Jul 29, 2000
South America
Here are some old concepts in the 2 stroke world that have been “updated”:
1) The carb needle and the main jet effects don’t overlap, they totally control the jetting for their designated range of throttle opening.
I think that oversimplified belief was designed to help out the simple minded. It’s like they thought all 2 stroke riders crashed their brains out on a regular basis. Dell'Orto says that that at full throttle the needle flow area needs to be more than the main jet flow area. (If you use the JD Jetting calculator you can also see that both the needle and the main jet affect WOT jetting.) So obviously the needle still does have influence on the gas flow at full throttle. As long as the needle is in the needle jet area then it presents a flow restriction. So if you change to a needle with a different taper angle then you’ll also need to change the main jet. If the angle is increased then the main needs to be smaller.
2) You can use a simple formula with an average wave speed to determine the correct length of expansion chamber the the top RPM you desire.
It’s much more complex than that because the gas temps in the pipe are reduced the farther they are along the pipe. That is because the walls of the pipe are radiating out the inner heat. And the wave speed changes with the heat. And the surface area of the pipe compared to the inner area becomes less as the pipe is bigger for bigger engines. And the temp changes with top RPM and jetting/timing. So ideally the exhaust gas temperature needs to be measured first.
3) There are standard gas/oil ratios.
The ideas of ratios started back before synthetic oils and they only had regular petroleum oil or castor oil. And all the engines were air cooled. So 20:1 was a common ratio for petroleum oils. Now that we have blends of synthetic and petroleum oils the most common ratio is 32:1. But in reality now the ratios need to match the type of engine oil used, the max RPM, and whether the cylinder is air or water cooled. Now we have a full range of oil types available that result in different oil viscosities. Viscosity is the most important piece of data about any oil since that is the first line of defense of an oil against friction. So a water cooled engine that revs to 11,000 will be good with these examples of oils and the ratios to use them at for this sample engine: Bel-Ray Mineral Oil 30:1, Motul 710 2T 36:1, Maxima Super M 43:1, Maxima Castor 927 48:1, Motul 800 2T Off Road 54:1, Motul 800 2T Road Racing 64:1.
4) There are standard port durations that change with the desired top RPM.
Well those charts were made back when the exhaust ports were “single”, not with auxiliary ports or bridged. The newer exhaust port designed allow the exhaust pressure to dissipate more quickly so less blowdown time is needed between exhaust port opening and transfer ports opening. And many race cylinders now have even more transfer port area which allows less port durations for the same peak power RPM. So really now a computer program should be relied on to figure out what port durations are needed for any desired peak power RPM.
5) Crank balancing is as simple as using a balance factor.
The truth is that “balance factor” has only one purpose and that is not diagnostic. There is nothing in that method that tells you exactly what each engine needs. It is just a means to communicate the needed balance for the same engine. For instance a ’90 YZ250 owner in USA could email an owner of the same bike in the UK that a balance factor of .35 worked good for him and then that UK owner could use that method to duplicate his results. But if there is no one to tell you what balance factor works good on your engine then you have to start from scratch. You could use the trial-and-error method but that is horrible because the cases have to be spit each time. Only a modern computer program can tell you exactly what each engine needs, one that takes into account all the centrifugal and inertial forces every 15 degrees of crank rotation and can graph the total result.
6) The bigger the carburetor the more top RPM power.
The truth is that bigger carbs do produce larger gas droplets which are better for high RPM power, but you can go too far on carb sizing. And carb sizing shouldn’t be your principle focus. Also very important are squish velocity, engine compression, and ignition timing. If you go too big on the carb then it will be hell to get the idle and low throttle opening jetting correct. Bigger is not always better.
Please check out my website which has in-depth info on these topics and my 2 stroke computer calculators:

Welcome to DRN

No trolls, no cliques, no spam & newb friendly. Do it.

Top Bottom