Right, as I say Dopey question, I'm looking to have my belts done on my 749, I've looked at few videos (youtube), looks complicated but, I just wondered, when the engine is stopped it must have been stopped when everything was correctly timed, so when it stops no matter which cylinder is at TDC etc, why can't you just take the old one's off and slip the new one's back on? I'd imagine that you' d have to loosen the tensioner but, as long as the cams don't move out of sync with each other and the crank Position then surely you could just slip the belts on and re-tension? Obviously I am mad, but can you tell me why? Thanks and just remember I've had a drink so that's my excuse!
The pistons have to be in the correct position relative to TDC to ensure that the belts are correctly tensioned. The belt needs to have the correct load applied by the cams to set the tension. (Also had a drink so may not make much sense to a sober person!)
I bow to your superior knowledge, just seemed that if it was running when you switch it off then everything must be ok, so other than being careful not to move the cams in relation to each other and the crank, then why not? Obviously the tension would have to be done carefully but..... Thanks for your reply.
There are springs on the closing rockers that rotate the cams to the closed position when the belts are removed.
The older bikes (Desmoquattro) have alignment marks on the cam pulleys, I understand later models don't? (don't know why?) If this is the case then simply mark them before removing the belts and it'll be good to go. As for not turning the cams, they may inadvertently turn plus it would be too easy to slip a tooth out of sync when refitting so unless you can physically lock them in place then there is too much risk. I also check the valve clearances with the belts off so will turn them many times, not to mention take the cams out to check the rockers so physical marks are only way to go.
Just to add, if you took the belts off at some random positioning and then a cam rotated, how would you know which position to put it back into? You may get called away by the phone, a cup of tea, etc, etc. If the cylinder is aligned at TDC then this becomes your reference point so you know how everything goes back together because of the timing marks, regardless of whether they move or not.
Regardless of the timing marks, I always mark all three pulleys and mark the belts in the same places (away from the timing marks)........I also mark each belt so I know which is vert and which is horiz and also the direction (then I take photos of them)..... When the belts are off, I count all the teeth between each of the marks and then mark each new belt to match........ Unless you can't count, it's idiot proof.
Very true! I went with yellow paint to highlight the marks on pulleys and casings and Tippex on the belts to match up with the marks. I think I did a couple of extra Tippex blobs on belts and pulleys too for good measure. Then you can mark up the new belts to match the old ones. It's better to be safe than sorry.
Very important to get it on TDC on the firing stroke, not the exhaust stroke. I put a timing disc on the end of the crankshaft with a fixed pointer on the crankcases as It's possible to get confused.
Various methods. One handy method is to screw a sparkplug with the central electrode removed and a tube inserted into the hole long enough to protrude above the cylinder head and then fit a balloon to the tube. Then when the piston travel upwards in the cylinder, if both valves are closed ( Piston on compression stroke) the balloon will inflate. Of course it wont do this if the piston is on the exhaust stroke as the exhaust valve will be open. That's why I fitted a timing wheel to the crankshaft, so the once TDC on the firing stroke has been found, the timing disc can be set to show TDC, (then when you are turning the engine over backwards, and then forwards to eliminate backlash) you always know exactly where you are. There are other methods of finding the correct TDC, but right at this moment I can't think of them . I'm sure someone else will enlighten us though.
btw I used the Ducatitech videos to change my belts for the first time as I found them the clearest. If you are going to tension the belts using the Sonic method let me know and I can lend you the microphone.
Just set the horizontal cylinder using the timing marks on the pulleys and the ones on the end of the flywheel, double check the piston is at TDC by carefully lowering a screwdriver or something similar (but blunt) and ensure that both the inlet and exhaust valves rockers have clearance - that way you know that they're both in the closed position and the piston must therefore be at TDC on the compression stroke. Then you turn the engine in it's normal direction of rotation about 270 degrees (look at the crankshaft pulley as a guide) and do the same with the vertical cylinder. Double check that the timing marks correctly align with the 'pointer' through the window of the left-hand engine casing as before and that's TDC on compression for that one.
i think its about 12degrees of crank shaft rotation at tdc use a dial gauge mark when stopped rising and starting to drop and mark in the middle. but shouldn't mater unless your building from scratch.
Right, thanks for the info Guys, so much knowledge! Lesson learnt, I'll probably get it done for the first time by a proper Mechanic, I'm very interested though in doing it from the videos - thanks again everyone.
Just follow the Ducatitech video. It looks more complicated than it turns out to be. I used an iphone with a guitar tuning app to get the correct Herz reading. Not quite as easy peasy as a two valve but easily doable with moderate knowledge
