So I'm having the biggest problems getting a 2003 RM125 to idle. It didn't run properly since I bought it secondhand almost two years ago, ran always rich. So far it has had two rebuild done by me (two pistons, one cylinder, and a new crankshaft assembly, all gaskets new every time). One problem we only recently discovered was that the flywheel-weight had a worn trigger? (block on the circumference). It basically was so worn that it didn't even have the correct size/shape so naturally it didn't ignite when it should (way to late I assume). Anyway fixed that with a new fww. I expected a big improvement in running, and was hoping it would idle. The noise is definitely better, but still won't idle. So I adjusted the idle screw and checked the air screw, got it to idle when the idle screw was wound in as far as it could (bad I know). Started swapping pilot jet's (from 37,5 to 30 and everything in between), didn't really make a difference and turning the air screw doesn't either (although once you turn it in farther then 1 turn it dies but that's only logical). I tried the stock and fmf recommended jetting. Still no joy. Checked the spark with a new spark-plug, looks good to me (ngk br9eix iridium). Bike gets maintained properly and looks tidy. Below are the jetting settings (150meter altitude, 20degrees Celsius) Mikuni TMX (the new version) Pilot: 37,5 - 35 - 32,5 - 30 Mainjet: 440 (tried 450 and 460 too) slide: 5.0 Needle: 6bgy28-77 Clip: 3 (tried 2 too) Air screw: 1,5 (adjusted too) Checked for air leaks, couldn't find one (all gaskets were renewed two times by me). I'm kinda fed up with this bike, put a lot of money in it, and it still doesn't run properly. What else can I try? I'm suspecting some kind of electrical problem (CDI?). The strange thing is I closed the pet cock, and it began to idle increasingly faster (to a point were I had to shut it of). Then I realized when the bowl goes empty it starts to run leaner. so should I try a 27,5 pilot jet?
Thats usually a pilot issues but looks like you have addressed that. Is the slide warn? Have you ever tried a different carb on it to see if maybe it is something not carb related?
I'm going to try a different carb, forgot to mention I cleaned this one very good. I'm kinda clueless.
Have you checked the float height? If set too high or the needle leaking the bike will run rich. Paw Paw
I haven't checked on the TMX, but on some models of MIkuni carbs, there is an O-ring around the float needle seat. If that is damaged or has dried out and shrunk, gasoline will enter the float bowl without being regulated by the float needle. That will certainly cause idling problems, to say the least.
Fitted a new throttle cable as the old ones had some cracks in it. I also swapped the carb over from a yz125 also with a mikuni tmxx, (that carb is good since the yz is running very good, it can idle all day). Naturally the jetting was different, but I mounted it any way to see what it did, still no idle (27,5 pilot). Also didn't respond to air screw changes. So I'm thinking the carb or jetting isn't the cause, but what else can it be. Reeds are good too. Piston doesn't show any blow by, compression is good. Spark is bright enough. I can get it to a very low idle by turning the idle screw almost full in (with lock nut removed), but shouldn't be like that since it bypasses the pilot circuit. I'm suspecting some sort of electrical problem, CDI perhaps?
It's the expansion chamber dented/smashed? Is the clutch dragging perhaps? I could be way off but if the ignition and carb are ruled out...
Or maybe a mouse made a nest inside the pipe? I've seen that happen, or little kids stuff paper or parts of toys into the end of the exhaust.
You don't have the air filter soaked with tons of filter oil or left rag in air box? Don't laugh cause I left a paper towel in there once! Will run like choke is on! Are your ends of carb vent hoses plugged? Sure you don't have one or both of power valves stuck with spooge?
Swapped the pilot from 27,5 to 37,5 (37,5 is stock), also with YZ carb. Vent hoses are open (cleaned it with carb-cleaner and compressed air). Expansion chamber isn't dented nor has any holes in it. Air-filter isn't excessively oiled, no obstructions. Power valve slides freely. Clutch can't be a problem when it's in neutral? Very frustrating this problem, I've rebuild and jetted a lot of 2strokes, only this one is really a tough nut to crack.
Piston in backwards? It's easier to do than you'd think. Leak down test is a good idea. What a pisser!
By leak test I assume you mean a compression test? Piston isn't in backwards position. I can see through the muffler, no loose parts.
Tell me about it, it's driving me nuts. Flywheel was bad, but replaced it so that's ruled out too. Compression feels good to me. I had transmission problems last year so installed new gears, shift cams, bearings the lot, installed a new piston (measured bore). Some hours later big-end was bad so replaced that, with new crank bearings too. Actually everything has been renewed except the electrical components. The problem seems mechanical, but I'm pretty sure I ruled everything out.
crank seals?? air/ignition and/or oil/primary drive side. at this point I would do a crankcase leakdown test. after all the other good stuff you did...electrical?
