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I am getting a backfire from the pipes about one every couple seconds. My plugs are dark like I am getting too much fuel. This bike was sitting for many years... Here are the knowns... New cylinders and pistons, valve adjustment had no efect on the backfire. Compresson is 121, 123. Timing change had no effect on the backfire. The carb has been rebuilt though I think it may be the sorce of the problem. Any Ideas?
 

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Starts but runs irregulary or misses

1. spark plugs in bad condition, or partially fouled
2. spark plug cables in bad condition and "leaking"
3. spark plug gap too close or too wide
4. Circuit breaker points out of adjustment or in need of cleaning
5. condenser connections loose.
6. Defective ignition coil.
7. defective condenser
8. battery nearly discharged
9. loose wire connection at one of the battery terminals, or at coil or circuit breaker.
10. intermittent short circuit due to damaged wiring insulation.
11. water or dirt in fuel system and carburetor
12. gasoline tank cap vent plugged or carburetor vent line closed off restricting fuel flow
13. carburetor not adjusted correctly.
14. weak or broken valve springs.

This info comes from the 70'-74' Sportster service manual
part no.99484-74

Hope something here might help you. I guess you have made sure the choke is open. I thought there was something on compression in the manual but no luck.

Buzz
 

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Discussion Starter #3
thanks, I thought that sounded familiar. I got the same manual. When I find the prob. I will post it.
 
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