Mech

The difference between your measure and the standard bore was minimal, you might have measured on a bit of varnish or carbon build up.

The increase of compression when you put the oil in the cylinder is enough to indicate it's time for a new set of rings anyway.

Ha.. Glad you found it.. Well done.

The 0.003" taper means the lip at the top will be 0.003" also. That will break a new ring. I don't think your measurements are quite accurate though because the standard bore should be bigger than your worn measurement. If your measures are right, then it seems that the piston to bore clearance is right at the wear limit(though we can exceed that and still get a reliable engine), but if the standard bore is slightly bigger as the book says, then there is a bit more piston clearance than they recommend for a guaranteed good engine. I could get that running though, and not needing any oil between services probably If it was me, for myself or for a customer, if they were agreeable, check the crank, I'd take the ridge off, check the piston to ring clearance, then fit a set of rings with a barrel base gasket, head gasket, and valve grind and seals. It would go for years, but only because I'd take care to minimise the ring gap and bed it in carefully. If the piston was worn and needed to be replaced, I'd bore it. An engine with taper makes the rings work harder to maintain a seal, they have to expand and contract every stroke, so if they get overheated, or don't bed in, then they'll let oil past.. If the rings are bedded in carefully in the first ten minutes after first start, and don't ever get overheated, then it'll be a perfectly good engine. We can generally wear our two pistons before we need to do the crank.. It's like Gw says, you could get it to go again, but it's not going to last as long as a rebore, especially if the work and care riding aren't all they should be.

It's likely that the rings are riding on a layer of oil after the oiled comp test. To get rid of the smoke you need to ride it putting the engine under a load at medium revs. Going slowly up a long hill should do it. Then... There is a way to test the oil rings. You get the engine warm and then in neutral you start gently revving the engine to about 1/4 revs and letting the throttle off as soon as it gets to the revs. You just keep doing that non stop, with the throttle open a bit less than 1/4 open and until it gets to about 1/4 revs, then you snap the throttle closed, as soon as the revs get down near idle, but without waiting for it to idle, you open the throttle again. You might need to experiment a little to get the amount of throttle right, and the revs right, but if the oil rings are bad it will start blowing huge amounts of smoke, and the smoke will get worse the longer you keep doing it. It doesn't need much throttle and it doesn't need much revs. My 1/4's may be more than actually needed. Once you get it right, it will make even slightly worn oil rings smoke bad. I've bought cheap machinery by doing it I'm ashamed now to say.. haha. If the smoke doesn't come, or if it's only a little smoke and not a steady stream of smoke, then it's more likely to be your valve guides that are smoking.

Ok. Good stuff. Clutch adjustment can be fairly critical on any bike, and once there is wear they can get trickier still.

The side play/rocking will be the same on the worn spot because the worn spot is just a narrowish band across the crank pin. If you are pushing down hard though as you rock the rod over the wear you feel it, and there might be the tiniest amount of endwards movement there, but mostly not, not if the side play is within limits. Side play/rocking of 0.1" sounds ok. Good even. I've put bikes together with more side play than that. That lip at the top is a measure of how much taper there is in the bore. Too much taper shortens the life of the rings, but a certain amount is ok. The thing about reusing a tapered bore(if it's within reason), is that when you put a new piston in, with new rings, the top of the new ring is going to hit that lip and break the rings within no time. To use a bore with taper you have to remove that lip, and if you think you'll be all fancy about it and use a "ridge remover', it's going to not work. The taper is worse in line with the rod swing, and if you use a ridge remover till all the lip's gone at the worse place, then you are going to have under cut the bore at the sides. What you have to do to do a good job, is use a triangular file ground on it's end so it's a three sided scraper with slightly curved cutting edges. Then you just gently cut/scrape metal away from there it's needed and no where else. It's easy to see what's scraping and when the two surfaces are flush.

When engines get high mileage the rings wear the grooves, then the rings start to flip and flop as the piston goes up then down again, they act like they were a disk getting flexed concave then convex.. that breaks them and then the ring groove breaks too. Rings also break if the ignition timing gets too advanced till there is "pinking". You need to test the rod bearing at the place the wear takes place, which is about twenty degrees after tdc. You just have to carefully work your way over that area checking for play or roughness in any spot about that angle.

I can't identify it by the frame and engine number, but that 12/91 will be the date of birth.. Which I think will make it the YFM350D.. Look in babbits yamaha atv parts.. Download several service manuals.. compare details..

Some will start if they are in gear, (or think they are in gear) if you put the brakes on. It can be a good work around for faulty neutral switches.. Ok, so if you are sure about the wiring, the book should explain how to test it all.. I think it even had resistance readings for the cdi unit..

Haha.. Interesting.. That chemical to dissolve carbon sounds like the power ingredient. And yeah, my old stihl's about close to forty, had a set of rings when son two and I were slabbing up some acacia once but apart from that still runs and cuts beauty.. Oh it's had a bar, and several chains. Maybe two bars.. haha.

