Yamaha SR250: A Perfect Donor?

The Yamaha SR250 is one of the world’s most popular donor bikes for custom builders. Today, Jake Snowdon from Sweden’s Jadus Motorcycle Parts — the SR250 experts — delves into the heritage, design, and specs of this beloved thumper, explaining what makes it such a great base bike for builders.

Words by Jake Snowdon:

Some might wonder why Jadus settled on the SR250 as the bike for the first range of parts offerings. Part of the reason is covered in the “Jadus Founder” page on the Jadus website, but there is more to it…

The humble SR250 – why it is a good builders base bike?

There are several contributing factors, obviously, but we can start with the engine, which is an absolute peach. So, why is the SR250 engine so simple and likable? Continue reading…

• Air cooled single cylinder (duh), wet sump, unit construction, 5 speed box, multiplate wet clutch – which is simple – no liquid cooling components to fail.
• A simple, two valve, single, chain driven Over-Head-Cam (with mechanical tension adjustment) head.
• Roller bearings at both ends of the cam (this is a huge advantage over say, the SR400/500 cam design which has plain bronze bearings which need oil pressure to operate without wear).
• Modern sphere and squish style combustion chamber with hardened valve seat inserts (no, no lead additives or octane boosters are needed) and matched domed pistons (with valve relief cutouts – interference engine).
• Easy valve lash adjustment – easy access covers and easy access to simple mechanism (rocker arms and tappets), similar to VW beetles and air-cooled Porsches of the era (and in-fact, many other engines throughout history – why change a great thing).
• Internal balancer shaft – killing a lot of engine vibrations – a rarity for single cylinder four strokes of the time and a huge improvement on the crankshaft balanced only design – also allowing the engine to be used as a stressed member.
• Engine used as a stressed member – meaning a simpler and lighter frame was possible, plus a tighter wheel base – having the swingarm pivot through the engine mounts.
• Hi-flow, low pressure trochoid oil pump and oiling system, plus internal oiling galleys (no ugly and leak prone external oil lines).
• Caged needle roller bearings – as opposed to plain bush bearings.
• Scraper lubricated little end – a hardened wrist pin runs directly inside the hardened connecting rod’s small end with a tapered, countersunk hole to collect oil from the pistons scraper holes and spread it over the contact surfaces.
• Very reliable electric starter, with possibility to install kick starter.
• CV carb… It is really, very easy to work on and is very forgiving to small tuning errors and changes in air density (altitude). We made a service video on youtube to help people at home navigate their way through the adjustments and serviceable components of it here.

Some of these engine features are actually thanks to a not-so-distant Yamaha sibling… A little digging into Yamaha history and the SR250’s architecture and you run across the famous Yamaha XS650 — a hard to miss classic and very crucial model for Yamaha that ended up being very influential — both at the time of release and later — with a comeback as a base for building all sorts of customs. So what is it that the XS650 and the SR250 have in common that makes them so good? Some of these details have already been mentioned above, but here is an elaboration on the most critical ones.

The use of caged needle roller bearings instead of plain bush bearings — this allows the use of a low pressure, high flow oil pump — meaning adequate oiling to critical components, without placing unnecessary pressure on all of the sealing components in the engine. This meant a relatively leak free engine (a revelation in the day of leaky Harleys and Triumphs)! Both the XS650 and SR250 engines are unit construction — the engine, gearbox and output shaft are all the same package (which is nice and simple) — as opposed to American and English bikes of the era with a primary engine unit and chain drive to a gear box.

These engines also employ a chain driven overhead camshaft — a vast improvement over the cam in block and pushrod norm of the day. Both engines have a pretty good electric starter — especially for the 80’s. The XS came standard with a kickstarter and the SR cases allow the installation of an aftermarket kickstarter — because of it’s very close brother models, the XT250 (the road-going cross bike) and the TT250 (the off-road only bike) sharing almost the exact same engine — only differences being the lack of electric starter and therefore, different crankcases and covers (XT and TT even had magnesium covers!). Most XS650 models even share the same constant velocity (CV) carburetor with the SR250 — the mighty Mikuni BS34!

