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The TTR250 FAQ web pages -> TTR250 -> Modifications -> RPMs go up REALLY REALLY fast after 50-55 mph
Post Info TOPIC: RPMs go up REALLY REALLY fast after 50-55 mph
JCW


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RPMs go up REALLY REALLY fast after 50-55 mph
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I have owned a lot of bikes through my 45 years of riding, but I have never seen one go up in RPMs so fast after 50 mph.   I don't have a tach on it, but the bike feels like it's about 2500 rpms at 50 mph, and 5000 rpms at 60 mph.   Why would there be that much difference?  I have ordered a 48 rear sprocket and hopefully I will have more top end room and still have enough power down low.  Is this common to TTRs, and if so any suggestions?  Thanks



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TTRfan


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The revs should go up incrementally so I am wondering if you have clutch slip?

A quick way to check is to ride at 30 to 40 mph in top gear, pull in the clutch, give the engine full revs and dump the clutch. If the revs don't instantly come down to match road speed then the clutch is slipping.

Brian



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JCW


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Thanks. I will try that. According to the odometer that looks totally factory, it has 2065 miles, and it really feels super tight. But, that could be it. I will check.


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JOHNYBOY


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is your cable adjusted correctly it should have slight play as you pull the lever in just before you feel the pressure of parting the clutch if not it will be having pressure on the clutch as if you were pulling the lever slightly & it will slip on high revs or you can put your front wheel up against a wall pull clutch in put in gear then let the clutch out with a few revs be carefull it should try to spin the rear wheel if it doesnt & the engine races up the clutch is slipping do not try this if you are not confident with this & as i say be very carefull

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TTRfan


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With such low miles it is unlikely to be a worn clutch so, as John suggests, check there is free play on the cable.....



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JOHNYBOY


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just a thought on this one eng oil is not too far up the eye glass possible overfilled just a thought as i am having a few tinnies fosters

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ridelimousin


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is there a super charger fitted? Might be spinning up as the charger kicks in?


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ttr steve


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JOHNYBOY wrote:

just a thought on this one eng oil is not too far up the eye glass possible overfilled just a thought as i am having a few tinnies fosters

 biggrinbiggrinbiggrin



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JCW


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Engine oil level is good. The clutch feels good, but I will check your recommendations tomorrow. I really appreciate the comments.

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JCW


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Fixed!!! The clutch and oil were fine. Opening the air box and cutting 8 extra 3/8” holes in the muffler baffle fixed it. It just needed to move more air. Thanks everyone for your comments.

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JOHNYBOY


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dont understand that im beat

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TTRfan


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JCW wrote:

Fixed!!! The clutch and oil were fine. Opening the air box and cutting 8 extra 3/8” holes in the muffler baffle fixed it. It just needed to move more air. Thanks everyone for your comments.

Can you explain a little further please? I am struggling to understand how giving the TTR more air slowed down its acceleration confuse

Cheers

Brian 



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JOHNYBOY


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come on jcw give us a clue cos im baffled you must know more than me but im still on a learning curve

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JCW


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I removed the rubber restrictor on the air box and cut the hole about 30-40% bigger. Then, I cut 8 ea 3/8” holes around the larger main exit hole in the baffle (Be sure to cut through both restrictor plates. The 2nd one is 1” behind the first one.) That’s all I did, and it feels about 10% stronger and it will continue to accelerate smoothly past 50 mph now. .??? I guess it’s something like a water hose that was too small to move a larger amount of water (air in this case). The little hose could handle a small amount of water, but it started to fail when the water was turned up too high. That’s my only possible comparison.

Before it accelerated nicely until 50 mph, and at that point the rpms would go up dramatically for every 5 mph past 50. Now it still accelerates nicely up to 50 mph. But, it’s about 10% stronger everywhere and it continues to gain speed smoothly after 50 mph. It runs low rpms at 55 mph now (50 before) and rpms increase fast (but much less fast than before) after 55. I don’t run WOT, but it would likely run 75 mph WOT.

FYI, Be sure to cut straight through the restrictor plates in the baffle. There is a spark arrestor screen on the outside edge of the baffle. Don’t let your drill bit move away from the center or you will cut the screen.

Sorry I can’t give a more scientific explanation, but that’s what I did and in my case it worked great. Thanks and good luck!

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JCW


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It did not slow down the acceleration (see my latest post below).   But, it can.  I bought a KTM 250 4 stroke years ago and it had no power.  I found out they used the same head pipe on the 250 and the 400.  I bought an aftermarket 250 head pipe (smaller) and the power was finally there.  You must have “some” restriction in the exhaust.  But that was not the issue here.  Thanks, John



-- Edited by JCW on Sunday 5th of November 2017 01:30:05 PM

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Moo


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You cut holes in the exhaust, not the airbox. I think that's what confused people

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JCW


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I removed the air box rubber boot and cut the plastic air box about 30% bigger in addition to the baffle holes. More air in and more air out.

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mossproof


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The crankshaft is connected mechanically to the rear wheel. For any given rpm there will be a fixed road speed, unless the clutch is slipping. I think that the solution of making the engine breathe better, coupled with the initial description of a symptom - revs rise really quickly above 50mph - is what is causing the confusion here

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