RMX250 problem

**matrox02** · 17th August 2014, 19:45

Hi lads and gents, I finally got my rmx completely together today and got her fired up, albeit it took a long time to start, I could each time I bumped the kill switch a few times it kicked fine which could be a coincidence, but one started the bike was hella smokey and was clearing up and slowly bringing the revs up too, so I was easing the idle adjuster down while twisting the throttle a few times for it to warm up when while i was bringing the idle screw down, The bike revved up by itself and the kill switch wouldnt kill it, at this point is wasnt smoking heavily, just looked like i guess it should but it was revving smoother and higher than I had managed to get it to up to at that point, felt like it was revving to the moon, then while I was looking to kill it some how else it then returned back to idle where the kill switch became functional again during the revving I think there was no smoke, its a 1990 rmx250 with FMF fatty and FMF can, 260 main and 45 pilot on recommended carb settings, can anyone shed light on what's happening? I noticed the breather pipe on the back of the barrel is also smoking a bit, is that normal? I cant see any air leaks around the carb, and i dont think the crank seal would have been at fault, but could be wrong?? or is it just a random act ?? I put it away after that happened because I dont want to blow it up after just spending big money getting it to a running condition, ( I really should learn to get rid of it and buy a new 4 stroker! )

Its a 1990 RMX250 btw

tldr : bike revved by itself and scared me when it wouldnt shut off with the kill switch, why did it do that?

**FJRider** · 17th August 2014, 21:55

Originally Posted by matrox02

: bike revved by itself and scared me when it wouldnt shut off with the kill switch, why did it do that?

Maybe the kill switch isn't properly earthing ...

Pull the plugs and check for spark with the switch on AND off ...

**matrox02** · 18th August 2014, 01:17

Originally Posted by FJRider

Maybe the kill switch isn't properly earthing ...

Pull the plugs and check for spark with the switch on AND off ...

Ill do that, but what happens if the switch gets fixed and the engine does it again, ie, running by itself??, I suppose the spark plug could be at fault if it is actually not sparking but getting a hot spot on the electrode or something...I guess its the only part other than the casings that i havnt replaced because it looked brand new :S ( there goes my assumptions biting my ass again! )

**Kickaha** · 18th August 2014, 07:50

Originally Posted by matrox02

bike revved by itself and scared me when it wouldnt shut off with the kill switch, why did it do that?

I had the same thing one time with my IT200 and it was an air leak at the cylinder base

**Grumph** · 18th August 2014, 13:55

Originally Posted by Kickaha

I had the same thing one time with my IT200 and it was an air leak at the cylinder base

+1 - You're onto it, but given the breather at the back of the barrel is fuming - and that's the gearbox breather, I'd suggest there's an air leak from the gearbox into the crankcase.

**matrox02** · 18th August 2014, 14:55

Originally Posted by Grumph

+1 - You're onto it, but given the breather at the back of the barrel is fuming - and that's the gearbox breather, I'd suggest there's an air leak from the gearbox into the crankcase.

I checked the base, its torqued down properly, and the head too, and the breather is o the power valve, maybe its the power valve leaking instead?? its not leaking around the case as far as i can see, the only leak is a coolant leak from a shitty hose/ hose clamp by the radiator ( i intend to remedy this )

**Grumph** · 18th August 2014, 16:22

Originally Posted by matrox02

I checked the base, its torqued down properly, and the head too, and the breather is o the power valve, maybe its the power valve leaking instead?? its not leaking around the case as far as i can see, the only leak is a coolant leak from a shitty hose/ hose clamp by the radiator ( i intend to remedy this )

Any power valve is on the exhaust side of the barrel. What you're seeing is characteristic of an air leak somewhere either cases,barrel base or a seal.

**Drew** · 18th August 2014, 16:34

Sounds fucked. Give ya a hunded bucks for it.

**matrox02** · 18th August 2014, 17:11

Originally Posted by Grumph

Any power valve is on the exhaust side of the barrel. What you're seeing is characteristic of an air leak somewhere either cases,barrel base or a seal.

looks like ill be pressure testing the seals them, I skipped this because " nah it ran fine before and its all new seals and gaskets now " < that right there is an assumption and im annoyed that i do that all the time now, So otherwise, i should check the crank seals the barrel base, head and the casings?? hard to check the inside of the casings , Id hate to pull it right apart again, just after a full rebuild...

**matrox02** · 18th August 2014, 17:23

Originally Posted by matrox02

looks like ill be pressure testing the seals them, I skipped this because " nah it ran fine before and its all new seals and gaskets now " < that right there is an assumption and im annoyed that i do that all the time now, So otherwise, i should check the crank seals the barrel base, head and the casings?? hard to check the inside of the casings , Id hate to pull it right apart again, just after a full rebuild...

though if you look at the barrel diagram, its only the power valves that could be leaking here... 23 is the breather pipe im talking about, and the arm that operated the valves is in there nicely, hmm I have a spare set of valves i may swap them out...

though Ill still pressure check everything else, someone else online also mentioned smoke out the vent and said he had to plug that off to get pressure testing the cases to work properly
http://a3.bikebandit-images.com/sche...SU0230_002.gif

**F5 Dave** · 18th August 2014, 20:30

Pv vent will smoke. If carb runs out of gas under no load the revs increase for a while. When revving that high the kill may be a little ineffective if the earth is not that good.

**matrox02** · 18th August 2014, 21:19

Originally Posted by F5 Dave

Pv vent will smoke. If carb runs out of gas under no load the revs increase for a while. When revving that high the kill may be a little ineffective if the earth is not that good.

Yeah I noticed that when I was free hand kicking it with the kill switch off then as you would expect it sparked fine, then when on it sparked once or twice then dropped off completely, so the kill switch is rather a bit snotty or not earthing right, buying a new one to be safe though , and I suppose the running out of gas bit does explain it a bit, I remembered seeing the same on rc gas's cars, revving before during... Well that puts me at ease for now, I'll make some more pre mix and fill her up as I had only put 2 liters in from the get go

**gwynfryn** · 19th August 2014, 07:57

Air getting in somewhere on inlet side; crankcase seals , cylinder gasket, carb boot, gasket either side of reed valve.
Carb idle set way high and or lean pilot circuit (air screw/PJ)
Throttle Stuck.

**cheese** · 19th August 2014, 19:53

RM seals are outside the bearing so they can be done without splitting the crank.

My 96 RM did the same thing once. It was fine and never did it again.freaked me out too. Turn the fuel off next time.