Yamaha MX250 Engine Rebuild

Ready to start work
Radio On
In the frame
Head and barrel removed
Engine of the bench

Clutch cover removed
Clutch centre
Removing crankcase bolts
Case splitting
Splitting the cases
Damaged gear selector
Removing the crankshaft
Inspection of the residue
Piston ring
Gear selector

Crankcase reassembly
Cases back together
Gear selection mechanism
Clutch baskets
Clutch side reassembly
Clutch side back together

Head and barrel back together
Engine back in
A running bike.

The bike started up third kick and ran, with a top-end rattle. However, nothing much under load, so something not right still, but it’s fuel or ignition rather than the engine.

MX250, back to the track

With work (and therefore travel), I’d not really had the chance to get the MX250 sorted since coming back from Belgium. The Yamaha had stopped running at the Lille track and I’d not had a chance to look at why. Also, with Llangorse Lake shut because of a blue-green algae outbreak, also not been able to go sailing. Time to spend some time in the garage.

I’d had a 0.50 oversize piston in the stock for a while, so got the rebore done Joe Lyon’s in Brynmawr (next day, £20.00 so no complaints) on one the barrels. I’d got 4 barrels, 3 of which had been polished so should therefore also be tuned.

Also, the replacement exhaust had arrived from Kevin Falle at Circle F in Houston (via Atlanta). If you can find a decent shipping route, these exhausts are reasonable ($200). I’d ordered them in February, some a longer than anticipated delivery time 🙂 Nice piece of work and decided to paint them with clear heat-resistance laquer. Not an easy product to find, and tracked down to Car Care in Monmouth; thanks to Chris Kent for the tip.

Circle F pipe fitted

The SC500 engine build still needed completing (more of this to come) but onward with the 250. Fitted the new barrel and piston, along with the cylinder head I’d been using. However, bit of kicking didn’t make the bike go; spark and some fuel, but nothing. The exhaust looked good, but nothing.

Hmm, frustrating. So after a cup of tea, re-laying some of the stone work in the back garden, decided to fit the other engine, with the barrel and piston I’d used in Belgium. The small-end bearing was a bit slack, but it fired up after a couple kicks. Exhaust sounded as good as it looked and engine ran okay, if not a little loudly. I’d swaped ignition, carb, piston, barrel, etc in the search of a work solution and having spare components around makes all the difference.

Yamaha kickstarts 1 Boots 0

Other jobs then got in the way and left it with the newly bored barrel and replacement piston in the bike, but with it not running quite right. Maybe it’s the reed valve or the head; ah well more research to come when I back in garage.

Off to Rome for work…and some sunshine at least.

Pre Season 2012: the week before

The MX250 needed a bit more work and continued on from the tuning work I’d started a few weeks ago. In the end, the modifications for the first Llanthony meeting of the season are as follows:

– top-end polish and rebuild. I ended up not altering the port heights, but just went for a removal of the anomalies on the inlet, boost and transfer ports. New Wossner piston (they list the one for the 76 YZ 250 and on, which is a basically the same).
– thicker base gasket (home made paper gasket) and Hylomar blue to replace the top end gasket; more compression. Gasket paper came from http://www.gaskets-direct.co.uk/ , who I can recommend.
– PVL ignition, more on this to come in a separate article, but have sourced what I think is the best ignition for the MX and SC bikes. Also reasonable price, including having a stator plate made up.
– new tyres, Dunlops to replace the somewhat erratic Michelins, which were also showing the wear after two seasons.The front was an awkward bugger to do, and shredded the old tube in the process.
– handlebar risers; also on the SC500 and something I should have done before to make standing up easier.
– foot rests, replacement
– petrol tank, acquired a plastic Clark tank from the US and the tank strap from Motolink; looks good.

Still to do is the exhaust and waiting on Circle F to fabricate and then ship, maybe by the start of May.

Anyway, eve of race 1 of the season for me, and looking forward to it. The MX250 needs a bit more running it, but sounds okay in the garage.

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.

Looking for performance

The main problems with bike were:
– the main mounting for the exhaust was missing and the combination of cable ties and lockwire wasn’t really doing a good job.
– the engine was running, but there was no low-down power at all and getting off the line was becoming a bit of a joke. Some playing with timing and carb didn’t seem to be doing the job.

So, prior to a trip to Belgium I spent a couple of hours stripping the bike down and prepping another frame, with a complete exhaust mount. Sprayed the new frame as before with aerosol black Hammerite Satin which seems to do a good job. Didn’t strip the engine, as I already had a spare engine following on from a rebuild during last season, so had one on the bench so to speak.

