I was heading out this morning and had just warmed the bike a little, when I was tipping into a roundabout at low RPM. Nothing more than 4k or so on the tacho and the bike gave a bit of cough and there was a distinct metallic noise and the engine stopped. I don’t think that the back wheel locked. Belt failure was my first thought, which is a bit of a pisser, as there were changed by a pro last year. I managed to get it home and ripped into it. The belts did not snap, thankfully, but the horizontal one was very chewed on the inside. One section had actually started to split, as can be seen in the pic. There was also a fair amount of fluffy stuff from the belts left inside the housing around the drive pulley. I took out the plugs and put it in top gear and tried to turn it over by hand with the back wheel, but it would only turn a little and then it seemed to lock. I did not want to force it, so I surmised that the horizontal belt must have slipped a tooth. I had an old set of belts in the shed, so I took of the horizontal belt and put on the old one and lined everything up and tensioned both belts by the quarter turn on the long run method. I was then able to turn over the engine smoothly with the rear wheel and there were no noises or evidence of impacts. At that point I reckoned that if there was a bent valve then it probably wasn’t going to get any worse. I put the plugs back in, reconnected the battery and decided to fire it up. It started no problem and seemed to run fine. There was no hideous smoke or crunchy noises from the engine, so now I am in two minds. Should I order a new set of belts and just let it go, or should I strip it down and have a look inside the head of the horizontal cylinder? Thoughts? A
The binding on the belt could be due to too much tension. Were the belts fitted recently and was the tensioner wound the right way when fitted? (wrong way would mean the belts can almost touch at tickover and consequently do under load). I always find fine tensioning the vertical cylinder more difficult to get it perfect as its awkward to get the same feel due to the frame tube in the way than on the horizontal, a double check is always recommended . Either way, you are lucky for sure. Personally I would fit a new set of belts and be done with. If the engine runs well enough, maybe a compression test and I would leave it at that IMHO.....
I had retensioned the belts in March, about 3000 miles ago when I was fitting a new oil pressure swith due to a leak. I took off the covers to clean up the oil leak and realised that the belts looked fairly slack, compared to most references I had seen. I used the Hz method, with a guitar tuner on my smart phone for tensioning. That said, they seemed pretty loose on inspection this time. The belts had been fitted by a pro last summer. Damn, I had just been complimenting myself on how smooth the engine had felt after the retension. And yeah, I was bloody lucky A
I think you may have fallen into the trap of thinking the belts looked too loose. Now that you have tinkered with the tension any reference to the previous mechanics work is irrelevant. Do a compression check if you can. If the bike ticked over smoothly prior to this event then if all is o.k it will now. If not it might indicate a valve seating issue which in turn and due to what happened would indicate a bent one. Fingers crossed !!!
That's a good point, a compression check should indicate if there's anything further amiss. Cheers, A
+1 Check the tensioner pulley is adjusted in the correct direction, also the pulley bearings condition as this could make the belt 'track' off the centre of the tensioner pulley and that belt is running at the back of the pulley.
OK, not what is meant here when saying the correct direction for the tensioner pulley? None of the procedures I have seen have indicated that it only goes one way. The bearing seems fine, in so far as the pulley does not seem to have lateral play in it. A
The pulley will turn and add tension in either direction but if you tighten it clockwise then it sits lower and closer to the fixed pulley, so close that the belt teeth can touch each other whilst running. Adjusting anti-clockwise takes it up and away from the fixed pulley, it's an easy mistake to make as it seems intuitive to go clockwise because of the way it tightens. I did read on a US forum someone claiming that if adjusted clockwise then the belt has a tendency to 'track' although I'm not sure why this would be? The belt shouldn't be running to the back of the pulley and the pic shows it there.
When you "retensioned" did you do it too 110Hz or to 80Hz ( I think) because 110Hz is for new belts only.
@Dukedesmo: Gotcha mate. That is indeed the case. I adjusted the pulley clockwise on the eccentric, as that is the intuitive thing to do. Arse biscuits. Oh well, I'll know for the next time.
When I retnesioned, as per the Hz tech note, I tensioned to 90Hz for used belts on a four valve head. The belts were on for a year and looked in good nick, so I reckoned that was the best. It was easy to set the tension and both came in easily with the 90Hz +/- tolerance. However, as per Dukedesmo above, I think I may have tiurned the eccentric adjusters in the wrong direction, thowing out the alignment of the horizontal belt run. Thanks guys. A
Although the bike starts and seems to run fine that doesn't mean that the valves are ok. Recently, due to an error on his part, my son had managed to get the belt timing out on his 916. When he corrected it he thought he'd got way with it as the bike started with no problem and idled OK but when he took it out on the road it ran badly below 3,000 rpm. When he did a compression test it showed the vert cylinder at over 150psi but the horiz down to 120 psi. We took the head off and found that the 2 exhausts had just kissed the piston and weren't obviously bent but were enough to prevent them from seating properly.
Check also the spigot that the tensioner pulley mounts to. It's quite common that it unscrews when you undo the tensioner nut but doesn't get screwed back in when you tighten......that throws the belt alignment out. If you've just checked tension and not removed the pulley, you wouldn't spot it.
@Derek: Thanks for that mate, I ahve a compression tester on the way for that very reason, good call. @Nelly: The veritcal cylinder is all lined up, belt OK and tension good. The issues were with the horitzontal belt. I whipped off the damaged belt and put on an old one from the last change and so I got it all lined up and marked before I fired it up again. Thanks for all the help chaps. If the compression test is OK, I'll check the pulley spigots before I put it all back together and hope for the best. Lessons learned all round! A
Well, the worst was confirmed last night. Vertical cylinder reads good, compression was about the 150psi mark on a not particularly accurate guage, but compression on the horizontal cylinder is effectivly nil. It is pumping but practically zero compression. So, I reckon there is a bent valve or two at best, and at worst a holey piston! Buggery fucksticks bollocks. I'll pull the head an have a look at the damage. A
