98 rm 125 wont get into high rpms

[db94z28]

just like it says my rm wont rev up all the way.

i just bought this bike a couple of weeks ago for 1000$. its very clean and has tons of new parts, thats why i bought it, when i test drove it i knew it wasn't running right but since i know how to work on stuff i thought this would be an easy fix, like a carb adjustment or something.

basically when i nail the throttle, it revs up fine but a little bit after the it hits its power band, it starts cutting out, to me it sounds like what a revlimiter sounds like on a car. rev then die rev then die over and over. it only happens at a certain rpm in every gear. also it wont idle to save its life, but i dont know if its related

so i started checking all the usual stuff
replaced spark plug
getting a nice fat spark
cleaned filter
cleaned carb
played with air screw- nothing
played with the jetting- no fix
checked reeds-look new
checked power valve- working fine
compression at 145 psi
checked for air leaks by spaying carb cleaner everywhere on seals
i even checked the clutch to see if was slipping when the power hit- it looks band new

i am stumped, could it be the coil, stator, or something like that, and how much do they go for?

sorry for the long post guys but i am stumped any help is greatly appreciated, thanks

 

[2-Strokes 4-ever]

You mentioned you messed with jetting.........are you absolutely sure the main jet is not clogged or too lean? That would be my cheapest and easiest thing to look into. Also, a gummed up silencer will not let things breath.

 

[db94z28]

yep i have had the carb off several times and the bowl, jet, and every circuit in there are clean, i took the silencer apart, it looked ok but i cleaned up anyways, still no change. do the computers or other elecrtronics go out gradually on these dirtbikes or does it usually ethier work or not

 

[monte]

my 98 rmx wouldnt rev out and i tried everything!
i checked if the power valve was working and it was when i moved the arm with my fingers
but after ruling out everything i checked if the power valve was working when the bike was running by taking off the right hand side cover on the barrel and sure enough it was not opening up when it should have

rebuilt the bottom mechanism of the power valve and its now sweet!

 

[db94z28]

yep i did that too, i took the cover off and put the pipe back on to see if it was working and of course it is. :bang: when you ride you can tell the power valve is working. you can hear and feel it. but right after it opens the SOB will cut out and miss really bad. im really on the verge of taking this thing out back and blowing it up.

 

[nikki]

If you're confident in the powervalve, it really sounds carb related. If it won't idle, that's more air screw/pilot jet adjustments. If it's cutting out at WOT, that's more main jet adjustments. Might as well check the needle and clip position against the stock jetting too, could be something throwing it off if it was drastically changed.

As for the coil/stator, if you have a manual (or find someone who does) and get an ohm tester, you can check each circuit against the readings in the manual to see if there is an electrical problem. It's pretty easy, manual should tell you which color wires to touch with the red and black tester, to get the readout, and the correct range that you should get.

The clutch wouldn't really make the bike cut out, more like sound WOT and not go anywhere. But you say that is good as well....

 

[sick 96 250]

I say main jet is to lean or your floats are a tad off and not letting enough fuel in fast enough

 

[db94z28]

Alright so now I have went ahead and put an entirely new top end, tried swaping CDIs, new reed valves, and more playing with the carb jetting, still has the same underlying symptom. Even the mechanic that did the top end has no idea. I have worked on dirtbikes for almost 10 years and never have had a problem like this. has anyone ever had similar symptoms on their bike and if so what was the solution.

 

[MeanorMX]

I have the exact same problem! However it's only in first gear it doesn't full rev. And my bike won't idle for **** either. Mine is a '98 too.

 

[db94z28]

Okay i was just riding my bike around to see if I could get it to clear up. I was riding around going threw the gears and all of a sudden the bike started running perfect, reving up just like its supposed to, then bogged out and died. I looked down and I forgot to turn the petcock on. So that means It has to be carb related right? I checked the float hieght and it was right on the money, so I moved the floats down(leaner) anyways, but I still didn't get a change. I even repeated this a couple of times to confirm it, it runs awesome right before it runs out of fuel!? Any suggestions?

 

[76GMC1500]

You say it runs good as it runs out of fuel. Did you try opening the petcock immediately afterwards and running it running it hard? It honestly sounds like the bike just needs to be cleared out/get the pipe hot.

 

[db94z28]

yeah after I open the petcock it goes back to running like crap again.

 

[Moparman1539]

It could be running way to rich. When your tank runs out of gas it starts to create air bubbles in the fuel line. And thus starts to make it run leaner. It could be very rich, so when you get into the high RPM's it would be like driving with your choke on until it gets lean.

Maby try going down a few on your main jet to lean it out a bit.

BUT im not entirley sure. If it is alomost the same as a 2-stroke Nitro RC car then it sound like the symptoms of running to rich.

Thats just my $0.02

 

[76GMC1500]

If you leave the petcock on and let the bike sit, do you lose any fuel out the overflows? My only other thought is that someone swapped carbs or did some terrible jetting. A service manual will give you all of the numbers you need to bring the jetting back to stock for a baseline.

 

[db94z28]

Actually i have left the petcock on before and it still starts on the first kick with no puddles or leaking. As for the carb its the stock Kehin (PWK i think) with the stock 175 main and 55 pilot and that what my Clymer says is stock.

 

[76GMC1500]

How about the needle and slide, there should be numbers on those as well. Also, if anybody has been cleaning the jets with a wire (mainly the pilot jet), the orifices could be enlarged. What position is the clip on the needle?

 

[db94z28]

clip is on the middle position and the number on the needle is 1470, I could'nt find a number on the slide. tommorow i'm gonna try to lean the jetting and see if it makes any difference, then i'll report in.

 

[76GMC1500]

Sounds good to me. Since it's such a pain to remove the needle and your jetting sounds like it might be way off, I would start by moving the clip up 2 slots.

 

[mox69]

When you left the petcock off you caused a "lean condition" this let in more air and less fuel.

I can almost guarantee you have a problem in your carburetor. Personally if it were me I would go out and buy a new jet kit. Who knows if some guy before you bored a jet out, or some other funky things.

Read up on how to do a "plug chop." This is a fairly straightforward way to tell which jets are lean or rich.


If you want the thing just to run and want to stop messing around, you might want to consider buying a whole new carburetor as well. Not much can really wear out or go bad, but again who knows what some ya'hoo before you did.

 

[2-Strokes 4-ever]

The 98 IS a little rich with the stock setting (will be hard to "clean-out" and spooge some, at........altitude 0-1500ft, 60-90 degrees). However, it shouldn't run as bad as you're indicating. We run 50p, 2'nd clip needle, 172m in those conditions. I'm sure you have something else going on.

 

[db94z28]

well i went out and bought a whole range of jets and i finally got the thing to run right, turns out 162 main and a 55 pilot did the trick. I am amazed the that the stock jetting would make it run so bad and that it had to be leaned out so much.

 

[84cr125]

since no one touched on this, what are you running your mixture as?

 

[Moparman1539]

yay i was right =P

Great to see that you have it running good! Now have some fun ;)