Prajhot Pednekar and I have been fiddling around with the power plant of my Hero Impulse to squeeze out some extra power. I’m trying to avoid replacing the engine with that of a Karizma. I don’t need an insane amount of power to pop power wheelies or go rallying. All I need is some more torque to climb steep slopes of trails without riding the clutch. That’s the only thing I can’t do with the stock Impulse and that’s the only thing I want to fix.
After a great deal of trial and error, we now have a set up which does yield some extra torque. Whether it will be enough I don’t know. One of these days I’m planning to ride up the same Pargadh trail where the Impulse failed miserably. The lethargic Impulse engine does feel a little peppy and purposeful now.
Reducing the size of the front sprocket increases the torque. But it also has the side effect of reducing top speed. But we hoped that our other mods would compensate for that. I asked around and Aditya Kumar recommended we switch the 17T stock front sprocket with the 15T front sprocket of the Honda Unicorn. It was a direct fit.
To get more power you need to burn more fuel. Fuel is sprayed into the cylinder using more than one jet in the carburetor. But the jet that controls the flow of fuel at the RPM’s that matter to me is the main jet. So we replaced the stock 115 jet with a bigger 132. We also experimented with different carburetors and managed to see a noticeable increase in power. But the mixing of air and fuel wasn’t smooth and the engine misfired at different places in the RPM range. So we decided to stick to the stock carburetor.
More fuel needs more air to burn properly. Otherwise you end up with a rich mixture and combustion isn’t ideal. This is similar to how you blow air into a barbecue grill to get the fire to burn more. So we bypassed the stock air filter with a K&N conical free flow air filter.
Although this yielded the desired results, it also reduced the level of water I can take my Impulse swimming. The solution was to place the K&N air filter inside the aix box that was connected to the snorkel under the seat.
If the engine has more fuel to burn and you are giving it more air to burn it with, you also need to make it easier for the exhaust gases to escape the cylinder faster. To do that you remove or reduce the obstructions in the exhaust pipe and let air flow out faster. If the obstructions are in place then it increases back pressure and some of the exhaust gases go back into the cylinder diluting the air-fuel mixture for the next explosion. This reduces the power the engine generates.
The main reason for the obstructions is to reduce the sound of the explosion. The principle is quite simple. A single loud sound wave is broken down into multiple smaller sound waves by making it go through a series of baffles inside the silencer. That’s how a very loud clap sound is converted into a much softer and bearable “doob” sound. Apart from baffles, silencers also employ packing material like glass wool to decrease the speed of sound waves and absorb some of their energy. This is similar to how sound proof rooms have padded walls. The padding on the walls absorb some of the sound waves and prevent them from bouncing off and hitting another wall. We opened up the Impulse’s exhaust pipe and found a cylinder of wool lining the inner wall. We removed it, but didn’t mess with the baffles. We didn’t think the changes we made to the jet and the air filter warranted such a drastic change in the exhaust pipe. If we were wrong we could also revisit this later.
The result of doing all this was a motorcycle that doesn’t feel sluggish any more. The power plant is generating more power and the resulting extra torque is being further increased by the smaller front sprocket. If this solves my problem, I plan on stopping here. If not then I may do other stuff like increase the size of the rear sprocket. Now it’s time to test.