Yamaha SR250 - A clean standard build

JadusMotorcycleParts said:

The problem I kept having (even after 3 tries) was that the tank mounting flanges (inside the tunnel) kept interfering with the brackets, no matter how thin I made them and no matter what orientation I positioned them in.

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OK - makes sense. As you say, the mount could be used with other tanks - just would need some careful pre-planning.

RE your tail light - do you think you would have enough room for more of a "classic" off-road tail light like this http://www.ebay.com/itm/Tail-light-Brake-light-Off-road-Enduro-Honda-Yamaha-Kawasaki-Suzuki-KDX-TS-XL-/131016091939? I think it would work better (on a tracker) than the smaller "bobberish" one you pictured.

Edit: RE your headlight - I recently converted my OEM headlight bucket/trim ring to accept a Virago 750 H-4 bulb/reflector as per the guidance on the Yahoo Groups page for better illumination. Have you seen the upcoming Yamaha XSR-700 headlight? Smaller diameter but longer nacelle (and presumably with a 55w/60w beam).

zap2504 said:

RE your tail light - do you think you would have enough room for more of a "classic" off-road tail light like this http://www.ebay.com/itm/Tail-light-Brake-light-Off-road-Enduro-Honda-Yamaha-Kawasaki-Suzuki-KDX-TS-XL-/131016091939? I think it would work better (on a tracker) than the smaller "bobberish" one you pictured.

Edit: RE your headlight - I recently converted my OEM headlight bucket/trim ring to accept a Virago 750 H-4 bulb/reflector as per the guidance on the Yahoo Groups page for better illumination. Have you seen the upcoming Yamaha XSR-700 headlight? Smaller diameter but longer nacelle (and presumably with a 55w/60w beam).

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I really like that tail light set up. I am already planning on building a more tracker styled 250 for my next build so that will be on the cards then.

That XSR-700 headlight is a really nice modern take on the vintage style headlight. How much of an impact did the H-4 light conversion have?

Btw, what do you think of the XSR-700 in general? Beats me why they decided to make that bike with the twin engine from the MT-07 instead of the triple from the MT-09. Personally I think the styling is a mess - there is no flow, cohesion and it generally looks like a mashed together hodge bodge of a job. They could have done a lot more for the design, and perhaps done something with that big slab of a radiator up front? There are many bikes (including my home built rat bike) that have good functioning under-seat radiators. This bike would have been a sitter for that treatment - and I am sure the Yamaha engineers are VERY capable of making the whole package work. It falls short compared to the Ducati Scrambler if you ask me - if that is its competition?

Converting to a 55w/60w H-4 bulb versus the OEM 35w/50w sealed beam is noticeably brighter (plus easier to change out - even to an LED unit if desired). If I had/was able to get a complete 7" headlight for the same price I may have gone that route but I got the Virago 750 lens/reflector at a real good price and it was minimal work.

RE the XSR-700: I kind of like it. I know that it is not a "cohesive" look - more like a Mad Max look - maybe that's why I like it. It does not have the same minimalist look the Ducati Scrambler does, but the Duc has a more narrow, air-cooled engine to start with. A radiator cover (or some high-temp black paint) and the XSR will look just fine to me. It also has some real nice-looking Yamaha accessories to further the "apocalyptical" look. If the XSR-700 does well, Yamaha might come out with a XSR-900 (and an XSR-300 for its new small twin). Most probably, some private customizer will beat them to it.

Yeah I have seen a ton of LED H-4 bulbs on ebay and alibaba recently. Must be a hit. Well its good to know it was worth it. It might be something I do after the build if I end up doing some night riding into the Autumn months.

I think you're right with the XSR-700, if you look at it as an intention to be all over the place with the design. I think it will be one of those bikes that divides people, whereas perhaps the Ducati is more easy to the eye and therefor is more widely appealing? Hadn't thought of the XSR-900 and 300 versions but now you mention it, that must be a very likely path for them to go down. Maybe to start with the 900 version would have been to much for entry level riders where it is targeted? I mean, that engine is a beast. Besides, with the 270 crank, the 700 has an amazing sound anyway ;D

Not too much to report on from the weekend but I did collect a few screen shots of an assembly I made comparing the different revisions of exhaust I made. Plus took a photo of the prototypes side by side. Its amazing what 1 degree here, half a degree there and a few millimeters there will do to the overall fit. These very small changes each time got me closer and closer to having it sit just right on the bike.

