Mech

No offense, but it's been overlooked before... have you checked the bulb ? The wires run up near the top right frame rail I think, under the seat.. I'd test the switch, there at the connector.

Yup, I'd try a different tool, possibly a torx if one felt like it might tap in reasonably tight. Next would be as suggested, try to tap it around, You need a small chisel that's not too sharp, so it will dig in if hit straight down, but won't cut metal away when we lower the angle to get the bolt to move. If not a chisel, then an engineers punch that is ground off nice and square so it it sharp and will dig in.-Then a couple of sharp hits downwards to get the chisel or punch to bite in, then one light tap to try and move that side around, then go to the opposite side and repeat, two sharp hits to get the punch to bite, then lower the angle and give a tap or two, then go back to side one and give that a tap, then side two... and so on. If neither of those works then drill it out in stages, trying to loosen it by taking the center out first, and trying plan two above, then drilling it bigger and trying an easyout, then drilling it right out and re-tapping if necessary.

Ok, well since it's now going back and forwards, it must have been in gear before, and the sprag clutch was what prevented it going forwards. The sprag clutch is a one way clutch that provides engine braking when the engine revs get low enough for the centrifugal clutch to disengage. Normally the centrifugal clutch locks up and crank and gearbox shaft turn at the same speed, but if you are going down a hill with the throttle off and the engine gets slow enough, the centrifugal clutch disengages and the bike tries to run away down the hill. If the gearbox shaft is turning faster than the engine shaft after the centrifugal clutch has disengaged, the sprag drives the engine shaft forwards and provides engine braking. At idle, before the centrifugal clutch locks up, the engine is turning faster than the gearbox shaft and so the sprag slips. When the engine shaft isn't turning because the engine is stopped, then any rotation of the gearbox shaft is faster than the engine shaft and so the sprag tries to turn the engine, which in low gears seems like it's locked up. I suspect you still have the bike in between two gears, but not in the proper neutral place. Between third and forth perhaps.. I think you need to keep rocking the bike backwards and forwards, while pushing the shift lever down, and keep rocking and tapping the lever till eventually it will shift down one gear at a time to the proper neutral. It will probably take a lot of rocking..

The load doesn't have to be too extreme, just normal riding really, but paying attention not to let it rev without load on it.. just keep using higher gears, and about a third throttle if that can be achieved. Revving it in neutral isn't a good idea. Idling isn't a good idea till it's had a good load and warm up. That breather hose has a restriction in it somewhere, a metal plug with a really small hole in it, inside the hose in some models. It gets gummed up.. Best to run the engine till it's warmed up, turn it off, wait a minute then undo the oil filler bung..

You should check the crankcase isn't pressurising, that could cause it too. They all have a bit of pressure but it should drop after a minute of being turned off if the breather is working. If the breather is blocked, and you wait for a minute before undoing the fill bung, there will be a puff of slightly presurised fumes. If the breather is clear there should be no pressure. That initial hard to start without the decomp might have been because the cylinder had a lot of oil in it, perhaps because it had been upside down, or perhaps because the oil level was too high, or, the exhaust valve has a worn seal and oil had dribbled down into the cylinder over time while being parked up. Once engines get oiled up rings they tend to keep sliding instead of scraping and so get worse if nursed along. You should try to get it under a bit of load, in higher gears, and avoid driving along in a low gear where there is no load on the motor. Try to keep the revs down and the load up.. it may come right. If it hasn't come right after a few decent rides, which would be a good idea to suss out any other problems before doing the motor up, then yeah, time to pull the head and barrel.

From the circumstances it sounds like it's in high and low at the same time....

And just to be quite clear about this Atv.. when it runs on starter, fluid it does run for more than a second right ? If you keep giving it a squirt every half second or so it will keep running.. ? If it does then it's a shortage of fuel causing the problem, but, if the one second is consistent no matter what it's burning, then it suggests there is an electrical problem. In that case you want to connect a strobe timing light to it and watch to see whether the light keeps flashing right down to the very last rotation of the motor, or whether the flashing stops and then the motor slows down to a stop.

