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2 stroke rev limiters

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Sometimes I hear people mention that two strokes don't have rev limiters. Seems like a two stroke CDI would still have a maximum RPM at which it will still deliver spark.

So what's the word? Two stroke CDIs have rev limiters or no?

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Four-stroke ignition retard the spark sharply and suddenly at a predetermined rpm level to reduce the chance of floating a valve into the piston. Two-stroke ignitons don't do this, as there are no valves to float. A two-stroke's rpm is limited primarily by it's breathing ability. The ignition does limit the two-strokes ability to rev beyond a certain point, but only because the spark advance, at a high-enough rpm, is no longer early enough before TDC to allow a complete combustion event to occur before the piston started on the down-stroke and the exhaust port opened. But the port timing, and the wave tuning of the intake and exhaust, will stop the increase of revs long before that point is reached.

A mechanically sound motocross two-stroke could safely rev to near 20,000 rpm before excessive piston acceleration caused the engine to fail. There is no need for a rev limiter, because you aren't going to damage it with revs.

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No 2strokes do not have rev limiters, they simply reach a point where the port timings and exhaust resonance will not allow it to rev farther.

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2 strokes have rev limits you know youve hit it when your topend blows:bonk:
And you garnered this from how many years of repairing two-stroke engines? :thumbsup:

You won't "blow" a mechanically sound two-stroke by over-revving it. If the engine is tight and within specs, you can hold the throttle open until the engine stops revving. They run out of breathing ability long before they run out of safe rpms.

In almost every case, if a two-stroke riders blows the engine, something was wrong with it. Loose crank bearings or rod bearing that allows the piston to contact the head at high rpms, loose piston-to-cylinder clearance that allows piston slap, detonation, things of that nature. Piston slap will kill an engine very quickly, two-stroke or four-stroke.

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Does a 250F have a rev limiter? I've always wondered if mine does...

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All MX four-strokes have rev limiters. They are to keep idiots that don't know when to upshift from slamming the valves into the pistons.

If you ride your 250F on the rev limiter frequently, expect to replace valves and valve springs with regularity.

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i have a KTM 250 sx and it DOES have a rev limiter and i know its not the max rpm it bounces the RPMS 

you dug up a thread so old and didnt even bother reading it did you

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Four-stroke ignition retard the spark sharply and suddenly at a predetermined rpm level to reduce the chance of floating a valve into the piston. Two-stroke ignitons don't do this, as there are no valves to float. A two-stroke's rpm is limited primarily by it's breathing ability. The ignition does limit the two-strokes ability to rev beyond a certain point, but only because the spark advance, at a high-enough rpm, is no longer early enough before TDC to allow a complete combustion event to occur before the piston started on the down-stroke and the exhaust port opened. But the port timing, and the wave tuning of the intake and exhaust, will stop the increase of revs long before that point is reached.

A mechanically sound motocross two-stroke could safely rev to near 20,000 rpm before excessive piston acceleration caused the engine to fail. There is no need for a rev limiter, because you aren't going to damage it with revs.

Chokey, how does the exhaust governor play it's part?  I'm only asking because most of my old school bikes don't have it, while others do.  I hear the word "governor" and it can only mean one thing, to "limit" an amount of bypass.  Wonder if that also plays a part.  Just asking, don't really know. :thinking::devil:

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I'm not sure I understand your question. Do you mean the powervalves?

At high rpms, the powervalve is at it's full open position, allowing for maximum peak horsepower and revs. When open the valves don't restrict the exhaust port, so they aren't a factor in the engine running out of breathing ability.

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Chokey, how does the exhaust governor play it's part?  I'm only asking because most of my old school bikes don't have it, while others do.  I hear the word "governor" and it can only mean one thing, to "limit" an amount of bypass.  Wonder if that also plays a part.  Just asking, don't really know. :thinking::devil:

It "governs" by dictating when the power valve goes from the closed position, to the open position. Usually, dependent on a ball and ramp sort of system. Granted, some manufacturers do it differently, though.

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It "governs" by dictating when the power valve goes from the closed position, to the open position. Usually, dependent on a ball and ramp sort of system. Granted, some manufacturers do it differently, though.

He's asking whether or not the powervalve system has any effect on the engine running out of breathing ability on top.

It doesn't.

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He's asking whether or not the powervalve system has any effect on the engine running out of breathing ability on top.

It doesn't.

I was just answering the part about an exhaust governor... I figured that the whole part about the breathing ability has already been answered.

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i have a KTM 250 sx and it DOES have a rev limiter and i know its not the max rpm it bounces the RPMS

It does NOT have a hard limiter built into the ignition like a four-stroke. What you describe is typically caused by reed flutter. Reeds that are soft enough to provide good flow at lower rpms can't close fast enough to keep up with the engine's demands at high rpms and begin to flutter, causing that "stutter" you describe. That's why you choose reeds that are designed for the type of power curve you want from your engine. Stiffer reeds are better for top-end, while softer reeds are better for low-end.

This is why Boyesen developed his multi-stage reeds. They use a soft reed on top of a stiffer ported reed. The soft secondary reed responds well at lower rpms, while the stiffer primary reed can close fully at high rpms.

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