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Author Topic: Yamaha SR250 Power and Temperature Testing  (Read 16117 times)

Offline JadusMotorcycleParts

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Yamaha SR250 Power and Temperature Testing
« on: May 15, 2016, 05:21:22 »
As some of you may know, I am building a 'standard' style (going for some timeless style) Yamaha SR250 and using it as the basis for designing and testing a range of styling and performance parts.  I felt that the infamous SR250 was grossly neglected on the aftermarket and thought I could step up and make use of my design/engineering background to make some cool parts for garage builders.  Needless to say, I have a pretty strong passion for these little bikes!

Anyway, I decided to start this separate thread for the testing alone so not to detract from the build itself in the original thread.  Also because I bought another SR250, bone stock, just for this purpose - to test the performance parts and to test fit the styling parts.  I am hoping to eventually make a series of instructional installation videos for the parts on youtube as well - I myself find it easiest to see people do stuff and do what they do, rather than reading instructions ;)  Damn left brain!

I intend to be open and honest and unbiased with the process, hoping that if anyone else were to perform the same tests, they would get the same or similar results.  Maybe not the smartest business move, but who wants to buy parts that dont work?!

Check out this poor little (albeit ugly af) test mule  ;D

Offline JadusMotorcycleParts

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Re: Yamaha SR250 Power and Temperature Testing
« Reply #1 on: May 16, 2016, 03:24:31 »
I thought I could start by establishing some base lines.

Many people might expect that the SR250 puts out either 17 or 20 horse power - according to Yamaha and many online documents.  As with many motorcycle companies, they tend to publish power output at the engine, rather than at the wheel - making the power figure sound much higher because it hasn't had any drive train losses.

I have found two independent period tests from two different magazines (one in Australia and one in Spain) that put rear wheel horse power at a tad over 13.  Sorry to burst anyone's bubble here!  That figure is a little underwhelming haha.  Power at the engine was however at 18 - only two less than Yamaha's claimed 20 in this case.

With regards to head and oil temperature, and oil pressure, I have not been able to find any information online specifically about the SR250.  If anyone else has, please share!  This means I'll have to try and establish some baselines of my own with the sensors I have installed and get the results from the stock bike first.

Offline JadusMotorcycleParts

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Re: Yamaha SR250 Power and Temperature Testing
« Reply #2 on: May 17, 2016, 03:41:24 »
First things first were to get the bike running properly and road legal in Sweden.  There were quite a few unpleasant surprises when I started working on the bike...

The stock exhaust (as with many of these bikes) had rusted out leaving a hole - so that had to be replaced.  Fine.  However, when taking the exhaust off to replace it, I discovered that it was attached to the head with one normal M6 bolt, great, then the other, a f*cking wood screw!  What?!  I was fuming.  Some muppet had snapped off the original bolt in the head, then tried to drill it out, drilled waaay crooked, missed the original bolt and ended up with another almost sideways hole in the head.  Then the hole was too mashed to tap properly so they used this wood screw.  Awesome.  I myself have messed up stuff like this, but when you do, it is important to do the repair properly.  Easy thing to break off a stuck bolt by mistake.  But the bolt should be removed properly, then a thread repair/helicoil set put in its place.

A few other items... had to replace the completely worn drive package, the speedo (faulty), the rear brake light switch, the rear wheel drive studs were stripped, rear tank mount was missing (due to mounting hole being stripped), sump plug was stripped (the hex part, not the thread thank god), the left side cover was held to the bike with two wood screws straight into the air box!  If that doesn't fuck up carb settings I dont know what would.  Fuel petcock seals shot, pissing fuel all over the ground when disconnecting the tank, oil super black (hadn't been changed in eternity), valve tappets super loose, no gasket used on the alternator side engine cover, three holes in the carb diaphragm (no wonder it was topping out at 95 on the highway), completely stuck air screw and rust particles in the float bowl.  I ended up just replacing the whole carb with a spare one I had.

I got to it and replaced/repaired most things.  I shrugged everything off apart from the wood screw.  Just couldn't let it go.  Anyway, I managed to clean up the hole and re-tap it with an M6 tap and got about 6mm worth of decent thread - just enough (I hope) to hold it in place on that side.  I think I will some day soon make a big bore kit for these bikes and then I'll be taking the head off anyway and will get it repaired properly.

