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Poor botom end

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Hi

I have a 250 2stroke, witch is running sluggish at bottom end.

I really need to wind her up when climbing hills and when using bottom end power otherwise she dies on me.

The top end runs fine and have a lot of power. Any Idea on what it could be??

:busted:

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Maybe Jetting/ or it needs an new top end or reed valves?? But it is a 2stroke!! Unless it a fairly new one like the ktm 300's? they really dont make good bottom end?

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Maybe Jetting/ or it needs an new top end or reed valves?? But it is a 2stroke!! Unless it a fairly new one like the ktm 300's? they really dont make good bottom end?

Incorrect answer IMO. You can get a 250 two stroke to make good low end power with proper jetting, use of powervalve timing, or say the use of a low end specific pipe. Pipe and reed spacers also help but even a 15 year old 250 will have plenty of low end power for trail riding. The 125s do not however.

You need to jet the pilot and air screw. Read about that here.

http://www.thumpertalk.com/forum/showthread.php?t=412662

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Did it have more grunt before and now does not? Or has it always been this way?

A properly jetted 250 2T will have more bottom end power than a lot of people think. But a MX bike is hampered by a light rotating mass and a tall first gear, so it's not going to climb like something along the line of a WR450.

Check your compression and reeds to make sure your motor is in good shape, jet it correctly, then if you still want more bottom end, look into a FMF Gnarly pipe and an 11oz or 13oz FWW.

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Yes OlhillBilly, It did had more grunt before lost bottom end power the last coupel of rides. The top end still seems fine have a tot of power and response.

Im going to cleen carb check jetting, plug and reed petals this weekend.

what is the chanses of the rings and piston been warn? what symtoms will that have on the bike?

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If it happened suddenly I would be looking for an air leak, probably LH crank seal. What does your plug look like? Pics.

Loss of compression will happen slowly so power loss usually isn't so noticable. Syptoms are hard starting and lots of splooge/ smoke out of the pipe and a rich looking plug on jetting that was previously good.

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Reduced compression from worn piston and rings can contribute to poor bottom end...A 2 stoke increases compression as the rpm rise....

Actually 2 strokes reduce compression as the RPM's rise via the exhaust valve. Usually around 11-1 to a 8-1 ratios from bottom to top end.

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Has the weather changed? My bike had a bog right when it cooled off off the bottom. It was fine up top. I had my tuner re-jet it, I think he just changed out the main, went up a size and maybe had changed the clip position. I would've done it myself if I would've read up on here 1st!!! Ahhhh

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Actually 2 strokes reduce compression as the RPM's rise via the exhaust valve. Usually around 11-1 to a 8-1 ratios from bottom to top end.

That's different to what my 2 stroke builder told me...2 stroke gain, 4 strokes lose as rpm increases....

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That's different to what my 2 stroke builder told me...2 stroke gain, 4 strokes lose as rpm increases....

Well don't let him touch your bikes if this is what he really said. A 2 stroke has an exhaust valve governed by springs/ celunoids, depending on bike. When RPM's increase to a certain point the exhaust valve opens dumping compression from the cylinder.

A 4 strokes compresion remains constant.

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Hey, great point! Two stroke motors are SO vastly different amongst the OEMs anymore. :busted:

Yes they are all SO simular! some case reed some piston port some cable opperated power valves some useing spring tension ect ect, could be real early and not even have a reed block! so I will sit and wait till I know what kind of bike it is as I am not a clever as the rest of you :bonk::worthy::busted:

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Actually 2 strokes reduce compression as the RPM's rise via the exhaust valve. Usually around 11-1 to a 8-1 ratios from bottom to top end.
Well don't let him touch your bikes if this is what he really said. A 2 stroke has an exhaust valve governed by springs/ celunoids, depending on bike. When RPM's increase to a certain point the exhaust valve opens dumping compression from the cylinder.

A 4 strokes compresion remains constant.

You do realize you're quoting static compression which is a measure of the volume at BDC over volume at TDC. This will remain the same with the powervalve open and closed.

But I see in your second post, you're speaking of dynamic compression.

Then I agree, 2T should drop compression as the PV opens. But because of the nature of the powervalve not sealing the CC, I would doubt the drop is very signifcant (2-3%).

The volumetric efficiency of the intake would yield more significant results, giving the 2T and 4T a rise in dynamic compression as the rpm increase, in the right circumstance.

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Well don't let him touch your bikes if this is what he really said. A 2 stroke has an exhaust valve governed by springs/ celunoids, depending on bike. When RPM's increase to a certain point the exhaust valve opens dumping compression from the cylinder.

A 4 strokes compresion remains constant.

Man..don't let him touch my bikes?????...I won 2 state road race titles on engines he has built...HE KNOWS HIS SHIT.....

Edited by CraigoDRZ400sm

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