Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp

teazer

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That's odd. I always rip CD's to my PC so I can listen to them while I work. Still a great way to get music onto a phone or other device so I can listen when there's no internet connection. Streaming is so last year.....
 

crazypj

Split personality, I fake being smart
There were all sorts of 'fair use' issues so they removed it after a year or so (maybe less, I forget) . The workaround was use a Sharpie to make a black band around outer rim of CD maybe 1/16" in from edge. They did take an unemployed single mother to court and have her fined the maximum ($250.000) for posting on Spotify or something. 'Just to set an example'
 

JadusMotorcycleParts

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I have spent the last month preparing for and executing moving the Jadus workshop. God what a mission and a load of stress. Not something to be repeated. Now I have a much more motorcycle friendly space and in an industrial area rather than residential - so I won't need to piss neighbors off any longer.

Anyway, before I packed up everything I did some final blasting and powder coating and came up with this... The parts look pretty sweet. Still deciding if I will use the blasted finish or go with brushed or even something a little more 'distressed'. Again, I want the bike to look raggidy :D
 

Attachments

3DogNate

"You Meet the Nicest People on a Honda"
Nah the blasted and/or distressed look doesn't really fit with the clean look of the rest of the work you've done. I'd go ahead and powder those parts too. Maybe bring in a matte or satin powder to balance off the gloss black.
 

teazer

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And there is scope for further development with lighter rockers and adjusters and nuts. And squish area has scope for further development too.

I has limited success with alloy tappet nuts - they come loose, but Ti nuts work. We also had sets of the screw adjusters drilled out and broached to take a tiny allen key to say a few more grams in one of our race motors.
 

JadusMotorcycleParts

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teazer said:
And there is scope for further development with lighter rockers and adjusters and nuts. And squish area has scope for further development too.

I has limited success with alloy tappet nuts - they come loose, but Ti nuts work. We also had sets of the screw adjusters drilled out and broached to take a tiny allen key to say a few more grams in one of our race motors.
Awesome to hear someone has done that! Any measurable results? I actually bought some adjuster bolts from Kedo for the SR500 to see how they were done - like how you described (hole with Allen detail). Although that might be out of scope for this project, Ti nuts certainly isn't... !
 

der_nanno

Faster!
teazer said:
I has limited success with alloy tappet nuts - they come loose, but Ti nuts work. We also had sets of the screw adjusters drilled out and broached to take a tiny allen key to say a few more grams in one of our race motors.
The latter are also available in various other Yamaha-models, e.g. Yamaha XVs
 

teazer

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JadusMotorcycleParts said:
Awesome to hear someone has done that! Any measurable results? I actually bought some adjuster bolts from Kedo for the SR500 to see how they were done - like how you described (hole with Allen detail). Although that might be out of scope for this project, Ti nuts certainly isn't... !
I didn't do a before and after comparison, but fully lightened motors tend to rev a little higher than those that are not as extensively modified.

Pops Yoshimura published an article decades ago about modifying CB72 and CB77 motors and he documented the peak revs that the motors achieved with different weight of valves. Anything that reduces reciprocating weight will allow the motor to rev higher before bad things happen but may not necessarily increase HP unless the motor has sufficient breathing ability. Another way to phrase that is that if a motor has more flow capacity than it needs at peak revs, then it should be possible to raise that peak and make more power.

In the case of valve trains, every little help, but only a little helps only a little bit. :)

You can also check your port shapes and compare them to AG Bell's relative shapes. It's not always possible to get it all 100% correct, but Bell's numbers give a good idea of where a port is restricted and where it's too large. If a section is cast too large, it can be reduced with Devcon F or increase the size of other sections including the valve if necessary to make the port more efficient.

One trick that people seem to underestimate is getting the squish right - or at least making the squish band work. That usually requires the head and piston and barrel to be machined which will raise compression a touch. The result of getting it to work should be more power everywhere in the range. Good squish allows higher compression before detonation sets in.
 

crazypj

Split personality, I fake being smart
teazer said:
One trick that people seem to underestimate is getting the squish right - or at least making the squish band work. That usually requires the head and piston and barrel to be machined which will raise compression a touch. The result of getting it to work should be more power everywhere in the range. Good squish allows higher compression before detonation sets in.
That is one of the things I've done since about 1978 as it's cheap plus improves fuel economy and driveability. On my CJ250 (@391cc) I used divergent angles between piston top and cylinder head to make sure gas could get where I wanted in a relatively small (250 head) combustion chamber for 56mm bore instead of 67mm head design. It was based on two stroke chambers I had worked on. I've modified several sets of CB750 pistons to fit 500/550 fours. (591cc) My mods are way removed from the published 'Gentleman's Express' 500F spec. and give about a 40% decrease in fuel consumption plus a 17mph increase in top speed. It's part of the reason modern sport bikes are so successful, the entire engine package was developed around known (and now much better understood) parameters. It may be a good idea to clay the piston top and do a combustion chamber assessment?
 

spotty

Vmax...why,yes i think i will
another lightening idea i just had, replace all the metal washers ( weight bearing and structural being allowed for) that you can with plastic/nylon/rubber ones

every .1 of a gram helps especially when you think of how many little washers there are on a bike

also, don't eat pie
 

JadusMotorcycleParts

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All good ideas! I am trying to keep a cap on the goals of the project at the moment - things have got out of hand from the first draft of the plan (as some may have noticed ;D) so I will save some extreme ideas for another build - or this build later if it fails the first time. Something I have learned through my job is to stick to what you know and feel comfortable with, then take in expertise when needed (read; be humble). Unless of course the goal is to learn something new and your employer is cool enough to pay for your learning experience. I figure I would apply the same thinking here - that is why I outsourced the head work and the cylinder boring and the piston design. My specs, yes, but someone with a damn site more experience doing the work ;)
 

JadusMotorcycleParts

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Some of you may have seen this?

https://www.bikeexif.com/yamaha-exciter-sr250-scrambler

Stunning bike, no criticism, only praise. BUT, 75kg? Surely there's not 25kg to be had in modern forks and an aluminium tank? Am I missing something?
 

Pete12

Member
No way it's 75kg, my YZ125 with plastic everything weighs around 90kgs. The stock SR was around 130kgs, alloy rims and rear guard don't remove 55kgs. To quote an Aussie classic, tell him he's dreaming...
 

irk miller

You've been mostly-dead all day.
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There is no way in hell that bike weighs 75kg. They're not saving any weight with those forks. The front drum, fenders, handlebars and rims are about it. The frame looks pretty stock and that's a stock steel swingarm. If that's an alloy tank, maybe they lose a pound or two there. They're claiming that bike weighs the same as a Montesa Cota 300rr trials bike. Yeah, right.
 
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