Hot Tips for the 75 RM125M?

[Graeme M]

I've really enjoyed racing my 125M, but next year I am focusing on dirt track rather than MX, mainly cos I am a pansy and can't hack all the bumpy bits in MX.

I want to get the little M hooting a bit better for that, so what's the tips for getting a bit more go? Can you chuck on bigger carbs and stuff? I was thinking of adapting the big carb, getting rid of the airbox and upping the gearing a bit. I've also got a nice S model head to slip on, are they any different to the M head?

Tyres wise I'll stick with my Dunlop 756 front and go to a Michelin trials on the rear, so I reckon that'll be fine, although a front more suited to hardpack might be a better option. Thoughts?

[oldyzman]

Hello Graham,
were you thinking a 34 mm carb or bigger??? I will be watching this thread to see what others have to say
Cheers Brett

[Graeme M]

Brett, I've got an S model carb (34mm) and the adapter plate, plus an S head. I assume the S cylinder was different to take advantage of the larger carb, but would hope that the M cylinder isn't too far out of the ballpark. Interested to see what others have to say too.

I'm not trying to make the M into a race winner, it'd need a faster pilot for that. I am simply hoping to be not too far off being competitive power wise with the CRs and RM-A/Bs... I *was* thinking of bolting an RM250B engine in there, but this will have to do for now! 

[oldyzman]

Graham, i reckon the OD of the flange of the VM32 and VM34 is the same, so shouldn't take too much to do it. I know that the s had a 34 the main jet is quite large. So dont run it too lean on the jets that may be in the 34. I did fit a 34 to RMA went back to the 32mm cause i lost too much low range. This may reflect my lack of experience...
Brett

[TM BILL]

Hey Graeme  Doc  and Dodgee are the experts on this but i think the only difference on the S clyinde is one less fin on the rear to accomodate the adapter plate. The heads are the same, and an S chamber works way better than an M chamber. I think from memory you are running a Vintage iron pipe these seem to work well like an S pipe. Does your airbox have the fluted vents and the smaller M air boot ? .

S and M gearing are the same i think  Bill

[Graeme M]

Hey Bill, yes I have a VI pipe. And it may be my imagination, but I was regularly hosed on the starts with the stock pipe, yet with the VI pipe I have actually pulled some holeshots. Could be coincidence but hmmmm...

I have the vented airbox and small boot, but as I am going dirt trackin I will toss the box in favour of a pod filter. I also don't mind losing some bottom end for the same reason. I read in VMX that the S could handle going up a tooth or two at the front over the standard gearing, so I will likely go up as well so that it pulls each gear a little longer, which should help acceleration off the corners you find in DT.

I won't motocross the RM again, but will definitely take it to CD8 for a flog round the Pre 78 track.

[211kawasaki]

get rid of the air box completly, this was the single biggest improvement to my TM which are all but the same.

[monaro308]

Why not use this instead? 
http://cgi.ebay.com.au/MITZA-/270682100583?pt=AU_Motorcycles&hash=item3f05e91367

[Lozza]

None of that will get you near the front in DT. PM me a daytime phone number as there is too much info to type.
Basicaly raising the redline rpm
Building a port/pipe and ignition combo around that.

[Graeme M]

Don't forget Lozza, this is on a shoestring budget! I get about 50c a week to spend on this stuff... And I don't need to be near the front, just don't want to be left too far behind when racing other old codgers at club days.

[Lozza]

I have worked with people with champangne tastes and beer budgets and people with beer tastes and tap water budgets, all have rode away smiling

[DR]

Personally Graeme, for DT I'd be dropping the RM down or looking to slip the engine into the lower TM or TS roller. If you do the later I can all but guarantee'd it will want to go sideways and predictably sideways at that!

I'd be looking at less weight also, tyres, chains, alloy rear sprocket, alloy bars..some weight can be trimmed if selective. Gearing is probably trial and error due to rider weight, style and differing tracks.

I have the 125S motor in my TS125



 

[Graeme M]

You'd probably be right Doc. But I don't really want to muck around too much. I like the little RM and just want to do some local club level DT. So my efforts are really just aimed at getting a little more power so the thing isn't too far off the pace. Off the pace it will be, but with a couple of extra HP I am hoping to at least keep them in sight!

That said, I actually thought the little RM was pretty low to the ground already and figured it'd slide pretty nice as is! I am guessing it'll feel way better that way than the 05 YZ125 that I will also race without any lowering at all...

[Rossvickicampbell]

if there is one thing I can say about Lozza (and actually there could probably be a few  ) is that he certainly tries to look after you - no dramas there!

[Lozza]

Weight reduction is twice the cost of making more HP and half as effective.
eg(figures arbitary)

Bike mass 75kgs
Output 20 HP
=0.266HP/kg

New mass 70kgs
output 20hp
= 0.285 HP/kg

mass 75kgs
output 22hp
=0.293hp/kg

There is an easy 2HP in the cylinder head alone cost=$0.00 just time on a lathe. Dropping 5kgs out of a 125 which weigh approx 75kgs will be very very costly.Optimising an engine doesn't really cost anything just time. As I said before Graeme, 1500 rpm more over rev is what you desparately need.With more over rev you can utilise lower gearing, still have top speed but accellerate fast off corners. Lower gearing also helps peaky engines stay in band.

Thanks for the kind words Rossco,must be christmas