Yamaha SR250 Power and Temperature Testing

I got the Jadus test mule 2.0 all registered, taxed and insured in Sweden now. It goes a treat! Feels a bit perkier than any of the other SR's I have ridden. Did they do anything different on the engine on these models does anyone know? Exhaust is the same(ish), air box is the same, stroke/bore is the same. Does it have the same valves and cam?
 

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When I was doing the testing of the finned valve covers I noticed how much they were shrouded from the wind flow by the frame and tank. Then I noticed how out in the open the cam cover is and saw how much clean air it gets over it. So I thought I would design a finned one just to see what it would look like and how much I could increase its surface area over stock.

Well, the finned valve covers have an extra 40% surface area over stock (easily calculated in CAD) and reduced total head and oil temps by a couple of degrees. This cam cover has around 45% more surface area than stock so I am hoping it will do something if I decide to prototype it in ally. The cool thing is, I took recordings of the cam cover surface temperature when I was doing all the testing, so I could test it back to back later if I wanted.
 

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I had a stuffed stock header in the workshop from the last bike I modified. It rusted out so had to replace it.

I figured I could cut it open and show how piss thin the stock header pipe is - the one running internal to the chromed exterior. Eventually I'll trim away the entire outer and sit it side by side the Jadus header to highlight the diameter difference.

The stock header will simply not support high flow of exhaust gasses, therefor will not support higher horsepower. It is good for power up to 4000rpm (because it promotes good exit velocity) but then it strangles the engine - this is evident in the various torque curves of period tests. Looking forward to cutting it open and seeing how funny it looks on the bike!
 

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I got the first order of ignition advance brackets in the post this week. They look awesome! But before I start selling them to people I wanted to design a version that fits the SR250 Classic as well - the set up is a bit different. I would also like to do a bit more testing to make sure they don't hurt the engine and only make it run more efficiently and give more power.

So here is some development - a CAD sketch to work out the exact timing advance from distance to degrees, some 3D printed prototypes and then some screen shots of some video I took that shows the actual effect on base and final timing with a strobe timing light! Also a shot of each timing set up on the different models.
 

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Btw, does anyone reading this already have a modified or well performing SR250 that wants to try these brackets and report back to me? I'll offer free shipping :)

Just to re-cap the idea/purpose...

The SR250 was designed and developed to be a commuter bike basically and to function in all environments and in all countries - meaning large variances in fuel octane and quality. For this reason, the stock ignition timing is quite mild - reduced timing helps reduce engine knock with poorer grade fuels and keeps cylinder pressures slightly lower - also reducing the risk of pinging.

Spark timing is very important to get right for a single cylinder (or any engine) and should ignite the fuel at a point BTDC so that the mixture starts burning and peak cylinder pressure is reached at between 10 and 20 degrees ATDC. If you ignite the mixture too early, peak pressure will come too early and will result in power loss. The same goes for igniting the mixture too late - you will miss out on potential power.

Now with modern standard to premium fuels in most developed countries (especially in Europe) the octane rating or RON is around 95-98. In Sweden 95 is everywhere and you cannot get less, and 98 is available quite widely. With higher octane you get a slower more controlled burn of the mixture. So igniting it too late, you miss out on some potential power.

So what is the magic number for initial and peak advance? There isn't one really, there are too many variables. On the SR the timing curve is fixed and controlled form the CDI box - peak advance comes n by 3000rpm. On modern bikes they have several more inputs that control the timing - with the addition of MAP sensors and throttle position as well - all to get spark at the right time for that precise riding moment. So on the SR, there is only one adjustment to be made - the position of the pick up (which is fixed from the factory) and the CDI doesn't care what the throttle is doing or the intake pressure, it just measurers rpm and releases a spark based on that.

I have read and seen performance engines gaining considerable power (5-10%) with optimised ignition timing, so that is why I thought it would be worth trying the brackets. I hope they work two fold - more efficient (cleaner, more complete combustion, leaving less carbon buildup in the engine), and more power - getting peak cylinder pressure earlier. I need to do more testing on its effects with engine head temp and air fuel ratios from the exhaust/O2 sensor, but with all the parts I designed installed, including the brackets, that first test mule made good power, so I am feeling confident. The only risk is the potential too knock but more testing will reveal or negate this issue.

The last reason I am trying them is because many engines, especially these ones that many have had a hard life, will loose compression over time from worn cylinder walls and piston rings, even slightly leaky valves. I am talking a reduction from say 180psi (I doubt its that high but the manual says so...) from the factory to 130 or even 120psi after 20-30 years of use. This means much less cylinder pressure - requiring the mixture to be ignited earlier to make up for it. Btw, if you're getting compression readings below 120psi, its probably time for an inspection and rebuild - 110 is really deaths doorstep.

So I would not recommend these brackets for anyone using less that 95RON, or who is getting compression readings higher than 140psi :D

Again, any early adopters willing to try???
 
