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Bizarre Yamaha SR125 problem.


Dirty_Andres
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Hello all.

I bought my SR125 last summer as a project bike and have yet to get it running properly.

To get you up to date on what' shappening here's a bit of a chronology.

Last year, bought the bike, front tyre flat, drums seized up, carb stuck open, crank flooded with petrol, indicators acting out.

In between, sorting out all that crap, dismantling, cleaning and reassembling the carb about a million times trying to figure out why on earth the engine

wouldn't run without the throttle in all the way, then running really fast and dropping of rapidly to barely ticking over then dying.

Last month, took the top end off, cleaned out the gunk, polished piston head, reassembled it all with new gaskets, checked valve clearances, changed spark plug for correct type(read the manual, duh!) started engine, rejoiced. Engine running sweet as, hardly any problems with idling.

The day after, started engine up went to adjust carb, revved engine a wee bit and PLINK! Both rocker arms snapped at same point in same way,

piston now has a lovely little stamping on it from where it whacked the valve, I'm assuming that it was the timing chain jumping, lots of white smoke before this happened. Checked head, loads of oil, so not a lubrication problem.

This week, finally got around to buying a new engine, swapped over starter motor from old eninge(this one didn't come with one) adjusted valve clearances(very fiddly, seems almost suspiciously hard to get it right and yes I'm using feeler gagues), timing is out by a bit so will need to take the chain off and stick it on the sprockets properly(waiting on a flywheel puller to do that) with the timing marks all lined up.

How do you split the cam chain? I've been looking for a link and can't find one, oh and how do you split the drive chain once the pin is on it? Can get it on, but not off!!

And now for the strange bit:

Both engines are from the same year ('89 as far as I can tell), so this may be an issue of the engine's manufacturing run.

On inspecting the cam chain on both engines, I've noticed that the sprocket end of the cam tunnel used to have a lip, I know this because I can see what's left of it on the old engine, however the new engine has none left, as it's been completely ground off by the cam chain, the old engine isn't so bad.

As you can imagine this is quite shocking, but my mind's been put to rest as I almost had a fit when I looked at the new engine's oil filter and saw that it was full of little bits of metal!!!!!

It would appear that the timing chains in these engines have been either slack and running for some time, or even worse that the camshaft's bearing is slightly on the wobbly side of things.

So anyway, i have now ordered a new cam chain to fit into the newer engine, as it seems that it's got the same problem aswell as the timing being off. I'm actually quite tempted to take the top end off to make sure that everything's ok.

Ah, and by the way how the hell do you guys get the cam sprocket off? I've been doing it according to the haynes book of lies and on the old engine the cam chain slipped and I don't want to go damaging anything. I'm hoping that the flywheel/clutch holder will let me undo that bolt more easily.

So yeah, quite a lot of time and money being gobbled up over the last year, if anyone has any idea as to how the cam chain could get so out of hand, then I would like to hear it.

Ta.

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How do you split the cam chain? I've been looking for a link and can't find one, oh and how do you split the drive chain once the pin is on it? Can get it on, but not off!!

The chain is endless no joining link, to remove undo the camshaft sproket and remove the stator plate/mag and the chain will pull out at the crankshaft end( dont forget to tie something to it so you can pull the new one through)

And now for the strange bit:

Both engines are from the same year ('89 as far as I can tell), so this may be an issue of the engine's manufacturing run.

On inspecting the cam chain on both engines, I've noticed that the sprocket end of the cam tunnel used to have a lip, I know this because I can see what's left of it on the old engine, however the new engine has none left, as it's been completely ground off by the cam chain, the old engine isn't so bad.

As you can imagine this is quite shocking, but my mind's been put to rest as I almost had a fit when I looked at the new engine's oil filter and saw that it was full of little bits of metal!!!!!

It would appear that the timing chains in these engines have been either slack and running for some time, or even worse that the camshaft's bearing is slightly on the wobbly side of things.

So anyway, i have now ordered a new cam chain to fit into the newer engine, as it seems that it's got the same problem aswell as the timing being off. I'm actually quite tempted to take the top end off to make sure that everything's ok.

