Hi, new on here, picked up a yellow 748E a month ago. The belts had been done 3 years ago so I'm in the process of fitting new genuine belts. I've done the obligatory YouTube vid watching, but have a couple of Q's. I can get both belts to 110hz measured on the longest run, but if I turn the motor over a couple of cycles and get it back to tdc on the horizontal cylinder the tensions have drifted, mostly on the vertical cylinder, often to ~125hz, so not subtle. Is there something I'm missing? I haven't seen anyone using vertical cylinder tdc which I thought could be the problem. Belt tention seems to vary by crank position from 90-125hz?! Secondly any recommendations for 24mm spanners to hold the adjuster still whilst tightening the nut? Standard ring spanners I have are an insecure fit, I was thinking of a cut down ring spanner perhaps ? Tensioner bearings feel ok. Bike has nearly 17k on. My first bike in 15 years!
Don’t worry about the change in measured tension after turning the engine over a few cycles. With the horizontal on TDC compression fit the belts. Take out the slack on the belts on both H & V cylinders then rotate the engine s few times to A) make sure all is ok and there’s no valve to piston contact - which there won’t be if you’ve put them on correctly, and, B) to settle the belts on the pulleys. Then go back to HTDC compression and set the tension in the H belt. Then turn the engine to VTDC compression and set the tension on the V belt. Torque for the tensioner pulley bolts is 25Nm. Then put the cambelt covers on and walk away. If you keep turning the engine and remeasuring the tension you’ll get a different result each time. You’ll be there for ever doing and re-doing it.
...to tdc on the horizontal cylinder the tensions have drifted, mostly on the vertical cylinder, often to ~125hz, so not subtle. Is there something I'm missing? I haven't seen anyone using vertical cylinder tdc which I thought could be the problem... I read that as you tensioned & checked both belts at horizontal TDC. In which case you must do each belt at their respective TDC positions. Or just follow the instructions from @West Cork Paul
Tensioned the vertical cylinder at compression tdc and that seemed to keep the tension more repeatable. Fired up and seems ok Getting the fuel hoses back on without the quick disconnect fittings a pita, tempted to look out for a non E fuel pump plate. Thanks for the help!
