Vesrah Suzuki’s Mark Junge helped me dyno my RM144 project bike this afternoon. I’m trying to get more mid-range power out of an engine that’s designed to make power at high rpms. Why more mid-range? So it’s better in the woods. The challenge might be insurmountable but it’s been fun trying to change the way the engine performs, none the less.
Test Protocol: carbs, venturi splitters, jets, pipes and wishful thinking
We had time for 6 test runs. I was playing hooky from work and I could only stand so much guilt before I had to bug out. We ran a baseline that gave us a decent looking power curve and 27.5 peak horsepower but as we suspected, all the power was above 10,000rpm. It’s impossible for me to ride this thing in the woods in that range. That’s why we’re here; move that usable power down in the rpm range!
The engine’s displacement has been punched out to 144cc from 125 and the porting had been massaged but the compression ratio and carb are stock. I’ve put an FMF SST pipe on to eek more power across the range and that seems to be working, based on the limited ride time I have on the bike so far.
Mark had to calibrate the rpms the bike was making with the dyno and then get the computer set up to collect data. When it was time to start the bike, Mark grabbed the kickstarter with his hand and gave it a push. It started. I laughed because who would have though you could do that but I was also very embarrassed; why am I trying to make a beast out of this teddy bear?
After the baseline we inserted a Power Now venturi splitter. It’s said to increase low- and mid-range power by increasing air velocity and decreasing turbulence in the bell opening of the carb. With the splitter in place we fired it up and got ready to take data. The bike revved up to 9,000 rpm or so then totally died. I immediately jumped to the conclusion that we’d blown it up. False alarm – it was out of gas.
Here’s a video of what a few test runs look and sound like:
The Power Now slip-in splitter
After fueling up, the Power Now splitter made some unfortunate results. It did nothing but decrease both mid-range and peak power. Bummer – I had high hopes for that thing!
Carburator swap
Mark called the helpful people at Sudco for some carb options. They sent us a 36mm Keihin carb that bolts right onto this bike. The stock carb is a 38mm Mikuni. Our thinking was that the smaller venturi would have a higher intake velocity and give more mid-range power. This is similar to what the Power Now splitter was supposed to do.
One of the best things about single, two-stroke engines (other than you can start them with your hand) is their relatively simply design. The Keihin bolted right into place and used all the same hardware as the Mikuni – same throttle cable and reed block and air box boots. But ultimately the best thing was that the bike started first kick after the carb swap!
The dyno / repair stand
Then the head scratching started. The smaller carb didn’t enhance the midrange at all but it carried the peak horsepower about 500 rpm farther down the line. It didn’t make more HP but it made it for longer. That wasn’t supposed to happen.
This got us going down the jetting road. Tony Pogue is Mark’s full-time mechanic. Tony has been doing race bikes forever and had lots of two-stroke knowledge to share with us. We pulled the plug on my bike and got Tony to “read” it. It was a lovely golden brown but, according to Tony, we could lean it out a little. He took a long look at the thing and pointed out that where the colors were and what the colors were was indicative of both jetting and timing.
Tony (right) reads the plug
Looks perfect to me but Tony says, "lean 'er out!"
Chasing jetting can take forever. I was going to run out of time, if I didn’t run out of premix first. We did change the main and raised the needle and got more peak power out of it but still didn’t increase the mid-range.
Mark called a friend about the jetting issue and why the smaller carb didn’t help the mid-range more. He learned that the stock carb has a throttle position sensor that we had bypassed. That might have been hindering the timing somehow so we plugged the electric solenoid from the stock carb into the wiring harness but ran the bike with the smaller aftermarket carb. Still no increase in midrange.
Singing along at 67mph and 12,000rpm
So what did we learn? First off, there’s something the smaller carb doesn’t account for in the electronics. The solenoid influences the throttle position somehow which in turn affects the ignition timing. Second, there’s lots of performance to be had in the jetting. This isn’t really new to me, just a reminder that the more you know, the more there is to know. Thirdly, Suzuki has the thing pretty dialed with that stock 38mm Mikuni carb. It’s going to be difficult to improve on how the bike is out of the box.
In the end, it might take some trail riding to get the best combination of parts and jetting. Have to wait until St Joe in late February for those experiments. I think there’s more to learn by fiddling with the jetting and I also think an adjustment to the timing could be good. I also think that becuase we were doing wide-open throttle runs the benefits of the splitter and smaller carb aren’t as obvious as they might be at 1/3rd or 1/2 throttle. Riding the bike in the woods is the only way to know for sure.
It was a fun day of trial and error for me, although I didn’t walk away with a better performing engine. I haven’t lost hope and think there’s still a lot to benefit from with the lighter RM125 chassis. Next stop: St Joe Recreation Area in St Louis, MO.
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