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I have a 1973 90cc G3 (vin # GA 359641). Approx 3500 miles. Appears to have good compression, but I have not put a meter on it. I have had it 15+ years sitting in dry storage shed. Took it out for my son and got it running. Replaced the fuel line and filter. It also needed a (crank?) seal replaced. Would run but smoked when throttled up (discribed to me as sucking oil from tranny into engine during throttle up?). Anyway, bike has run great for approximately 45 min-1 hour. Stops running. Is not seised up, but just like you turned the key switch off. However, the engine has visible spark when plug removed and cranked over with kick start. Lights do not work, I did not get a new battery (wanted to see how it runs before becoming further buried). It acts like no spark-i.e. no coughing nothing when it is kicked over. Fairly sure it is getting gas. Have taken plug out and blown off. Makes no difference still acts like the key switch is turned off. Would the spark plug still show a spark if the key switch failed? Is there a way the timing on these engines can jump or suddenly change? Checked throttle cable and is working. Would a short on the electrical system cause this? My understanding is 2-strokes don't have valves, could it not be getting fuel from the carb to cylinder (2-stroke equilvilant of valves not opening/closing)?
 

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OK having transmission oil leak into the crankcase would mean you had the main crankcase gasket replaced. Crankshaft seals would be replaced mostly if the bike seemed to be running poorly. These old bikes will smoke somewhat by their very nature if the oil injection is working properly, the possible exception being my old Suzuki that uses an entirely different oil injection system. That aside I sure haven't seen it all and I'm hoping that your little bike gets working good again.

I'm somewhat familiar with the old Kawasaki two strokes and they can admittedly sometimes seem to take on human characteristics sometimes, seeming to be "stubborn" on some days and other days "pleasant" to be around. Let me give you some steps to try as I'm not really sure of your experience level.

Before step one, try push starting the bike. Push starting in second gear is even advisable in this case as it will give you some time to try a few things like choking, throttle, and might generally help the magneto generate a hotter spark. After doing this once or twice do the following:

1) Start by spraying a small ammount of starting fluid into the air cleaner. I say small but don't be too timid or you won't get anything. See if you can coax the bike to "pop" on starting fluid. If you can get it to run really well on starting fluid but won't run without, then you have a fuel problem (doubtful in my book). If it doesn't run much on starting fluid then it's probably an ignition problem. I'm betting it won't run on starting fluid so the next steps are for this case:

2) Try to evaluate the stregnth of spark here. It should be a nice blue spark. If it isn't try adjusting the points (behind the flywheel is my guess).

3) Try replacing the condesner if you're sure the points are correctly gapped, cleaned, and dressed.

4) After that check the coil wire - sometimes wiggling the coil wire is enough to get the bike running again. If that be the case, replace the coil wire. :)

5) Dum dum dum. After that you should test the coil and hope it works out OK with an Ohm meter. If it doesn't, they're expensive if they're available at all for vintage jap equipment.

6) Assuming you're checking the wires going to all these components as you're going, you start looking at the stator and magneto coil for the problem. Probably the least likely scenario here.

I would start here with ignition related problems. Typically when you see a carburetion related issue, you don't get a "sudden death" symptom. It tends to start running bad, then it stops running, in that order. :D Seals don't just give out on a whim.

Carburetor problems:

1) Verify the carb is getting gas down the line.

2) Remove bowl and plug in fuel hose and turn fuel valve on. You should get good gas flow. Push the bowl up against the body and it should shut off the fuel flow. Let it back down and flow should resume normally. If it doesn't, then it's a clogged needle valve.

3) After that you take the carb, remove the float, remove the needle and seat and clean those out thoroughly. Then remove the pilot jet and clean it out (it requires a very small screwdriver to get it out). Then do the same for the main jet, being careful not to lose the little washer on the main jet.

The bottom line though is that these are extremely simple little "round slide" carburetors. It's rare to see an engine stop running because of one. If the above repairs don't work, then I'd go back to the ignition system unless you are certain that the ignition system checks out.

So that's what I'd do. My bet is ignition. I chased a problem like this for a long time with spark always testing OK, or intermittently testing OK, but it was the coil wire itself that had failed. It made the spark intermittently too weak (eventually it would intermittently fail altogether). Your specific questions:

1) The ignition switches don't fail often, and if the switch is off, then there is no spark on this bike, plain and simple.

2) A short somewhere in the electrical system is unlikely to cause this as most of the lights, wiring etc are run off of two lighting coils and the battery. The ignition system is isolated to its own rather limited wiring, although there could be an intermittent short somewhere in the ignition system.

3) Rotary valves I believe I covered. Basically you have "Piston port" two strokes that have no valves. They're uncommon after the early '70s bikes. Reed valves are still king in two strokes - two reeds open and close based on fluctuations in crankcase pressure. They widen the power band on a two stroke. Rotary valves are more reliable than reed valves that can "break off" if not properly cared fore.

Rotary valves were largely introduced by Kawasaki. I'm not sure they were ever used on a multi-cylinder engine. It's a disk that rotates with the crankshaft and has holes in it, essentially timed to the same fluctuations a reed valve uses. They are in place to widen the power band.

4) No, the timing doesn't usually just "jump" out of place on an engine like this. Timing is related to the position of the magneto coil on the stator in the case of this bike, or at least I think it is. Very hard to adjust and very difficult to "come loose".

5) For the record, battery or no battery makes no difference, although you'll probably burn out the tail light and neutral indicator if you use it much without a battery. Not to worry, the tail light is probably 3 bucks and the neutral indicator about $0.80 if you choose to replace 'em. :)
 
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