Another disaster…

The heavy showers, made the track at West Atherton slippery and boggy in places, though it was good elsewhere.

Rob Griffiths...

With the SC500 offline with a transmission failure, I just had the 250 to look after, though practice was all I managed. Near the end of the session the big decided to just stop and not restart. Without Malc and his quad on the scene, there was plenty of pushing out of the mud going on, but luckily I’d broken down on the final corner. Still knackering to get it back to the van.

With another rain shower going over, I checked the bike over; spark, fuel, but not running. Changed the carb, checked the LH crankseal, direct fuel in the pot, etc and still nothing. 60 minutes later decided that was enough and loaded the bike into the van.

Return from Narberth

The lawn looking less than perfect

With a lot of work travel coming and Hawkstone on 5th May decided to get back and work out what is wrong and get it running. Back in Bwlch and the washed and cleaned by 5pm and a deadline of 9pm to have a running bike, or I’ll withdraw the Hawkstone entry.

The spare engine, ready to fit

The plan was to go back to the 2011 season bottom end and swap the new top end. Firstly took the top end off the old engine and made a new base gasket using the old one as template. Took the other engine out of the bike (continuing to clean the Narberth mud off the bike) and then stripped the top end. The two-meeting old Wossner piston wasn’t looking too good. I’d checked the crown when I took the head off following Clyro and all was in order. However, the small metal fragments across the piston crown wasn’t good.

Not looking good

Therefore, into the piston selection box and found a 0.25 (1st oversize) that matched another barrel from the collection. The ring was worn, but also had a replacement in stock. The advantages of OEM Yamaha pistons and rings are:
– they are available, they may not be listed as MX250, but the later YZ250 pistons fit
– they are cheap, USD 30-50 for piston and 20-30 for a ring, about the same in GBP and in the UK.
– you can get the oversize pistons, up to 1.00mm (4th oversize)
The Wossner and Wiseco pistons are also relatively reasonable (compared with other pre74 kit).

Honed the barrel and look to find the best head and carb mounting block. Moved the reedblock across to the new barrel and then onto the crankcases (with some hassle getting the ring past the top of the boost port).

Back into the frame and the reconnection (re-timed the PVL ignition, exhaust remount etc). Petrol tank back on and it fired up third kick and it sounded okay. Good news and it was 7.45pm. Time for a Spag Bol and then back to finish up with some cleaning and fastening of cable ties. All done by 9.15pm and to get a Blanche de Brussel from the fridge. I’ll be at Hawkstone, but need to prepare a backup piston and barrel, so now need to locate a company that still does rebores.

Finish, at 9.15pm

Reliability this season hasn’t been brilliant, which after two years of pretty good performance. It does make you wonder about getting something else (CZ ?) however an evening in the workshop makes you realise that the Yam is easy to work on and having a stock of spares is cheap and easy to arrange. That’s for the 250 though, the SC500 is a different story.

Leave a Reply

Your email address will not be published. Required fields are marked *