Restore it, then thrash it

The bike was ready and after checking the bolts one more time, headed the 5 miles down the road to Howton Court and the Pontrilas MX track and one of their practice evenings.

Ready for action

Warm evening and plenty of dust at what is a modern rather than a classic track.

Youtube link to a lap of the track

The bike fired up perfectly and managed an initial three laps before the rear brake disappeared; takes a while to learn the track, lots of dust and deep sand on the outside of corners. The practice session was pretty quiet but even a few bikes (all modern) meant lots of dust.

The rear brake arm needs sorting; its not a solid arm and keeps coming off the spline. May need some new shoes also, front works well (as only one I had for the last 5 laps).


Good evening, though the dry conditions and the dust literally too some of the shine off it.

Getting there with the YZ

However, some time at home meant time working on the bike (along with other projects, like the DT175 ). I’d got a rolling chassis after getting a new rear wheel rim made up in Belgium and put in the bottom end of the engine.

Graphics done, ready to ride

The piston’s for the YZ250 are indeed the same that I’d used for the MX250 (the later 509 piston, with the cutaway rather than the boost ports)

Nearly there....

The jobs that got done:
– barrel and piston; there is now nowhere in Hereford that can (or wants to) rebore a two stroke piston. Hereford Rebore has retired (he’d checked out a while ago though) and Phoenix Engineering doesn’t have the kit and are also down sizing. Nearest options are Ledbury or Brynmawr. Anyway, stuck with the original Wiseco piston for now as their didn’t seem to be too much noise.
– no spark on reassembly and spent 3-4 hours checking and cleaning connectors; also cleaned up the stator and rotor which had seen some water ingress at some point. Going for a cup of tea and coming back to find a spark was nice and reinforces the ‘take your time and don’t stress approach’.
– fitting the cables; I’d borrowed the ones I’d order for the MX250 as both the clutch and throttle had become a little worn. This turned out to a bit a faff to get the adjustments right and just took longer than expected
– ended up making a new bracket to go under the seat to hold the rear mudguard in place. Some expensive (well I though so) black coated aluminium from B&Q did the trick. Used some nice bolts from the written-off KTM 950 as the fixings (doesn’t this mean its not twinshock eligible now?)
– the carb and air filter box went together okay though the float height still needs doing.
– got the SuperTrap exhaust back off Alan and will use this on the bike for now to see how it goes.

It runs well though

Managed to get in a few laps of the field and the bike seems good. Need to set up the front forks a little better and change the oil, but the fork tops have been mangled in the past, so might be a challenge.

Other jobs:
– get the airfilter to fit; the one I have is from a 78 YZ250 E and needs some modification
– do the plastics, vinyl and numbers.
– iron out any other small items, like the cable routing and ties
– check all the bolts and make sure they are tightened and correct.

Completed bike

PS: ended up getting up at 6am to finish off the bike and now pondering whether to give it an outing at Pontrilas this evening as they have a practice session today

Looking good and a relatively straightforward project that seems to have taken ages.

Further work on the YZ

Firstly decided to clean up the frame prior to painting (using the Hammerite solution). First problem of the day was a cracked frame to the rear of the main top tube. One of the side stays had cracked at the base of the tube as it connects to the welded end. Needs fixing, obviously !

The frame tis cracked

Starting stripping the engine and first problem is that the drain plug had seen some action from the molegrips. Managed to hammer an 11mm socket onto it to undo it, but thats going to need to be replaced.

Off with the head and barrel and though the bike was running okay, with plenty of power (I didn’t check the compression), the rings are worn and there is clear evidence of them blowing down each side of the piston. No scoring on the piston, though the gudgeon pin holes are worn to an oval shape (?) Piston was at 70.25mm, therefore

Clutch side first and no problems with the getting the clutch bolt off (with an airgun !). The primary drive nut is a 26mm special, but managed to get it off with the long bar and a thin sided hex 27mm socket, saving another special trip to the tool shop.

The clutch basket has plenty of notching from wear, though the plates aren’t original and in pretty good condition.

Clutch basket wear

Removing the flywheel / rotor should have been straightforward, but like a few other things on the bike, it needs a specific M18 x 1.5mm RH thread puller. Alas the ones listed in most kits are solid and like the puller for the MX250 it needs to be hollow for a depth of around 22mm. Therefore drilled my puller with a 12mm bit down to 20mm and this did the trick, which amazed me. Off for a celebratory cup of tea.

Drilling the puller

Onward, and time to split the cases, still thinking about why the engine kept slipping out of neutral. No problems with the puller and again like the MX250 there are two handy M6 threads so the puller can sit on the primary drive end of the crank. Cases came apart easily, though they’d been put together with what looked like contact adhesive.

As it was apart, I’d got some new main bearings (as OEM, Koyo) and crank seals both of which were provided by Keith at Motolink. Heating the cases, popping out the old bearings with a couple of swipes of the hammer on a large socket and then more heat to the cases and the bearings just dropped in; easy. As seems to be the norm, as couple of shims were found on the bench under the engine. Did some checks on the parts diagrams online at North West Vintage Cycle Parts and managed to get it wrong a couple of times. Some input from Gavin and there still seemed to be too much movement in the clutch shaft, but I think is taking up by an external shim underneath the basket on the outside.

