Everything2Stroke Forum banner

1 - 2 of 2 Posts

4 Posts
Discussion Starter #1
Here's the steps to making a moderate output engine into a high output engine:
1) first check the ring end gap and replace the rings if the gap is too big
2) with the cylinder off, move the piston up/down to check for free play in the con rod bearings and replace them if there is any
3) grab one end of the crankshaft and see if you can detect any free play by moving the shaft. Replace bearings if there is any.
4) check jetting at high RPM by checking for chocolate brown color on spark plug. Cruise at mid throttle and listen to the exhaust note. If it sounds congested or irregular and it likes it when you quickly crack open the throttle then the needle needs to be replaced with one with a lesser taper angle (and then you'll also need a bigger main jet). If the power drops when you quickly open the throttle then the needle has too small a taper angle and it needs to be replaced with a larger angle needle.
5) check the high RPM ignition timing by looking at the end of the center electrode (not one of those fine tip spark plugs, but a regular plug) and if there is a "clean band" the last .5mm then the timing is right. If the band is longer then retard the timing. If there is no band then advance the timing.
6) measure the squish clearance using plumbers solder and use a squish velocity calculator to know the squish velocity. Gordon Blair said that the max velocity at peak power RPM shouldn't exceed 20m/sec but many people have exceeded that max with high octane gasoline and rounding off all sharp corners, and also making sure the mid throttle jetting isn't lean. Higher velocity is needed with higher RPM.
7) Decide on a new top RPM (because more RPM = more power) by the piston velocity and the carburetors available to you and that you know you can mount. 25m/sec average piston velocity is generally considered the maximum for good piston life. Download the spreadsheet that contains a piston velocity calculator from www.dragonfly75.com/moto/various.html Use the carb size calculator there to make sure the RPM and the carb you have in mind are a good match.
8) Find your engines peak power RPM (PPRPM) and add 1000 to that to know the top RPM you should currently have without making any porting changes. Use a porting calculator to find the current PPRPM: and what porting changes are needed (raising the transfers and exhaust port and widening the exhaust port to 70% of bore if it is single).
9) After you have decided on top RPM and porting changes then redesign your exhaust pipe to have the desired top RPM since it is the pipe that determines that (although the engine may not achieve that if the porting isn't adequate). Here is the formula for determining the pipes top RPM: top RPM = (exh duration x 1606) / length in inches from piston to beginning of baffle cone
10) Once it is all together and working then decide if the pre-powerband power dip is too much. If you are skilled enough then when racing you can always keep the RPM within the powerband but sometimes there is an exception to this and you may want to use Yamaha's YEIS system to correct that power dip.
1 - 2 of 2 Posts