Four-stroke physics and fixes.
Please note: This is about 2-strokes four-stroking, it is not about 4-stroke engines. Sorry for the confusion.
Scroll to the bottom if you dont care for learning why four-stroking happens and just looking for solutions
Four-stroking is a term when a 2-stroke motor cannot fire every revolution, so it instead fires every second revolution (like a 4-stroke motor), reducing power drastically and increasing vibrations. But what's really going on? When I first got into these motors I was told it's because it's simply too rich, and to jet down. But that's not the case, though it can make it worse.
What the problem is: Really the issue is dilution of your intake charge with previous exhaust gases that failed to be extracted via the 2-stroke's normal means.
In laymans terms: not enough exhaust gases are being pushed out.
Why?: Your engines capacity to extract exhaust gases becomes insufficient at high revs, the higher the revs, the harder it is to extract them (because time time the motor has to do it is shorter and shorter).
What exactly happens?: Your carb is sized for the motor's current power band out of the box. Modifying it will change the characteristics. The carb is big enough for your 66cc motor to run at around 6000 RPM without an issue. Your carb gives a set amount of gas per stroke determined by your jet size(s) and/or needle (dellorto SHA and Runtong carbs dont have adjustable needles or bowls). The motor will attempt to suck in an amount of air equal to it's displacement. at mid RPM's (we'll say 5000), this is easy, the air has lots of time and room to move through the intake and into the case, remember though the next rev needs to be prepared too, and each one after that, simple enough. the carb lets enough air through. However at a higher RPM (we'll say 8000) the motor is furiously trying to suck air through that intake and carb. If you've ever attempted to suck a lot of air through a tiny straw, you'll note its difficult. Go out and get a mcdonalds straw (a bigger carb AND intake tract, just like a straw, your intake is limited by the SMALLEST area available), and suddenly it can breathe again. Block the straw (install reeds), and it's a bit harder again... solution? bigger straw. You get it.
So why does it four stroke? well we're getting there. So you're speeding along at 8000 RPM, and you hear that tell-tale "Pop pop pop" of the four-stroking. As the motor is trying to suck in air through your tiny intake, it can't. so instead of a nice, full 66cc of intake charge, you're getting 50cc. what does that do? nothing good. less air in the case cant push as hard into the combustion chamber. less pressure means the transfer has to wait until the combustion chamber pressure drops BELOW the pressure in the transfers, the longer the exhaust gas is in there, means more heat. This means your motor has even less time to get the exhaust gas out before it squashes it. So the transfer gases get in there, and the HT's terrible transfer angles sends lets say 10% of your fresh intake charge straight out the exhaust, no effect, so now you're playing with 45cc of intake to try and clear out a 72cc (depends on your head) chamber. what does that mean? It means you've got a whopping 27cc of exhaust gases just hanging out doing nothing. Your transfer pressure dropped, not giving it enough speed to use inertia to clear the cylinder either. So what happens? the piston rises, closing off all the ports leaving nearly 40% of the exhaust gases from the last stroke mixing in with your intake gases. so instead of being 14% gas and 86% air, it is now 9% gas, 54% air, and the rest unburnable garbage. What happens? well it doesn't fire
Next stroke comes along: 40% of the above remains, meaning 20% air, 4% gas, and 16% exhaust gases from two strokes ago, now mixing in with the perfect 60% of the new charge, 14% gas, 86% air, giving you now 12% gas, 72% air, and 16% exhaust gases. What happens this time? Well, you're close enough to the stoichiometric ratio to fire, although poorly. BANG!, assuming an even burn, all that above is now 100% exhaust gases, to start the gong show all over again.
Why does changing jet size stop or start four stroking? Because you're changing that ratio. A ratio that is too far off, simply wont fire. Running rich already? expect to four stroke earlier
NOTE: all of the above numbers are off the top of my head, and are exaggerated to make the picture clearer. It also does NOT take into account intertial forces and weights of particles, nor port timing, I know a 66cc piston port will never draw anywhere close to 66cc of air.
so the important question:
What can I do to stop it?
Expansion chamber: This can easily influence your four-stroking point, its suction waves and return waves give the motor an extra little bit of breathing (meaning instead of drawing 66cc of air, it could draw 72cc), and then at the perfect RPM, the return wave will cram any extra intake gases in the exhaust, back into the cylinder, giving you maximum power, however depending on the design it may not help much for power at the low end.
Bigger carb: When you can't breathe through a small tube, you get a bigger tube. This is simple logic
Reeds: I know, above I mentioned reeds restrict breathing ability and it does. However the pros far outweigh the cons (provided they can breathe enough, use Arrow reeds, they're big and ugly, but work). What are the pros? Well, when you port for reeds, you should add a boost port. This is a third transfer linked directly to the intake port stretching upward. What does this do? When your side transfers come in, they come in flat and toward the intake and middle of the cylinder. When they collide, they have nowhere to go but up and toward the exhaust. Simple but inefficient. The boost port blows the intake charge upward and toward the exhaust more directly, the drawback is you're losing pressure, assuming a perfect world, about 33% pressure drop. So stuff your crank case if you reed port.
They also have the advantage of allowing your expansion pipe suck that extra bit of intake straight into the cylinder, rather than into the case and through the transfers. If you cut a hole in the piston for reeds it also runs fuel and oil by the wrist pin, lubricating it and extending its life, as well as lightening the piston, helping your bearings, reducing vibrations and increasing rev capability
Transfer angles: The drawbacks of epoxy use doing this are debatable, but filling the exhaust-side of the transfers with a substance or material (usually JB weld) and letting set at an angle that will fire the side transfers more directly toward the intake side of the cylinder will help prevent short-circuiting (getting shot straight out the exhaust), increasing efficiency significantly. A base gasket with tall, angled transfer towers to correct this would be the best option for the community. I've been trying but I lack CAD skills, if someone (fred? arrow? dax?) is interested, I can send a drawing showing what I mean.
Porting: The fabled porting, scary at first but actually quite easy, but also easy to screw up and take way too far. Basically speaking, the bigger your exhaust port, the easier it will be to get the exhaust gases out. It will also let your expansion chamber suck more intake with the reverse wave, and if you raise the port height, you'll change your power band potentially giving you more top speed, and also allowing an exhaust pressure drop sooner, allowing the transfers to be more effective (however this will lower your low end power because now the transfers have much more time to sneak out the exhaust). Making the exhaust port too wide also increases the risk of short circuiting (intake being blown straight out the exhaust).
Last edited by mew905; 08-24-2013 at 12:52 PM.