De-tuning the MX250

The experimentation with a clip on filter at Narberth hadn’t proved to be too success and whilst the engine was better and there was some better starts, it wouldn’t idle and required a bit of tuning between races.

I decided to revisit the carb tuning, going back to stock and then working forward. Armed with two stripped carbs, a carb kit and a 1974 MX250B, MX360B and SC500 Service Supplement manual , time to get the bike into racing trim.

I experimented with the following settings. This is with the airbox fitted and Uni filter in place (more on this later)

|Component|Standard Settings| Setting 1|Setting 2|Setting 3|
|Main Jet|270|270|270|250|
|Needle slot|2|3|4|2|
|Pilot Jet|50|50|50|50|

* from Keystar carb kit

Some comments on the different settings; basically they were all a bit crap. Some go in 1st, but no pull in 2nd at all. Tolerable and no worse than at some of the meetings earlier in the season.

Tuning the MX250

After a while, and a few laps of the field to annoy the neighbours, I failed to start it and the process of trying to restart, managed to flood it. Used the opportunity to take the head and barrel off. Not good, with loads of fuel in the crankcase and in the exhaust. Further inspection and a second opinion later it was clear that:
– the piston ring was knackered. Massive end gap and at least 1mm off the thickness on the exhaust side
– main bearings; very noisy and shot
– big end; lots of play and shot

Revealing the horrors

The reason for the damage, well there are a number of possible reasons
– dirt intake; all of the meetings (with the exception of Teifiside) had been really dusty and the air filter hadn’t been perfect
– the rebore; the tolerances provided by Yamaha are on the slack side
– the cylinder bolts coming loose, may have hasten the demise of piston rings
– the big end may not have been perfect from the outset
– oil premix may have been on the low side, with not enough oil getting to bearings. Time to re-visit the 2 stroke oil and ratio of the mix

All up, not good and with less than 20 races. Classic scrambles is a bit tougher on engines and bikes than trials, without an engine strip on the Beta in 2 years. It may however, be an indication that its a completely different ball game (or bike event). Things like the air filter and oil mix are much more critical, as is engine performance.

In the subsequent few days, I managed to pick up a spare bike, an engine and also a crank / big end kit from Keith Alderman at Motolink .

Engine #4, attempting to remove the front sprocket

The spare engine needs some work and although it seems to run, the exhaust manifold bolts are non-functional and it needs a new gasket. The cases are ‘nicely’ polished and need returning to normal. The engine from what now must been seen as bike 2 seems good though bit of a risk running it as is.

Swapping it into the running bike and yea, the power is back. Bit of a top end rattle, but will see how it runs for now (and see what else I can break)

Overall ‘Bike 2’ is mainly complete (well until I took the engine out), with only the rear shocks and cables really requiring replacement.

More info and (hopefully) race reports to come.

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