Whe you try to start it, and it doesn't start, if you pull the spark plug out then is it wet ? That will tell us whether it's getting fuel or not.

Did you try undoing the drain screw on the bottom of the carb and checking fuel flows right through ? Did you take the jets out and clean them, blow through them, and look through them when you sprayed it with carb cleaner ?

Ha.. ok.. forget that then.

If you look up spares online it will probably tell you what make and model of carb it is, or a service manual will definitely have the information.

If it runs on starting fluid then the spark is ok.. You need to work on the fuel supply.. Is fuel getting to the carb, is it getting right inside the carb, is there enough of it to run the bike ? If it's getting plenty of fuel right to and through the carb(undo the drain screw to check), then the carb needs cleaning inside.

Well I don't have any experience with them but I've read that if the distance between the engine and the gearbox isn't correct the belt can flap and hit the case and self destruct.. It will all be messy now but you might be able to see a polished or rubbed bit inside the case to indicate it had been hitting for a while..

Yeah, the cheap carbs have inaccurate jet sizes, and they use threads of their own sometimes. Stahlwille use to sell bike jet measuring and reaming kits which went down to the small sizes, but they don't seem to make them any more which is a pity. I should have got one years ago.

The cable or lever was only if it had reverse. It won't have one. So now it's in neutral, but it only pushes backwards, right ? You can't push it or drive it forwards.. right ? I'd guess that would be a brake playing up. You could drop the drive chain off it and try turning the back wheels then. Or jack the front end and try turning that wheel.

I've looked at several years now and no reverse.. Seems more likely it can only be pushed backwards, which could be because it's in gear and the over-run/sprag clutches won't let it go forwards, or, the brake is playing up, loose lining on the shoe perhaps..

Oh.. The manual I just looked in says that bike doesn't have reveres. Does it have reverse gear? And are you saying it drives in reverse ? Or that you can only move it backwards ?

It's been a long time since I've seen one of those, but it will be as Gw says and something in the shift mechanism rather than the clutch. First off I'd check there isn't some lever or cable that has to be used to put it into reverse.. That part might be playing up and stopping the shift lever from moving.. perhaps. If it's not a designed shift lock, then I'd try rocking it backwards and forwards while trying to get the shift lever to move. Sometimes we have to push the bike quite hard in one direction or the other to get the engine to turn and allow a shift to happen. Decpmpressing the engine will help, or, jack the back wheels up and turn them while trying to move the shift lever. If that doesn't work, describe how the shift lever behaves/feels.. is it solid and not move at all, does it move freely but do nothing, does it move the usual distance both ways, does it self center ?

You can also buy feeler strips from engineering supply shops. They are about eight inches long and can be bent or cut to requirements.. good for doing valves running so the good feelers don't get hammered. For regular feelers I like the ones with the taper to them. Taper in width that is. The finer tip is slightly more flexible, and doesn't get obstructed so you can go in at an angle if needed.

Ha.. yeah that too Gw. I've got spare jets in the packet that were all different numbered, but looked to me like the same size. The fuel jets alone don't dictate the tune, the fuel jets, air jets, emulsion tube, discharge tube, slide, and slide needle all have to be chosen to work together.

Yeah I reckon it must have been too clean and needed a spray of wd40 or something after the soap

Yup five and seven.

You need to check the valves are flat on their ends where the adjuster presses. If it's worn dished you have to allow for that. If it is flat then the proper(accurate) way to use feeler gauges is to use two at a time, one a thou bigger than you want, and one a thou smaller than you want, then adjust it so one gauge slides through without any hindrance and the other just drags.

If this is a belt drive, does that mean the clutch isn't in the engine oil ? If they're prone to wearing the cam, I think I'd put some supper slippery in the oil. Cams have always been prone to oil shortages, because they are a long way from the pump at startup, because of air bubbles in the oil, because it's the first place to run short of oil when there isn't enough in the engine. The additive I'd add would have moly-whatever it is, in it.

I thought those parts were suspiciously cheap.. I think I agree with Gw about the wear having to be pretty bad to cause a flat spot , If the cam is wearing though you will get more valve clearance not less. I'd be pretty sure the running would come right with a bit of a carb tune..