A running bike

The engine rebuild was finished a couple of weeks ago and was actually pretty straightforward. Compared with Montesa or SWM (Rotax) engines, the Yamaha engine went together pretty well and on the first attempt.

I changed the main bearings, the crankshaft seals (which can be done from outside the cases) and got the gearbox back together in such a way that all the gears were there.

I got the spare barrel with the broken fins welded up and then rebored (by local rally specialist Russ Tyler Motorsport, with lots of attention to detail). New 2nd oversize piston from Motolink fitted, along with new gudgeon pin and little end bearing.

Yamaha MX250 after a couple of laps of the field

The clutch plates were serviceable so went back into the bike as is. I painted the crankcases and covers with Projekt Barbeque paint from B&Q, which is heat (but not totally petrol) resistant.

I put the engine back in the frame without the piston and barrel, just to make it a little easier and then fitted them later.

I then jury-rigged up the bits I needed to see if the engine was okay, and put the carb on the engine having not stripped it all. After a few kicks nothing, so dropped some fuel directly into the cylinder head and hey-presto she fired up for 15 seconds or so. Engine sounded fine during the brief run up,

Took the Mikuni carb off and gave it a good clean and also adjusted the float height as there seemed to be very little in the bowl. Still wouldn’t run without direct fuel injection through the spark plug hole. After a few iterations, the bike then fired up and ran and my attentions turned to the fuel tap, and that ‘reserve’ may not be working, but ‘on’ was fine.

Not too pretty, but running

A bit spluttery at first, but it cleared itself after warming up. My field has a tight corner at the bottom which meant dropping down to first then a quick shift up to second and then opening up heading to house. Timing and fueling seemed ok and the front wheel was happy to lift slightly under power. Excellent !

Some work still needs to be done, including:
– carb, needs more fettling or replacement of jets. Seems to run ok, but not idling too well and need to check if the mixture is right. Its a Mikuni and my formative bike years have been mainly with Dell’Orto’s.
– seat recovering, should be straightforward as has grips built into the seat base
– electrics, these were rigged up temporarily to get the engine running, including the fitting of the Yamaha-style kill switch. Need to get the solder and heat shrink out to finish the job.
– new chain, have re-used one of the old ones for now
– side panels, need to be fitted along with the front number board, vinyl ovals and numbers
– front brake, currently not ideal as re-used one of the cables I had. New brake pads and springs might be also a good idea. Back brake seems fine
– suspensions, works but the front seems a little hard, but maybe because I’m used to a trials bike

Hopefully finish the bike off this coming week and get the entry in for the first race of the season, April 18 and the Llanthony Classic meeting at Clyro.

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