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Post by Clinton Cool on Jun 30, 2017 21:55:40 GMT
I posted a while back that I was thinking about buying an 80's Yamaha RS125. Well I bought it, spent lots of time trying to sort out its problems since then. I've nearly lost the will to live trying to get a replacment float needle and seat, but that's sorted now. I won't bore you with all its other problems but, I have a question:
I suspect the right side crank seal may have gone. It throws out white smoke for a few miles before clearing up. I thought I was using gearbox oil but now I'm not sure, as I've found a small hole in the right crankcase cover. That's going to get the JB weld treatment. Thought I'd ask though: if you crank seal has gone, would you expect the white smoke to be continuous or does the fact that it clears up after a while suggests there's a small leak that enables a fair amount of oil to enter the crankshaft area when the bike is laid up, but when that's burned off, the amount going through is negligible. What do you guys think?
Also, if the seal does need doing, can this be done by removing the crankcase cover and clutch assembly or is it an engine out and split the crankcase job? My Haynes manual is about as clear as mud on this.
Cheers for any replies.
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Post by Telemachus on Jun 30, 2017 22:43:27 GMT
Yes does sound a bit like the clutch-side seal. If the bike had been unused for a long time, the seals tend to go hard. It could of course also be caused by shot main bearings but hopefully just hardened seals. Sorry, not familiar with this specific engine to know about seal replacement but I would imagine it's a splitting crankcase job. Bearing in mind the bottom end has both positive and negative pressures I'd imagine the seals were in a groove, not just pushed in.
edit: yes the oil seeps in when the bike isn't running, so you get a lot of smoke for a while after start.
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Post by peterboat on Jun 30, 2017 23:52:43 GMT
2 strokes need good oil seals, does the clutch run in oil? they used to so that could be a source as Nick says, I cant remember whether these are oil injected or a mix? if oil injected is it leaking in when its stood? If its a mix are you putting to much oil in as the new synthetic 2 stroke oils seem to be at a lower mix of oil to petrol than yesteryear.
Good luck with your Bike
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Post by tomsk on Jul 1, 2017 6:28:10 GMT
Bite the bullet and rebuild it.
They're good bikes and worth saving if possible.
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Post by Deleted on Jul 1, 2017 7:17:58 GMT
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Post by Deleted on Jul 1, 2017 7:34:34 GMT
It will likely be a strip down, there may well be a retaining plate to stop the seal moving.
Piece of cake engine to work on, the biggest hassle will be if it still has the nasty Philips head case screws still in it, an impact driver is needed to undo them. It's always worth replacing them with Allen screws.
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Post by Clinton Cool on Jul 1, 2017 8:26:16 GMT
Cheers for the replies. I'm not really confident enough for a full strip down, taking the carb off and apart is about the limit of my ability. It's going to be a bike shop I think. I don't mind spending a few hundred quid getting it right, it's a rare bike these days, would be a great shame for it to die a death.
By the way, assuming the seal has gone, does it need doing urgently, or would a few hundred miles before doing it likely cause no further harm?
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Post by Telemachus on Jul 1, 2017 16:30:16 GMT
Cheers for the replies. I'm not really confident enough for a full strip down, taking the carb off and apart is about the limit of my ability. It's going to be a bike shop I think. I don't mind spending a few hundred quid getting it right, it's a rare bike these days, would be a great shame for it to die a death. By the way, assuming the seal has gone, does it need doing urgently, or would a few hundred miles before doing it likely cause no further harm? Probably OK with the seal on that side, just keep an eye on the oil level. If the seal is leaking the other side, it will suck air in and weaken the mixture and thus maybe hole or seize the piston. If it starts fine and idles fine, its probably OK. If not, I'd worry about weak mixture. |
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Post by Deleted on Jul 1, 2017 17:00:41 GMT
Cheers for the replies. I'm not really confident enough for a full strip down, taking the carb off and apart is about the limit of my ability. It's going to be a bike shop I think. I don't mind spending a few hundred quid getting it right, it's a rare bike these days, would be a great shame for it to die a death. By the way, assuming the seal has gone, does it need doing urgently, or would a few hundred miles before doing it likely cause no further harm? Probably OK with the seal on that side, just keep an eye on the oil level. If the seal is leaking the other side, it will suck air in and weaken the mixture and thus maybe hole or seize the piston. If it starts fine and idles fine, its probably OK. If not, I'd worry about weak mixture. You're half right. A leak on the generator side will weaken the mixture. A leak on the clutch side will weaken the mixture..... It's unlikely to go bang, particularly if you don't ride it like Phil Read. |
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Post by Telemachus on Jul 1, 2017 21:27:08 GMT
