Smoking Yamaha moto 250

[Mech]

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.

[Need-help-with-quads]

Alright I’ll take apart the top end and look and see what it needs but I can’t really try that because it cuts out if the rpm aren’t high enough but I’ll see if I can’t do that 

[Mech]

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.

[Need-help-with-quads]

Ok so I took off the top end and obviously I’m getting rings but the cylinder walls had a bit of light wear with up and down scratching(I don’t remember what the real term for it is) so I’ll hone the cylinder and put new rings in also the piston was a tiny bit loose in the cylinder but when I looked at the piston the thing looked brand new in terms of wear on the side, there was a bit of gunk on it but there wasn’t and wear on the sides so should I get a piston too? I’m gonna try rings on the original piston first but if that doesn’t solve it should I try a new piston?

[Gwbarm]

I would put the rings one at a time in the cylinder and Mic the ring end gap to see how badly they are worn, should be around .019 i would also Mic the cylinder and piston to see if it is within specs if not get a new piston. If the cylinder has scratches the piston should have matching scratches unless they are from a previous rebuild job.

[Mech]

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.

[AhSoSpice]

Probably valve seals

[Need-help-with-quads]

Ok I should be able to put in everything this weekend hopefully so I’ll let you guys know how it goes I was pleasantly surprised when the gasket kit came and valve seals were included so I’ll replace them too

[Need-help-with-quads]

Ok so I replaced the rings and all the gaskets and the valve seals and yet it’s still smoking what the heck do I do now, I’m thinking about selling it

[Gwbarm]

How long have you run it, may take the rings a little bit to seat. Did you check the new ring gap on the cylinder before installation. Cyllinder may be worn also.

[Mech]

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.

 

[Need-help-with-quads]

Ok I’ll try that tomorrow but also it does run better but as soon as I drive a little while and then stop it wants to stumble and not idle

Edited December 11, 2024 by Need-help-with-quads

[Mech]

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 ..

[Need-help-with-quads]

Alright so you were right about there being oil in the exhaust it doesn’t smoke unless I hit really high rpm’s and even then it’s just a little bit which might just be a little left in the exhaust but the stupid thing won’t idle at all I tried messing with the carb and adjusting everything and I even had the idle set all the way up and yet the idle didn’t go up and it still wouldn’t idle but other than that it runs ok

Oh and it pops a lot almost like a backfire and it does it even at the slightest of throttle

And to respond about the ring gap it was good also I don’t remember exactly what it was but the gap was good also I readjusted the valves and you were right about that as well

[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.. 

[Need-help-with-quads]

Alright I’ll do that, do you know what the jet sizes are for it

[Mech]

I could look up the original carb's jet sizes, but an aftermarket carb might have a different emulsion tube, and/or needle, and might need different main jet, and/or main air jet..  That's the trouble with after-market, if they aren't the same design, then the jetting is just guess work.

You should try changing the slide needle setting..

[Need-help-with-quads]

Ok I’ll try that but I do have a mikuni carb for it I’m pretty sure it’s the carb that goes to it, it’s not the original one for it but I got it off of a parts quad that was a 250 moto 4 so it should be the same

[Mech]

Well check the manifold for air leaks.

[Need-help-with-quads]

Alright so I check the manifold and it was good so I dug into the carb and I cleaned it good it was already pretty clean but then I brought the needle up and it finally idles I didn’t even think about the needle so now it’s running and idling really good I will take it for test drive soon and make sure that there’s no overheating or anything

[Mech]

Then you could try one step higher to see if it's too much... after the first test ride.

[Need-help-with-quads]

Alright so I think it’s running pretty good I’ve just got to mess with carb adjustments I think but one thing is that I can’t push start it for some reason, I’ll kick it into first and the thing just rolls like it’s in neutral and that’s for every gear, any idea what that could be? Not really that interested in fixing that but just wondering what it could be

Edited December 21, 2024 by Need-help-with-quads

[Mech]

The sprag/oneway clutch in the centrifugal clutch isn't working. You'll have no engine braking once the engine revs get low enough for the centrifugal clutch to disengage.

When you were testing it before you must have have not let the bike get slow enough down hill for it to do it, but if you go down a steep slope in gear and leave the throttle off till the bike's almost come to a stop, it'll accelerate and start rolling away again uncontrolled once the centrifugal clutch disengages.

If we know the bikes got the fault and take precautions when going slow and don't rely entirely on the engine braking it's not too dangerous, but people that haven't known their bike had the fault, have been hurt or given a scare  when their bike having almost stopped, then started rolling and rolled off a bend or into a gate.. 

[Need-help-with-quads]

Ok well I’d have to split the crankcase for that right?

[Mech]

No. Just a side case.

[Gwbarm]

Yeah and depending on which side you have to pull the clutches on one side or the rotor on the otherside. But all good still a ton better than splitting the case.

[Mech]

I'd imagine it can be done with the engine in place ? Most can.

The service manual says what can be done with the engine in place.

[Need-help-with-quads]

Ok I finally test rode it good and found that it runs great at low rpm’s but then I get to 1/2 to full throttle and it pops and sputters so much that I can’t hardly pick up speed, I didn’t pick up on that the first time I test rode it because I only had time to putt around the yard so I tried all the settings on the needle and nothing worked so I tried the mixture screw and I couldn’t get it to work, so I ordered an oem needle because the one that’s in the carb is a cheap rebuild needle all the jets and everything is oem so I’m hoping it’s the needle but that said, anything else I should look for?

[Mech]

Depends where the "pop' is  coming from. If it's under load and out the exhaust it may may an electrical misfire. If it's out the carb then it's probably a lean mixture.

But...  the no idle and popping could be caused by valve clearance or valve timing.

You really have to know one part of the system is absolutely right before you start looking at the next possibility. Valve clearance and timing is simple and doesn't take any discrimination or experience to judge. A new spark plug is simple and cheap to replace. The carb, which is from the sound of the description (symptoms and circumstances), the most likely, needs to be right for that bike. If the carb's been set up to be mounted flat it may not run well on a bike where it's mounted at an angle. You need to use some discretion when checking the jets and settings. If the float level's set different by the manufacturers because it's for another model bike, then the jets may all be suited to that float level. I always check the float level seems right for the way it's being mounted, then ride it taking careful note of all the symptoms and the circumstances they occur under trying to figure what part needs adjusting or changing.  I only ever change one thing then go for another ride taking careful note of what effect that had to the fault, and whether it's made other ranges of throttle better or worse. The three jet systems in the carb, idle, mid-range and full throttle, all overlap, and there are special features that help with the transition from one system to the next. The usual way to set a carb up is, get it idling and running, test ride and set the main jet system, then recheck the idling system, then set the mid-range system. Sometimes we have to go around the cycle doing all three again if it was a really badly chosen or set up carb. It takes time, a lot of careful test riding, and a good understanding of what's going on..  That's why it's always best to stick with the original carb. 

[Need-help-with-quads]

I’ll go through timing, valve and the carb but here’s the video I took before I sent that last message