The reason behind the gearbox dropping out of neutral was also becoming clear; for the 1979 and 80 models, the neutral selector bolt and the drain bolt were combined (they were always separate on early and later bikes) and as mentioned above the one with this bike is knackered (internally and externally).

For torque settings:
|Cylinder Head bolts|25|
|Cylinder bolts|33|
|Clutch Nut|75|
|Drive Sprocket|75|
|Flywheel / rotor bolt|35|

Nothing listed in the Clymer manual for the primary drive bolt, which is interesting…well not really. I went to 75Nm for this also.

Cases split

Further inspection of the piston and ring showed it to be a 1st oversize Wiseco in old money (that 20 thou oversize, rather than metric), so again on the phone to Keith and a 2nd oversize piston kit with little end bearing acquired. Also, had a look at the Wossner options, which list a piston for the MX and YZ250’s; cheaper that Wiseco, available and seem pretty good.

Hammerite sprayed frame

Sprayed up the frame and got the compressor working a lot better than I’d done in the past, in particular managed to lay the paint quite thickly without the excessive overspray. Much happier with the result than from when I did the MX250 a couple of months before.

Time to order some bits, get the rear wheel sorted (a bit of a special saga all of its own) and move on to completing the bike. All great fun !

YZ250 strip

There were some obvious problems to be resolved:
– the rear wheel rim is cracked
– plastics need to be replaced
– clean up and possibly repair the petrol tank
– gear selection problems (doesn’t stay in neutral) and engine rebuild

Ready to be dismantled

The strip revealed a few other interesting things:
– the rear frame loop seems to be an ‘aftermarket’ bodge with a nice strip of aluminium bolted to the plastic
– an interesting spattering of 11mm bolts, along with the ‘normal’ 12 and 13mm selection.
– the ding in the exhaust was sizeable but tolerably. The end can was a Supertrap, probably off some four-stroke Honda.

Yamaha went monoshock in 1975

Took a couple of hours to strip the bike, taking photos and notes as I went. Managed to pick up a Clymer manual and wasn’t planning to strip the engine just yet.

Ordered some spares (like cables, bearings, seals and gaskets) from Keith Alderman at Motolink.


The wheel rim was going to be more of a problem that I thought as it seems not only are Central Wheels busy people, they seem to have a bit of a monopoly on the supply of rims, spokes and in particular drilling rims. Replacement Morad rim ordered, but there is a 4 week (minimum) wait for drilling and delivery.

CDI connectors

Rebuilding starts here.

Well at least it runs

One of the anomalies of the Twinshock class, that after 1975, Yamaha’s went monoshock, with the De Carbon unit (okay there was the Geboers senior unit on a Suzuki before then), but the MX250 B of 1975 was the first production monoshock.

Yamaha YZ250 F

Picked up the bike from Rupert across the other side of Hereford, and he assured me that it ran, despite its refusal to start on the night. After the SC500 ignition saga, I was a little skeptical but hey, it will be alright. Rupert already has a YZ250 E (1978) and it was interesting to look at the differences (though subtle) between the two bikes. The F is longer but 5cm or so and this is done to a longer swingarm, which is also slightly modified. Engines are basically the same, though the plastics were very different, with the number boards moved further back from that year.

Much taller than the bike from 1975

The US bikes were yellow, whilst the European ones were white and this an import (via Governor’s Bridge motocycles, where the SC500 came from).

Got the bike back and after 1 beer and 10 minutes, Gavin was on the bike and away, as it decided to fire up. Some smoke, a sticking throttle, but it went and pretty well too. The engine sounds find and not too rattly for a Yamaha.

The following morning saw me take a closer look at the bike and start to list what I need. It shouldn’t be a complex restoration but with upcoming travel and work, this is going to be the biggest restriction on getting it done. Will order all the spares I need upfront so I can work on the bike as I need to.


The bike chucked me off in the field after I didn’t take into account the slick tyre on the rear, but managed to confirm bikes going power and that is a nice riding position. One thing you notice straightaway is how light the bike is. Amazing compared to the smaller, but heavy MX and SC’s.

The list of things to do:
– strip the chassis, and clean up and paint, probably won’t shot blast but go with Hammerite Satin method
– the engine sounds good, crisp and yes it does go well. However, the problem with the engine coming out of neutral and general good practice means it’s going to be best to strip it and see what it’s like inside
– rear wheel rebuild, the rear rim is cracked along the spoke holes, which is pretty interesting.

Spares needed:
– complete set of plastics, can be provided by DC Plastics , includes front panel and the different for that year only side panels.
– rear exhaust, silencer. The whole exhaust has taken a battering but only a rear silencer needed
– new rear wheel or rim. Hubs and wheels maybe hard to come by, but can put on an OEM or replacement rim. Back to Gerard in Caerphilly for the rebuild. Keith at Motolink has a rim.
– seat cover, MXM provide a decent one.
– tank and side panel graphics, MXM do one for the tank, but DC Plastics kit also include the ‘250’ for the side panels
– handlebars and controls. I have a new Renthal replacement for the bars, which seem to have been chopped in the past. The throttle is knackered as is the clutch lever. Will check the cables, but will get replacements from Keith at Motolink
– tyres: probably from the local L G Racing and my normal two sets of tubes
– gaskets and bearings : gaskets hopefully from Keith also, with the bearings from Simply Bearings or from my local supplier Brammer.