Ah, and by the way how the hell do you guys get the cam sprocket off? I've been doing it according to the haynes book of lies and on the old engine the cam chain slipped and I don't want to go damaging anything. I'm hoping that the flywheel/clutch holder will let me undo that bolt more easily.

Ta.

would the valve timming be about ½ a tooth out with the chain in tension ? If so this is quite normal and is just wear in the chain but anymore than that its out & needs attention. If you set the valve timming with a worn chain/tensioners you may need to set the timming a tooth out when slack as when tension is applied the timming will become correct, if set correct slack the timming will be out when tensioned which can cause the valve to strike the piston.

When fitting a new cam chain it is good practice to fitt new tensioners also, as the adjusting tensioner may have been bent to far for too long and has become weak.

To undo the nut on the camshaft a good strap wrench around the mag or the correct flywheel holder will do the job.

But as you stripped motor #1 how did you undo the nut that time as it would need to be undo in order to reset the valve timming.

Regards Jim

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Hey mate, cheers for the quick reply.

On the first engine I had to jam the sprocket with a alan socket.....not the best way of doing things.

Seeing as how the engine is about as old as the last one, I may as well change out the timing chain guides, if they're worn it may well have

let the chain wobble and grind off some metal......any thoughts?

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i dont think the timing chains have a link. when i changed mine i had to lower it down from the cam cover and loop it over the crank pulley.( a bit fiddly but can be done).i also tied a peice of string around chain in case i dropped it inside engine.this way you can just pull it back out without stripping down. good luck anth

Hello all.

I bought my SR125 last summer as a project bike and have yet to get it running properly.

To get you up to date on what' shappening here's a bit of a chronology.

Last year, bought the bike, front tyre flat, drums seized up, carb stuck open, crank flooded with petrol, indicators acting out.

In between, sorting out all that crap, dismantling, cleaning and reassembling the carb about a million times trying to figure out why on earth the engine

wouldn't run without the throttle in all the way, then running really fast and dropping of rapidly to barely ticking over then dying.

Last month, took the top end off, cleaned out the gunk, polished piston head, reassembled it all with new gaskets, checked valve clearances, changed spark plug for correct type(read the manual, duh!) started engine, rejoiced. Engine running sweet as, hardly any problems with idling.

The day after, started engine up went to adjust carb, revved engine a wee bit and PLINK! Both rocker arms snapped at same point in same way,

piston now has a lovely little stamping on it from where it whacked the valve, I'm assuming that it was the timing chain jumping, lots of white smoke before this happened. Checked head, loads of oil, so not a lubrication problem.

This week, finally got around to buying a new engine, swapped over starter motor from old eninge(this one didn't come with one) adjusted valve clearances(very fiddly, seems almost suspiciously hard to get it right and yes I'm using feeler gagues), timing is out by a bit so will need to take the chain off and stick it on the sprockets properly(waiting on a flywheel puller to do that) with the timing marks all lined up.

How do you split the cam chain? I've been looking for a link and can't find one, oh and how do you split the drive chain once the pin is on it? Can get it on, but not off!!

And now for the strange bit:

Both engines are from the same year ('89 as far as I can tell), so this may be an issue of the engine's manufacturing run.

On inspecting the cam chain on both engines, I've noticed that the sprocket end of the cam tunnel used to have a lip, I know this because I can see what's left of it on the old engine, however the new engine has none left, as it's been completely ground off by the cam chain, the old engine isn't so bad.

As you can imagine this is quite shocking, but my mind's been put to rest as I almost had a fit when I looked at the new engine's oil filter and saw that it was full of little bits of metal!!!!!

It would appear that the timing chains in these engines have been either slack and running for some time, or even worse that the camshaft's bearing is slightly on the wobbly side of things.

So anyway, i have now ordered a new cam chain to fit into the newer engine, as it seems that it's got the same problem aswell as the timing being off. I'm actually quite tempted to take the top end off to make sure that everything's ok.

Ah, and by the way how the hell do you guys get the cam sprocket off? I've been doing it according to the haynes book of lies and on the old engine the cam chain slipped and I don't want to go damaging anything. I'm hoping that the flywheel/clutch holder will let me undo that bolt more easily.

