Ton up SR250: 100mph, 100kg, 30hp

John Murray

Been Around the Block
Sounds like quite a successful session. I think you have enough hp to crack 100mph. Team Afflick over here holds quite a few records in smaller classes, mainly 100cc. Here's what he once wrote "To go 100mph without a fairing you will need 23-25 rear wheel HP. We ran 94.5mph with 21hp and then 106mph with 28hp unfaired." Keep in mind this was with a bike that was physically smaller than yours. But if you drop the nose down and keep it as clean as possible drag-wise I think you'll do it.
 

crazypj

Split personality, I fake being smart
Nice to see your still going on with things after the last few years. I just got older and tired(er)
 

JerryAssburger

Been Around the Block
CONGRATS!!

Sounds like you've pretty much extracted every last bit of "Ummmph!" possible out of an air-cooled, two-valve, naturally aspirated engine of that size. The fact that it's peaking at 9000 rpm pretty much shows you've hit the design flow limit. Gearing for your target-top-speed@ peak rpm (160kph@9000 rpm top gear) should be your best shot.
You've done an amazing job of eeking out everything, now the Gods of Top Speed just need to smile upon you that day.
 

sav0r

Coast to Coast
Yep, she is going to run. Lube up, shave the leg hair, smash a beer, and hope for the best.
 

JadusMotorcycleParts

Over 1,000 Posts
DTT SUPPORTER
I had already put the testing off once, so this time there was no backing out, I was fully committed. I booked a small enclosed trailer for the 4.5 hour road trip and packed a whole bunch of tools, spare parts, extra fluids etc and built a wooden ramp to get the bike up on the trailer. It was a pretty drama free trip and the only hassle was sticking to 80kph the whole way.

Once arriving at the event in the morning the next day, nerves turned to focused excitement. You turn up, check in, pay for special one day racing insurance, get a race number etc. Then it was time to unpack, prepare the bike, and head over to the drivers meeting. Once we were all on board for how things work, it was time to line up at the start line! I had bought a full leather racing suit of really good quality second hand online and it fit surprisingly well, so it was on with that and the rest of the safety gear.

Heading out from the paddock towards the start line there was a pretty decent headwind, which did not inspire a lot of confidence. A quick check of the weather app indicated a direct head wind of 6m/s (21kph). Later, as the wind picked up to 8m/s, some of the car guys getting up to 250+kph were saying it felt like slamming on the brakes between gear shifts just letting off the throttle. The start/timing system was really set up for cars, so it was a bit tricky to get it right with skinny spoked wheels - the laser would just pass right through them. This meant that of all 12 runs I did, only 4 got captured/timed. Anyway, my first run started alright and I got up to 142 kph. The bike felt like it had plenty more to give but just ran short of track and I had to throttle off. It also spluttered a little near max rpm on a couple of occasions right before shifting, so I thought something was off in the jetting (after a lot of thought, after the fact, I now suspect it was something else - more on that later). Even those speeds will move anything on the bike that is not 1000% secure and the gopro mounted on the handlebar did the old flipperoo and pointed skyward after getting to 120kph haha. Needed a new solution there.

I thought I would try gearing the bike down a little to see if I could get to speed faster, so I swapped the 15t front sprocket to a 14t - moving from a calculated 8500rpm at 161kph to 9100. At the same time I changed the jet needle to a richer one thinking it was going lean near the top. The next few runs resulted in similar terminal speeds and the bike was just struggling with the wind and the short distance. I still had some sputtering/hesitation high up in the rpms, so I tried swapping main jets and needle clip position a couple of times and gained no better results. One run I forgot to turn the fuel back on so half way up the track the float bowl ran dry and the bike started spluttering. I switched the fuel on mid run and hopefully no engine damage occurred due to running lean. Duh! I even swapped to shorter rear shocks to lower my profile - to no avail/no better results. I should have tried lowering the front as well, but in the heat of the moment I was focused on getting the engine to run right. I was sure that without the wind and about 1k of track at wide open throttle, the rpms and speed would have kept climbing and I would have gotten to the ton. At this point I was a little disheartened. So much build up for such an anticlimax. I couldn’t stop thinking about how I could probably get to 161kph if only heading in the other direction…

Then one run I raced another bike, a CBR1000RR (a Repsol spec bike). He of course wheelied past me and got up to like 230kph over the 804 meter distance. Then, when following him on the way back, along the taxi strip, he blasted off… so I decided to go after him! Once the revs started climbing I realised this was my chance. Luckily the gopro with gps was on (and in a new, better fixed position) and I gave it the beans for a good 1km, eventually getting to 161kph! The revs were still climbing and I probably could have gone just a tad further and a tad faster, especially if I swapped back to the 15t front sprocket. But I wanted to be respectful of the rules there and gave plenty of room, like 600m to slow down. The rules were that you were not allowed to race each other on the way back from your run but you could ‘come up to speed to cool your engines’ - so that was a bit open for interpretation and obviously the guy on the Honda thought he needed some more air through his radiator to cool his engine ;) But the whole area was a closed track so I deemed it safe to wring it out there.

