Mech

A little leak up at the head to exhaust pipe join, or anywhere in the first few inches of pipe, will make them backfire.

Tried these guys.. They have always been good exhausts at a fair price.

https://www.musketmufflers.com/

A bad misfire can cause a sooty black plug, but it has to miss a lot and for a while.. I'm thinking you would notice that.

If it only goes rich occasionaly, and if it was only when you throttle off hard as would be the case if the choke was stuck on, then you wouldn't particularly notice the richness..  That could also explain why the idle mixture doesn't respond to adjustment.. -

Old worn low compression engines do get a build up on their plug, but it's generally a healthy tan or grey hard build up, not sooty. Worn rings burning oil, or leaking valve stems letting oil in to be burnt, the plug goes black and oily looking .. Black soot is always rich mixture I think..

Start the bike and operate the choke.. You should be able to hear if it's working, or doing nothing. The choke only sucks fuel up when the throttle's closed. Soon as you open the throttle the vacuum drops in the choke circuit and it doesn't lift the fuel right to the top of the venturi  where the discharge for the choke is. So it could be on and only go rich when you throttle off hard.

It does seem unusual to have fluffy dry soot that fast. That's definitely rich though.. Oil looks wet black. You have got fresh fuel haven't you ? And a good grade ?

The compression is probably meant to be nearer 175 Lb but that's warm and with a full open throttle and cranking at the right speed. If it comes up to it's 125 in the first couple of compression strokes it's a good sign. If it has to be cranked a lot and slowly creeps up with each compression stroke it's a bad sign. You could put a teaspoon of oil down the plug hole, give it a quick spin with the starter or pull start to distribute the oil, then do a compression check. If the oil raises the compression a lot it points to rings, not valves. Having a tight or leaking valve doesn't normally cause  dry fluffy sooty spark plug. Leaking rings doesn't normally cause that sort of soot either.. Only fuel does that.

Some bikes say what size the jets are in the parts manuals. You can look that up online, or in a service manual.

Sounds like the charging isn't working. Check the fuses and the connection between the regulator and the battery, both power and earth connections, then the next check is that the stator is putting AC power out down at the engine, or even better, that the AC is getting to the regulator.

#41 I'm not sure which sensor is fitted where on the bike. I only know one has one wire and one has two.

If putting power into the fan makes the fan run then the fan is ok. My test procedure is testing which place along the wiring system the disconnect is that's preventing it from going.

The most common sign of a blown head gasket is bubbles coming up in the radiator.

Steam could be caused by the air being very cold ? Warm water always looks like it's steaming if the air over it is cold enough. Even a farm pond can look like steam's rising up off it.

If the engine is really overheating in fifteen minutes of idling it could be the thermostat, the water pump, the head-gasket, a blocked or dirty radiator.

Have you looked at that wiring diagram I posted for you at #32?

There are two temp sensor switches, one has one wire and the other has two. They use different coloured wires.

If you follow the recommendations/test procedure in #34 it should find the fault with the fan, if there is a fault with the fan.

Most engines won't overheat in fifteen minutes of idling from cold. It may be that you have a blown head-gasket, faulty water pump,  blocked radiator, or a radiator that's dirty on the outside. If you take the radiator cap off and let it idle with a full to the brim radiator, there shouldn't be any bubbles coming up in the filler. I'd check that.

Dry black soot sure sounds like too much fuel. The mixture adjustment doing nothing is always a sign of something wrong in a carb.

Since it's running good, no mention of missing or hard start etc I'd doubt the ignition is at fault. Check it's got a reasonably fat blue spark. Ignition systems with resistor caps or leads can have a skinny spark, but it should be blue. If it looks yellow it's a bad sign. If the soot is only on the porcelain tip it might pay to try a hotter plug. If it lost it's spark suddenly it might have a wet plug, but it wouldn't have black soot.

I'd make sure the fuel tap was good and not filling the sump with fuel. And if it's got a vacuum controlled tap check that isn't letting fuel into the vacuum line.

Then I'd start looking at the mixture. Choke's going right off, check that first because the choke only works when the throttle's shut so they run ok with a stuck on choke, till you throttle off, then air-filter, intake piping isn't crushed or blocked(mouse nest), carb breather tubes, fuel tank breather. It might be that the richness is intermittent, or caused by heat or the time run. So check the carb breathers, and tank's not presurising. Then It'd be off with the carb for another look. Check the float height(can be done before pulling the carb off) and that it isn't sinking, check the idle mixture screw tip and the seat the screw goes into for damage, check the float needle seat is sealed to the body, the breather drillings are clear, all the drillings are clear, the jets are the right size, the slide needle is the right one and attached to the slide, the slide isn't sticking. Reassemble and check the idle mixture is responsive. If it isn't then there's still something wrong in the carb.

Probably on the outer side of the  lower frame member just in front of the front of the gear shift lever.

