Hi. I've got a elderly Yamaha Yb100, as a project. It's in a pretty grotty state, and its more a nostalgia driven thing than a genuine good idea. I was amazed when I connected it to a small header tank of fuel with the ease it ran, and ran well, ticking over like a sewing machine, pulling as well as I'd expect, stopping on the key, restarting first or second kick. . Then it stopped, and hasn't run since. Ive checked the spark, which is very good. I've checked the fuel pipe, and checked the carb. I whipped the plug out and squirted fuel down the hole, then replaced the plug, and on kicking it over it fired then stopped, presumably as it used the fuel in the cylinder. Figuring it to be a carb snag, I've had the carb in bits looking for grime and dirt, found none and rebuilt it. Still nothing. Had I not had it running I'd have lost my patience by now and stripped it for spares, but I've heard it run, and actually ridden the thing. It was run on premix, as the oil tank is disconnected at present. Thanks, Ian.
sounds like you might have a crankcase leak somewhere that maybe stopping the charge of fuel from being drawn in. You can confirm it's fuel by (carefully) using Bradex Easystart to spray directly into carb mouth and then trying the starter immediately after.
I tried spraying neat unleaded into the carb mouth when kicking it over and got nothing, but ill have a look under the crankcase for leaks... Thanks!
I'm not up to speed on the YB, I'm guessing like most Yams it's reed valve intake. Is the carb getting fuel? Try a fresh spark plug, don't rely on cleaning the old one off. If it ran sweet and ticked over nicely I'd doubt it's a crankcase or crank seal failure. The crankseals can harden and give you problems but if it seemed ok it probably is. What premix % are you running? Have you checked for a nice fat spark? Little 2 strokes are so easy to work on it won't be much work
crankseal failure can happen at anytime and there's not always any rhyme or reason in it - poster spoke of checking testing for fuel and spark but no luck so symptoms certainly agree..
Crank seal failure will always in my experience lead to an increased idle and reluctance for the revs to drop down due to the weakened mixture.
Almost every time I've had a running issue on a carb'ed bike it's been a blocked pilot jet. I know you said you've checked it, but it's something I'll check 5 times nowadays before moving on as I've spents days in the past just to come back to it and find the issue.
