Backfiring problem

 

Hi all,
    I have a 1978 Yamaha XS750SE. Last fall I started it, and while warming
up, it quit. When I try to start it now, it backfires (loudly). I would like
to know what the causes of this might be and what to look for to cure this.

Thanks

 

I had a similar problem it was dirty carbs,however it can also be caused by
faulty ignition

warming

 

wrote:


dump out the shit gas. flush the lines with clean gas. flush
the tank with a little clean too. then clean the
carbs..yeah, all 3.
then check the points to make sure they are good and set
correctly.
then go ride.
btw, check them sidewalls...are the tires ok?

 



Sounds like flooding, the carb needle valve is sticking , need to drop the
bottoms of the carbs to check!

 



Backfiring is generally caused by one or more of three possibilities:
- Raw gas hitting a hot exhaust system.
- Timing either firing the spark plug while the intake or exhaust valves
are open.
- Bad valves.

You can diagnose for bad valves with a compression test.
You can diagnose for timing by checking the manufacturers specs. for
degrees off top dead center.  Knowing if it is backfiring through the
carb. or the exhaust will help indicate timing.
Raw gas in the exhaust will indicate carbs.

Good luck.
Serge