Belt idler pulley

[Douglas]

Hi

I suffered a terminal engine failure on my '97 748 Bip in Mid Wales yesterday. This happened at fairly low revs (about 4k), and the engine was still running on one cylinder. The bike was recovered home.

When I got the bike home I removed the front cylinder cambelt cover to find that the belt idler pulley had failed. The stud holding it on had sheared in the head at the top of the thread (overtightened by someone no doubt?). The pulley had embedded itself in the cover. The belt was still engaged on both cam pulleys, and looked to be still driving the cams, but of course very slack.

Has anyone else had this happen; is it a common problem? Also any ideas on what carnage I am likeley to find inside?

[rockhopper]

It is quite common, the idler pully is known to seize and do exactly what yours has done. Over tightening isn't the problem. The pully itself should be checked each time the belts are changed.

Worst case is a number of bent valves and possible head and piston damage. Anyhow the head needs to come off to have a look.

[Douglas]

Thanks. The pulley is running quite freely on its bearing now, but I wonder if the seizure usually clears itself?

[rockhopper]

Not sure. I know its a known fault but i don't actually know anyone its happened to if you get what i mean.
Most of the guys are away at Cadwell right now so you might get some more info on wednesday when they start to arrive home.

[Douglas]

Thanks. I look forward to hearing the wisdom of the others when they return, as i am anxious to know what caused it to prevent a repeat on the other cylinder!

[Karl]

Did you find the cause Douglas?? If you checked the idle pulley and that's ok then the other possibility would be, the stud failing due to the belt tension being too tight and putting stress on the idle stud. would check the conditon of the tensioner pulley and replace the locknut just in case.

[Douglas]

Karl, I havent really got to the bottom of it. I had wondered aboout an overtightened belt, but I set them very carefullly using the Hz meter method (110hz both belts), and they had done nearly 5000 miles when they failed, by which time the other belt had slackened off.

The other mystery was the very low revs at which the failure occured. Infact I hadnt used many revs on the bike for the last 1500 miles, as I was rideing with the wife, who rides pretty gently!

Any other ideas anyone?!

[rockhopper]

Its too early for me to work out the physics behind it but belts and things like that often tend to fail at low revs becasue the loadings on them are higher. My dad had an old Rover SD1 2.6 and that seiezed its cam shaft at tickover (twice actually but thats another story).
Years ago when i used to build Mini engines with hot cam shafts they needed to be run in at highish revs otherwise it was possible to knock the lobes off the cam.