- Sep 18, 2019
Then explain to me why when I heightened and enlarged the exhaust port of my 66cc HT, which was jetted at 0.61mm with the stock carburetor (extremely lean for HT and jetting below that would make the engine rev out of control and stop from being too lean) and had a free-flowing exhaust my 4-stroking almost disappeared compared to before, when it 4-stroked at all RPM ranges nonstop. I'm not making this s*** up, why would I anyway I have nothing to gain here lol. A rich air/fuel mix will make an engine 4 stroke but there's a reason why 4 stroking only occurs at high RPM and load. Most people who have issues with 4 stroking will say that it only happens beyond 3/4 throttle. If the issue was solely the air/fuel mix being too rich then why would it only happen at high RPM and load??? The air/fuel mix should be similar at all RPM ranges and if anything at high RPM the mix would become leaner because of the increased volume of air passing by the jet. The reason is because the messy porting on HTs is insufficient at high RPM and load to properly extract exhaust gasses and inject a fresh air/fuel charge. Therefore, at low RPM and load the engine will not 4 stroke since exhaust is leaving and fresh charge is entering properly but at high RPM and load the exhaust gases begin to build up inside the engine while an insufficient amount of fresh charge is entering the engine, leading to excessive dilution of an insufficient intake charge with exhaust gasses which prevents the charge from burning. An extra stroke is required to extract the exhaust gasses and suck in enough intake charge. Leaning out the air/fuel mix is one way to fix 4 stroking since the increased amount of air mixed in with the intake charge allows it to burn more easily even when diluted.Wow dude I know you're just trying to help a guy but you need to get your fact's straight other wise you will just confuse him.4 stroking has nothing to do with port volumes,the design or construction.4 stroking is solely due to being rich/over fueled and the spark not being strong enough to burn the complete charge within it's cycle.A 2 stroke engine having only 2 strokes to complete all the functions required cant retain left over charge and add more air in the next stroke,while the charge is introduced to the combustion area via the transfers the piston is on the down stroke of it's cycle and is done so with the pressure created by the descending piston,this pressure helps to expel any exhaust gasses left in the chamber weather fully combusted or not,it's impossible to add more air to it on the next stroke.So 4 stroking is solely due to being over fueled/rich and too weak of spark and the best way to solve this is by having the correct jetting so that the air/fuel mix can be fully combusted cleanly.
If you don't believe me, just look at this post: https://motorbicycling.com/threads/four-stroking-physics-and-fixes.49842/