The initial run of the bike was dominated by a noisy top end. Whats going on ?
During the initial fire-up of the Jumbo, I didn’t have the mid and rear box of the bike attached; just the front pipe. Though it was a little loud, as Gavin my not too near neighbour heard me fire it up from inside his house and it had my ears ringing after, you could tell there was some noise from the top end.
The first run around the garden confirmed that there was a bit too much noise and Gavin confirmed the diagnosis as piston slap. So, 24 hours after getting it all running, the engine came out of the bike for a top end strip, which took me around 45 minutes. Don’t think you can get the piston out with the engine in the frame, as you wouldn’t get the barrel off.
Confirmation that the little end bearing and the gudgeon pin were worn, but I’d seen this during the rebuild. More seriously was the gap between piston and barrel was way to large and more and the 0.08mm tolerance. Now, I’d looked at this on reassembly and both piston and barrel looked good ok, with no wear marks on either, though confess to not checking the ring end gaps, nor the piston to barrel clearance.
Out with the vernier; with the barrel at around 84.10mm. The piston is marked at 84,17mm (note continental decimal point) and is an Eckoh piston, made in Austria. The piston is in good condition, but not the right size.
An email to Peter Knight later, Rotax man in the UK and it seems that the original piston size is 82mm (?), with 4 possible overbores. These are 82.25mm, 84mm, 84.25mm and 84.50mm. There for this barrel was somewhere between 3rd – 4th rebores.
Therefore decided to go for a new piston and 84.25mm piston, with rings, new
gudgeon pin and small end bearing all ordered from Peter Knight. So waiting
for delivery of these before reboring the piston and putting the bike