Tuning the MX250

After putting the bike together in 2010, and with attention focused on the SC500 and the YZ250 for 2011, time to spend some time preseason on the MX250.

Therefore with the 2012 season looming and my first event at Clyro on 1st April, time to get cracking in the newly refurbished garage.

I’d spend a trip to Germany reading [ A.Graham Bell’s two Two Stroke Performance Tuning bible ->
http://www.amazon.co.uk/gp/product/1859606199/ ] decided to build a complete new engine and take the engine from last season and leave it as back up on the bench for now.

The MX250 in no way measures up to the YZ250A or B of the same era (1973-4); its heavy and hasn’t got the engine power of what was an out-and-out dirt bike. If you do the comparisons and analysis (and it’s something I might do during the season), the other Japanese and European bikes were more motocross focused. Making what was a relative cheap dirt bike competitive isn’t straightforward.

I had a bottom end, which (from memory) I’d rebuilt the engine during last season, following the engine implosion during the 2010 season. This has the original conrod but has had limited use so all should be good.

Engine for 2012 (?)

So the work on the engine and the bike included:
– polishing of the boost/inlet, exhaust and transfer ports
– cleaning and polishing the head, in particular removing any anomalies in the squish chamber
– strip and carb clean
– fitting new piston
– new reed valves
– new electronic ignition

Some  port chamfering helped get the barrel in place

Barrel and head

Mainly focused on the barrel and the head, as mentioned above I didn’t strip the bottom end. It may need doing but time wasn’t on my side. I focused on polish the main ports with a dremmel [[Actually it’s a cheap Argos alternative, but which came with all the right attachments]] and took out the ridges and obstructions.

I didn’t change any port heights (well yet, I’ve two other spare barrels to play with later in the season) and then some guides in the period Clymer manual of some interesting modifications.

I didn’t measure the squish band in the end (though had the modeling clay ready for that purpose.

Strip and clean Carb

The Mikuni carb (NOS Yamaha supplied painted black) has done two seasons and was pretty mucky so stripped it down and took a lot at the jets and set-up.

Also, from reading ‘the book’ determined that float height was more critical than I thought so also carefully measured and set up it. Interestingly both the Yamaha Service and Clymer manuals have the place to measure wrong (I think). Turn the carb upside down, let the floats sit on the fuel needle valve and then measure on the mid-line of the carb, from the float arm down to the carb body.

Given the relability of the Del Orto on the SC500 (and numerous SWM’s from the past) I’m considering getting one for the MX250. Just thinking for now.


Reed Valves

Acquired some Boyesen reeds from ebay.com and they arrived prompty, but with £0.15 and an £8 handling charge ! Fitting took a couple of minutes and two sheet approach makes the original metal petals look a bit old fashioned. I didn’t adjust the petal stops, but this is an option for later tuning and development.

New Piston

I’ve got a collection of NOS Yamaha piston’s I’d acquired over the last two years, but decided to a get a aftermarket one. Basically for the MX250 (and 360, but not for the SC500) it seems there there are two choices, Wiseco or Wossner. Both come as kits, with gudgeon pin, circlips, rings, but little end bearings. The also come in 0.25, 0.5, 1 and 2mm oversize it seems.

The latter, German made, Wossner pistons are usually cheaper in the UK (I paid around 85 quid) and are very nicely made, with teflon coated sides.

Full carb clean

Electronic Ignition

It must be said that I’ve not had any real problems with Hitachi ignition units I had with the MX, however talking to others about it and having had problems with SC500 unit , and having seen the issues with Chris Kent’s MX360 staying with them doesn’t seem a good idea.

I’ll do a separate article on the electronic ignitions for the Yamaha MX’s and also do a report on their relevant performance. Needless to say there are some cost effective ways of getting a decent ignition unit sorted.

Engine back in frame

Engine Assembly

I’d made up a base gasket after finding some thin gasket paper in ‘Top Gear’ in Brecon . Like all small towns that luckily haven’t got a Halford’s, it still possible to find these little goldmines. Also good range of gasket sealant. Using two pieces of paper meant I was able to raise the barrel slightly and therefore increase the compression. Yet to be tested.

I also decided to forgo the copper cylinder head gasket and opt to Hylomar. Shall see what this also brings.

Further work to be done:
– fitting and timing the electronic ignition
– new exhaust, from CircleF though it’s going to be mid-April before it makes an appearance I think
– new tyres, two seasons on the cheapo Golden Tyre option, so gone for a set of Michelins.

Haven’t got much time to do alll this and run it in when I get back to the workshop, but optimistic of having a runner for Clyro.

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