I was pretty stoked but wanted to do it again to be sure. I finally got a run where I ran on my own (no one in the right lane) and got up to 161kph again, a repeat, not just a fluke! I managed to capture the speed with both the gopro and a gps app on my phone. Both showed a terminal speed of 100mph the first time and 101mph the second time. It felt pretty good, but almost bittersweet and I am not 100% satisfied… Full disclosure, the speed was not recorded by a third party and is therefore not official, plus the speed was obtained with a pretty decent tail wind. So that is the bitter part. However, I have now ridden this bike 100mph, twice and that feels like the goal has somewhat been achieved, that is the sweet part. I am certain that, as mentioned earlier, on a completely still day, on the flat, with a 1km straight, this bike would do the ton again and again. I wonder how factories used to do their top speed tests. On those big velodrome circuits? If this bike was able to run on one of those I am certain it would top the ton easily.

Anyway back to the mystery spluttering issue. I wondered if it was fuel starvation from the narrow opening in the quick connect connector I had installed. I decided to remove that and run a straight tube instead. This seemed to help but hard to know decisively. Then the day after, with all the time to think about it on the trip back, I came up with a theory. When dyno testing, we got perfect jetting and didn't have any of this spluttering, but we always throttled off right after reaching peak rpm. Whereas doing these tests involves sustained high rpm running. So I thought maybe I was draining the float bowl and starving the main jet and the fuel line was not able to fill the bowl quickly enough. Would that be possible? Sounds plausible. To overcome this I would either need to vent the tank better so fuel could flow at the petcock easier. Or install a vacuum operated fuel pump off of the vacuum pick up on the intake boot.

All in all, it was a sweet trip to do and would have been even more fun to do with a buddy on a similar machine - some kind of older motorcycle. If I were to do it again, I would try to enjoy the event a bit more and take time to watch the other cars and bikes and talk to more people. I was so focused on wrenching and trying to improve the bike I feel I missed out on some other cool stuff.

Whats next for this project?! Fix a few details that aren’t working great - speed sensor position/bracket and rpm pick up for example, get some decent photos of it, then ride it and enjoy it! A few more posts/updates to come yet!
 

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John Murray

Been Around the Block
Great story and pics! Don't be disappointed, that's a very good result for a first outing. Fuel flow can be easily checked, and is best done by measuring the fuel flow over a minute. If at all possible you should include the float valve in the flow path by measuring the flow out of the bowl drain plug. A good rule of thumb for this type of engine would be 320ml of gas per hp per hour. This works out to 160ml, add a safety margin and call it 200ml/min. Be aware that conditions on a moving bike are different to on a dyno; air moving at 100mph or more over the bike can affect the fuel pressure at the main jet depending on where the tank breather and fuel bowl vent are situated. Consider the settings that gave best results on the dyno as a good starting point for track work rather than absolutes. Great work!
 

pidjones

Over 1,000 Posts
Congrats! And, a well done for getting good leathers before the attempt. Vacuum operated pumps suck. (Why did I include that?) They work on fluctuating vacuum, and I doubt their reliability at high RPM. Consider a small low-pressure electric. You should have the reserve electrical power to handle it.
 

John Murray

Been Around the Block
I doubt you'll need a pump for such a small volume of fuel. Most motorcycle carbs can only hold back a very low level of pump pressure (like 1.5psi or less) before the needle is overcome and the bowl overflows. It's generally more productive to use a larger bore seat without a pump and check that the petcock and lines can keep up. You shouldn't have much trouble at all with 200ml/min, I've used non-pumped setups that flowed over 600ml/min using a ballvalve petcock, 8mm ID fuel line and a 3.5mm seat. Like you said, check the tank vent is sufficiently sized.
 

crazypj

Split personality, I fake being smart
I think you may be right about fuel flow.
As mentioned, you need to check flow amount actually through the carb although I think it may just be the fuel tap bore or filter and it's easier to check that.
You may need to change tap for something off a different model, maybe DT250 or something?
 