Steady progress there Gw.. Well done you. Those bolts, and everything else is amazingly clean.. Grass juice is corrosive ..Better put some of that marvel oil on the brake arm.. I've never heard of that stuff but I used to use something called "liquid wrench", thin as diesel and very penetrating. That used to surprise me sometimes.. I never called it a marvel though.. haha. And since this is the garden corner.. I've been up tree three and four today, cutting branches off with my new stihl battery saw. It's a bit weak but wonderfully convenient, safe and easy to use.

Didn't see the vid of the bottom end. Slow internet so I'll try again later. Getting the engine out isn't so bad. Once the body work's off you take out the left rear suspension and wheel and all in one piece, then undo a frame member and gear shift, and the engine slides out to the left. Getting it back in you have to line up the right rear drive shaft but that's simple. That rod, if it moves four mills it's probably ok, but six mm would be too much . Six mm is 1/4". The most play, and any roughness, will be when the crank's about twenty degrees past top dead center.. If you tip the rod back it will be about right. If you check for endwards movement of the rod there, you shouldn't feel any..haha. It shouldn't rock more than about 4mm, and if you push and rock the rod on it's bearing it should feel smooth.. Cranks aren't too hard to do if you have a workshop with a press or good pullers. My sons used to do their own cranks when they were twelve and fourteen.. maybe even earlier. They did lots and they never had any problems. I've done lots. I've never had a problem. If I was doubtful about it's being worth doing, I'd probably fit a top end/fix it, and have a listen to how it sounds, then pull it out and apart if I had to. Just saying.. It's old as you say. The manual will tell you how much rod movement they think there should be. My 4mm is just a rough figure.

Does the clutch slip if it's cold and you are going up hill ? If the plate(adjustable) clutch is slipping ever, whether hot or cold, it should slip more under power than it will when it's only providing engine braking.. Engine power is far greater than engine braking and should cause slippage going up hill.. And yes if you have too much oil or the wrong grade oil it can cause slipping clutches, and dragging clutches too.

Did your wiring colours and the plug shapes match any of those wiring diagrams Bobbles ? Have you identified which model it is absolutely ? have you checked the output from the stator is getting to the cdi? Trying parts can be expensive, and frustrating !

I think the right side of the engine, and you should see it if you look just below and in front of the body panel.

If you give me the engine number I may be able to narrow it down...

If you decide it's going to be one of those year ranges then I'd go to Babbits and open a new tab for each of those years of the model, then by clicking from tab to tab your eye will hopefully spot a flicker when one set of pictures is different to the tab before(your eye does notice the difference as a flicker), then take note what the difference is and go confirm which your bike has. Then you might find there is a tab or two with your version of that first difference you spotted, so close everything except the two or three tabs that have that part on and start comparing those tabs for any flicker/differences. That normally finds the model you have.. Then it gets tricky because even though you've found the year and basic version they use different electrics and carbs in different markets, so to be really certain that the service manual you download is right you really need to study the wiring diagrams and make sure that the wiring is exactly the same as your bike. A good place to start is by looking at the cdi and noting how many wires it has, what shape the plugs are and what colour the wires are. Every detail of the cdi and wiring has to match.

I'd ignore the hunterway part, that's probably a local market name. You might find some reference to that name in a part listing site that's local(aussie or here). Here are the years they were called moto4 overseas.. 1986 1987 1988 1990 350 YFM350 YFM350ER Moto-4 YFM350ERT Moto-4 YFM350ERW Moto-4 YFM350ERA Moto-4 YFM350ERU Moto-4 1991 1992 1993 1994 1995 350 YFM350 YFM350D Moto-4 YFM350ERE Moto-4 YFM350ERG Moto-4

Hi Kiwi.. I'm in the far north..

Nearly there...

Filters sometimes have valves in them... Some have anti-drain valves and some have bypass valves in case they get blocked. Just those specs mentioned before the "etc", ain't always going to be enough.. be cautious people.

Yup, what he says.. But.. what do you mean by "to no avail", ? Did the engine respond to the mixture adjustment ? Could you get it to go too rich, and too lean ? Does the idle speed respond to it's screw being turned ? And I'd just clean and check the original carb if it's the genuine one for the bike. The aftermarkets can need a bit of tuning.

I think this is the one you need. Compare the ignition circuit diagrams in the ignition section, with the bike to be sure..

So just to clarify.. You got the bike in pieces, and assembled the engine, right ? So the valve timing might be out, but at this stage we don't have any reason to suspect any prior faults, such as a broken key in the flywheel, it should just need the cam timing redone.. So.. tell me how far out the cam timing is. If it's only out by a half tooth then you should still have reasonable compression.. Are you sure the valve clearances are right, and, is there any sound of air escaping out the inlet or exhaust when you are cranking it over ? Tell me what tests you have done on the ignition and I/we might be able to suggest what you've missed. Some makes and models of cdi can be checked using an ohm gauge, others not. If you read the service manual it will tell you whether it can be done, and how, for that bike. All of them say to check the resistance of the two windings that run the cdi, and I always suggest people actually check there is some output from the two windings because even though the resistances may be correct, if the timing tag has dropped off the flywheel, or the magnets have flakes of steel stuck to them, the windings may not produce any voltage.