Hi,

Posted a while back on this, got some serious vibes unable to solve and only getting back to it now.
Was recommended to check flywheel forward for the source, so far, found the following:

With my shiny new DTI, I measured the following:
Engine shaft runout at tip: 0.07mm
Adaptor bolt runout at tip (bolts over the engine shaft): 0.3mm

Flywheel on the 25cc is a Phelon (45?)
Flywheel on the 30cc is a Walbro (G62)

The Phelon has been machined down but seems much more out of balance than the Walbro (unmachined).

Questions:
1) Is the runout sufficient to cause significant vibrations? I think not as I used the same crank/flywheel/shaft on the 25cc without disturbing it (just changed the piston and conrod) and it was fine, no vibes.

2) Should the flywheels balance or is there an offset to counteract part of the piston assembly weight?

3) If there is an offset built in, is it possible that there is a variation according to the piston used? If so, one would expect the Walbro to have greater imbalance, not less!

4) Is the crank web the same across capacities? I was told that it is interchangeable but I dont see how a bigger piston (and therefore heavier) can be effectivly counterbalanced then.

5) Can I eliminate the runout by tapping the end of the shaft with a hammer once its all screwed together?!

Im going to try running the Walbro flywheel on it without the prop conversion (if I can work out how to start it) to home in on the vibes.

Any help greatly appreciated!

Thanks
Mark