JadusMotorcycleParts

Over 1,000 Posts
DTT SUPPORTER
Thanks all! After the dust has settled I am pretty chuffed all round.

Interesting thoughts on the fuel supply situation. I have a spare carb so I can test how much fuel flows through the float valve with the float bowl off. Then I can test how much fuel the petcock flows as well. The good thing is, this problem does not occur during normal riding. So the only pressure to fix it is when I want to do another speed test. But from you suggestions it sounds like I have some options to remedy it!

First prio now... sort the damn speed sensor out!
 

teazer

Over 10,000 Posts
DTT BOTM WINNER
It helps to "port" the fuel tap and the area of the carb above the needle valve and to fit a larger needle valve if one is available. And try it without the quick disconnect just to make sure thee are no fuel flow restrictions. You can also dill a slightly larger breather hole in the filler cap.

All in all, that was a great result. Congratulations. Your description of the day is like any race day. Anticipation, weather not perfect, making changes and forgetting to change other things. It all gets better with practice but that's the nature of the beast. Next time you could try narrower bars to get you arms in tight against the tank and perhaps a different seat - lowe where you sit and with a bump on the rear to try to smooth the airflow and reduce drag.
 

JadusMotorcycleParts

Over 1,000 Posts
DTT SUPPORTER
It helps to "port" the fuel tap and the area of the carb above the needle valve and to fit a larger needle valve if one is available. And try it without the quick disconnect just to make sure thee are no fuel flow restrictions. You can also dill a slightly larger breather hole in the filler cap.

All in all, that was a great result. Congratulations. Your description of the day is like any race day. Anticipation, weather not perfect, making changes and forgetting to change other things. It all gets better with practice but that's the nature of the beast. Next time you could try narrower bars to get you arms in tight against the tank and perhaps a different seat - lowe where you sit and with a bump on the rear to try to smooth the airflow and reduce drag.
Good ideas! And thanks for backing up the 'race day feelings'. Certainly felt like a rookie but nice to know others who do it often experience the same.
 

JadusMotorcycleParts

Over 1,000 Posts
DTT SUPPORTER
These are the classic full leathers I got hold of. Really nice stuff and so happy to get hold of them second hand and in such good shape.
 

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gt alex

Been Around the Block
Hi, in my limited experience lean mixture usually feels more like the ignition has been turned off and richness more like spluttering unless really rich then it dies.
I agree dynos are good but the real run can still find gremlins.
Is your float bowl breather connected at the mouth of the carb so the engine air intake and float bowl are seeing the same pressure? Lower intake pressure would make a richer mixture. Sometime a longer ramtube or bigger pod filter will run a little richer buy not allowing the fuel fog to be lost when flat out WOT
Is vibration foaming the fuel in the float bowl?
Just a few ideas
 

JerryAssburger

Been Around the Block
I know I told you about this, but don't know if you found this article.
https://static1.squarespace.com/sta...4962694774/CCW+Heist+250+LSR+Tech+Article.pdf
That's an interesting article, and kudos to them for taking an engine that never was meant to be much more than a commuter engine and getting that kind of power out of it. Too bad the rules don't allow them to go with a smaller headlight and drop that front wheel down to an 18" or 19" skinny for better aero... the last picture kinda shows what I'm talking about. They probably could've hit 100 with those mods.
 

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pidjones

Over 1,000 Posts
That's an interesting article, and kudos to them for taking an engine that never was meant to be much more than a commuter engine and getting that kind of power out of it. Too bad the rules don't allow them to go with a smaller headlight and drop that front wheel down to an 18" or 19" skinny for better aero... the last picture kinda shows what I'm talking about. They probably could've hit 100 with those mods.
Wonder if the rules allow the headlamp to be reversed?
 

JadusMotorcycleParts

Over 1,000 Posts
DTT SUPPORTER
Very little done on this bike since the summer but have started prototyping a better speed sensor (hall sensor) bracket for it. It picks up the M8 bolts on the rear sprocket/hub. A few more tweeks, probably 2-3 prototypes and I rekon it'll be ready to order in billet aluminium. I had a few ideas for how to mount it but it came down to this one being the easiest to install and remove and will always follow the sprocket bolts position - even with sprocket changes and chain tension changes (which was the problem with the previous fixed position on the swingarm).
 

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