| 11 Changes I Did To Make My 48cc Climb Hills (at 5000 ft altitude) 1. raised ports for exhaust duration of 171 and transfers of 1222. over 120 psi engine compression by lathing metal off of head mating surface 3. large idle jet which boosted low/mid range 4. smoothed the slide bottom with JBWeld [page] 5. Iridium plug (NGK BPR6HIX) 6. 56 tooth rear sprocket 7. custom expansion chamber w/holed baffle 8. Jaguar CDI 9. Intake extension 10. put on 16mm Mikuni but according to my carb page it should have a 14mm carb (and the large engine should have 21mm) 11. I created transfer ramps on the piston 12. put on JRL piston rings which have a small (.2mm) ring end gap, unlike the stock rings which are close to 1mm. Here's the rings for the larger engine. A small end gap is necessary to not lose compression and power. This engine had already been converted to having a reed valve intake. I don't consider that a mod for increased low RPM power because I did a full comparison between the two intake types and they produce the same power. Of course this depends on correct intake duration for the piston port engine which needs to be around 110. The only real advantage of using reed valves is that all aftermarket carbs have their clearance between needle and needle jet set for use with reed valves. Piston port requires a smaller clearance or else the idle mixture isn't right and you need a leaner needle (lesser taper angle). 1. The higher the exhaust port is, the smaller the true engine size, the trapped volume. But if the port is too low then it won't rev out. So my advice is to raise the ports to get around 170 exhaust duration and 122 transfer duration. Here's my page on finding port durations. 2. At high elevations the psi goes down. So I had around 1.2mm lathed off the top of the cylinder which gave me over 120 psi. This is with a standard head I modified to have a squish band but really a squish band is not needed on a low reving engine. 3. With this Mikuni a #15 or #17.5 idle jet is what gets it needing around 1.5 turns out on the air screw. But while testing different sizes I found out that #20 with 2 turns out gave the best off idle power and it could even climb hills better. 4. I found that making the slide bottom have a smooth flow transition gives a better idle mixture and also affects mid range. I did this using JBWeld. Click here to read more about this mod. 5. Iridium plugs spark easier when there is low spark energy which is the case with these cheap chinese engines. They don't have good spark power till high revs. 6. I increased the rear sprocket size to 56 teeth for climbing streets but I prefer 36 tooth for good speed in a flat city without hills. 7. Expansion chambers increase intake flow because of the return suction wave from the diffuser cone. At top RPM the baffle cone creates a reverse pressure wave which increases the engines dynamic compression. But below the 2000 RPM pipe powerband the baffle wave interferes with the intake. So by putting holes in the baffle and enclosing it with another section I reduced the return baffle wave which aids low and mid range power. Read more 8. My Jaguar CDI mostly helps with high RPM because the stock CDI doesn't retard the ignition timing enough at high RPM. But due to low manufacturing standards for the stock CDI sometimes it also gives the wrong timing at low RPM so my CDI gives a near perfect timing curve that assures ignition is correct at all RPM. Read more 9. Adding on a long intake tube allows the return wave from the intake suction pulse to arrive back at the engine at the right time for low and mid range revs. This increases power for climbing hills. Although this is the dorkiest looking mod I do consider it one of the best. 10. For a 48cc a 14mm or 15mm carb is ideal. Before I knew how to size carbs I had bought this 18mm Mikuni and so now by decreasing its flow area with JBWeld I have essentially reduced it to 16mm which increases flow velocity which aids in gasoline atomization. 11. Transfer ramps smooth out the intake flow into the cylinder which increases it. A rounded corner doesn't create the obstruction of turbulence that a square corner does. The resulting transfer port duration was 122 which is good for low RPM power for these engines. Read more |