After a week in Belgium (good trip over on the KTM), back in the UK for the royal wedding and more importantly a whole day of bike rebuilding. A few issues needing resolving as a rebuilt the bike:

– steering head bearings, needed a new set
– clutch arm, during the strip I’d seem to have misplaced the connection from cable to clutch arm. Faffing with this and nicking one of the one of the TY250’s took about 1.5 hours, which is a pain.
– exhaust, the mounting of the silencer etc

Checked the compression of the old and the replacement engines, with old showing 75 psi, whilst the new 125 psi.

Existing engine, reason for poor performance

Removing the cylinder head and the barrel from the used engine revealed why there was loss of compression and power. The piston had cracked from the skirt up towards the rings (there are two rings on this piston). Might account for the problem getting from the line, but amazed that it was a least a little competitive and that it hadn’t completely broken up in the engine.

Starting up the new engine, it had slightly more than just a nice top end rattle with a metallic knock also. Though the bike ran okay and seems to have some power, decided not to continue the running in until I’d had a chance to resolve the noise.

Wire insert kit

Going through the piston and barrel selection (this new piston and barrel are 0.25 oversize), gave me a couple more options, but decided to stick with the existing piston as this had a new conrod, little end etc. Tried a different barrel (which seemed to a closer fit), but which needed more faff time to get ready as someone had butchered the exhaust bolt threads. Out with the new 6mm wire thread inserts, but took a couple of attempts, as the threads needed to go deeper than the tool would allow.

Used the same piston, ring, little end and gudgeon pin, the latter two of which were new…but same top end clatter. Time for a think and for a barbeque.

The thinkended up carrying on for another week and a trip to Madrid and Antwerp and with just the afternoon to go before the meeting at Abbeycwmhir decided to have another look. Some inspection of the top end components by neighbour Gavin identified the problem as being a worn gudgeon pin and a quick 15 minutes later, then engine was running again, though still with a slight knock. Also changed to another 0.25 piston and a fresh ring.

End can

Some further fettling and modified the exhaust rear can as it seemed to be restricting the engine. A quick run round the wet field seemed to indicate that there was some power and response. The carbueration was a little off, and I retarted the timing slightly and all seemed okay.

Will see what happens at Abbeycwmhir tomorrow….

MX250 Engine Reassembly

Basically, the best order for rebuilding the engine is probably as follows;

– heat the cases [ I’ve never tried the oven approach, though my [ kitchen is still fairly new .]] and drop in the new bearings, I used SKF 6306/C3 bearings, they are both the same size.
– place the left hand (LH) crankcase (ie flywheel side) face down on some blocks (I’ve made a simple frame which works for most the two stroke engines I’ve worked on
– slightly heat the bearing in the LH case and just drop the crankshaft in; this saves any need to tap it into place and therefore damaging the threads on either end (it isn’t a good feeling if you do mash these with a hammer, which is possible even with the nut on the end.
– place the new seals in the right way round over each bearing. The LH seal is 32-62-10 and RH seal is a 40-62-10, the only thing to note is that the OEM Yamaha seal has a castled top, to hold it clear the bearing, whilst an ordinary seal won’t have that; not sure what impact this has on assembly.
– drop in the gear cluster; you can put in both shafts together at the same time and put in the selector forks and drum in afterwards. Slacken off the selector both through the case and make sure the shafts are seated correctly. Put in the selector forks, but pull the shafts for the forks out, allowing the drum to be put into place, before swinging them in an locating the shafts. Put the drum into the neutral position and you can then check if the shafts run correctly before proceeding.

I wrote these notes a few weeks back and haven’t had a chance to run the engine in a frame yet but should be all okay.

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.

Completed Bike and notes for restorers

Completed bike

Some final adjustments during the day, in particular setting up the carb, as the bike felt lumpy as it pulling away.

