04 RM125 tuning info

[marcusgunby]

Let me know if they do please eric:)

 

[georgieboy]

Like Bruce i wanna know how the pv opens. I feel over a 500rpm but is that true? Is it on/off switch?

 

[marcusgunby]

well you dont need a electronic pv to get a smooth curve on the dyno-the pv operating range is only a few hundred rpm-i opened it by hand fairly abruptly and it didnt show at all on the graph, so i conclude you dont need a slow opening of the pv at all.Its the rpm it opens is critical.

 

[marcusgunby]

well i have lots of good dynos for you all coming soon, end result is 35.7hp with a std 03 KX125 making 31hp on the same dyno(04 Cr125 is 30hp)
got rid of the dip in the curve by removing 0.2mm preload from the governor spring.Swapping to a 42.5 pilot(from 37.5 AS 0.75t) and AS 1.5 turns gave 1hp more on the bottom.It has as much torque(over 17lbft) as any gp bike KP has tested and has it at a lower rpm than most of them ,it wont put out the 37hp they have or rev to 13000rpm but then only 0.5% of riders can use that, it has more bottom than most of the gp bikes anyway.All this without porting mods, im want to test a boyesen 3 on it next. BTW tried a(sleeved down 38mm) 36mm carb and it lost 2 hp from the top and gained 0.5hp on the bottom.

 

[bclapham]

Originally posted by marcusgunby
BTW tried a(sleeved down 38mm) 36mm carb and it lost 2 hp from the top and gained 0.5hp on the bottom.

is that the same carb steve is going to bash you around the head with????? :confused:

i still think you should but a stuffer and try those YZ reed cage- it will cost <$10 and you can mount it up with that chopped up manifold- stepper angle and 6 petals has got to be worth a try before you spend mega money on the Boyesens.

 

[marcusgunby]

bruce i think it was actually a sleeved down kp carb(identical carb as mine) as he has one bored out to accept different inserts-im not sure and will check,
i will try to test the yz reed setup,steve reckons he even has the boyesen 3 setup improved opon.

 

[georgieboy]

Marcus, knowing you now as being a very sensitive guy(grin), i am just curious about it.Wld you have been able to get the same result by seat of the pants feeling. And then the 1 hp increase by raising the pilot. Is is noticable by just plain riding the bike?

 

[marcusgunby]

I would say the pilot jet for me is one of the most critical-it makes a huge difference to my corner speed as without controllable power a 125 stuggles to keep momentum.Not sure if i could have felt the dip in power(it was 0.25 seconds long) but i do believe that on a slippy corner that big step in power is likely to make the biek loose traction.So feel -probably not, faster lap times -yes.

 

[steve125]

Originally posted by bclapham


is that the same carb steve is going to bash you around the head with????? :confused:

Bruce his carby was not the same as the STD. 36mm that i use and his jetting is way different. I doesn't suprise me that a sleeved down 38mm did not work for him. He's ridden my CR139 and KX144 with a&nbsp;36mm and knows it's makes good power! Not to mention all my dyno's&nbsp;that back it up, it's all in the set-up really.

 

[marcusgunby]

Just to add to steves comments-the carb (if its KPs) has been used on many bikes and its made more power than a 38 sometimes(as has a 39), however more and more KP is finding the std carbs are hard to beat.The one on the 04 CR250 is a horrible thing to work on, but it makes more power than the others, he has tried the Kehin 39mm like on a KTM, on many bikes and it makes big big numbers, however it was very hard to jet at low throttle openings, sound familiar bruce;)

 

[elf]

marcus what would you say was the best way to ajust your air screw. Of course with the right pilot jet in the carb. I mean how you do it at the track without the dyno?

 

[bclapham]

we should dedicate a whole forum to these damn carbs! i know the one steve is talking of, i was just having some fun- its the 36mm version of the cresent TMX.

as for sleeved carbs, i think there is much more to it than that since the opening is always bigger on the venturi side and so a 38mm sleeved to 36mm with a stock slide is still really lean on the slide compared with a proper 36mm carb- ive found the slide effects more than jetting at the lower settings since if the carb doesnt get the velocity at the lower settings it wont pull the fuel through the main/needle circuit and the bike will run lean and lame....... bwaaaahhhh!

so why did honda run that TMX on the 250- its been nothing but trouble since 01 and now they have stuck a powerjet on it- the Keihin PWK powerjet seems to work well on most other 250s. KX, RM, YZ, KTM and most these bikes are all making good or better power compared with the CR

as for the PWK airstiker on the ktm125, is it a good carb compared with that little Mikuni cresent TMX??? i know its a little longer so the slide is around 10-15mm further back from the engine, how does that effect performance? i still cant believe how well that bike pulls that 39.5mm carb-

my rm144 wont pull the 38mm airstriker without the real rich #4 slide- this last few weeks ive made good progress getting it jetted, i am back on a 1469 needle and its just the pilot and AS that is left- i usually set the pilot and aiirscrew by holding the bike at a fast idle and then turning out the AS until the revs peak- are there any better/ easier methods??? this wasnt working with my new slide but i cut a little slot in the bottom of the slide so it will now hopefully idle.

at the end of the day, carbs can make power, but they are of no use if they are a major pain to jet- i am begining to think i should go and buy a slick tire and find a local dyno- seems to me that the final touches that you can do on a dyno with respect to tuning are more important than all the other stuff put together.

