Q: I installed a performance exhaust (MLM people’s pipe, circuit pipe, or other) and my Honda Urban Express really doesn’t go much faster than with the stock pipe. What gives?
As you can see from the video above, I experienced the same issue. Top speed at the time was around 35 mph and I believe that was with the BX31 belt. I was also running all the stock weights in the variator. I had a 90 main jet in the stock carburetor at the time. Well, I made all the mistakes. Here’s why the new MLM pipe had little effect on acceleration and top speed.
1. Improper jetting. I was torn with where to start as there are several factors inhibiting the pipe’s performance. But I think we always have to go to jetting first. A 90 main jet was way too rich. The stock 85 main jet is way too rich for this setup as well. I later went to an 82 main jet but I bet an 80 main jet would have been fine. Too rich of a main jet causes your pipe not too hit. Use a temperature gauge if you are concerned about soft seizing your piston. On a stock setup with a performance pipe, you probably want to keep it below 385F.
2. Too much weight in the variator.. On a stock setup with a performance pipe, you want to have 33 to 36 grams in the variator. Stock variator weight is 78 grams. Just running three variator weights (39 grams) is still too much. And, yes, there is a huge difference between a little too much weight and the perfect weight. Less weight in the variator will cause the bike to rev up quicker and higher. The MLM pipe needs high rpms to hit – probably somewhere close to 8000-8500 rpms. You generally experience a “shift” at about 20 mph on the Urban Express. I believe this is caused by the belt going to a different part of the pulley or pulleys. When it shifts, your rpms drop way low again until you build them up in “2nd” gear. Less weight in the variator will allow the rpms to spin up quickly again and to a higher level and hopefully to the point where the pipe will hit.
Too little weight in the variator will negatively affect top speed. You want to find the perfect balance so that you can maximize acceleration and top speed.
3. The wrong belt.. The BX31 belt is a popular belt because, for about $10, you should get a 5-8 mph top speed increase. I got about 6-7 mph on an all stock setup. You get this speed increase because the BX31 is probably the longest and fattest belt you can run in the Urban Express. Yeah, that was a nice $10 deal! Of course, low end suffers immensely. You can counteract that by using a washer between the end of the boss and the outer front pulley cheek.
I’ve added a show and tell picture. By adding a washer between the boss and the pulley cheek, you are basically increasing the length of the boss so that the belt can sink down lower in the front pulley resulting in better acceleration. Of course, the belt won’t travel as high on the pulley now so you will lose a little top speed. Hey, but that’s the trade off, right? You just need a washer that will fit over the crankshaft and is about the same diameter as the boss.
Another downside to the BX31 is that it won’t sink as far down into the rear pulley as thinner belts. Picture a ten speed bike or a 21 or 24 speed bike, for that matter. Top speed is reached when the chain is on the highest sprocket of the front sprocket and the lowest one on the rear. Same concept applies to the Urban Express. You want to be as high as possible on the front pulley and as low as possible in the rear pulley. The BX31 belt is so wide that it can’t sink as far down in the rear pulley as would be ideal.
But the biggest downer about the BX31 is that it is too long and too fat resulting in lower than ideal rpms. You’ll hear it slap the cover at idle because it is too loose. You have to accelerate a bit to get it to tighten up on the pulleys. Once you get close to top speed, the belt is positioned in the pulleys so that it is in a relatively high gear. Picture your ten speed bike, if you are in the highest gear, you are actually pedaling slower (less rpms) than if you were in lower gears but the bike is going at or near top speed. Same thing here, your Urban is going faster on less rpms. And less rpms work against the pipe hitting.
So once you correct the jetting and the variator weights, you will be better off running a shorter, thinner belt. Why? That seems all wrong, right? We want top speed so we need bigger, longer belts. We want the double bonus. The double bonus? Yeah, the triple word score. Whatever you want to call it. In other words, we may have to take a step back to go three steps forward. I suggest taking a step back to an AX31 belt or even a 9325 belt so that you can increase the rpms, get the pipe to hit and go three steps forward.
I experienced this same conundrum on my Hybrid NCU50 express (has an urban engine mounted to an nc50 frame). I had the Malossi racing gears installed, but I removed them and installed them on another bike and went down to the stock gears on the ncu50. Surprisingly, I lost quite a bit of top speed (I originally thought the stock engine couldn’t drive the taller Malossi gears). I went from high 30s to about 33mph or so. I was running the BX31 with a washer (as described above). I was about to remove the washer and just run the BX31 but I decided to try a 9325 belt that I had laying around. I also put in a boss that I had sanded 1.3mm off (yeah, I had to go and complicate things by sanding the boss down – if you don’t want to do this, just run an ax31 belt with stock boss). Amazingly, I gained 4-5 mph with this setup because I was finally starting to get into the pipe. The 9325 belt is 1.12″ shorter than the BX31 and almost 2mm thinner. By sanding off 1.3mm from the boss, it was only 0.7mm thinner on the front pulley. On the rear pulley, the 9325 sinks way lower because of this 2mm difference.
So back to the ten speed analogy, the shorter 9325 belt doesn’t travel as far up on the front pulley but it travels quite a bit lower on the rear pulley. So we are in a slightly lower gear than with the BX31 belt. In that lower gear, we are pedaling faster (higher rpms) but we are below the top speed of the BX31. However, the higher rpms at this speed cause the pipe to hit and we experience acceleration and top speed greater than what could be achieved with the BX31. So we took a step back (shorter, thinner belt) to go three steps forward as we exceeded the top speed of the BX31 because we brought the performance pipe into play (where before it had less impact on top speed).
By addressing jetting, variator weight, and belt size, you can get your new performance pipe to hit and go three steps forward. Other factors having some impact on the pipe hitting include compression (or lack thereof), spark plug selection, and exhaust port size. I’ll get into these later. Stay tuned for a video on my NCU50 and its new 9325 belt.