jetting or electrical or something else all together?

[gregorious77]

Just got an 84 rm125
Well I took it out today for the first time.
Was not running right all day.
Started off bogging down with a lot of smoke so I adjusted the needle to the top clip to lean it out.
The engine was bogging really bad until the revs picked up then it took off like a rocket, it seemed to be ok as long as I kept the revs up but bogged badly if I let them drop off. this made riding pretty dificult and pulling off was a fine balance of throttle and clutch.
Later in the day the engine lost power and was hard to start, once started if i pinned the throttle in neutral the engine was reving itself as if I was pushing the kill switch with big puffs of smoke between revs, the plug was fouled so I cleaned it and the bike ran ok at high revs again for about 10 mins then went back to bogging and being hard to start, the plug was again fouled.
The carb is setup as per manual 270 main jet, 45 idle jet, running 32:1 premix.
any ideas? I have put a different carb on which is from a RM250 and jetted it the same as the 125 carb but so far have not tested this setup.
Any ideas out there on the probs??

[gregorious77]

Just tested that setup and it was very smokey again and a total ***** to start so I tryed a different pilot jet only one I had was a 24 the one it is supposed to have is a 45 now it starts fine and seems to go ok but has a loss of power in the mid range so im guesing its jetting. probs could do with a bit bigger jet than a 24 but ill see how it goes.

[dstollys]

almost sounds like your case half gasket is done. if you jet it from a 45 to a 24, your topend is gonna be nothing, not enough fuel. check crankcase oil level after running it for a while to see if youre consuming it, if you notice it getting lower and lower, more and likely , its youre case half gasket and/or main seals.

[gregorious77]

I was thinking the crankcase may have been sucking in gearbox oil but that does not explain why jetting it leaner has stopped it smoking and bogging as it would surely be still sucking in oil from the gearbox. previous owner said he replaced piston and liner and done crank seals. I dont have any way of knowing if this is true unless I do a tear down which I might have to do but first I think ill try a pilot in between the 24 and 45 like a 35 or something like that. Might be something to do with using a 250 carb on a 125 I dont know really. I guess there is no way of checking the crank seals are ok unless I strip the bottom end, looks like its a clutch basket off jobby.

[gregorious77]

Ok this morning I pulled the mag cover off took the stator off and removed the rotor.
There is play in the crank its got a fair bit of end float and a tiny bit of up down movement.
The seal does indeed look new on this side and has been weeping a little oil into the mag casing. the seal was flush with the face of the recess it sits in but looked like it was not in square so I tapped it in and now it sits about 5mm into the recess does this sound right? looks like the crank shaft has some pitting where the seal was sitting.Not sure if it is worth the cost to do the crank bearings and seals as it looks like it will cost me about £110 and the bike cost me £300 plus im not sure if the pitting can be sorted out which would mean new crank which is not worth doing.Might look at the clutch side seal because I dont think the previous owner would have bothered to do it even though he said he did. might be woth replacing that and seeing if it sorts the problem.

[Spikester]

Sounds like you knocked the seal in too far, they should be flush with the cases and not pushed inside. Jetting a 125 with a 38mm 250 carb will be a pain and if you have issues with worn out crank bearings and pitted crank surfaces where the seal is riding it will effect the way it runs. If you are useing a 38mm carb with jetting from a 32 it will be lean because you are letting in more air for the amount of fuel you are using, that big bog and then reving out sounds like overcarburation.

[gregorious77]

The thing is it behaved exactly the same with the 125 carb on setup as per manual i.e. 270 main jet and 45 pilot. I think it has to be the bottom end and im not sure if it is worth spending out the money sorting it out.
Think I can do the main bearings and seals for about £50 shopping around but not sure how to sort the pitting in the crank shaft, might clean up with some emery cloth I guess.
I am going out to the garage in a bit and im gonna strip the right hand side down and take a look at that seal. If it looks bad ill replace just the seal and see how it runs depending on how much play there is in the bearing. anyone happen to know the standard end fload tolerance of the crank as it doesnt say in the manual plus I know the bearings are a clearance fit to allow for heat expansion.

