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Author Topic: Yamaha SR250 Power and Temperature Testing  (Read 23188 times)

Offline JadusMotorcycleParts

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Re: Yamaha SR250 Power and Temperature Testing
« Reply #20 on: Jun 01, 2016, 03:11:00 »
I guess the next big questions are, is this normal?  OK?  Optimal?  What would be optimal?

With regards to head temperatures here's so much discussion on the subject on various motorcycle forums and so much miss-information out there.  Especially from forum 'gurus'.  I like to get information from as reliable sources as possible.

I found this great diagram about air-cooled aluminium engines and the relationship with temperature and strength from the book by Pratt and Whitney called 'The Aircraft Engine and its Operation'.  I actually found it in a post from a guy on a motorised bike forum: http://motoredbikes.com/threads/maximum-safe-cylinder-head-temperature-cht.40994/

I trust a diagram like this.  Pratt and Whitney engineers would have known their stuff. 

This means that with the SR's head temps getting to 214C on the highway in 26C heat its in the safe zone, but what would the head temp be on a hotter day?  In warmer climates than Sweden's pathetic summers?  Maybe the carb would just need to be jetted richer...  Attached is another cool diagram showing the relationship between rich/lean conditions, temperature, power and economy.

Offline JadusMotorcycleParts

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Re: Yamaha SR250 Power and Temperature Testing
« Reply #21 on: Jun 01, 2016, 03:21:28 »
And with regards to oil temp, there is an equal amount of misinformation out there, so I have been reading up on my bible by A. Graham Bell again.  Here is an extract:

'An engine should never be driven hard until the oil reaches 70C, while the ideal operating temperature is 95-110C.  It can go as high as 130C for short periods, but oil breakdown and excessive oxidation will take place above this temperature.  In a race engine, bearing failure is a possibility any time the oil temperature goes past 130C, and generally, HP is lost over 115C'.

Again, the SR is pretty safe here, but if ambient temps were higher, head winds stronger and hills steeper, perhaps these temps would be exceeded...


Offline JadusMotorcycleParts

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Re: Yamaha SR250 Power and Temperature Testing
« Reply #22 on: Jun 06, 2016, 05:39:49 »
Before I did some more testing yesterday I installed the oil pressure gauge - in place of the temp sensor post oil filter.  Really interesting!

I sure am glad I did some reading about the different oil pressures of plain bearing engines vs roller bearing engines before I installed it.  Otherwise I might have been very concerned! 

Results:

From cold <50C
Idle: 2-3psi
On road (3-6000rpm): 9-12psi

Operating temp >90C
Idle: <0.5psi (pretty much unregistered on gauge)
On road: 0.5-1.5psi

So there you have it.  Very similar readings to what others have documented on Yamaha engines with roller bearing cranks.  No wonder they nickname these things 'worry gauges' though.  Absolutely best not to install one haha - just an unneeded distraction on the bike. 

Btw, has anyone else seen or heard of another person to have installed an oil pressure gauge on an SR250?  I'd be curious to see if they got the same findings. 

Offline JadusMotorcycleParts

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Re: Yamaha SR250 Power and Temperature Testing
« Reply #23 on: Jun 06, 2016, 05:45:44 »
I have been discussing the temp testing scenario with my girlfriend - who is a mathematician come software engineer.  The conundrum, as mentioned earlier, is the number of variables in the equation.  She gave me some really good input though - with that many variables, the best thing you can do is take a lot of samples and average them out.  Makes a lot of sense.  Just do the same test many times in many different conditions with different riding styles and try to establish a more reliable/stable baseline (in comparison to one test for the baseline).  I'm happy to do this because it just means more riding!  ;D

Offline coyote13

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Re: Yamaha SR250 Power and Temperature Testing
« Reply #24 on: Jun 06, 2016, 13:10:02 »
I would have to agree with your girlfriend/matrhematician.  Operating in a completely controlled environment will give you a great deal of accuracy at a fixed point, but only for that fixed point, resulting in a static set of data which will not account for differing conditions, riding styles, or even different static throttle input.  What you need is good dynamic data, reached by averaging the data from a number of conditions.