There are two temperature sensing switches. One is wired back to the dash with a violet wire, and the other one has two red wires. The switch you are looking at is in the lower left of that wiring diagram I posted. 

The power for the fan comes from a fuse to the temperature sensing switch connector in a black wire with a red stripe (blk/red), Then the switch's wires are both red, and one of them comes back out of the switch and goes to the fan motor when the switch closes. It goes through the fan motor and comes back out on a black wire as far as the fan's connector where it turns into a black wire with a white stripe as far as the diode, and after the diode it's a black wire connected to earth.

And yes, the diode has to conduct to earth for the fan to operate.

Yes, a lot of people do add a bit of oil with the fuel as an extra safety measure, and a having a two-stroke running with a bit of extra oil going through it does seem to make them last longer.

Diagram up further. The diode's between the fan and earth.

Is this backfire making a load popping or banging sound, or is it just blowing fuel fumes back out the carb ?

And, is the bike idling ok with this backfiring ?

If you give it throttle what happens ? Does it come right or get worse ?

If the diode was open circuit it would stop the fan coming on.

Check power is getting to the temp switch with a test light, not a gauge, then unplug the switch and bridge across the plug on the bikes wiring with a wire. That should make the fan go. If the fan doesn't go then, check there is power getting to the fan's red wire using a test light. If there is power to the fan, bridge across the diode with a piece of wire and see if the fan goes. To bridge the diode you can just put a wire from black wire of the fan straight to a good earth.

It's a two-stroke engine but you don't need to premix oil in the fuel. It has an oil tank and it auto mixes the oil as it runs. You need to top the oil tank up every so often. I think the oil tank will last longer than a tank of gas for safety reasons, but you need to monitor it for a while to be sure about that. It may not last two tanks of gas !

There should be a manual for it in the manuals section..

Difficulty getting them out of gear if they were in gear when you switched off is quite common, and rocking the bike is the usual procedure.

Most bikes will start in gear if you have the brakes on well enough to make the brake light go. It doesn't actually need the brake light, it just needs the brake light circuit to be energised to bypass the neutral safety feature.

I think they are a bit light for a sprocket circlip, but they won't break.

My pair are more than fifty years old and been used a lot and are still doing the job.

Oh yeah. I've got a set of those. I think you'll find they are fairly small tips, good for piston circlips and brake cylinders, circlips about fifteen mill diameter. They'll do a sprocket circlip though.

They'll be 0.047" which is close as to 1mm.

Gw's suggestion will be the way to test the fan.

The fan and the temp light though both have their own temp sensors, so if it's just the light playing up, it might be it's switch.

output.pdf

0.47" pins would be close to half inch diameter.. 

Yeah well plastic gears are not going to like thicker oil on a cold start.

Since all the lights went off when it made the clunk noise and didn't start, it means the voltage dropped very low, which is why the starter didn't work, and since it started two hours later the battery can't have been flat, so it must have a bad connection somewhere. You should check the battery cables and the smaller wires going to the fuses and switches are tight everywhere, on the battery, on the body or engine, positive and negative. The battery cables, and wires, can get bad contact where the wire crimps into the metal terminals as well, and to find that you can inspect them, and wriggle them, and/or do what's known as a "voltage drop test".

To do the voltage drop test you attach a volt gauge from one end of the cable/wire to the other end of the cable you want to test, and when you press the start button that gauge should stay reading zero volts. If it shows a voltage when you are pressing the start button then it's because there's a bad connection somewhere and some of the power is going through the gauge. Because the bad connection might be in the crimped terminal or the connection between the terminal and the bolt it's attached to though you need the gauge right on the bolt/engine/starter solenoid stud, and right on the battery post.

It's also possibly a bad/dirty switch, either key or kill switch. To do a voltage drop test across them you attach the gauge with the key/switch off and it will show battery voltage, but when you turn the switch on it should bypass all the power going through the gauge and the voltage should drop to zero then.

It would pay to use an ohm/continuity gauge and check what the internal connections inside the old switch are, then check the new switch has the same switching options.

Is this bike like a car and have left and right indicator lights, and a separate switch to make them all flash in an emergency ?

Looks nicely made.

I just use a hammer to knock the pins out of master links and a pair of channel-lock pliers to put them together. If you position the link on the back sprocket the pins can be tapped out easily.

Modern car engine's have minimum amounts of oil in their sump, extreme temperatures in the engine and under the bonnet, especially if they have a turbo, they have incredibly fine clearances everywhere, shell bearings, high stresses, require high oil pressures from cold to hot, and they rev high, and from cold often. They also have stringent emission requirements.  They need good oil, and synthetic oils are particularly good with the high temperatures.

Bikes on the other hand, even air cooled bikes, are low temperatures, low stress, roller bearings, mild power, low oil pressure, generous oil capacity, few emission requirements. They have very simple and low spec requirements for their oil compared to car or truck oil. In my opinion mineral oil is plenty adequate for them.