[gregorious77]

Well stripped it down clutch side. not a lot of play in crank so im not worried about that anymore and the seal is new and looks to be perfect. this got me wondering then I saw the engine number F hmm thats 85 not 84 looked in the book and its supposed to be jetted a lot different 35 pilot jet for starters. looks like it is a jetting problem after all. gonna get a 35 pilot and try that. the mag side seal is not leaking either I sprayed some WD40 round it while she was running and no reaction.
Ill keep all who are interested informed of the progress.

[gregorious77]

Well the plot thickens.
Jetted her today as it is supposed to be. kicked her over and it started first kick.
Reved up nicely so took it out in the alley to get it warmed up, then it started bogging again loads of smoke etc I got frustrated reved it and it quit.
Took plug out it was wet so put it in gear and pushed it up and down the alley to clear the cylinder then it seized solid. wouldnt move back or forward so I had to strip it down after all.
Good job I already ordered a gasket set.
following are the results of the strip down (photos).

[gregorious77]

bottom broken off piston



bits of piston



barrell off



crank



left crank case



right crank case



All looks ok apart from the piston obviously. glad it broke when i was pushing it and not when i was riding it. bore is ok crank and cases look ok too. might as well do the main bearings now and seals as well then see how it goes. reeds look ok too. still not sure what was wrong unless the piston skirt was cracked or something. it broke on the inlet side BTW.

[gregorious77]

Still waiting on parts in case anyone is interested.

[gregorious77]

Well I have now sent the barrel off to the people who re sleved it for rebore etc, sent the old piston too to see if they could find out what happened.
The guy phoned me and said there was a huge amount of wear and it looks like sand or grit had got into the engine causing the piston to become so worn it failed, this engine had around 3-4 hours running!! also the piston is a flat top which is wrong as it should be a domed top.The engineer said the flat top could cause the bogging due to lower compression.
He is going to see if he can bore it out and get back to me.
another thing I have noticed is that the holes where the crank seals sit are scored so I am going to clean those up with some emery cloth.
Fingers crossed he can save the cylinder and I can get her rebuilt soon.
I have repacked the silencer too which was so badly splooged up it took me and a mate a lot of pulling to get the old packing out which was one solid lump of gooey carbon.

[gregorious77]

Is anyone interested in this thread?
seems like its not worth me putting this stuff up and I am talking to myself.
Thought there would be some comments and suggestions.
Anyone know of any dirtbike forums which are more into the older bikes?

[MXtras]

I think we are simply watching and listening - you are doing a fantastic job of documenting your progression and diagnosis. Thanks!

I vote to keep us informed.

Scott

[gregorious77]

No probs, should have the cylinder back soon.

[gregorious77]

Well got my cylinder back friday bored 1mm over size with the correct dome top piston kit.
Reassembled the engine friday using new main bearings new oil seals new small end bearing wrist pin and piston/rings.
put the engine back in the bike this morning filled her up with coolant and oil. connected the fuel up and kicked her over. ring ting ting ting.
She is running sweet just need to run the engine in now when the weather gets better, its a bit wet and windy in the uk at the moment.
put the old 125 carb back on jetted to the 1984 correct settings and all seems good also filing the grooves out of the clutch basket have stopped it dragging which is good.

[Spikester]

Congrats, good to hear, try to post some pics if you can.

[mynewcr250]

Originally Posted by MXtras

I think we are simply watching and listening - you are doing a fantastic job of documenting your progression and diagnosis. Thanks!

I vote to keep us informed.

Scott

bingo

[gregorious77]

Ill get some pics taken when I have a chance. It needs some tidying up so it will be good to document the restoration. Ill start a new thread for that one.

[xlr8motox]

well done m8 all that hard work paid off!