The prototype with the green tape on is the latest and final version (is also the green version in the 3D screen shots). This will be the design for the production run 8) When its all up and running I might pay for a dyno run to see what affect the pipe has on performance. Should be a healthy, but not crazy boost.

I have an XS400 headlight with a cheap set of chrome fork tabs from china. Great $8 purchase. I cut the burnt bulb out and jb-welded in an H4 that is relayed to the battery.

I tried an LED bulb and it fried right away. The polarity was correct etc. Since I have a box of H4 bulbs, it was easy to give up on the LED.

Dropping the headlight is the key on this bike, not always the size IMO, but drop it enough that it looks right but watch that the fender doesn't strike the light on full fork compression.

I received all of the magazines I ordered from the 80s with SR250 reviews and road tests ;D

If you're interested, I scanned them all hi-res and added them to the homepage:

http://www.jadusmotorcycleparts.com/#!other/cqh1

Been working on some sticker designs too. Not the most important task right now but I would call myself a 'creative' and have a hard time prioritising my time and often end up doing the tasks I think are most fun! Rather than getting into some of the more laborious but very necessary tasks

Got these in the post today. CNC machined prototypes of the valve/tappet covers I designed. They look awesome, quality is top notch. I specified a heavy bead blasted surface - to remove the machine marks and to get the finish to match the rest of the cast engine a bit better. A rough surface also increases surface area for better cooling. The mistake I made was to have the whole part blasted - the inside/underside should be raw machined - after all, it is a mating surface. Not only does it need to form a seal, it will also wick away more heat from the head with more contact surface area.... So I will set up some wet and dry sandpaper on a sheet of glass and sand those faces smooth myself.

I rekon they look pretty good. Hoping they are a hit if priced right. I have also ordered a head from an SR125 and the famous TW200 because I have a feeling these covers will fit them too 8) Even if they dont have the same part number, I am hoping the hole patterns are the same. We'll see.

A couple notes about the design of these parts... I have since done a lot of research on air-cooled engine fin design and heat sinks. Maybe I am looking into it too hard for some silly valve covers but its fun. Most of the hard math and physics/thermal dynamic stuff goes a bit over my head, but there is a lot of good 'garage theory' out there around the subject from racers, engine tuners and car guys in general - talking about inter-coolers, oil coolers, heat exchangers etc.

A couple of things I did right: The fin spacing and thickness is absolutely optimal for an engine that is to dissipate its heat in open air (rather than in controlled airflow with a fan and cover) at 1/4'' give or take - which was a fluke, but then again, I did just take the spacing from the existing head so you would expect the Yamaha engineers to be spot on. Plus the rough surface finish is about the best you can get for shedding heat.

A couple of things I will modify for the next prototype and eventual production run: Change the material spec from 6061 to 6063 - it has similar machining properties but has a 20% better thermal conductivity rate as long as the temper is left alone. Plus I think I will increase the internal cavities (volume) of both covers to catch more oil and hold it longer (to give it more of a chance to cool) before it drains back into the head and down to the sump again. I am hoping it will shed a couple degrees with its contact with the covers. The last contemplation is what finish could be best. As said, the raw blasted surface is optimal, but might corrode over time. It is advised to clear/silver anodize the parts to stop corrosion, but it does create a small thermal barrier. Then I also read that anodizing heat sinks black might increase heat transfer by 3-8% but in some cases it can become a thermal barrier instead of radiator. Either way its nominal and if I do get some anodized in black, it would be more for looks than performance.

Lastly, just when I thought this was an obsession, I found this video of a TW200 running with its valve covers off, and just as suspected, there is an incredible amount of oil being spat out each tappet - even at idle. So imagine the volume at full tick. There is also a really cool video of a V8 hot rod with clear valve covers where you can see how much oil spits onto them and the chance it would get to cool

https://www.youtube.com/watch?v=x6NelDmVCOc

https://www.youtube.com/watch?v=svjytSjd6g0

Jadus, I really appreciate the attention you're paying to the sr250. I'm sure I'll buy some bits off you to keep the little aftermarket stuff for it going

cosworth said:

Jadus, I really appreciate the attention you're paying to the sr250. I'm sure I'll buy some bits off you to keep the little aftermarket stuff for it going

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Cheers cosworth, really appreciate that. I'm so passionate about the SR250 myself I think it blinds me sometimes, but its a relief to see other people just as enthusiastic about it. I'm trying to make parts that I felt were missing when I built my first SR Many aftermarket parts companies seem to be focusing so hard on the SR400/500, the XS650, CB350/750 and W650/800 but forget the SR250!

