Question about 1972 Yamaha XS2

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Dead pw 50 and sad boy.

Bike was running but would quit every once in awhile and then we reved it up and quit like it ran out of gas. took tank off and cleaned took carb off and cleaned.have fuel and spark but wont fire.could it be reed valves?how do you check without tearing them out? has compression

Posted by 5eikums on


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Many things could cause the problem. Let me see if I can help out.
You have spark, but is it enough spark? A weak battery can rob power from the coil so only a weak spark is present. Get a new stock NGK spark plug to eliminate the possibility of a problem plug.

It is good that you cleaned the carb because that would be my second target. Reset the air and idle screws. IMPORTANT > do not tighten these two screws down. Only screw these in until they LIGHTLY seat. Now turn each screw one and one half turns outward. Clean or replace the air filter and be sure the bolts are tight on the carb manifold. INSTALL AN IN-LINE FUEL FILTER. Let the float bowl fill then start the engine. Set the idle speed with the idle screw.

Remove the cylinder head. run the piston down to the bottom of the cylinder. look inside the cylinder. On th carb side (intake) you will see an intake port > a large hole that lets gas into the cylinder. Now look at the exhaust port. It is possible the exhaust port is pretty plugged up with carbon. The intake and exhaust ports should be approximately the same size. Are they? If not then remove the muffler and take a screw driver and plastic hammer and knock the carbon into the cylinder, thus enlarging the exhaust port to its' proper size and allowing the exhaust gases to escape. Vacuum the carbon out of the cylinder. De-carbon the piston top and the cylinder head.

Get back to me and tell me what you find. If more is needed we will go further.

Posted on Apr 12, 2009

  • tombones Apr 16, 2009

    Is the bike running now? What did you find was the problem?

  • richardgwynn
    richardgwynn Sep 18, 2012

    tombone id quite agree nice answer. !!



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