Why do 2-stroke bikes require constant throttle pinning?

Question

Back when I was 12 my first bike was a KX80 and they told me I had to rev the engine high at all times, and shift when I hit the powerband, they said 2-strokes were meant to be rode like they were being raced, why exactly is this?

I'm 22 now I work on street motorcycles and I'd like to get a dirt bike but I barely fit a 125cc 2-stroke and I want recreational ride, like trails and powerlines, and there's no medium-size 4-strokes in the world besides the new ones from honda.

Answer 1

The 2 strokes you are talking about are tuned race bikes, KX,CR,YZ or a RM don't run properly at lower rpm riding. They are designed to put out big power for their size. Two stroke trail bikes are not like that, they don't have arm stretching power but they have a even power curve.

Answer 2

2 stroke engines will NOT seize when ran at a constant rpm. Neither will they overheat. That's a myth perpetuated by people who know little to nothing about mechanical theory, but use it as a way to explain why their engine broke. Just because something happened to them, they think their experience applies to all.



For decades Lawn Boy used 2 stroke engines on their lawn mowers and there was no adjustable throttle on them. They had two speeds, off and on,that was it. Those mowers would run all day long and never seize or overheat. The same way with 2 stroke street bikes, you could run them at a fixed road speed for as long as you wanted, day after day with no problems whatsoever. There are Suzuki GT 750 "Water Buffalo" bikes around with way over 100,000 miles and they have never seized or overheated once. Water cooling helped, but the air cooled GT 380 and 550 would not overheat or zeize up either.



I won't go into the mechanics of of 2 stroke engines because most people here wouldn't have any idea of what I'm talking about. Suffice to say, 2 stroke power delivery is dependent upon rpm and intake-exhaust resonance. Port a given engine in one manner and you will have a very tractible engine producing usable power over a wide rpm range. Take that very same engine and modify the shape and size of the intake and exhaust ports with a Dremel tool or die grinder and you will have an engine that won't pull the hat off of your head at lower rpms, but will produce tremendous power at a very narrow, high rpm range. It all has to do with the shape, size and location of the ports, along with the configuration of the expansion chamber.



Good examples are the 500 and 750 Kawasaki 2 stroke triples and the Suzuki triples of nearly identical displacement. The Kaws were powerhouses that were hard to keep from wheelieing and were therefore difficult to ride. They were a real pain to ride around town. You either had no power or an uncontrolled wheelie with nothing inbetween. Conversely, the Suzukis were docile, mild mannered, easy to ride anywhere, but you had to force them to wheelie. Nothing unexpected. It was all in the porting.

Answer 3

If you shift when you hit the powerband you are missing the best bit – you have to change to make sure you do not come out of the powerband. 2 strokes rev higher than 4 strokes (generally) but you do not have to keep them pinned to keep them going.



There are some 125s which will “fit” almost anyone, like the WR125 by Husqvarna, top enduro riders prefer the lighter, smaller displacement bikes over heavier less nimble larger displacement 4 strokes.

Answer 4

2 strokes do not require throttle pinning...



you can ride a 2 stroke under the power band with no problems..its just more efficient when the bike is running in its optimal band for producing power... so 6000 to 9000 RPM depending on the bike & how highly its tuned..



2 strokes do have issues with running constantly at one speed.. so if your riding a long way on the open road....in top gear at 7000 RPM.. say 60 MPH... occasionally you should increase or decrease the RPM... maybe change down to 5th for a short time so the revs increase to 7500RPM.



http://www.dansmc.com/seizure.htm

Answer 5

I think Dan's MC that there link pretty much answeres your question. Yea they don't HAVE do be floored all the time, but they sure as hell don't mind it like most engines do! They just don't have much power at the bottom and have flat spots down there but then the RPMs come up the power comes, just the way they was designed.



And yep you's got to vary them RPMs on them there 2 stroke bikes, they will usually start to overheat and soft seize but sometimes hard seize and require a rebuild.

Answer 6

2 stroke engines not only burn gasoline, they burn oil in the premix.

As 2 stroke engines begin to ware, they lose compression.

The lowered compression reduces the ability to burn the oil in the gas as

adequately as a fresh engine.

Reving the engine burns the oil more efficiently and prevents the spark plug

from getting oil fouled.



2 stroke engines are low on power at low RPMs.

When you shift at idle speeds, it takes a small amount of time

for the engine to build up speed and hit the power band.

The power band is not a part, it is a range of RPM where

the engine produces maximum performance.

When you shift at slow speeds, the engine will load up and foul the spark plug.

Answer 7

I grew up in the day when two stroke motorcycles were the norm.

They were fun and simple and cool.

Not so today.

That sucks, but that's just my opinion.

When Walmart starts selling time machines I'm going back!

Answer 8

I blew my kx85 keeping it at constant speed, it seized. Piston was all ****** up