gasgas1 Posted September 25, 2008 Report Share Posted September 25, 2008 Hi my jt 250 won't start it just backfires every 5th or 6th kick,anyone help Quote Link to comment Share on other sites More sharing options...
jse Posted September 25, 2008 Report Share Posted September 25, 2008 Hi my jt 250 won't start it just backfires every 5th or 6th kick,anyone help I'd check these things first: The fuel/premix oil mixture is fresh, absolutely fresh, as fuel will turn "stale" (loses it high-end aromatics that aid starting) when stored in a vented bike fuel tank (or a plastic fuel tank in the garage) for any length of time. The plug is new, not just "look good", and set to .020" (for the older GG's with MotoPlat) or .024" (for the newer Ducati or Kokusan ignitions). The flywheel Woodruff key has been checked and the flywheel fastener is torqued to about 32/34 ft lbs. When starting, flip the "choke" lever on the carb up and do not open the throttle when starting as opening the slide will defeat the enriching circuit function. The carb has been cleaned, jets blown out with compressed (from the end with the slot for the screwdriver as that's the narrow end of the "funnel shaped" jet and a piece of grit sometimes will only be removable from the "big end" of the jet hole). I'm guessing that it probably is a sheared key on the flywheel hub or clogged pilot jet, but when giving advice for "won't start", one has to make a bunch of assumptions as to the conditions of a lot of other components. I'd try these first and let us know if the engine makes good sounds. Jon Quote Link to comment Share on other sites More sharing options...
peterb Posted September 27, 2008 Report Share Posted September 27, 2008 Hi gasgas1, After doing all of those things that Jon suggests, particularly with respect to the possibility of a sheared flywheel key, it may be the HT coil/ign unit has expired. These are the same as fitted to quite a few bikes of that era, a Ducati system. After all the previous checks, you can try a friends unit if possible, to check out. The other area to look at is the trigger unit on the stator plate, this is the same as supplied to Vespa and is not available as a separate part through GG, but is a Vespa part, I think it is called a G200 and goes straight in/on - if not G200, it is very similar and not expensive. Bye, PeterB. Quote Link to comment Share on other sites More sharing options...
gasgas1 Posted September 27, 2008 Author Report Share Posted September 27, 2008 Hi cheers 4 that ive had the flywheel off & tinkerd abit & the flywheel key luks ok but i noticed the key doesn't stick out very far anyway i put it all bk together & 3rd kick it started but only ran 4 bout 30seconds so i tried again & its bk 2 backfiring again so i think the flywheel could be slipping rnd on the key. Quote Link to comment Share on other sites More sharing options...
jse Posted September 28, 2008 Report Share Posted September 28, 2008 Hi cheers 4 that ive had the flywheel off & tinkerd abit & the flywheel key luks ok but i noticed the key doesn't stick out very far anyway i put it all bk together & 3rd kick it started but only ran 4 bout 30seconds so i tried again & its bk 2 backfiring again so i think the flywheel could be slipping rnd on the key. The key does not hold the flywheel to the tapered shaft. It is only used to align the flywheel when installed. What keeps the flywheel from slipping is the special machining of the tapered shaft and flywheel hub. The flywheel hub is bored with a slightly (.001") different taper so that the hub is what's called an "interference fit" with the taper on the crank, which is why you need a puller to get it off (you wouldn't need a puller if just the key held it on). Sometimes, when the hub spins on the shaft it will ruin the surfaces of the tapers, and my guess is that if you look closely you will see the machined surfaces now rough rather than smooth, which means that, besides the loss of the inteference fit, there is less contact area. Short of replacing the flywheel and crank, you could try this: take some fine grinding compound and put it on the taper and install the flywheel and turn it to smooth out the surfaces until the contact areas are relatively uniform (bed the hub and shaft together). Then some very thin copper sheet (almost like foil) can be cut to put on the taper (this will fill in the irregular contact areas), use some red Loctite and torque the flywheel to about 40 ft lbs and let the Loctate cure. I'm sure others have some good ideas as to how to repair the situation also. Jon Quote Link to comment Share on other sites More sharing options...
peterb Posted September 28, 2008 Report Share Posted September 28, 2008 Hi Jon, You beat me to this one! I was just going to suggest that the woodruff key may have already been sheared as there should be approx 3mm sticking out proud from the shaft when fitted. I just had this problem with my project 330 bike. I ended up lapping in the flywheel to the shaft using a fine grinding paste and oil and used a new key, all ok. Make sure that the crankshaft nut has the correct locking washer fitted or the nut can vibrate loose with high revs. Bye, PeterB. Quote Link to comment Share on other sites More sharing options...
jse Posted September 29, 2008 Report Share Posted September 29, 2008 Hi Jon,You beat me to this one! I was just going to suggest that the woodruff key may have already been sheared as there should be approx 3mm sticking out proud from the shaft when fitted. I just had this problem with my project 330 bike. I ended up lapping in the flywheel to the shaft using a fine grinding paste and oil and used a new key, all ok. Make sure that the crankshaft nut has the correct locking washer fitted or the nut can vibrate loose with high revs. Bye, PeterB. Hi Peter! The locking washer is important. Also, sometimes, after lapping, the tip of the tapered crank will stick out a little too far from the hub surface due to the fact that it is now imbedded further into the hub and the fastener (nut/bolt) will bottom out against the step at the bottom of the threads of the crank tip without exerting enough "pulling pressure" and the flywheel will come loose quickly. You could use a washer with a center hole larger than the hub hole that will, in effect, extend the outer surface of the hub and the pressure will be transfered to the taper rather than the step where the threads meet the taper of the crank. The thin metal sheet/foil trick (brass will even work better) will "restore" the thickness of the taper so the tip will not be extending out too far and it will also make the surface contact area better as often the hub and crank surfaces will be really chewed up with a lot of pits and gouges, which greatly reduce the contact area. The thin sheet metal sleeve is an old trick from the 1950's as we often used 30's and 40's Ford "banjo housing" differentials in our race cars and they had axles with tapers on the end that fit into the rear hubs with shear keys and were noted to often spin the tapers in the hubs under high-horsepower applications. Jon Quote Link to comment Share on other sites More sharing options...
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