RM125x poor running when hot

[RMJJ]

My mates RM125x (81) starts and runs fine when warm but after a few laps when hot starts running as if the choke is on.
We just can't fathom it
All new electrics, jetting is spot on.
Its on a new 56mm bore.

It runs perfect, then when really hot it'll come out of a corner in second gear for example running as if the choke is on.
Its not bogging, he is a fast rider, if he slips the clutch it makes no difference, BUT if he goes up to 3rd gear  it'll stop doing it instantly
and the engine will pull ok although it will labour as its a gear too high then.

We've run out of ideas, changed things for change sake and its still the same.

It does'nt missfire as such its more a running mega rich type symptom but its jetted spot on.


Help

P.S. this is a brand new ground up rebuild, the engine is all fresh, its not an old nail.

[Lozza]

I would say you have an electrical problem, at a guess I would say a coil. A plug chop when the symptom is occuring would tell you a lot.

[Noel]

My 2 cents,
Is the choke seating properly ,
check all your earths,
check plug cap for arcing at the plug 'termina'l end,
hth,
Noel

[SUZUKI311]

my 125X had the same problem when it was about 12 mths old, turned out to be a faulty stator, replaced it and it was brand new again. i see you said it has new electrics, ( stator?)were they rewound or new oem items?

[RMJJ]

Yeah thanks fellas.
Weve checked all those things
It had used electrics and ran fine, then started buggering about and now has NOS elctrics, coil stator cdi. kill switch dissconnected
and its still doing it. The carb is NOS from Crooks suzuki.

Heres a link to the bike on my site, http://www.suzuki-rm125.com/html/friends_bikes.html its the twinschock works replica.

Its just started doing this. he changed nothing so its not as if he altered the jetting or anything and then it started.
It only does it when its got fully race hot after about two laps.

 I thought of the choke not being seated properly but weve checked that, and then again, it would do the fault hot warm or cold wouldn't it.

it really is a mystery. we are both thinking rotor but thats just permanent magnets, what could go wrong with the rotor ?
Rotor hasn't moved by the way its still on the key correctly.

[Rossvickicampbell]

something simple - check the plug - heat range?

[RMJJ]

Cheers Ross,
yes its running the standard plug at the minute, B8egv we are going to try a B9egv this afternoon. Its 3pm here.

thanks

[DR]

311, my RM80 played up like this also when less than 12 months old. Turned out to be a stator coil. When the shop tested the ignition it all seemed fine took it home and it was still up the shit..took it back where they tested it again and as a last resort they ordered a new hi speed 'primary' coil for the stator since it was only playing up at high rpm..problem solved  

[oldfart]

Change your fuel supplier   shell to caltex          It's a long shot                   

[LWC82PE]

sounds as though you have checked most things but have you checked the resistance values of the NOS electrics? Parts could be dodgy from the factory.

somethings that come to mind

- poor sealing rubber on choke plunger -  will cause a richer mixture all the time
- CDI getting hot
- if the rotor is old and stored incorrectly it will loose magnetism although i dont think its your problem. I have had problems with rotor magnets but it usually causes weak spark or no spark at all. Soloution, remagnetise the magnets or get another rotor.

- a component in the carburettor coming loose when hot. I had that problem with an Amal once. It was a bitch to find. When the carburettor got hot and expanded the little brass bit with the shroud on it in the middle of the carburattor where the needle goes in ( i cant think of the name for it but it wasnt the needle jet, its a part that is ment to permanantly stay in the carby and not be removed)

anyway when it got hot it would come loose and upset the mixture and cause a rich mixture and foul plugs. I only found it when after running the bike, took the slide out and felt inside the carby when it was hot and found the brass bit with the shroud for the needle to have up and down movement. I knew straight away it was the problem. I used Hi -temp Loctite on it and put it back in and its been as good as gold ever since.

Also as soon as it starts playing up, go back to the pits/shed and stick a strobe light inline with the plug and pulg cap and observe what it does and see if the ignition is breaking down.

[RED ALERT]

Also check the reeds. All it takes is one corner to be chipped and the bike will run rough on the top end. But that doesn't explain why it runs fine before it's hot.
Back in 83, I had a new RM80 that just didn't seem as fast as the other bikes I raced with. With a few races left in the season, the stator shit itself. When we put a new one on....WOW, was like the bike had 5 more hp.
Must have had a dud one all that time.