After adjusting the float height

Panels in the right place

|Bike 1|Acquired from a garage in Tredegar from Kev, running but needing some attention (ie complete rebuild)|
|Bike 2|Second bike came AP Motorcycles who are serial importers of cars and bikes, and the good thing is they know their dirt bikes. |
| Piston Kit | Got an original 0.50mm (2nd oversize) piston kit from Keith Alderman at Motolink . There are plenty of OEM Yamaha pistons and Wiseco piston kits on eBay in the US, though you need to take into account shipping and the exchange rate. |
| Crank bearings, seals and gaskets | Also from Motolink as well as lots of information on the MX250. |
|Tank Decals| These came via Speed and Sport Yamaha’s ebay shop and were the only place I could find who did them. |
| Plastics |The front and rear mudguard as well as the side panels came from [ Reproduction Decals ->
http://www.reproductiondecals.com/default_frameset.html?content.html~main ] in Canada however these came from DC Plastics who indeed sell a a complete plastics set for the 1974 MX . The parts from Repo Decals were cheaper than the set on the DC site (I think?) |
| Front number plate | I got from Yuniparts who are mainly on ebay UK and specialise in XT/TT 500 parts. Luckily the TT parts also fit MX250/360 and SC500’s. |
|Seat Cover | Came from Pit Replica in Thailand, and arrive in 10 days |
|Handlebars and Levers| I already these from an incomplete trials bike project. I used standard Renthal bars and a pair of the excellent Domino levers, which are cheap but are pretty durable. These originally came from BVM in Stroud, but available from anywhere. I also got a set of pattern copies of the original fitment bars, again from Yuniparts as they are like the TT/XT fitment of the day. |
|Throttle and Cable| A nice quick action set from Motolink with the appropriate cable to match |
|Clutch Cable | A pattern copy of the original cable, again from Motolink|
|Brake Cable | After some searching and thoughts about modification, ordered a good value one from Yuniparts (for £7.65) though it was listed for the later MX250B. It fits the earlier MX250’s equally well. Yuniparts also stock pattern brake shoes (front and rear) for the MX250, as they are the same hubs as for the TT500. |
|Spark Plugs| From the ebay seller in Alderney, both standard and Iridium plugs|
|Tyres| From my local offroad supplier in Pontrilas, LG Racing Gone for medium compund set of Michelin Cross AC10 |

Did the numbers last


As well as the parts, I got some of the work done, ie the bits I couldn’t do myself (like the rebore and the rechroming). The rebore was done locally (to a very high standard by Russ Tyler, whilst the forks went to [ A M Philpots ->
http://www.amphardchrome.co.uk/ ] who do an excellent collection and delivery service.

I decided to paint the frame myself, rather than powder coat it and did this with Hammerite Satin paint (from aerosol cans). I was really pleased with results and would do this again with any bike I could get the approx paint colour for.

The wheels were respoked (with galvanised rather than stainless) by Gerard Pettit done in Caerphilly and again excellent service for reasonable money. I’ve heard that Central Wheels are pretty good and they can also potentially polish the hubs etc if you want (I didn’t)

Some more notes of restoration

What’s good about the MX250 and the MX250A is they are pre74 eligible (the MX250B was monoshock) and though never imported to the UK there are loads of bikes and spares in the US (and also AUS and NZ it seems. There are common parts with the Yamaha DT250 and DT400 of the same period (not the later ones), both engine and cycle parts and lots of cycle parts are interchangeable with the Yamaha TT500 (lightweight version of the XT) which are currently massively popular in the UK (and silly money).

Restoring a silver MX250 (1973) or a yellow MX250A (1974) should both be straightforward and plastics and decals are available for both. I must admit the Yamaha US yellow wins for me.

Make sure you get hold of the right MP7 flywheel puller M20 1.5mm RH thread as you’ll need it and won’t get the flywheel off without it. It fits only a few CDI / small flywheel bikes and I had to order one from the US, which turned up in 10 days [[I can now recommend international eBay to anyone after completing this restoration, with parts from the UK, US, New Zealand and Thailand being ordered successfully without hassle ]].

Before my first race, I’m already happy with the project and working on the bike. Its easy to work on, and the engine strip was relatively straightforward (to a four-stroke and a Rotax trials engine) and would make a good first project. Spares are not a problem and you just need to plan up front on what you need and when.

Maybe a MX360 next (have some spares now) but lets wait till the first meeting and see how i get on.

Nearly there…

Some more photos of the nearly completed bike. The side panels are done, though the numbers I have are too large for the black vinyl ovals. The seat has been recovered (which was straightforward).

Still need to improve the the attachment for the exhaust rear box and trying to work out the best way of doing that, as will need some engineering modification.

Some tweaking to the carb stopped a fuel leak and the idle screw from the ‘spares’ bike was longer (??) and now seems to work correctly. Plenty of go on main jet though will clean up the rest of the spare carb just in case.

99% complete

The field wasn’t in such good form as the previous weekend with all the recent rain and the bottom corner had cut up pretty badly. Need to find out the best way of getting grip !

Hopefully the rider can match the looks