If you could use the bigger pulley on both the idler and tensioner then I guess the belt would still be 'central'? Not sure if you'd get the belt over them though? Maybe 2 (custom made) larger than the black but smaller than the gold pulleys would be a good solution...
Timing belts are synchronous and as long as a enough tension is maintained so that the belt cannot jump a tooth, then the routing makes no difference to the timing relationship, unless the belt has stretched.
Whilst moving just the tensioner pulley will keep the timing the same, altering both by fitting a larger idler pulley will increase the amount of belt on that side of the 'Y' shape belt setup. The adjustable pulley will of course take up the slack, but will need to take up less so that more of the belt will sit to the fixed pulley side and therefore less of the belt on the adjustable side. Having a longer belt run on one side (and therefore a shorter run on the other) will move (advance?) the position of the camshafts in relation to the lay shaft meaning the timing will be altered. It wouldn't be by much, but then neither is using an offset woodruff key which is the 'traditional' way that the cams are degreed.
Dude, its one tooth out, one tooth in, as if it were gears. Only stretch or tooth jump can vary this once the timing relationship is set. The tooth spacing is set by the manufacturer.
Yes once set it won't vary but because the extra belt length is taken up on 2 sides there is the variable if you alter the amount on the fixed pulley side where previously you couldn't do this. If you had 2 adjustable pulleys then it would be nigh impossible (without a degree wheel) to set the timing consistently which is why there is just the one adjustable pulley (and it is on the backside of the belt run) so that the 'pull side' is always a constant length and therefore timing remains constant, changing this constant alters the timing. With a larger pulley but with the pivot point of that pulley still being in the same place there needs to be more belt length on that run, therefore allowing you to move the adjustable pulley less for better distancing, this means the length of belt between the layshaft and the camshaft on the 'pull' side is greater meaning that the cam will sit in a slightly different position, and as this extra is taken up inwardly it pulls the cam further forwards. Hypothetically let's assume that the length of belt from camshaft to layshaft on the 'pull' belt run over the fixed pulley is 100mm and we fit a larger pulley that then makes the run 102mm but the pivot points remain the same, the adjustable pulley then needs 2mm less tensioning but, importantly it means the cam pulley will sit 2mm further round in relation to it's previous position therefore moving the timing. Hopefully this explanation makes sense (and is correct)?
The timing is governed by the length of the belt on the fixed pulley side. If you make that length longer by 1 whole tooth, no problem, but if you make it longer by 1/2 a tooth How do you fit the belt? Ah! You rotate the cam till the teeth line up. Ie. You change the valve timing. The tensioner side makes no difference as you are just adjusting out more, or less, slack in the belt.
Is there anything you guy don’t know about these engines. But you have to remember, this is an old brickie asking so most of this is way above my head. I can build your house no problem. So from what I can make out even changing the adjusting pully on it’s own for a gold one just to give the belts more clearance is a no go?.
Unless you want to check and, if necessary adjust the timing then it's not a good idea. As it happens, I do have some brickwork needs doing though...
I think your current clearance is 'tight' but not unworkable, changing the adjuster pulley wouldn't cause an issue with timing but I think is probably not required. Double and triple check your belt tension to make sure it's not too tight after rotating the engine a few times
I’ve just downloaded the gates carbon drive app. So I’ll get to grips with that and see how it goes. Thanks for all the help so far lads it’s very much appreciated and reassuring.
Thanks for that .... but now I can't decide where that difference is actually coming from . Different diameter of the fixed and movable pulleys on the ST4 ? .... that would seem the most likely , but it makes me wonder why they did it
Exhaust cam is 10 mm lower on testa bassa heads (ST4, some 748), rumour has it that Ducati wanted to put the weight further forward on the ST to compensate for luggage and passenger - only Ducati would think to that level
Thanks for that .... it makes things clearer There'll be different geometric arrangement of angles and distances , in relation to the other pulleys .... if that makes any sense ?