I figure its worth busting a myth about these bikes at the same time while on the topic of fuels...

The SR250 was never designed to be run on leaded fuel!!! Two SR's I have bought the previous owners have told me I needed to add octane boosters or fuel additives to the tank on each fill up. Nonsense! All bikes from Japan were designed to run on unleaded fuels as far back as the mid 70s, perhaps even earlier. So just because some of them were manufactured in the 80's does not at all mean they need the booster.

These bikes run as they are intended on unleaded fuel, period. A fuel additive or octane booster would only be needed if you're using real low grade fuel. Sure, adding it won't hurt, but you'll be wasting your money. Unless, for some reason your engine is pinging (which it shouldn't be), then adding some might help.

Lead in the fuel didn't increase the octane rating, rather it reduced the risk of knock (allowing higher compression ratios, thus increased power) and was kinder to the wearing surfaces of the engine. But the SR250 has hardened valve seat inserts in the head anyway!

Rant over haha
 
I have been meaning to offer jetting tips/guidelines for those who have bought the Jadus header for a while now. I thought the best way to do that would be to get concrete lambda results so that air-fuel ratios where not guessed, rather they were measured and exact.

Although the set up I had for the XS750 with the Oxygen sensor stuffed up the tail pipe with an adaptor and jig worked well, it did not work at all on the SR250. The exhaust pulses are too far apart and the O2 sensor reads 100% lean because it was being affected by the oxygen coming in from the end of the tail pipe too much.

So to be able to get a proper reading, I decided I would sacrifice one of the headers (one I was using for dyno testing anyway) and weld in a bung for the O2 sensor. I checked the position of O2 sensors on other modern EFI singles and decided on a location. The nice thing with this long bung is that the sensor does not end up interfering or disrupting the flow of the exhaust in anyway. Once its all installed, it sits pretty flush with the interior wall of the header.

Yet to be installed and tested but hoping it'll work ;D
 

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I also put on all of the finned covers I have designed for the engine to see what they would all look like as a family. I am considering getting a set made up in black anodised aluminium. The finned oil filter cover also has an updated design to give better cooling. I'll cover that later...

I think the covers really make the engine look aggressive and tuned! Completely changes the look of it I reckon.
 

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Hey man! The finned covers look really nice.

If you don't have any takes to help gather data on the ignition advance brackets let me know and I may be able to help. The issue is that we're heading into winter here and I won't have a running bike for quite a while. That means the baseline data and test data could not really be gathered for several months, which may not be in the timeline you're expecting. PM me if you want to discuss this further.
 
Looks amazing as usual! Would be interesting to see if the oil filter cover fits the SR400/500. If you want to try it I'm happy to help out.


Sent from my iPhone using Tapatalk
 
Thanks guys. To answer your questions...

sbruton, yeah man, would be great if you wanted to test. Give me a bell when your bike is ready and I'll ship them out to you. I am going to do some more testing hopefully before the end of the month - before it gets waaay too cold. We dropped down to 10 (C) and below this past week :(

Eleganten, yep, the oil filter cover fits these models: SR250, XT250, TT250, SR500, XT500, TT500, SR400, SRX600 :D
 
Something I have been wanting to share for a while...

After the first dyno testing sessions I realised the gains a tuned intake could have. Most times, with other engines, there is not much to be gained because the intake is already somewhat tuned from the factory - at least a balance has been achieved for packaging and harmonic benefits. So modifying the length of the intake tract will only move the power band rather than giving any flat-out gains. This is not so with the SR's intake. It is at such a length and design that it doesn't take advantage of this phenomenon and therefore doesn't get the benefits. So when adding a strategically designed stack, it actually creates a gain.

The 3rd harmonic pulse gives a pressure wave boost of 7% and the 4th gives a boost of 4%. So if distanced correctly, it can affect the torque curve through the rpm range quite a lot.

Anyway, I knew the idea worked in theory (and in the tests) but wanted to optimise it if possible and if I will take it into production (big start up cost/investment) it should be as good as it can be. So without access to a flow bench (which would be awesome!) I decided to utilise my CAD skills and run some flow simulations. I don't know enough to get concrete, close to real world results, but I know enough to be able to get a base line, then run the same exact test with all the same inputs on the other designs, and have a good set of comparison data. This way you can calculate percentage wise any affects the design has had. Each design was optimised in itself to have as close to tangential curvature as possible (which aids in a smooth surface transition for better air flow), hence the analysis tools for each design - curvature check in both the 2D sketch and in 3D, then zebra stripes to check for continuity.

The results were pretty cool and its amazing to be able to visualise the effects in the software. I was surprised by a couple things but generally the theory adds up and proves out. In the pictures you can see the flow through a standard opening, then you can see the flow through 3 other different designs. These designs are a compromise to be able to fit within a decided space and flow as much as possible.