Offline JadusMotorcycleParts

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Re: Yamaha SR250 Power and Temperature Testing
« Reply #3 on: May 17, 2016, 03:45:58 »
I also swapped out the silly semi-ape bars that are so uncomfortable for me being 6' 1''.  I put some lower profile ones on and now its much more comfortable to ride.  When changing bars on the SR250 you end up with copious amounts of extra cable length - black spaghetti everywhere!  So I took the opportunity to test fit the short cable kit I am working on for these bikes.  They fit pretty well for first samples.  I wrote up some notes/revisions and hopefully the next samples will be spot on - ready for a production order.

Offline dualero

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Re: Yamaha SR250 Power and Temperature Testing
« Reply #4 on: May 17, 2016, 10:34:56 »
Good writeup! This project is really really interesting, keep it coming dude!

Sent from my GT-I9300 using Tapatalk


Offline JadusMotorcycleParts

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Re: Yamaha SR250 Power and Temperature Testing
« Reply #5 on: May 17, 2016, 17:32:41 »
I ended up getting the bike to run really nice after the full service.  Pretty much a brand new stock exhaust from my other project, new re-furbed carb I had lying around, oil and filter change, valve tappets set, cam chain set, etc etc.  Now it almost pulls 100 merging onto the highway in third!  These bikes love revs haha.  It cruises pretty comfortable around 110 but struggles a bit after that.  It can do that 110 in 4th or 5th though!  5th just runs out of steam a bit on a hill or head wind so you just chop it down and ring it out some more.  Makes me realise that perhaps the other SR250s I have owned I haven't rung out hard enough  ;D

Once I got it to that level I took it in for inspection and it passed with flying colours.  Yay!  Now it is registered and insured in Sweden with a sweet Swedish plate!

Offline JadusMotorcycleParts

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Re: Yamaha SR250 Power and Temperature Testing
« Reply #6 on: May 17, 2016, 17:46:27 »
I also installed the sensors I have been talking about for months!  One neat head temp sensor from trail tech - just bolts on with the spark plug (remove crush washer and replace with their copper sensor ring) and has a handy foam tape backing.  One oil temp sensor/probe tapped into the sump plug to give me a really nice accurate oil temp.  Then one more tapped into the clutch side crank case - right after the oils journey up through the oil filter.  The idea is to be able to get a temperature comparison between these two pints.  And hopefully establish a correlation between oil and head temp, if there is any in this case.  All seem to work great!  I did a semi decent wire harness for the sensors so they handle all the testing.  I soldered in a couple of alligator clips so I can just connect and disconnect straight to the battery without messing up any of the stock wiring.  Then its also easy to take it off later down the track when testing is done.  The gauges themselves are held on by an M4 bolt that I drilled and tapped into one of the handle bar mount bolts! 

I installed one of these oil filler cap temperature sensors to see how accurate they really are  ;)

Offline JadusMotorcycleParts

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Re: Yamaha SR250 Power and Temperature Testing
« Reply #7 on: May 17, 2016, 17:51:33 »
Good writeup! This project is really really interesting, keep it coming dude!

Thanks!  I'll certainly keep it coming!

Offline Extreme_250

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Re: Yamaha SR250 Power and Temperature Testing
« Reply #8 on: May 17, 2016, 23:43:40 »
whoa, that rear fender though. XD Looking good man, keep it coming.

Offline JadusMotorcycleParts

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Re: Yamaha SR250 Power and Temperature Testing
« Reply #9 on: May 20, 2016, 04:49:16 »
Well I have done some quick test rides with the sensors and they seem to work pretty well.  The one installed after the oil filter seems to cut out after a while and completely dies on the highway. At first I thought it could be because its so exposed - sitting out in the open air and chilling right down to 40C which is the lowest point the gauge will show.  That is why I tried to insulate it with exhaust heat wrap.  Stupid me!  When that didn't help, I tried wiggling the sensor wire at 100kph haha.  That seemed to get some flashes of temperature readings.  So I thought it was the connection...  Re-did that to no avail.  Then I thought it might be a faulty sensor...  But why does it read correct sometimes?  Figured it out...  The sensor relies on getting its earth through the connection to the engine cases.  I figured this out after looking at the wiring diagram again.  The crank case cover perhaps does not have the best connect to the engine - due to the gasket.  Then with this bike, the earth is connected to the frame rather than the engine, so perhaps that contributed as well.  Anyway, I rigged up a second earth straight to the battery and that seemed to solve it!  Happy testing days ahead  ;D