03 RM 125 starts, but won't restart.

I am the mom of a 14 year old son that rides an 03 RM 125. The engine was completely rebuilt recently - about 12-15 hours on the bike since the rebuild (done by a professional). We bought the bike just before the rebuild... and it appears to have been very well taken care of by the previous owner. Currently, we are at an elevation of 4300 ft, (we usually ride at 500-1500ft) and the bike starts first kick and runs beautifully. Until you turn it off. Then it takes several hours to be able to start it up again. It won't even bump start. We've replaced the spark plug and checked that it is getting a spark. The air filter is clean. The gas is new - premium, 40:1. The carb outlet tubes are clear, no blockage. Could it have anything to do with the elevation change? This is our first year with the 125, and never had any trouble with the RM 85L we had. What things can I check myself...with my limited knowledge?

I am a mom...not a mechanic, but I am learning and don't mind getting my hands dirty! There is no male mechanic in the picture (aka dad), so it's up to me to figure this out. I'd love any advice you had to offer, just remember I am a true newbie :)

PS. My son's friend is on the trip with us and he has a Kx 85 that won't start at all. Being on a trip with two teenage boys who can't ride, while surrounded by dirt trails, really sucks! Lol!

Does it do the same thing at 500ft? Elevation does make a difference. As you go up in elevation it will run richer and need different carb jets.

Edited by night_stalker

We've only had the bike a couple of months. All I can say is that it was running perfectly for 12-15 hours (at 500ft). Then we traveled here, to the higher elevation, and it wont restart. We did take out the carb today and found a large piece of rubber debris inside one of the intake holes (lol, sorry, don't know the correct term). It was such a large piece, that we thought surely that was the cause of the problems. We cleaned out the rest of the carb and put it all back together... and, nothing.

We've traveled here before with my son's RM 85, and never changed the carb jets and it always ran fine. Would this 125 be more sensitive? In regards to having to change out the jets? Do you know what size jets should be used for each elevation?

does it start easier with the choke on


jetting should be changed at every 2000 ft altitude change and every 15 degree temperature change


this includes the needle clip, pilot jet needle screw, main jet,


make sure there is no moisture on the filter


and make sure the needle valve isn't sticking inside the float chamber

Thanks for the Reply. Usually, the bike starts first thing in the morning, because it hasn't been started since the previous day. Yesterday, it started and just as my son took it out of the barn, it died ...then if it dies, it won't start again with or without the choke, and wont start if bumped either. Again, we've changed the spark

I appreciate the thoughts on changing the jets. I hadn't understood those numbers. But, I'm still not sure how what sizes to go with? How do you make the decision on what sizes are necessary for each elevation and temp change? I didn't see that in the manual, but maybe its there. What would you suggest?

Is the needle valve the the one with the rubber tip that is sort of connected to the float? When we had it out yesterday, it appeared to be in great shape and we saw no indication of it sticking.

http://www.iwt.com.au/mikunicarb.htm. this is a good read explains carbs and jetting. Also has a conversion chart to help determine proper sizes for different conditions.

Thank you night_stalker for the link. At first the link didn't work...said the page wasn't found. But, if you take out the "http://" it works. - www.iwt.com.au/mikunicarb.htm -

Great resource page! Thanks for the recommendation!

Turns out we have another issue. Now the bike isn't starting at all. Could it be that the gunk we found stuck in the carb caused an electrical issue? Or, is it just a coincidence that we are experiencing these issues together?

Anyway... no spark. What to check now? Any other ideas? Ugh...

I had this issue before with an older bike


is the fuel tank clean check inside the fuel tank there shouldn't be gunk going into the carb bowl at all, if you're getting gunk into the carb bowl you're just clogging the jets each time you fill up your carb bowl.