The compromise is between ideal flow gains and optimised air filtration. If you have a huge, elliptical bell mouth opening you can gain a lot of smooth air at the carb, but it is very difficult to be able to filter the incoming air in an efficient and decent way - ideally you would have a large air box with the stack extending inside of it. Many people use mesh/guaze filters but these are pathetic for the street and your motor will be toast if you get into any loose stuff. Then there is the sock filter option, which offers better filtration than the gauze, but hinders flow a lot (minimal surface area). So the next best compromise is some kind of clamp on filter. I searched far and wide and found one by ram-air in the UK that had a really large neck diameter and was still quite compact. I love it. I have used ram-air filters before and like them a lot. I prefer foam filters over paper (K&N) type filters in a big way (that's a discussion for another day).

Well that was a lot of info but check the images, they say it better ;D
 

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Oh yeah, as you can see in the last pic of prototypes, I am developing two intakes simultaneously... One of them is the harmonic intake explained above, the other is a stubby bell mouth one for those that like the look of a filter right off the carb. This one is designed to go inside a pod style filter but have the benefit of improved flow into the carb mouth. There are solid gains to be had here as well - anywhere between 5-10% increase in flow compared to an open carb. Further testing will confirm or deny ;)
 
Thanks Jeff!

The O2 sensor and air-fuel gauge installation went pretty smoothly. Now I can see exactly what carb settings I should be running. Very, very surprisingly, with this larger diameter header and an open reverse cone mega (17" shorty version), the bike is running slightly rich?!! Go figure. Under load it dips into some pretty rich 11 territory. Would ideally like to get that up to at least 12 - 12.5. I'll try lifting the needle next because it seemed pretty close on idle and WOT, just in the middle was the issue. More testing to come ;D

The connection for the sensor and gauge is some alligator clips so I can still transfer the set up to another bike and just plug off the O2 sensor bung.
 

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Well, to adjust the carb I actually took it off the bike for the first time. Surprise, surprise, the PO had stripped the pilot jet slots and the thing wasn't moving. Damn. Short of getting in there with a left hand drill bit and using a bolt extractor I decided to try one technique I saw some dude pull off on youtube. It worked a treat! With one small modification...

The process:

_Spray some good penetrating oil onto the pilot jet
_Heat up the area where the pilot is lodge with either a low heat flame or a heat gun. This should break the cold weld and let the oil seep into the threads
_Bash an allen key or torx head screw driver (this was my modification to the process) into the pilot jet hole. It is very important that the fit is tight. The allen key or torx head needs to be tapered/chamfered slightly with a grinder or file so it leads into the hole nicely. When you punch it in hard, the edges of the bit should bite into the sides of the soft brass hole.
_Slowly screw out the pilot and replace that bastard!

This may not have worked, in which case I would have done the first option, but it thankfully worked a treat ;D

Cred to this guy :D :

https://www.youtube.com/watch?v=657tEj60VPo
 

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While I was in the carb I noticed a pinhole in the diaphragm as well. The bike worked fine with it and it is probably too small to have an adverse effect. But seeing as I am trying to run tests and get semi accurate results from different jetting settings, I wanted to make sure the slide was lifting and closing under vacuum as it should. So it was time to replace it. As in the other SR I am building, JBM Industries came to the rescue with their replacement part...
 

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Today I finished cutting up the old stock exhaust system I had. Now you can see how small the actual header is - the internal pipe. Crazy! Then a comparison shot ;D
 

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I was shocked at the double wall header pipe when I cut the stock muffler off mine... I assume that is a heat/ noise level issue, but it really must choke it up - (as you have proven in your testing!) Its fascinating to see them side-by-side.
 
sbruton said:
I was shocked at the double wall header pipe when I cut the stock muffler off mine... I assume that is a heat/ noise level issue, but it really must choke it up - (as you have proven in your testing!) Its fascinating to see them side-by-side.

Yeah it was the same for me when I first cut up a stock pipe to attach a new silencer (years ago now). I think the tiny inner pipe is for a couple reasons - one is like you say, for noise. One period tester described the exhaust like this: 'the SR was handicapped by exhaust arrangements which had something of an old-style racing megaphone look but stifled the note to no more than a polite cough'. Haha Brutal.

Another reason is looks - the outer pipe and its chrome are much less prone to discolouration from heat in comparison with a single walled header - chrome 'blueing' and stainless 'bronzing' if you will.

There may be one other reason, that is it actually provides a chunk of torque (being so small) right at about 2500 to 3000 rpm - maybe a good thing for new riders and city riding? After that though, power and torque are very unremarkable.

You can tell a lot about the design compromise just by looking at the SR's enduro and off road counterparts - the TT and the XT. Just look at the pipes on those things - not huge, but certainly larger diameter than the SR and they made much nicer power and a much better exhaust note ;D

Love the sound of this one:

https://www.youtube.com/watch?v=ySbs3zfjlJA
 
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