Cam Shaft Chain Snapped / auto decompression

[Henry74]

Hi all,
Need some good advice from you
after 25 000 km my Scorpion tour engine made some clanging/tapping noise.

To begin with my motor has Solid balance shaft gears (without springs). The motor is open and it has NO loose nuts/woodruff keys.\
What i noticed is a snapped Cam Chain guide and scouring on rocker arnm where the Decompression pin hits it "momentarily"

First picture is the snapped Cam chain, I measured it and got 25.3 cms, does anyone has a new/used and measure it for me so I could determine how much it s shortened and if it was the cause ?

Could it be the cause after all ?

Could it be that the auto decompression pin is not retracting back, i noticed scouring marks , picture below.
Is this normal I ride the bike almost every day so I start the MZ twice daily every day(deco only works on startup hence an idea of usage)

Looking forward for replies so ill order parts and start reassembly of the engine and bike(I just prefer it than the heavier B12)

[edfmaniac]

The chain guide appears to be normal length and the decompression pin has to hit the rocker arm to perform it's function. It only retracts once the engine fires and the rpms come up to idle speed. Did you check the valve clearances? They can make an awful lot of racket.

[Henry74]

Thanks edf, no I did not, I should have. On the other hand take a look at this pic since the tip looks broken. I don't have a new one yet so I can't know for sure.

[edfmaniac]

Lots of pics on the web to compare with. Looks pretty close to full length from where I'm sitting.

[edfmaniac]

I'm curious if anything performance wise has been done to your engine considering the balancer drive gear has been upgraded to a solid one. If you plan on riding the bike hard, you may want to delete the decomp system all together as it can fail at high rpm like the balancer drive gear. It's very easy, however, I don't know how much cylinder pressure the stock cam builds during turnover. The Megacycle X4 that I put in my bike turns over very easy without the decomp system. Let me know if you want instructions.

[Henry74]

Thanks again.
Am turning a blanking plug to keep oil pressure up on the can but which can still feed oil to the lobe and bearing surface near the auto deco.
PS if anyone have a used/ new chain guide and could send me the measurement, that would be helpful.

[edfmaniac]

Thanks again.
Am turning a blanking plug to keep oil pressure up on the can but which can still feed oil to the lobe and bearing surface near the auto deco.
PS if anyone have a used/ new chain guide and could send me the measurement, that would be helpful.

There's an easier solution but plugging the end and the pin hole will work too. Good luck with solving your noise issue.

[Henry74]

Which is ? I went for some relaxing fishing session, so I postponed the turning bit.

[edfmaniac]

All you need is a tap and set screw. Tap the hole where the pin exits and thread your set screw in to hold the OEM part in place, making sure it's just below flush so it clears the rocker arm. The set screw will plug the pin hole and the OEM part stays put to keep the oil pressure up. Just make sure you clean the parts really well and use a good high strength thread locker. I have 1000+ miles on my modified motor since doing this so it's holding up.

viewtopic.php?f=3&t=6620&p=34440#p34439

[Henry74]

That s brilliant and easier. Though I think I d go by it differently. My idea is machine a stepped plug flush to the deco plug, tap it M4 then machine a short round stock ending with an M4 thread to lock with the plug. I usually leave an open option to revert to the old system if Iencounter starting problems.