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Author Topic: Not another bloody SR250?  (Read 1930 times)

Offline ManxKat

  • Posts: 18
  • Older but no wiser
Re: Not another bloody SR250?
« Reply #15 on: Jan 01, 2017, 11:35:14 »
So, were you good last year? Did Santa deliver?

Apparently I must have been exceptionally good because the long suffering SR got a Christmas boost in the shape of a pair of prototype side panels and valve covers from Jake at Jadus.

As Jake has mentioned elsewhere, the finish isn't exactly glass smooth on the side panels (being 3D printed) but that's easily fixed with some sanding, primer and time. What will take more time to sort out will be the angle the bike sits at - the side panels fitted perfectly are exactly in line with frame rail and leading edge of the seat but, as you can see from the pictures, they don't sit anywhere near horizontal. Not sure now whether to drop the forks even more (the steering is already borderline skittish with the 19 inch front tyre) and/ or raise the back end (seat height is not exactly an issue!).     
GRAMMAR
The difference between knowing your sh*t and knowing you're sh*t

Offline zap2504

  • Posts: 326
Re: Not another bloody SR250?
« Reply #16 on: Jan 02, 2017, 00:04:24 »
I think he laced up a 16" rear wheel for the front on his bike.

Offline ManxKat

  • Posts: 18
  • Older but no wiser
Re: Not another bloody SR250?
« Reply #17 on: Mar 12, 2017, 06:27:59 »
Making some real progress now but, with every two steps forward, there’s one going the other way. So, if you’re planning on increasing the rear ride height on your SR250 (or just fancy a snigger at my expense), read on.
The shocks you can see are 340mm eye to eye and, coupled with a drop of 30mm to the front forks, I think they’ve got the bike sitting just about right. However:

i)   First problem is that the swingarm now fouls the exhaust “box”. OK, that’s not a huge problem as most SR owners will be looking to rid themselves of that restrictive, ugly, heavy POS anyway. I decided to keep the stock downpipe (but welded up the inner and outer pipes where I’d cut off the silencer to stop the inner rattling around and destroying itself) and bolted that up to a cheapo silencer (which looks a bit like the stock item) and an angled joint (aftermarket part for modern Bonnevilles apparently). Job done? Not quite.

ii)   Without the silencer, the centrestand will be flapping about (and with the increase in shock length, both wheels would be touching the ground when you tried to use it anyway) so that’s another candidate for the scrap pile. We’re not done yet though.

iii)   Attempts to use the rear brake lever will result in a very stiff action and a nasty grinding noise. That’s because the rear of the lever is fouling the swingarm and will need some delicate reprofiling to get things back to operational. While you’re grovelling about under the bike you’ll probably also notice the swingarm is bashing the brake lever’s return spring mounting. As I didn’t have access to welding gear, I went old school and shaped/ bent a suitable bolt, drilled/ tapped a hole in the bottom of the rear footrest loop and bolted it all up. So far, it’s stayed put but a dab of weld might be needed if it does start moving.

Other (non-visible) progress was to throw out my earlier efforts at fixing the battery box and use some scrap stainless sheet to make a more solid version which, more importantly, completely hides the battery and sundry electrics behind the new side panel. Oh, and I've started hacking a spare top yoke about to lose the stock Virago "wings".

Next step? It’s definitely going to need a proper speedo for the “stock” look I’m after and I’m hoping to be able to mock up some full length headlight brackets (i.e. that run from the top to bottom yokes) to give it the proper ‘70’s Yamaha image. Then ride it for a while to check it all works, then rip it all apart for a final polish/ paint/ clean. Then get it all filthy dirty again.   
GRAMMAR
The difference between knowing your sh*t and knowing you're sh*t

Offline zap2504

  • Posts: 326
Re: Not another bloody SR250?
« Reply #18 on: Mar 12, 2017, 15:27:58 »
I put a set of Progressive Suspension shocks on mine that were originally for a '83 XS650 Special for both longer travel and greater load; around 13" (330mm) and the rear tire just barely lifts off with the center stand. I will triple-check but did not notice any swingarm interference. Did not change the front suspension at all yet (might go to stiffer springs) as I did not want to change the rake. I'll probably create my own side panels to fit the tracker-style seat I'm going with.

RE: headlight mounts - I know you're thinking of a "full length" mount between the triples, but your new geometry may still make that headlight location look kind of high. Can you move the current clamp-on mounts lower against the triple? Or even below the triple (but not interfering with the fork), or a mount fastened to the center of the lower triple? Might make the headlight even with the tank.
« Last Edit: Mar 12, 2017, 16:29:07 by zap2504 »

Offline ManxKat

  • Posts: 18
  • Older but no wiser
Re: Not another bloody SR250?
« Reply #19 on: Mar 13, 2017, 04:45:21 »
Dammit! You're absolutely right, the headlight sits way too high. I'll try dropping the mounts but I have a nasty feeling that I might need to investigate a bottom mounted headlight so that it will sit against the bottom yoke - which of course means also finding new homes for the indicators and ignition switch. What was I saying about steps going the wrong way.....
GRAMMAR
The difference between knowing your sh*t and knowing you're sh*t

Offline zap2504

  • Posts: 326
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: 18
  • 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.....         
GRAMMAR
The difference between knowing your sh*t and knowing you're sh*t

Offline zap2504

  • Posts: 326
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: 18
  • 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.

       
     
GRAMMAR
The difference between knowing your sh*t and knowing you're sh*t

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