Hi, I've searched the forum and can't find this particular topic. I've changed the belts for the first time on my 2000 748. The belts I've put on are the same length and part number as the ones I've taken off (so not the shorter 2001 748 belts). The problem is the horizontal belt is not tight enough (get more like 90 degrees of twist) even though the tensioner pulley is on max tension. The belt I took off was loose as well. The vertical belt is correct tension. Any ideas?
The only thing I can think of is possible bearing failure on either the idler pulley or the tensioner pulley, but that should have been obvious. Steve
Maybe not that obvious to he uninitiated ? Is it possible the tensioner is being turned the wrong way? I know that can cause slightly different problems with the testastretta
I think you might have it old rider, The tensioner should be rotated anticlockwise to achieve tension but it is possible for owners to erroneously rotate clockwise. Never stripped one myself and discovered this, but I know people who have. Two things result from this, you can't achieve as much tension (just as you have found) and the belt becomes perilously close to touching itself. Below is correct so you can easily check: –
Yea agree with Chris above, changed mine 2 weeks ago and noticed this too. Need to turn anti-clockwise.
So it is similar to the Testastretta motor in that respect then. The other thing you need to be wary of with theTestastretta is that the two spacers under the bolts are sitting flat. You check this by placing a straight edge across both. If you get this wrong the belts will not be aligned and will try to come off the pulleys, leading to shredding.
I just found it odd that the OP has got the vertical one right and not the horizontal added to the statement that the original horizontal was also loose, just a coincidence then. Steve
What are you using to tension the belts? Twist method isn't too reliable. I've seen what you're describing many times, but the reality is that using the correct tool to tension shows the belt can be set right. If you don't have a tensioning tool to hand, try measuring the belt deflection centrally between the two cam pulleys. You're looking for 5mm when pushing the belt down from its rest position.
I agree but as it's his first time changing the belts I presume a bit of research will have flagged up the need to rotate the tensioner pulleys anticlockwise, all the videos and even the Haynes book highlight this. Steve
Come to think of it, that could make sense. If the original horizontal was loose, it could be that the tensioner had been turned the wrong way originally. Then, when the op came to retension, he simply turned it in the direction which appeared to make it tighter, which would be the wrong way again.
The complex part of this job could be seen as the counting teeth and marking, once that's done and the belts are in place, tensioning up and replacing covers etc could seem like the home straight, so less concentration on procedure required. If the tensioner was not fully slackened off and was already rotated the wrong way, then rotating in the correct direction would appear to slacken the tension and rotating in the wrong direction would appear to increase tension.
I don't bother counting and marking the teeth, if all the timing marks are lined up there's no need, a full tooth out moves the timing marks a long way off. Steve
Thanks for all the advice - I've stripped the idler and tensioner pulleys out to check the bearings, they all seem to have the same amount of float - all the pulleys rock slightly on the bearings, but seem ok otherwise. I swapped the tensioner pulleys over, and horizontal belt was still looser than vertical (using both twist and 5mm deflection method). I've taken the idlers off, next step is to swap them too, and measure pulley diameters to see if they've worn - just need to get some threadlock to put them back on. I will try the anticlockwise method, I was doing it clockwise, but I can't see how that makes a difference, it's down to geometry surely?
Indeed, I don't see the point in the counting business either? The number of teeth between pulleys is not variable so as long as you've got the right belts (you'd know if there were too many or not enough teeth) and the dots line up then it must be right. As said above, a tooth out is a lot and definitely noticeable.
Tightening clockwise will make a big difference, for a start the inside of the belt may rub against itself on the other pulley. If so it will shred the teeth off and cause you a huge problem when the cam timing jumps. I have also heard of belts tensioned wrongly 'walking' on the pulley so that it wears the edge away plus, as you noticed you might not get it tight enough...
