motorbikemike45
Member
Any opinions on the following will be read and considered with care. I've been a motor head for like forever, but my experience with micro engines is limited. Most of my experience comes from auto and motorcycle engines. So jump in with advice/opinions.
I have 2 robin engined MBs, the old Staton friction drive and the new GEBE bike. This gives me the oportunity of trying some things with the old MB to get a little more from a Robin. Higher compression? Hmmmmmm. I can shave the head but should I?
I intend to make a new lower back pressure exhaust. It should be a piece of cake since it is a straight shot out the back. I figured to cut the flange off the stock muffler, slightly hog out and polish the exhaust port a little, match the exhaust flange to the port, silver solder on a length of pipe gently bent 90 degrees towards the ground, and find a suitable muffler to attach.
The carb seems like an obvious way to punch things up. The stock carb is a Walbro diaphragm type with a 9.5mm hole. I could try a 10mm carb of the same type as stock. I would port the intake slightly, only enough to match the larger carb and also polish the intake port. The air cleaner can be replaced with a higher performance unit to allow freer breathing. One thing worries me; Higher top speed is only a small part of the goal, higher torque is the summit. Putting on a bigger carb may increase power at high rpms, but reduce low end punch, hurting overall performance more than it helps. By trying a little ram effect manifold length tuning, I might be able to bring the low end up. A longish intake tube could be added between the carb and the head. Likewise, the exhaust could be made to almost any length that would help with the bottom of the power curve. My problem is, I have no idea how long to make the intake and exhaust to bring the power on at the lower end. Are there formulas for 4 stroke micro engines regarding ram tuning, IE length to power/torque gains and at what rpms, or is each engine model unique due to port/valve/combustion chamber design.
A cam regrind is more than I want to try. I don't want to try to make a dragster from a 33.5cc engine, after all, just to maximize the scoot factor. LOL.
I have 2 robin engined MBs, the old Staton friction drive and the new GEBE bike. This gives me the oportunity of trying some things with the old MB to get a little more from a Robin. Higher compression? Hmmmmmm. I can shave the head but should I?
I intend to make a new lower back pressure exhaust. It should be a piece of cake since it is a straight shot out the back. I figured to cut the flange off the stock muffler, slightly hog out and polish the exhaust port a little, match the exhaust flange to the port, silver solder on a length of pipe gently bent 90 degrees towards the ground, and find a suitable muffler to attach.
The carb seems like an obvious way to punch things up. The stock carb is a Walbro diaphragm type with a 9.5mm hole. I could try a 10mm carb of the same type as stock. I would port the intake slightly, only enough to match the larger carb and also polish the intake port. The air cleaner can be replaced with a higher performance unit to allow freer breathing. One thing worries me; Higher top speed is only a small part of the goal, higher torque is the summit. Putting on a bigger carb may increase power at high rpms, but reduce low end punch, hurting overall performance more than it helps. By trying a little ram effect manifold length tuning, I might be able to bring the low end up. A longish intake tube could be added between the carb and the head. Likewise, the exhaust could be made to almost any length that would help with the bottom of the power curve. My problem is, I have no idea how long to make the intake and exhaust to bring the power on at the lower end. Are there formulas for 4 stroke micro engines regarding ram tuning, IE length to power/torque gains and at what rpms, or is each engine model unique due to port/valve/combustion chamber design.
A cam regrind is more than I want to try. I don't want to try to make a dragster from a 33.5cc engine, after all, just to maximize the scoot factor. LOL.