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Power Upgrades for 1998 RMX250

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I would like to know of any upgrades I could install to give my Rmx250 some more power....It's very powerful now but I would just like more. The bike has a fmf pipe already and has had the plastics converted to a more of an rm look...IMG_3928.JPG

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I know that motor is based off the 89-95 RM engine. It's likely the cylinder and head from one of those bikes will bolt on.
Back in the day, my buddy put a Boysen reed valve in his 89 RM250 and it really woke it up.

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^^^ The RMX engine is indeed based on the 89-95 RM engine. However the RM cylinders changed from 91 onwards to using an O ring headgasket system with other differences to the cylinder head and PVs, exhaust port / exhaust header size difference, so interchangeability can be done but has implications on sourcing other peripheral parts.

Basic mod for the RMX is the head gasket can be changed for a slightly thinner RM one. It's important that the engine is in good shape and optimised / tuned correctly to start with.

The 2 stage RMX PV system is prone to wear so needs to be checked and serviced to ensure it is operating correctly and is not about to fail and cause extensive and expensive damage.

The standard Keihin 38mm PJ carb can be replaced with certain types of either 36 or 38mm PWK which consensus suggests are better to jet and set up for improved performance. NB> There are different styles of PWK with long and short bodies and electronic connections so care needs to be taken when selecting a suitable replacement for the PJ.

Another thing to consider is that the RMX uses a wide ratio gear cluster compared to the RM. Depending on intended use the closer ratio RM gearing may be desirable.

I'm sure zig or jeremy will chime in at some point to add some input as they both have experience.

Edited by rmxdan

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rmxdan is right, the places to look at for RMX bolt on power is the head gasket, reeds and carb.

I've tried the Boyesen RAD valve but the Pro Series reeds seemed to give it a more "complete" power band (if that makes sense).  Top end is better with the RAD valve but the low end suffers to the point where the midrange is only used to help you get to the top.  The Pro Series helps the entire power band.

Head gasket.  A common trick is to use a RM head gasket ('89 thru '91), it's thinner which increases the compression and it will put a smile on your face.  However, if you look up the head gasket for your RMX you'll notice that there's an "optional" head gasket.  It's as thin as the RM head gasket but it's made specifically for your bike, and it appears to be made of better materials ~ yup, it also costs more.  To me it was worth the extra cost. 

Optional gasket on the left:

RMX_Gasket_2.jpg

Power valve.  Switching over to an RM250 PV (from '92 thru '95) will make it "feel" more powerful on the top end but it will cost you low end power.  This can be off set knowing that the RM PV is more reliable.  If I had to rebuild an RMX PV I'd do it with RM parts (mostly because the RM PV is cheaper), otherwise I'd leave it alone.

Carb.  If you have the carb skills get a PWK, most likely a 36mm will do the job, a 38mm if you really have the skills to pin it and hold on well into the top end. 

295cc Kit.  Eric Gorr use to offer a big bore kit for RMX's, cost between 600 and 700 dollars.  If you really want more power this will do the trick.  http://www.eric-gorr.com/index.php/services/big-bore-250s/services/big-bore-250s

Edited by zig06
added gasket picture.
  • Like 2

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