 

[marcusgunby]

Bruce i think if you use the rising revs technique, you should wait till peak revs and then richen it by half a turn, i seem to remember that from some distant memory.

Elf thats a good question, i like to go too far on the lean side and work backwards to a good setting, the rm came with a 45 i think and was rich, so i put a 37.5 in and it was better but not right, i knew it was too lean as it wouldnt start even with choke(AS 1.5 turns) so i put the AS to 1 turn and it started but it didnt run well.On the dyno it bogged hard on low revs, AS 0.75 turns helped but it wasnt right, so i sent upto a 42.5 pilot AS 1.5 and the dyno liked it, i havnt ridden it as yet.

So my tip is go lean and get it to bog and work richer, once you get a bog you are not far from the correct pilot setting.Just one size up maybe perfect.You cant do it the other way(rich towards lean) as you never will find if you have got it lean enough, you will just get it to run well(compared to rich) and stop trying.

 

[bclapham]

so it sounds like a pliot that is just lean will give peak revs on the airscrew less than 1 turn-thanks- ive got a bit more to play with now

 

[steve125]

Bruce you want it to run as clean as possible on the bottom without bogging. I never use a high/low idle when setting the air screw. Unless your needle setting is too lean, just go lean on the pilot untill it bogs on a quick throttle opening. Next slowly richen untill it's gone and your set. This can all be done in the garage, hope your neighbors don't mind!!!

Also&nbsp;I have a big fan to keep the engine from overheating and to clear the fumes! :thumb:

Another note, always set the air screw on the rich side for the advanced rider when riding a SX like track. Better a bit of blubber than a bog on a whoop or jump face.

 

[bclapham]

thanks steve- i just found out the price of a couple of runs at my local dyno- i think at this point its going to be cheaper to fly you out and show me how to put the finishing touches on this carb!:)

Big fan??? who is he? i had visions of Marcus stood in your garage waiving a piece of cardboard! :confused:

 

[steve125]

Originally posted by bclapham


Big fan??? who is he? i had visions of Marcus stood in your garage waiving a piece of cardboard! :confused:

Yeh! but that wasn't for the engine, he's quite the gassy fellow and proud of it I may add!!! :eek:

 

[elf]

Exellent information guys! I hope you dont mind me poping in and asking questions. Thanks.

 

[bclapham]

Elf- if you get the pilot dialled, drop back and let us know how it worked out- i might try a 1470 needle if i get chance also- i suspect this may have a bit of effect on how the pilot runs.

 

[elf]

It will be a while........ its 8 below zero here right now.

 

[marcusgunby]

Heres the latest dyno-see the dip is almost gone, maybe another 0.1mm less preload(for a total of -0.3mm)&nbsp;on the spring would have been perfect.

So here you see std(green) then the 2C pipe(red), and then the 2C pipe,head mods(1mm squish), retarded timing(2m),pv mod(blue).

 

[bclapham]

its funny that on the first dyno from the mag that you showed there was no PV dip.

check out this dyno, i just found it when clearing my inbox- looks like the 03's- on this one the rm125 has a dip, but look at the yz- it really takes a crap.

 

[marcusgunby]

yes bruce thats a 03 by the looks of it, heres a good one-full husky gp bike vs mine-mines better all the way to 10K, the gp bike didnt always last a race at 39hp, this is more than all the ktms had including dobbs works bike.40hp 125s&nbsp; mxers dont exist.

 

[steve125]

Very cool Marcus! Now you have good motor.

I see the 2c pipe added to the "dip" and in all fairness this may be specific to this pipes design. It's possible say a Bills or PC may have the same dip or less. The good part is you found a way to tune it out :thumb:

Bruce that dyno you have posted shows a massive dip on the 03 YZ125. Unlike on Marcus's RM, the YZ needs more spring tension with an additional shim or updated spring off the 04 model.

 

[marcusgunby]

Steve the HGS was by far the worst of the pipes in terms of the dip-i suspect they will all have it to some extent-shame i never went for 0.4mm less preload and them used shims to bring it to the correct tension, however it took a long time to get 0.2mm off the spring cup.Live and learn:)) imagine if i can get 1hp more using a boyesen 3?? i will never hang onto it:))