Mech

Well the no idle and backfiring could be dirt in the carb or an air leak at the manifold, or a carb breather pipe on the wrong place or blocked. Those are the most likely. I think I'd just pull the carb off, check the manifold for cracks, then clean the carb and put it back on knowing that part was all right. You need to clean the carb thoroughly though, and check the float level etc, so you do know it's right..

If a pin has dropped out or broken(never happens) from the shift barrel, then it wouldn't go into the next gear up. The ratchet wouldn't move the barrel far enough to engage the next pin around. If the ratchet/self centering mechanism between the shift lever and the shift barrel was damaged I don't think it would move up into the second gear. If the original description in post#1 is right, it's blown a cog.. I'd try some more tests to confirm it, and/or check the oil for metal.

If you go to Babbits spares they have pretty good exploded diagrams of all the parts, or a service manual will have much the same exploded view diagrams.. If the shifters are moving nicely but it's not engaging gear, it might be time to inspect the oil for metal chunks ? Have you ridden the bike recently ? Did it get into low gear then ? It might have blown low gear, and that other gear you say it will go into might(presumably) be second.. ?

Not sure what you are calling a nozzle. There is a long brass tube the main jet screws into the bottom of, and at the top of that tube is where fuel discharges into the air stream. The tube is mostly called an emulsion tube. The slide needle drops into the emulsion tube. You can sometimes change the emulsion tube, but it has to be matched to the slide needle to work properly. Sometimes the threads are different on the jets and/or tube. If it was my bike(or job), I'd do up the original carb.. It's set up just right for the bike. The easy thing for you to try now is to change the slide needle height and see if it makes it better or worse.

The bad idle might be the mixture needing adjusting, or the valves might have adjusted themselves after the first start and might need readjusting. Try not to let it sit idling though, that can bugger up the rings seating and give a less than satisfactory ring seal that's hard to cure. Ride it with a load on ..

Hmm. Well 3/4 throttle is about where the carbs changing from having it's fuel metered by the slide and it's discharge tube, and being metered by the main jet, which is what those sizes you mention will be. I think that the simplest thing for you to try is to take the top off the carb and pull the slide and it's needle out, and then change the groove the circlip is in the needle. If you raise the clip on the needle it will make it leaner, and if you lower the clip it will make it richer. It's easy to do as long as you don't drop any bits.. haha. Try it either way and see if it's better or worse, then move it again based on that first test. You can do that easy and undo it if wanted, and if that cures it then the main jet doesn't probably need any other adjustment, but if you pull the whole carb off and change the main jet, you might need to start adjusting the slide needle afterwards to get that right for the new jet. Try the easy thing first.

The exhaust is likely to be full of oil too.. You should run it at moderate revs but with a decent load on it(up a long slope), that will seat the rings in. Get it good and hot and see if the smoke doesn't start decreasing aft6er an hour, or, take the exhaust off and go for a putt.

It sounds like the main jet's possibly the wrong size, or the needle in the slide is wrong. Are you sure this carb is made and set up for your bike ? Is it a genuine yamaha carb ?

Yeah, check the reverse cable's good, adjusted, and has some spring tension from the lever. Check the lever moves.

Yup, 2HR engine number means 87-88. -0000101 was 87, -0050101 was 88. So perhaps late 88 for -007969.

If it's air cooled, that local heat might have come from the exhaust's front pipe..

I've seen worse. One thing that's a bit strange is that despite the skirt getting hot, the heat doesn't appear to have come from the top of the piston. The rings haven't seized/been jammed in the grooves. Just the skirt got hot. Water cooled engines that have the lower part of their cylinder cooling passages full of crud(used to be common), do that. Unusual these days to have a water jacket full of rusty deposit..

Ya see that bottom ring there Gw....

Well we can take side cases off without loosing the oil by jacking (and blocking) the bike on one side. It's hard to see what you could have done wrong inside the case since you say the shift feels normal. It couldn't be that the plate clutch adjustment has got out again could it ? The clutch is slipping so bad it feels like neutral..