Probably OK with the seal on that side, just keep an eye on the oil level. If the seal is leaking the other side, it will suck air in and weaken the mixture and thus maybe hole or seize the piston. If it starts fine and idles fine, its probably OK. If not, I'd worry about weak mixture. You're half right. A leak on the generator side will weaken the mixture. A leak on the clutch side will weaken the mixture..... It's unlikely to go bang, particularly if you don't ride it like Phil Read. Obviously it depends on the gearbox oil level, but on some engines it's above the seal on the clutch side thus it doesn't pull air in unless the oil gets low. If the oil is seeping into the crankcase when the engine is shut down, the level must be at least above the bottom of the seal and could easily be above the top. So you are half right, on some engines with some oil states, a leaky clutch-side seal will weaken the mixture, but mostly not. It's only a 125 so to ride it fast enough to hole a piston with weak mixture is not hard. |
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Post by Mr Stabby on Jul 1, 2017 21:57:18 GMT
To be honest, this is a question probably best asked on a forum for motorcycle mechanics rather than a forum of sweary, chip shouldered bigoted weirdos.  |
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Post by Deleted on Jul 2, 2017 6:19:14 GMT
You're half right. A leak on the generator side will weaken the mixture. A leak on the clutch side will weaken the mixture..... It's unlikely to go bang, particularly if you don't ride it like Phil Read. Obviously it depends on the gearbox oil level, but on some engines it's above the seal on the clutch side thus it doesn't pull air in unless the oil gets low. If the oil is seeping into the crankcase when the engine is shut down, the level must be at least above the bottom of the seal and could easily be above the top. So you are half right, on some engines with some oil states, a leaky clutch-side seal will weaken the mixture, but mostly not. It's only a 125 so to ride it fast enough to hole a piston with weak mixture is not hard. Extra oil in the air fuel mix leans the ratio off. You are wrong this time Dont be fooled into thinking it is only air being pulled in that will lean the mixture. Too much Oil does the same. But, as He's unlikely to ride it like Bill Ivy across the top of the mountain it will likely be OK. |
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Post by Clinton Cool on Jul 2, 2017 7:11:00 GMT
I've tried a couple of motor bike forums but had very little response, much better here. There seems to be no limit to the knowledge of Thunderboat members The other problem I have is that the float valve and needle are slightly worn. I've given both a thorough clean but when I watch the action of the needle it has some sideways movement, as well as up and down. From time to time either the needle sticks down, petrol pours from the carb overflow, or up, the engine starves of fuel and dies. The only repair kit I could find in the U.K. was for the 76 model (mine's 82). Bought it but the thread diameter of the seat is different. I'm now faced with the problem of sourcing a part with no legible size markings on it. Bit of a nightmare! |
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Post by Deleted on Jul 2, 2017 7:16:10 GMT
I've tried a couple of motor bike forums but had very little response, much better here. There seems to be no limit to the knowledge of Thunderboat members The other problem I have is that the float valve and needle are slightly worn. I've given both a thorough clean but when I watch the action of the needle it has some sideways movement, as well as up and down. From time to time either the needle sticks down, petrol pours from the carb overflow, or up, the engine starves of fuel and dies. The only repair kit I could find in the U.K. was for the 76 model (mine's 82). Bought it but the thread diameter of the seat is different. I'm now faced with the problem of sourcing a part with no legible size markings on it. Bit of a nightmare! Look at DT 125 MX bits, they were in production until '82 I'm half certain they used the same motor. |
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Post by Telemachus on Jul 2, 2017 7:21:35 GMT
Obviously it depends on the gearbox oil level, but on some engines it's above the seal on the clutch side thus it doesn't pull air in unless the oil gets low. If the oil is seeping into the crankcase when the engine is shut down, the level must be at least above the bottom of the seal and could easily be above the top. So you are half right, on some engines with some oil states, a leaky clutch-side seal will weaken the mixture, but mostly not. It's only a 125 so to ride it fast enough to hole a piston with weak mixture is not hard. Extra oil in the air fuel mix leans the ratio off. You are wrong this time Dont be fooled into thinking it is only air being pulled in that will lean the mixture. Too much Oil does the same. But, as He's unlikely to ride it like Bill Ivy across the top of the mountain it will likely be OK. I can't see how oil being pulled into the crank through the seals could lean the mixture. Could you explain the process? Too much oil in the petrol, yes I can see that would lean the mixture, but in the case we're talking about the correct mixture will be pulled through the carb, to which is then added a small amount of gearbox oil. Nope, I can't see how that could lean the mixture. And bear in mind that as described, it's not pulling in much oil, the smoke is coming from oil that's seeped through the seal during hours and days of being stopped. |
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