Before going into further detail about the 250, it is worth mentioning early on that despite a lot of alleged similarities to other SR models — there is an SR125, SR175, and of course, the infamous SR400 and SR500 models, they couldn’t be any different to each other. One might think they at least share some details, but there really are very few. One of the major differences to the SR400 and 500 models has already been mentioned about the engine — the use of different camshaft bearings types. A couple of others are: the 250 has an internal balancer shaft, the bigger SRs’ do not; the bigger SRs’ use dry sump lubrication, the smaller SR’s have a wet sump; the bigger SRs’ have only ever been sold as kickstart only, while the smaller SR’s have all had electric start. The frames of all models are also very different, so unfortunately, tanks, seats and other styling parts cannot be used interchangeably — what a shame!

The SR250 engine really is super reliable and can take a good thrashing. There was only ever one service bulletin for the engine throughout its production life, and that was this: because of lack of adequate oiling to the cam and rockers, the flow needed to be reduced to the crankshaft and in turn force more flow to the head — alleviating the starvation problem. This was achieved by inserting a roll pin in the crankshafts centre oil galley which restricted and reduced the flow by creating a smaller passage for the oil to pass through. From the available information, it seems that this modification was only necessary for the first 249cc models, because after that, it seems that all 239cc models had this modification from the factory. But every owner should double check this before thrashing the engine! See the technical bulletin here.

Onto some other good things with the SR250… The chassis. The frame is a very simple design and lends itself well to customisation. It is also relatively light because of the aforementioned engine being used as a stressed member. The only downside of the frame is the sloppy and therefore ugly factory welds — most SR’s look like a toddler got hold of the MIG gun and sprayed some molten metal while half asleep.

The front forks are nothing to write home about, employing a very basic damper rod design and using very soft springs — giving a mushy unresponsive feeling. The same goes for the rear shocks — these too are often too soft and are not the most attractive in design. The good thing however is that the rear shocks can be swapped out for many other aftermarket alternatives which look and perform much better. Unfortunately at this point there is very little that can be done to improve the front forks, other than rebuild them — draining and cleaning them, changing the seals and adding fresh fork oil.

It is also possible can also lower them through the triple tree very easily which gives more options for desired stance and handling characteristics (changes front end geometry) and buy new heavier weighted springs for them. Jadus has also been working on and developing an SR250 fork improvement kit for some time and there is just some final testing remaining before it can be launched to market. The later model SR250 Classic actually has roughly the same fork — internally at least. The only difference being the lack of an oil drainage hole and the brake mounts – the newer models have mounts for a single pot disc brake. Yes, brakes…

The SR250’s brakes are adequate, nothing more nothing less. Believe it or not, there are three different front brake types that were offered on the SR through the models over the years. The first and most recognisable is the single sided single pivot drum brake. The lackluster performance is saved only by the fact that the bike is so light, otherwise they would need to be upgraded – something Jadus has done on the 100mph bike. The other drum version was a double pivot double sided drum brake — which was only ever an option on the European market. It is difficult to find reviews of the performance of this brake, but the extra brake pad contact area indicate that it is marginally better. The best offering of them all is the later models SR250s with a disc brake — a single disc, single pot set up. Even this does not deliver stellar performance but does bring the bike to a stop with confidence. All SR250 models received the same single sided drum brake on the rear.

The SR250 tank lends itself very well to customisation. The classic shape suits all sorts of styles — from cafe racer to street tracker to scrambler and brat bike. The seat however is a different story — the SR250 Custom seat has simply got to go. There are few motorcycle seats from this era that are uglier, so throw it away! It is not even comfortable despite being so big and cushy. Same goes for the mirrors, handlebars, dash, blinkers and tail light — they are all very ugly 80’s design that have not aged well, so throw ‘em in the trash. The SR250 Classic on the other hand has a much better style from the factory. The seat is still a little too large, but it is much more attractive, the side covers are pretty simple, unobtrusive and cool and even the handlebars are pretty decent. Same goes for the dash — the round retro twin chromed clocks are pretty cool.

Ok so what are the exact differences between the SR250 Specials and the SR250 Classics then? Not a lot really! But before going into more detail, let’s clear up one thing… The SR250 Classics were only ever produced in the Spanish Yamaha factory. A Yamaha factory in Spain? Yeah, the humble little SR250 was so popular in Spain (most sold motorcycle model in Spain several years in a row), that Yamaha agreed to build a new manufacturing plant in Spain to produce them.

