence to the next problem. Your motor will tell you what to do next.
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I agree that the ignition should be tuned first. After that and assuming the problem is still there then yes I would get serious about the carb. Could be the accelerator pump shot is either not there or it could be anemic. The backfire is a typical sign of going lean. The jets could just be plugged some too. Could also have an air leak at the manifold. You can see the accelerator pump shot by just looking in the throat of carb and snapping the throttle. It should shoot straight at the needle.
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Not familiar with the Bendix, but if it has an accelerator pump, that would be my first check...and I usually pay most attention to electrical issues.
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"old condenser" could be an issue.
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Quote:
Originally Posted by doodah man View Post
"old condenser" could be an issue.
Not likely, capacitors either work or they don't, and when they don't they burn points almost immediately because their sole purpose in life is to eliminate the arc from points opening. Kind of like a sponge for catching the arc. You are better off not replacing a capacitor that is working, because it's more likely for a new one to fail right out of the box than have a good working one fail.
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I'm thinking drag pipes
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Quote:
Originally Posted by wedge View Post
Not likely, capacitors either work or they don't, and when they don't they burn points almost immediately because their sole purpose in life is to eliminate the arc from points opening. Kind of like a sponge for catching the arc.
In over 50 years of wrenching, I only found one bad 0.22 mfd capacitor.
It was on a positive ground MG.
One spark, then no more.

It is possible for the foil layers to short, as with a coil, but very rare.

Motor start capacitors are used for starting surge and run capacitors for efficiency.

Ignition point capacitors actually have a dual purpose:

1. As Wedge already stated, to reduce the effects of arcing as points open. Interrupting DC is not as easy as AC, since there is no sine wave going to zero 60 times per second. So the capacitor (condenser) absorbs that.

2. When the points close, to energize the coil quickly.
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steve lewis steve lewis is offline
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Thanks Guys,
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Thanks Jerry. I tend to forget your point #2.

In practice, like yourself I can only remember finding one bad capacitor and if memory serves correctly, it was the same as you describe, no spark at all from the coil. My assumption is that it must have been internally shorted to ground.

Back in 72-75 I worked for American Optical in New Hampshire and I used to do sample testing on the electronic parts we brought in. That is where I saw a large number of capacitors that were bad from the factory. After that, as a Mechanic I quit replacing condensers automatically with a tune up.

Bottom line here Steve is that the backfire is leading us in the direction of the bike going lean when you hit the throttle.
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