When my 76 XS750 is cold, it will only start on the starter. No luck on the kick starter.
When it's warm, it will only start on the kick starter. No luck on the starter!
How odd is that?
Any ideas?
It seems to turn on the starter at a reasonable pace. I'm going to clean up my grounds and see what happens.
Well, this is awkward. Someone asked a motorcycle blogger about idle issues on his XS750 and described what is happening EXACTLY! Unfortunately, the blogger didn't really have much to say. I can't blame him, it's a hell of a question.
http://www.motorcycleclassics.com/tech-corner/1977-yamaha-xs750-electrical-problems-zbmz12jfzbea.aspx#axzz33zhEINrz
Q: I own a 1977 Yamaha XS750 with about 25,000 miles on it. For the past two years the bike has gotten more and more stubborn about starting, and the idle is getting rougher, too. The starter turns the engine just fine, but it won’t start the bike. However, when I kick it, it starts right away. Until it is fully warm, it will idle at 500rpm, stuttering and stalling unless I keep the rpm’s up with the throttle. Once warm, it will idle at 1,100rpm, but if left alone for a couple of minutes it will slowly stumble and stall.
Sounds like a combination of fuel and spark.
Fuel when cold. Not rich enough to fire cold when kicked over, but close enough that the extra cranking on the starter gets enough fuel to fire it. That could also be the roughness until warm.
Spark when hot. I would look at the coil(s) and check resistances both warm and cold. The coil may be marginal, and the resistance could be increasing when hot. The reduced voltage due to the coil while cranking may not be enough to generate a good spark. When kicking over you don't have the voltage drop from the starter.
The coil could also cause the problems when idling, The longer dwell at low RPMs can cause the coil to get hot and start to miss, but higher RPM with shorter dwell times allow it to cool off hiding the problem.
Cleaning the carbs is cheap, checking the resistance on the coil should be easy with a multimeter. Also check for cracks in the coils, moisture could cause the roughness when cold.
You could also use a spare plug to check for spark when cranking it over when warm. That way the plugs stay in the engine and the starter still sees the same drag during cranking.
jstand wrote: When kicking over you don't have the voltage drop from the starter.
This is a brilliant observation.
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