Haha.. That's a work-around I like..

That key must be from the flywheel surely ? It looks a bit too rounded to be a key though.. Wrong proportions too.. haha. Are you sure the gear problem isn't just the lever on the case, or the adjustment if it's got a cable or rod ?

That roughness ? The piston nearly seized. By the looks of it it was caused by overheating. See just in front of your thumb, on the corner of the skirt, there's a dark spot.. Well that looks like the sort of hard surface pistons get when they overheat. The black bit will be hard and hard to scrape off if it's that.. otherwise it's just an oil spot.. haha. Try scratching the black bit with a scraper or screwdriver. It's some sort of aluminium anodising or something that happens and it's like that surface they put on aluminium doors and windows. Sometimes there can just be a tiny amount and it seems like the piston could be re-used, and they can, but they have a sharp tap when they would normally have piston slap noise, and they can tap/rattle a lot in use.. just because that little spot is too harder metal.

Ok. Most the stuff in the service manuals will be the same in most respects. When we look in the supplement for a specific year it shows that they've changed the size of a bearing or shaft or gear or some part, and/or, and most commonly what effects us when doing work, that the ignition or carb has changed. Most of the small model/year changes don't make any difference to us working on the bike, but be real careful if referring to the manual about the ignition or carb.

Hi Qcm. No mention of the trouble light coming on.. The trouble light should come on if the TPS has a too high or too low reading, and it should set a code of course. Also, I'd expect the engine to stumble if the TPS had a fault, but not perhaps die completely and instantly as I think(from the description), that this is doing. Fuel pressure or that throttle cable switch seem like more likely things to me. It would be good to know what engine we are talking about.. haha. The eight hundred engine might have different software I think, in which the throttle switch might not be a suspect.. That switch does cause problems though, and it and low fuel pressure both don't set the light.

I'd check the throttle cable adjustment, and the operation of the switch in the throttle hand piece that the cable operates.

Cool. That's handy if the engine stops in second or more gear.. Or on a steep hill in gear and you can't get it out of gear.. Pull the handle and go.. And you could try, just so you know, and me, put the front brake on so the switch is closed, then try driving.. If it has the safety feature to protect the park brake then it will start cutting out and then going again.. like a rev limiter.

It's quite likely that the next size up from standard is +0.020".

If the scratches will hone out they will be ok. If there is a lip/ridge at the top of the bore, about where the top ring gets to, then you need to remove the lip/ridge or it will hit a new ring and break it. You will be able to feel a lip if you run your finger up the bore front and rear. You should really measure or calculate how much taper there is causing that lip. Too much taper makes the new rings work harder and shortens their life. You should also check the side ply between the new ring and it's groove in the piston. That's the clearance above or below the ring. Too much side play lets oil past, and eventually breaks rings. You should fit new valve guide seals also to be sure to stop the smoke.

On some models it would put a brake light on as well. it's also possible, as a safety feature, that it might hamper the running of the engine if the handbrake/parkbrake was on. So you notice the brake's on and don't burn it out.

I'd got confused between this and another online engine job that's going on. I'd forgotten about the broken rings. Slapping new rings in is not a viable option. Since you need a new piston, then it would be worth boring the cylinder. You'd have to see what the new piston's availiable in. They will come either 0.010" oversize or 0.020" oversize. The clearance will be set by the guys doing the boring and it should be to the book. You could fit a new piston and rings in the worn bore, and the engine would(assuming the crank is ok), be good for years. A car like that I'd patch up for a customer and guarantee it to be reliable, using no oil to start with, and perhaps after the four years it may need a top up between services.. If I was doing a rebore and new piston though, then I'd have to be confident the bike crank was going to be up to it, and the gearbox etc, so I'd strip it entirely and clean it and then do the engine work if it was going to be a good unit. I wouldn't buy anything or make any decisions though till I'd stripped, cleaned, inspected and priced everything..