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problems with my 98rm250

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well where do i start...bought bike about a yaer ago from very well known english bike dealer-mechanic.was built for enduro-hare and hounds etc and as a result has had lots of work done to improve the torque and smooth power delivery out.problem is i did not ride the bike before buying and its actually a bit flat and boring for me as i love the two stroke rush, bloody thing won´t even wheelie offa the power.like the bike apart from that as it has factory suspension and some good aftermarket parts on...does anyone know what the builder may have done to the powervalve etc to chill the thing out?and how easy might it be to reverse the work? i would ask the guy myself but after i had bought the thing he became very unhelpfull and rude, total w`+ker really but thats another story...

by the way great community here chaps and seemingly free of idiots! all the best, MM.:applause:

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check the power valve. being that its such an old bike it could be totally gummed up, it sounds like that might be your problem. also there could be a massive flywheel weight on it to make the power more tractable, never heard of a flywheel weight stealing that much power tho. check the power valve!

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cheers for your reply, the mechanic that developed the bike did a lot of work on the powervalve to smooth out the power for english wet grassy surfaces which is where it was destined to spend most of its time.he told me this when i was first looking at the bike.at the time my intuition radar spiked up as i thought the guy seemed cagy, having not run a d.bike for 20 yrs. however i was dazzled by the bling factor on the bike and stupidly did not demand a test ride...so what can be done to a powervalve to even power out? the bottom, mid and top just all seem to blend into one with the top signing off very early.makes the bike extremely tractable for a two popper but dull.oh yeah there does seem to be a extra large flywheel weight on the thing as it has a thick gasket on to help casing cover flywheel.cheers chaps, MM.

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Depends on what he did to the powervalve and if he only altered that or if he actually did work on the cylinder head to open up the transition as I've heard of that, too.

Do you have a manual as maybe he just changed the settings on the powervalve and did nothing physical to the powervalve which you can easily adjust:

http://www.thumpertalk.com/forum/showthread.php?t=395472

Or he might have powervalve spacers in there that you can simply remove and which you can see here:

http://motocrossplus.net/products.asp

Here is what they do as far as engine 'tuning':

"As I understand it, the function of the subport chamber is to simulate added pipe volume close to the cylinder (think Gnarly pipe) and improve power at low RPM when the subports are open. Seems reasonable that adding even more volume to the existing subport chamber could further boost low RPM power. Since the subports are only open while the powevalve is closed, there shouldn't be a compromise to high RPM power.

Altering the exhaust volume will alter the wave tuning, thereby altering the power curve of the engine. Adding volume to the exhaust of a two-stroke can (not always, it of course depends on the existing state of tune) add low-rpm torque by retarding the arrival time and reducing the amplitude of the returning negative pressure wave."

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thank you for your message man , very informative...if you have time could you explain how the mechanics of how the powervalve works? i totally understand the theory of them just how this particular one does its thing would be really usefull as i have yet to dissasemble the thing and dont have a manual(getting one would be very difficult in this spanish speaking country!) sorted out yet...i suppose ultimatly i could get around all these issues with buying second hand powrvalve and cylinder assembly.wonder how much that might cost what with shipping from the USA as that seems to be where all the best-cheapest mx bits come from. many thanks for your time.MM

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In the TT thread I posted above, read the last post in that thread and it tells you what they did as it appears it's a guess maybe even if you have a manual depending on what else they've done to the engine as you will see.

I thought about it more and it could be they also have a huge flywheel weight in there in the 14oz range which would slow the hit.

They may have also removed any spacers between the reeds and the case as a 'shorter' intake improves low end and bigger spacers improve the hit and there is one sold for yamaha which does this (speeds up the air flow from the carb to the reeds)

Even if they don't have a non-stock powervalve cover he may have added spacers between the head and the pipe which can also do the same thing (increasing volume) to a degree.

I thought I had posted this on here, but this tells you about reeds and powervalves and while it hypes his computer stuff, you can do some by yourself as well as learn how things work to affect the performance:

While most reeds claim they don't change the low end, fact is that it does and here is an good site to learn about how to 'really' mod your 2s for what you want:

http://tsrsoftware.com/redesign.htm

If he raced it, I doubt the powervalve would be gummed up and if you see splooge in the exhaust it might be, but if no splooge it should be clean.

First, be sure to check compression as that could explain why it's dead across the entire range.

What I would then do is check to see what flywheel weight he has on there. If it's EXTRA large and you take it off and get an 8oz or 6oz or the lightest one you can get, whatever is done to the powervalve with spacers or however might still be DESIRABLE to keep as low midrange power with the 'hit' coming back to some degree.

That, however, might not be the 'cheapest' way, but it will be the quickest and save you much frustration if you do things to the powervalve which are correct, but still cannot overcome the flywheel wt.

I have the 96-02 manual and it says the 98 powervalve should be 1/2 turn clockwise. (You have to loosen the two screws, turn, hold it and re-tighten) This is just spring pressure which pushes on a rod which then keeps the exhaust valve from opening or not. (There are other springs associated with the exhaust valve itself so whether this adjustment makes it easier or harder for the exhaust valve itself to open would be a guess and we can see in the other thread where somebody was told to turn it one way and it actually had to go the other way so it doesn't matter because you turn it and then it gets better or worse so who cares). For reference, 96, 99-00 are 1 full turn clockwise and 2001-2002 are 1/4 turn so there's a wide range to monkey with.

AFter that, if you want to check the exhaust valve/chamber for being gummed up or having spacers or being larger than stock you likely will have to remove the exhaust which will then enable you to check for exhaust spacers, too.

As to how all this works, it's all on pressure. By putting in spacers it increases the volume in the given chambers and lowers pressure on the exhaust valve thereby changing how it opens and transfers, etc.

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just removed the zorst and with all the muck around pipe did not see that its spooged up! no spacers in it mind....my mates gonna take the barrel off for me at the end of the week as it has to come off to repair a head stud shear.we will go through any info you supply me with as its the best i have so far by a long way.he is a bit of a genius with the bikes (my background is heavy plant!) so together we can get to the bottom of this and sort it out.flywheel weight is a big un for sure as the casing has been shaved to accomodate the shifter...will the flywheel weight have its size stamped on? or i get the leccy scales out? many thanks mate your a good un...Andy

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The flywheel might have the wt on it, but ones like steahly don't so you'll have to remove it and weigh it.

The normal position for the powervalve is 1/2 clockwise as I mentioned above and you might want to see where it is befofe taking the flywheel wt off.

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I once had a '98 that lost all of it's power.

Turned out that the exhaust silencer had collapsed on the inside. (or maybe filled with spooge?)

Installed a new silencer and it ran fine again.

Just another thought for ya.

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