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[Enceladus]

The eccentric on the top idler roller isn't right. The point should be at around 11 o'clock and it seems to be more like 2 o'clock. So it's moved. The bolt must be loose. So the belt has lost tension between the inlet cams.

Take out the plugs if necessary and turn the crank to TDC on cylinder one but be careful and stop if you feel resistance.  Take another picture of the tensioner when at TDC. When the crank mark is at TDC then the timing marks on the cam pulleys should line up with the timing marks on the backplate. Ignore any markings on the belt itself. Check both banks with the proper timing gauge.

[YZ250]

The eccentric on the top idler roller isn't right. The point should be at around 11 o'clock and it seems to be more like 2 o'clock. So it's moved. The bolt must be loose. So the belt has lost tension between the inlet cams.
……….

Surely this depends on the backplate fitted. If it was a like for like backplate with the same marking, and with the 2.6 kit having a fixed lower idler, the tip of the direction arrow being sat at 7 o’clock/8 o’clock is about right. It’s certainly how my 3.2 was set up whenever I changed mine anyway. 

I do agree that this is a potential issue when fitting the belt in the first place though, as the adjustment nut goes one way and the bolt goes the other way, hence the importance of holding the nut whilst tightening the bolt. 

[Dave G]

Hi, after very carefully turning the crankshaft to align the camshaft marks, here are the photos.

This is the 1-2 pulleys - they line up perfectly with the backplate dents - any variation in the pic is down to parallax. Nothing is obviously loose - everything is tight

Highlighted the marks.


Similarly with the 3-4 pulleys. There's some muck in the centre idler, circled.


The only mark I can find on the crankshaft pulley is this - I rubbed some chalk into it. It's obvioiusly not needing a mirror to see it, so is this the right mark?

Anyway, it's to the right of vertical by maybe 10 degrees - relevant?


(Wasted 2 hours trying to locate a water pump bolt and a socket that I'd dropped.)

[Doctor Gollum]

If you remove the crank, is there anything obvious a miss there?

There's a mark at 6 o'clock on the crank. But so far no smoking gun re timing.

Did you remove the plugs? And if so, what state were they in?

[YZ250]

Hi, after very carefully turning the crankshaft to align the camshaft marks, here are the photos.
……….
The only mark I can find on the crankshaft pulley is this - I rubbed some chalk into it. It's obvioiusly not needing a mirror to see it, so is this the right mark?

Anyway, it's to the right of vertical by maybe 10 degrees - relevant?

That’s not the timing mark, it’s behind that big crank aux pulley as shown in this video clip at 3.45 minutes.    The big pulley needs to come off but for ficks sake DON’T try to undo the big bolt in the middle.  Undo the six bolts holding it on and it will tap off, the timing mark UNDERNEATH the crankcase cog should align with the crankcase notch, hence the need for a mirror and a torch. 

https://youtu.be/YCF1feeUyc0?si=WWLjd7psqXmA1J54

The cam notches and top idler look ok, the lower idler is fixed.    Does the crank turn a full revolution, obviously with the plugs out, with no resistance.    If it does, we could be chasing a red herring here and the fault could be elsewhere. 

[YZ250]

......
Parts removed (scary - for me anyway)

Fair play for having a go at investigating, you're doing a cracking job so far. 

[Dave G]

Thanks for that YZ250...

Now here's a pic of the bottom sprocket - nothing turned since the cam sprocket pix earlier. I assume the line at about 4:30 should be vertical to align with the notch on the back plate?


And here's another - spooky one...


These bits were in the space under the cam belt sprocket...
Two MORE 12mm pins (now I've got 4), a stiff-looking piece of something, and some torn rubber bits. So those pins from further up the topic must be relevant...

[YZ250]

Shit, the timings slipped. Not sure what the other bits are (looks like part of the cam cover)  but what would be really annoying for you is if those bits fell into the path of the belt and got between the belt and bottom pulley. That’s shit luck that is. 

Now the question is, what damage has it done. We’d have to guess that the clattering was the bits flying about in there until the belt slipped and stopped the engine. If the crank will go to TDC without force, you’ll be able to see roughly how many teeth it has jumped by the cam positions.

[Dave G]

That’s exactly what I was thinking when I saw the next two pins.
They would seriously derail the belt…

[Dave G]

So am I right saying that line should be vertical??

[YZ250]

So am I right saying that line should be vertical??

Sadly yes, it should be.

[STEMO]

Where are the pins from then ?

[YZ250]

Where are the pins from then ?

I’ve tried zooming in to the steel backplate as the only thing I can think of is the fixing lugs.
Edit: They still look like dowels to me. 

This is a kicker as it appears that nothing has let go, but more like the loose parts have derailed the belt.

[Dave G]

Here you go.



No threading, slightly bevelled ends - very shiny.
Centre sections quite dull as if they were load bearing?

Whatever they've come out of there must be more - nothing with a roller bearing has just four rollers...??

Does anyone know if either the belt tensioner or idler uses this kind of thing?
Or maybe someone has a scrap one to take apart to check??

[YZ250]

Searching for engine breakdown drawings now.