Ok, onto differences… The first models to go to the US were true 249cc engines, all others were then 239cc — the difference being a 75mm piston vs a 73.5mm piston. This in turn affected the compression ratio as well – from 9.2:1 to 8.9:1. Some European model bikes and all Classics have a tachometer with a pick up off the cam via a worm drive — running through the top of the head. However most US and Australasian model bikes did not have a tachometer — the cylinder heads differ slightly. Yamaha used the same tooling for the head but used a ‘blanking’ insert for the non-tachometer heads — which fills the hole with aluminium and leaves a smooth lump there instead — unnoticed by the untrained eye.

 

Both bikes use a very similar and simple wiring setup. A little about the spark control of the SR250’s: all of them share the same brain — what is usually called a CDI (Capacitor Discharge Ignition) box, is, in the SRs’ case actually a TCI (Transistor Controlled Ignition) box — but for some reason the names are used interchangeably despite being quite different. The best explanation for the difference between the two types and how they work can be found on this chaps website here: https://www.jetav8r.com/Vision/IgnitionFAQ.html#a7p0. The Japanese produced SR250’s (all of which are ‘Customs’ or ‘Exciters’) all have the same type of flywheel and ignition pick up mounts. The Spanish built SR250’s (there were several different models) all have another style of flywheel and ignition pickup bracket/set up. All models had a very simple and similar wiring diagram – the items making them different being the aforementioned dash units (single – speedo only – for the Specials, dual – speedo and rev-counter – for the Classics).

The head of the Spanish built SR250’s has the newer style hardened pad rockers with weird M7x1 valve tappet adjusters and a hardened cam to suit, whereas the Japanese built SR250’s have the old style rocker with a hardened surface straight onto the cast rocker. These rockers have a more standard M6x1 thread for the adjusters. These different cam and rocker combinations are not compatible with each other — people have tried! So these components should be kept as families so to speak — to make sure the metallurgy works in harmony as intended by the engineers. All other major engine components are the same across all models.

The other major difference is the rear part of the frame. The Spanish built SRs have a similar rear seat bracket to the Japanese models but it is welded to the frame further back along the frame rails and has more holes in it (purpose of holes is unclear). The frame rails themselves are also slightly longer — extending further to the rear than the Japanese built frames. There is a series of videos on the Jadus Youtube channel that show the differences quite well. The last frame difference between the Classic and the Special models is down where the rear brake mount is. The Classic models have a different mount to be able to position the footpegs further back and achieve a slightly sportier riding position — which is still quite comfortable and allows a bit better cornering clearance and an easier to achieve highway “ducking position.”

Wheels should really be covered under the “chassis” paragraph but they get their own section here. All Japanese built SR250s (all Specials and Exciters) have a spoked 16’’ rear wheel with a 120/90 tyre. For some reason, some models got an 18’’ spoked front wheel, while others had a 19’’ spoked wheel, but they both came with a 90/90 tyre. One of the most common questions on forums about customising the SR250 is tyre choice. A lot of this comes down to personal preference and what style of bike is being built, so do the homework, read as much as possible – the information is out there, and buy the tyres that suit the style of bike.

But to answer one simple, very common question; the widest tyre that can be fitted on the front without modifying anything is 120mm and the widest rear is a 150mm, but only with a 16’’ diameter – obviously when increasing diameter, there is no longer as much space in the swingarm for such a wide tyre. Plus, when increasing tyre size up front, it will throw out the speedo readings slightly – so that is something to be aware of. The last thing on SR250 wheels, there were even some models that came with cast wheels from the factory! Some of these versions were sold in Italy and there is very likely more that were sold in other European countries as well.

That about covers the basics of the bike and why they are so popular and likable. If anyone has any complementary information to what is presented here, or would like to correct us on any mistakes, don’t hesitate to get in touch, we are always trying to expand our knowledge on these little rippers!

Tyre size reference: www.motomucci.com/1981-yamaha-sr250.

Jadus customer builds: www.jadusmotorcycleparts.com/customer-builds

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