Edit:  If you included a dyno-style setup you could do this indoors in a very controlled environment, and still get dynamic data since you would be able to actually run the bike under load
« Last Edit: Jun 06, 2016, 13:14:01 by coyote13 »
Half the fun's in the get there...

Offline cosworth

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Re: Yamaha SR250 Power and Temperature Testing
« Reply #25 on: Jun 07, 2016, 16:23:21 »
This engine needs as much oil as it can get up the head bolt passageway to the cam. To the point that I overfill my sump a bit and change the oil often.

These numbers jive with the temps I get from my Ducati monster that is air cooled and has an oil temp gauge. I'd say it looks good.

To maximise the oil feed to the cam we need flow, pressure, and optimal viscosity. The flow can only be helped by drilling a small hole through the access bolt from the head bolt shaft to cam bearing. And drilling the cam holes a bit as well. This changes flow but affects pressure, of which there is very little. And every SR250 motor should have the flow limiting roll pin in the crank too.

So really that leaves viscosity. Or quality of oil. Which is a subjective can of worms.

The real solution long term for anyone running this motor is for there to be a roller tappet. They are available for the SR500 but do not fit. I have contemplated making a prototype but it would be my biggest engineering feat to date. I'm not that fancy.

Drill head, good oil, roller upgrade. That could keep these motors running for 50-100 years as long as people keep making XS650 pistons which are the same.
1981 SR250 café hot rod (sold)
1976 XS360 collector plates
1980 XS400 (on the bench)
1981 XS400 (in pieces)
2017 Husqvarna 701 enduro
Livin' the dream in western Canuckistan

Offline JadusMotorcycleParts

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Re: Yamaha SR250 Power and Temperature Testing
« Reply #26 on: Jun 08, 2016, 15:48:58 »
If you included a dyno-style setup you could do this indoors in a very controlled environment, and still get dynamic data since you would be able to actually run the bike under load

Yeah I think getting some good dynamic results will be good.  I'll write everything up in a spreadsheet to be able to analyse and compare later.

But yes, doing the same test on a dyno or 'rolling road' would be a good way to get a pretty controlled environment and be able to do back to back tests to get a stable result/comparison.   :D

Offline JadusMotorcycleParts

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Re: Yamaha SR250 Power and Temperature Testing
« Reply #27 on: Jun 08, 2016, 15:50:26 »
Drill head, good oil, roller upgrade. That could keep these motors running for 50-100 years as long as people keep making XS650 pistons which are the same.

Thanks for the input man.  Always appreciate stuff from people with experience/who have tried and done some tests themselves.  Really valuable!  Cheers.

Offline JadusMotorcycleParts

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Re: Yamaha SR250 Power and Temperature Testing
« Reply #28 on: Jun 08, 2016, 18:07:54 »
Rummaging around in some old parts bins and found this from an abandoned project!  New Barnett clutch with stronger springs!  Guess I'll try the XT250 clutch on the next bike  :D

Offline JadusMotorcycleParts

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Re: Yamaha SR250 Power and Temperature Testing
« Reply #29 on: Jun 16, 2016, 18:28:58 »
I decided it would be worth while testing the use of an oxygen sensor with an air fuel gauge to take the guesswork out of jetting - especially if I want to test all these different parts. Its not a cheap set up, but I figure its a worthwhile investment if I'm wanting to get the jetting spot on, be able to give jetting guidelines for others, and for when I work on any other bikes in the future - for example this xs750!

I had some perfect size tube lying around that fit into the end of these reverse megaphone exhausts, so I welded the bung into that (ignore the shit welds, its airtight ) and I will do as I have seen others do and attach this set up to different bikes with some large ss ring clamps. Seeing as it is only ever meant as a temporary thing just to get the jetting right at the completion of a build. We'll see if it works and is a useful tool shortly!