Get a cheap fuel filter mtgD5w_EvsYN8HEjAiJL3Lg.jpg



and check the inside of the fuel tank for debris dirt gunk watever might be there


also when you get it started again shut the fuel petcock off after you start it then let it idle til it dies out give it a few more kicks with the fuel petcock off again then turn the fuel on wait for it to fill up the bowl and see if it starts again

So I also have an 03 rm 125. I just bought it. The guy I bought it from had said it has less then 8 hours on a complete rebuild. It was bored out to a 133cc. V force reeds, and brand new pro action suspension. So I bought the bike and rode it the next weekend at a track. I noticed that it wasn't near as ballsy as my 06 rm 125 that I also have. So I kept going and finished the lap and she started blowing smoke like crazy.. Had a smell like it was gonna blow. I shut er off and started thinkin about the power issue. The main but that screws onto the crank that holds the clutch all in place was finger tight loose. I tightened that up to spec and went to start it. And there's my problem. It won't start. So we checked for spark. Have spark. Took apart the carb cleaned all the jets, made sure everything was clean. All was good. Put it back together and tried again. Still no luck. Won't even bump start. I am ridiculously confused. Has spark and it's getting fuel. Any ideas?? Anything helps? Please

To the mother - my suspicion is that your fuel petcock is clogged (you can start it but once it dies it takes a few hours for the petcock to drain enough fuel into the bowl for the bike to start)

If you take it off the tank there's a good chance it's able to be dissasembled and cleaned. If you haven't fixed it yet haha

To the newest member of this thread, I would be willing to bet your loss of spark was coincidence or you knocked a wire loose while working in it

It has spark tho..

In such a case I would investigate around your repair, see if you stuck the prover valve open or left a crucial bolt loose or created some sort of air leak

Everything on the carb is fine. It's getting spark and smooth gas flow. When I check the spark it seems like there's a little less then what should be there. My guess is some sort of electrical problem but I can't figure it out.

Did you check the magneto/coil/ecu with a multimeter and a shop manual?

Hello all!


Thanks to everyone for the input!  I seriously learn so much from you all!!


The problem on our suzuki was the wiring to the kill switch.  The wires were worn, would short out causing the bike to stall or not restart.  Sometimes we would get spark, but it was weak.. sometimes no spark, because the wires were not connecting effectivley.   For now, we have just disengaged the switch.  But, I guess now that leaves me with a new question...  Are we doing damage to the bike if we aren't using the kill switch?  My kid is just letting the clutch out when the bike is in gear when he needs to shut it off.  Is that ok?  We plan on getting the switch fixed, but in the mean-time, are we causing damage?

Hello all!

Thanks to everyone for the input! I seriously learn so much from you all!!

The problem on our suzuki was the wiring to the kill switch. The wires were worn, would short out causing the bike to stall or not restart. Sometimes we would get spark, but it was weak.. sometimes no spark, because the wires were not connecting effectivley. For now, we have just disengaged the switch. But, I guess now that leaves me with a new question... Are we doing damage to the bike if we aren't using the kill switch? My kid is just letting the clutch out when the bike is in gear when he needs to shut it off. Is that ok? We plan on getting the switch fixed, but in the mean-time, are we causing damage?

Nothing serious I can think of.

Wiring is easy though, I suggest fixing it yourself

No I have not. But I just took the cylinder off and found that the exhaust valves were completely stuck. Wouldn't move more than 1/8 of an inch. Would that cause it to not run?

to kubalek:  no you wont hurt the rm125 by not having the kill switch but it is risky safety wise.  if something such as a bent, misrouted or other wise damaged throttle cable condition were to occur, hanging the throttle wide open, then this suddenly becomes very dangerous. 


  be prepared to pull in the clutch(which isn't good) or shut the motor down with the rear brakes. 


get some electrical tape and cover up your bare spots on the kill switch wire.  replace it asap, most of them work the same; grounding out the coil to the frame when pressed.    


and no, letting the clutch out to kill the motor wont do any damage.     

Cleaned the valves. Put werything back together. Still nothing. Checked for spark again and still have plenty of spark. Like I said I have gone through the carb 3 times now and it is totally fine. I have never gotten into such a stump. It's getting plenty of gas and has plenty of spark. No idea what to do. Any help? Please. I'm beyond confused. Simplest motor out there and I can't crack this code for the life of me.

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