I have a 1977 xs 750 yamaha 3 cyl. was having trouble starting it . so i change the plugs and points and cond. now some times it will starts and some times it wont , but when it does run it back fires I have a 1977 xs 750 yamaha 3 cyl. was having trouble starting it . so i change the plugs and points and cond. now some times it will starts and some times it wont , but when it does run it back fires and when it geys hot it will not start in till cools down

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Hi, Harry and the usual suspects are:
1. Spark plugs in bad condition or have an improper gap or are partially fouled.
2. Spark plug cables in bad condition.
3. Battery damaged or nearly discharged, should have 12.5 volts.
4. Check battery terminals for damage or corrosion, check the battery cables at "BOTH" ends for loose, corroded, or broken connectors, "INSIDE" and outside the cable harness, perform connector wiggle test and check cables with an ohmmeter if necessary.
5. Throttle controls not adjusted correctly.
6. Water or dirt in the fuel system.
7. Water or dirt in the carburetor.
8. Faulty fuel pump.
9. Intake air leak.
10. Fuel tank vent hose and vapor valve plugged, or fuel line closed off, restricting fuel flow.
11. Enricher valve inoperative.
12. Engine lubricant to heavy (winter operation).
13. Ignition is not functioning properly (possible sensor failure).
14. Faulty ignition coil.
15. Valves sticking.
For more information about your issue and valuable "FREE" downloads that you will need please click on the blue links below. Good luck and have a wonderful day.
http://pnwriders.com/threads/xs750-engine-wont-start.49752
1977 Yamaha XS750 Electrical Problems Tech Corner Blog Motorcycle Classics
Yamaha XS750 2D Supplemental Service Manual
OEM parts for Yamaha
http://mybikemanuals.com/yamaha/yamaha-xs-owners-manuals
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