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Author Topic: Not another bloody SR250?  (Read 3379 times)

Offline zap2504

  • Posts: 345
Re: Not another bloody SR250?
« Reply #20 on: Mar 13, 2017, 18:45:10 »
FWIW - I learned some time ago (in a reply to Jadus' build) that the "32mm seat post clamps for bicycles fit the fork tubes. It made it really easy and clean to make headlight ears with. Cut some ally plate and hand finish with a file and it turns out pretty nice."

Offline ManxKat

  • Posts: 26
  • Older but no wiser
Re: Not another bloody SR250?
« Reply #21 on: Mar 14, 2017, 13:26:28 »
Sigh. Perhaps that 535 front end wasn't such a good idea after all (fork tubes are 36mm)!

And a quick test run last night revealed:

- I seem to have stumbled on a half way decent suspension/ ride height set up. It's hardly MotoGP standard but it is predictable and the shocks/ forks work well together.
- I also seem to have lost about 20mph off the top end. Round town it's fine and picks up well but on the open road you get to around 60 and, well, the engine loses all interest in adding much more speed. Given how restrictive the stock silencer was (and the weird internal gas flow it used) that's not entirely unexpected, so there's some dyno time coming to restore the missing mph. A larger main jet beckons.....         
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Offline zap2504

  • Posts: 345
Re: Not another bloody SR250?
« Reply #22 on: Mar 14, 2017, 14:36:42 »
You would do well to read-up on Chiel's and Cosworth's SR250 projects. You should also check your compression as low compression would cause lower top speed.

Offline ManxKat

  • Posts: 26
  • Older but no wiser
Re: Not another bloody SR250?
« Reply #23 on: Apr 09, 2017, 14:53:28 »
The dyno never lies (but it can be persuaded to bend the truth)!

As mentioned earlier, the mighty SR seemed to have lost some top end with the fitting of a new silencer. A thorough going over yielded two interesting contributory factors:

i) there was no tappet clearance at all on either valve but that's not entirely unexpected with a bike that has never been apart for any form of servicing - even the oil drain plug was pristine before I got my hands on it - so those were set to factory spec, and
ii) the speedo was lying! Who would have thought that a cheap and nasty push bike speedo might have been inaccurate? To be honest, it turns out I must have buggered up the spacing between the sensor and the trigger when the original glue gave out and I didn't look too closely at where I put the sensor back. Bottom line was that the sensor probably wasn't picking up the trigger with each revolution of the front wheel and hence the speedo was giving weird readings.

So, with a thoroughly cleaned carb (all jets on stock settings), a healthy compression reading and spot on timing, it was strapped to the dyno to see what the engine could do.

First run was surprisingly solid but, as expected, it was struggling a bit at the top end. We decided to remove the air filter's foam outer cover to see if that improved air flow and another little quirk came to light. Although the foam looked fine where it could be seen, on its inner surface it was badly deteriorated and chunks had been breaking off and heading into the carb - not optimal.

So, what was left of the foam was removed and the dyno fired up again. Bingo! Slightly rich at the bottom end but the exhaust gases were bang on for the rest of the rev range and a maximum reading of 15.5 horsepower! Not bad considering Yamaha quoted 18 - 20 horsepower at the crank when new. What I did find interesting was how little tweaking a healthy engine needed (i.e. pretty much none) to compensate for the stock silencer being replaced with something much less restrictive.

If the dyno printout looks a bit rough that's because it's the "raw" data before the software smooths it all out.

Basically, we're all good so it's time to sort out the headlight and speedo (again) to get the looks nailed before the final beautification process gets underway.

       
     
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Offline JadusMotorcycleParts

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    • Jadus Motorcycle Parts
Re: Not another bloody SR250?
« Reply #24 on: Apr 16, 2017, 05:11:45 »
This is really cool!  Pretty much backs up the results I found with two different stock-ish SR's (plus over 20 hours of testing).  But I also took a closer look at the air-fuel ratios too (down there at the bottom of your graph) and even that is spot on to what I found.

Now to test some Jadus performance parts  ;) ;D  Bikes looking great btw.  I might be a little biased though

Offline ManxKat

  • Posts: 26
  • Older but no wiser
Re: Not another bloody SR250?
« Reply #25 on: Jul 22, 2017, 13:49:43 »
OK, the Yam has been stripped and parts scattered all over the place for chroming, painting and welding.

First major reassembly were the forks. I took 25mm out of them by shortening the springs and machining up 25mm damper rod spacers. As I've mentioned earlier, these forks are off an early 535 Virago and if you're considering a similar exercise it's worth checking that the damper rod spacers don't cover up the rebound bleed holes (hence the spacers were actually made into two halves) - later forks may have different internals. 

Next up was the engine which now looks a little tidier than the crusty lump that was prised out of the frame. After an internal health check didn't throw up any horrors, some external bling was applied in the form of a good polish for the outer cases, a finned oil filter cover and some really nice valve covers from Jake at Jadus.

Next stop: a freshly repainted (and pruned) rolling chassis.   
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Offline ManxKat

  • Posts: 26
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Re: Not another bloody SR250?
« Reply #26 on: Aug 14, 2017, 13:50:51 »
OK, first mock up after paint/ polish and I think that's definitely heading in the right direction!
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The difference between knowing your sh*t and knowing you're sh*t

Offline zap2504

  • Posts: 345
Re: Not another bloody SR250?
« Reply #27 on: Aug 14, 2017, 16:35:38 »
Looks nice! Would like to see it with the Virago front fender too as that fender hugged the tire real well.

Offline ManxKat

  • Posts: 26
  • Older but no wiser
Re: Not another bloody SR250?
« Reply #28 on: Aug 15, 2017, 03:07:56 »
The Virago front fender will be bolted on as soon as it gets back from the chromers (together with the exhaust downpipe and headlight brackets)!
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Offline ManxKat

  • Posts: 26
  • Older but no wiser
Re: Not another bloody SR250?
« Reply #29 on: Oct 15, 2017, 14:30:43 »
OK, I'm calling this one done (at least from a visual point of view, there's some carb fettling needed and the speedo to be calibrated but otherwise it's finished). The original brief was to turn an SR250 into something that looked like it rolled off Yamaha's production line in the late '70's or early '80's - I think it's pretty close and, anyway, it's certainly a whole lot better looking than what I started with! For me, that's a win.

If you're remotely interested, the build includes:

535 Virago front end chromed mudguard, forks shortened 30mm, speedo drive blanked off, top yoke reworked to lose the stock "wings", satin black paint

Triumph T140 headlight, aftermarket brackets

Aftermarket clock, idiot lights and indicators

RD 250 bars (I think)

SR 250 controls (shortened throttle and clutch cables)

Stock tank lowered with a Jadus kit

Jadus valve covers

Aftermarket oil filter cover

Jadus side panels

Jadus seat and rear frame loop

LC 250 tank and side panel badges

Reprofiled chainguard

Aftermarket silencer/ stock downpipe

Hagon shocks - 30mm over stock

Delugged frame, side stand extended 25 mm to avoid unsightly droop when parked

Aftermarket rear light

Time to go and get it filthy.
GRAMMAR
The difference between knowing your sh*t and knowing you're sh*t