Has anyone else had trouble trying to find shorter control cables for the SR250? I remember searching far and wide for some for my first SR build. I needed them to fit the clip-ons I installed. The original cables look all loopy and ridiculous with shorter bars and especially clip-ons. I have seen a few otherwise nice builds ruined by these big pieces of black spaghetti out the font of the bike! Anyone have any suggestions?

I think I ended up finding a clutch and brake cable from a KX80 or something like that fit? But was stuck with the throttle. I think I might have bought a cut to length kit? Where I soldered on the appropriate end fittings? Cant remember.

Otherwise I will draw up the specs for some shorter cables in a CAD drawing and specify all the correct Yamaha fittings, then place a batch order and add them to the webshop as well. Think it would be helpful to some. I have seen these kits for the SR400 but it doesn't have the same fittings :

Figured I might as well go ahead and do the drawings anyway - good to have. I think making an order for these shorter cables wouldn't be a bad idea anyway.

Started with the clutch cable, then will do throttle and brake.

Major distraction... I picked up a used but very complete Ducati 600ss rear cylinder head on ebay for 30 quid. The plan would be to adapt it to run on top of the stock SR engine - depending on how much modification would be required to either run the cam via chain drive as in the original system, or rig up a 2-1 reducing gear to be able to run the belt drive system of the Duc. This idea will not be happening for this bike, but perhaps for a future build, would be fun.

Its amazing how easy the cam is to turn to operate the valves with the desmo system (by hand!) - no wonder Duc's have good torque, there is almost no power sapping resistance from the valve train.

On that note, does anyone know if there is a rev limiter on the SR? Via the ignition system/CDI box? Or would it only be limited by mechanical design - valve float perhaps? If the latter, maybe the Duc head could raise the rpm ceiling of the engine? Long way down the track for this kind of experimenting!

"Its amazing how easy the cam is to turn to operate the valves with the desmo system (by hand!) - no wonder Duc's have good torque, there is almost no power sapping resistance from the valve train."

Actually, Ducati engines are not really known for their torque - they are known for their horsepower (which is achieved by higher revs due to the lower valve train inertia). Really large-displacement, slow-reving V-twin cruisers are known for their torque. Torque increases are usually from volumetric efficiencies (intake/exhaust scavenging), compression ratio (higher=more torque; also includes forced induction from super/turbochargers) and displacement (more=more torque). The better (rated) motorcycle engines have a higher/flatter torque curve, starting early in the rev range and continuing through redline and proper transmission gearing to keep the engine in max torque.

I would think that you would be better off trying to fit a Ducati V-twin into the SR250 frame than trying to change the SR250's head to include desmo valve action (not really - too much power for the SR250 frame and brakes).

zap2504 said:

I would think that you would be better off trying to fit a Ducati V-twin into the SR250 frame than trying to change the SR250's head to include desmo valve action (not really - too much power for the SR250 frame and brakes).

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Haha, true, would be damn near disastrous. Good notes on torque vs hp though. Should have thought harder about that one. Torque on the cruiser v-twins is also achieved from the rather long strokes they often have - as apposed to over-square high revving twins. Which the SR is not, its quite over-square itself. In fact, it shares the same bore and stroke as the many of the 1000cc 4-pot sports bikes

The XS400 and XS360 motor fits. You mod the swingarm mount and fab a front Y to split and miss the oil filter.

Wiring it into the SR harness is easy with the twin coils. You have to fiddle with the kickstand to fit a pipe on the left.

From 20hp to 35hp and most people have removed far more than the difference in motor weight. EBC shoes up front, a drilled vent, and a new front cable would allay my fears of stopping it.

It also shares the same rings as the XS650. Not in theory, in practice. I have 500kms on the new XS650 rings in my SR250 motor.

cosworth said:

It also shares the same rings as the XS650. Not in theory, in practice. I have 500kms on the new XS650 rings in my SR250 motor.

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Sweet, good to know. Is that with stock bore? Then one could presume that the overbores would work too? On that note, how different are the pistons side by side?