I've recently bought an RM 125,
It seems to have quite a lot of black unburnt premix coming out of the exhaust and reasonably oily plug, although the tip wasn't overly oily.
I plan on running 98 octane super unleaded (UK) with 156ml of Motul 800 (32:1 as per manual)
Always remember as a teenager running any 125s we had a 40:1, so this sounds like a lot of oil to me, but it's what's in the manual, so plan to stick to that.
I'm not going to be changing the jets each time I ride I would just like to find a base, and set and forget
I'm a fair weather vet rider, and the average temp will be 18C (65.5F), and the altitude will always be around 100m (350ft) if it's super cold or hot I'll take the 350 out if temps affect jetting that much.. not sure??
I'm am not overly clued up when it comes to jetting, just done a bit of research online, but I plan to set it to Pro Circuits spec which seems leaner than current, which I'm hoping will deal with the unburnt premix and make it run a bit cleaner / leaner
The bike is bone stock with a full FMF (I may also add a vforce)
the Pro circuit specs are:
Main - 370 (currently 390)
pilot - 40 (currently 42.5)
air mixture screw 1.75 turns out (currently unknown, didn't count them back in)
needle position - 2 (currently 3)
Does this look like it'll be a good setting based on currently being too rich?
I can see on the pro circuit site it says their specs are with race fuel (c12 i think) not sure how much this impacts jetting, but the fuel on the pump here is quite a high octane at 98 anyway.
Thank you in advance to anyone who has any help or advice for me with this
The difference between 32:1 and 40:1 is less than the difference in a single step in jet sizes.
Fuel octane in the UK is advertised as Research Octane Number (RON), in the US it is advertised as Motor Octane Number (MON), they are not the same 98 RON octane in the UK is 89-90 MON octane in the US.
Do a pressure/leak test before you start messing with jetting, it may be sucking gearbox oil, and/or sucking air!
Recommend changing one jet circuit at a time rather than all three. Try to determine which throttle range feels too rich and start with leaning that jet out.
I could be wrong, but C12 should be on similar jetting as premium pump gas/stock.
I recall my rm125 being quite a PIA to jet, then I found a leaky crank seal. Leak down test would be the top of my list. Also, you mentioned a FMF pipe, so here is a chart for FMF recommended specs.
https://www.revzilla.com/assets/0000/9539/fmf_gold_series_fatty_pipe_su…
Thanks for the advice, I didn't realise about the MON and RON.
I will see if my Local shop can do a leak-down test and go from there,
If it's holding pressure, how do these specs look, or is it just try them one by one as suggested and see how I get on?
I saw those FMF specs but they only run to 2003 and this one i've bought is 05.
The Shop
Motion Pro make a leak down tester...
https://www.motionpro.com/product/08-0071
or you can make your own leak down tester
Yeh thanks, I didn’t want to spend £300 on a tool I’d very rarely use so I’ve just bought some bits to try make my own should only come to around £40. Will see how I get on
I got my local shop to do the leak down test and was all good,
when I ran the bike using the pro circuit specs in my initial message the spooge has reduced however,
the pro circuit specs said I should be 1.75 turns out on the air mixture screw that was making the tick over run WAY to high it’s now out to about 3.5 turn to make it tick over sensibly, what does that suggest?
the plug now looks a nice golden brown but there is a little drip of oil from the power valve overflow.
Also still a small amount of oil out the end of the exhaust (this could just be blowing out what was built up in there)
thanks in advance for anyone who has any suggestions/thoughts
"the pro circuit specs said I should be 1.75 turns out on the air mixture screw that was making the tick over run WAY to high it’s now out to about 3.5 turn to make it tick over sensibly, what does that suggest?"
Air screw adjustment is optimal when the engine idles at it's highest speed.
Adjust the throttle stop screw to lower idle speed.
i run the pro circuit jetting, and it runs spot on for me, i only live about 20 miles from you buddy (crewe cheshire) so sea level and temps are about the same, im a fair weather rider now a days due to age, but when i bought my bike new, its a 2003 yz 125 i started with motul 800 at the recommended manual amount, and it drooled like yours but i also threw plugs all the time, i changed the jets the same time as i added the pro circuit pipe, went to 40 to 1, and been happy days since
Motul is a 50:1 oil. That is part of your problem.
This will tell you everything you need to know about jetting your bike. All you need to know if it's rich or lean. I would be happy to help with any questions.
Thanks for this will try dropping to 40:1 and check the plug
Thanks for the chart that is very helpful,
what oil would you suggest/reccomend over the Motul?
Thanks
If you are re-jetting motul fine. Just mix at 50:1 and dial from there.
Another thing worth looking into if you have the mikuni tmx carb is the jet block gasket.
Most seem bee bad not sure if its from factory or just over time but it makes jetting easier you end up chasing your tail otherwise.
Also if you do need a new gasket i think its jd jetting who makes an uprated version with a thin sheet of metal between the gasket layers so i can't deform or shrink.
Definitely don't stress about powervalve overflow spooge, that's normal and happens on most bikes.
I wouldn't even worry about a little out of the pipe. If your plug is a golden brown as you say then it's a sign you are somewhere near (especially on the main jet). Try going from clip 3 to 2 on the needle, this may eliminate it. If it makes it too lean you will be able to tell as the bike will become 'zingy' and the revs will take off when slowly turning the throttle in neutral on the stand.
Thanks for this,
it’s already on clip 2, I think I’m somewhere near going to try 38:1 of oil instead of 32:1 and monitor closely next time I’m out
The less oil you run, the richer it will run. Just sayin'. Just put the Motul in at 50:1. You will never get the Motul to combust because there is more of it than it was designed for. You may not have to re-jet at all.
My CR125M was 20:1. I ran Bel-Ray MC1 @ 50:1. Never touched a jet, never fouled a plug and never smoked excessively. Best of all my buddy and his '79 RM125 couldn't beat me drag racing on the road or track.
Ok thanks, maybe I’ll rethink that one then
Back in the early 90's I had buddies that rode Honda 250's. They all ran Motul @ 50:1 and I don't think they ever changed any jets but they loved the stuff and they where hard on bikes. They bought new ones every year because they where so tore up from riding Blackwater 100's. They also rode/raced Moto.
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