So yeah, quite a lot of time and money being gobbled up over the last year, if anyone has any idea as to how the cam chain could get so out of hand, then I would like to hear it.

Ta.

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Hi,

so what is the reply ? <_<

I think its in there somewhere, but bollox if i'm trying to find it :D

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I think its in there somewhere, but bollox if i'm trying to find it :D

Hi drewp,

I did find it, the top paragraph in the quote, but I guess this just mirrors previous posts

Regards Jim

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  • 2 weeks later...

Update!

I've just got a 16 mil bolt in today(to take off the flywheel), waiting on my mate to turn up so that we can put on a new cam chain :)

It just looks like the cam chains haven't been adjusted at all in the past, gah! How can people even get away with neglecting these machines so badly?!

I'll let you know how things work out, bit skint at the mo, so it may be a while till I get my bike MOT'd and taxed.

Ps. Anyone wanting to buy a 97 VW polo with a years MOT in scotland, send me a PM, can offer a decent price.

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Update!

I've just got a 16 mil bolt in today(to take off the flywheel), waiting on my mate to turn up so that we can put on a new cam chain :)

It just looks like the cam chains haven't been adjusted at all in the past, gah! How can people even get away with neglecting these machines so badly?!

I'll let you know how things work out, bit skint at the mo, so it may be a while till I get my bike MOT'd and taxed.

Ps. Anyone wanting to buy a 97 VW polo with a years MOT in scotland, send me a PM, can offer a decent price.

hi,

Dont forget the valve timming issues in the topic !

Regards Jim.

50p for the Polo sounds good :D !

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Update!

I've just got a 16 mil bolt in today(to take off the flywheel), waiting on my mate to turn up so that we can put on a new cam chain :)

It just looks like the cam chains haven't been adjusted at all in the past, gah! How can people even get away with neglecting these machines so badly?!

I'll let you know how things work out, bit skint at the mo, so it may be a while till I get my bike MOT'd and taxed.

Ps. Anyone wanting to buy a 97 VW polo with a years MOT in scotland, send me a PM, can offer a decent price.

hi,

Dont forget the valve timming issues in the topic !

Regards Jim.

50p for the Polo sounds good :D !

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  • 1 month later...

Update!

Hello all, so here goes the hopefully last update to this page.

Polo has been sold, I now have some skittles.

Bolt was a peice of shit, so I got a flywheel puller and changed the cams.

Started up engine and still had the same old problem, finally got around to buying some new jets.

Adjusted float height properly and installed new jets.

Cranked engine over, would only run with the choke on.

Cleaned carb about a million times, still would not run properly.

Checked fuel, filter, full of crap, checked air filter POS!!!!

Ordered carb from scrappie, ordered new air filter.

Cleaned and installed carb from scrappies, took out airbox, cut that ugly useless thing off with a dremel,

installed air filter.

VROOOM!!!!!!

Rejoiced.

Currently waiting on in-line fuel filter, just to make sure there's no more crap going into the carb.

Bike is running sweet as!!!

I'm so happy, but so tired, spent last 4 days trying to get the old carb working with a "repair kit", which it turns out had the wrong bloody jets in it. Useless shop monkeys.

So yeah, all that's needing done now is for me to fix the ugly plastic fairing that holds the headlight in place, seeing as how it's broken again and she'll be up for her MOT. :)

If anyone has one of the newer yamaha sr125 speedos and headlight sets then please pleaase get in touch.

YAY!

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  • 2 weeks later...

Bike has now passed its MOT!!!!!! YAY!!!!!

Absolutely chuffed, everything's working peachy, only problem is that the crank-case halves seem to be leaking a very small ammount of oil. I doubt it's the breather tube being blocked. Tried torqing some of the screws down, but the metal's too old and it's getting chewed up by the impact driver.

Anyone have any ideas on how to stop it?

So far I'm thinking, undo the relevant screw/bolts and replace them, then torque the buggers down.

Does that sound reasonable?

Not 100% sure where it's leaking oil from :(

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