Linearity was nice on the school bus you used to go to school in, but linearity on a motorcycle this side of the economuters can mean only one thing, boredom. I’m not suggesting liter class SBKs or ported two strokers for your daily dose of gonad juices here, I’m talking about how your daily ride 150cc motorcycle can put a smile on your face every time you whack open the throttle. Kicking off this tech post, I’m going to elaborate and review a simple mod on the CV(constant velocity) carburetor that can make your everyday commute just that tad bit exciting.
Let’s begin with understanding how CV carbs work, in a layman’s perspective, and why your engine doesn’t die out as you whack open the throttle on your 150cc Pulsar/Karizma or Unicorn. First of all, you need to remember that the advent of the CV carbs was simply to make the riding experience simpler and kinder by giving the carburetor more control than your right fist. It is therefore of no surprise that the dirt bike crowd simply shun the sluggish CV carbs for ultra responsive flat slides. The CV carb basically works by it’s slide operating on differences in vacuum created by the engine’s piston, instead of being connected directly to the throttle cable.
This is in complete contrast to round slide and flat slide carburetors, whose slides are connected directly to the throttle cable thus responding instantly to the throttle inputs your right fist gives it. The CV, on the other hand has a diaphragm atop it’s throttle slide that will regulate the speed/rate at which the throttle opens or closes. This, it does by gradually regulating the vacuum inside the carburetor and thus the movement of the slide. Hence, you can whack open the throttle on a CV carb throttle and still manage not to cause the engine to starve, which otherwise would have been the case if you did the same with a round/flatslide carb.
CV Carb Cutaway
Coming to the throttle slide on a CV carb, there usually are two or three ports/holes/orifices on the bottom of the throttle slide. On of these holes, the one which is in the center, accommodates the needle of the carburetor. The other hole(s) are basically vacuum ports, which are extremely important as they are the ports which are responsible for vacuum changes, and hence directly impact the working of the CV carb. The throttle slide modification in question is directly related to these ports.
What we essentially do in this modification, is we drill out the vacuum ports to a slightly larger diameter. For example, the vacuum ports on the Mikuni BS29 CV carb measures 1/16th of a inch in diameter. What we do is, we drill out the vacuum ports to 1/8th of an inch, effectively doubling it’s diameter. This doubling of diameter increases the flow of air through these ports by 4 times. Thus, when you whack open the throttle of a motorcycle that has a ported throttle slide in it, this time, the rate of change in vacuum will be a lot quicker than that in the stock slide. As this rate of change in vacuum is quicker, your throttle response goes up massively and your motorcycle feels much more livelier. Of course, you won’t be going any faster but you’re motorcycle will certainly feel a lot more agile and more responsive to your throttle inputs.
Chinmay Dangre or CorePower Motorcycles sent me a ported throttle slide which I fitted on my Stage 1 Royal Enfield Bullet Machismo LB500 last Sunday. Riding down the Airport road in suburban Bangalore, I immediately felt a marked difference in the way my Bullet responded to throttle inputs. As I run on a richer pilot mixture to prevent popping from my goldstar muffler, my initial throttle response is usually sluggish/lumpy on the stock CV carb. But with the ported throttle slide in my carb, the initial throttle response became a lot more smoother and more controllable. I could literally control the throttle down to the last mm of it’s play, which was hitherto an impossibility on the more sluggish stock CV carb.
Since my motorcycle runs on a free flow intake and exhaust, I have ample low end torque. This ported throttle slide has enabled me to control and use this torque more efficiently. With the ported throttle slide, I can now ride the torque curve for say till the first 1/4th of the throttle opening, like say on a STD350 and thump along nicely, and that is quite a statement to make. Once I whacked open the throttle beyond the initial 1/4th throttle, my bullet simply lurched forward, quite unlike the measured, linear way in which it used to gather speed before. All in all, my Bullet now feels a lot more lively and I’m enjoying the improved throttle response to the hilt.
For people interested in porting their throttle slides to make their bikes more responsive, you can do so by
1> Drilling out your existing throttle slide. This requires a very steady drilling hand as it is extremely easy to burr the slide and thus destroy the vacuum ports beyond repair. Also, care must be taken to make sure that you handle the diaphragm atop the slide very gently as the extremely delicate diaphragm is prone to tearing if mishandled.
2> Contact Chinmay Dangre at [email protected] and specify your motorcycle model. Chinmay will then port a new throttle slide and ship it over to you. Then, you can ask your mechanic to do the install of the ported throttle slide or if you’re a DIYer, you can do it yourself if you’re confident of your way around carburetors. For people in Pune, the fitment can be done at Chinmay Dangre’s workshop. For pricing details, please contact Chinmay directly.
This mod will work on all motorcycles that run on CV Carbs, and these include, but are not limited to, Yamaha Gladiator/Fazer/FZ16, all Bajaj Pulsars including the Pulsar 135, Bajaj Discover 125 and 135, Honda Unicorn, Suzuki GS150R, Hero Honda CBZ Extreme, Karizma, all Royal Enfield Bullet UCEs exceot the Classic 500, Royal Enfield Thunderbird LB350 and Royal Enfield Machismo LB500.
I ride, I blog and I trip. This apart, motorcycles, vintage mechanical watches, people watching, camping and traveling are my other interests. Incidentally, I hold a engineering degree in electronics and work experience in computer networks and professional blogging. I live in Bangalore currently and you could get in touch with me at [email protected]