Author Topic: RM125S questions for the Guru's  (Read 15419 times)

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Offline Doc

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RM125S questions for the Guru's
« on: January 11, 2015, 10:51:52 PM »
2 questions put to me that I'm at a loss to answer ???

Was the 'porting' the same for both 125S and 125M?

Was there an engine number sequence designated specifically for the RM125S?
« Last Edit: January 11, 2015, 10:55:05 PM by Doc »

albrid-3

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Re: RM125S questions
« Reply #1 on: January 11, 2015, 11:00:46 PM »
I was sponsored on a RM 125 S, and I was told that the S was much more developed than the M model, the  Rm S model was sold as an expert bike had a larger carbie and porting was more improved over the RM 125 M.
« Last Edit: January 11, 2015, 11:05:44 PM by Dave #14 »

Offline pokey

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Re: RM125S questions for the Guru's
« Reply #2 on: January 11, 2015, 11:01:45 PM »
The S were hand ported wernt they doc?
if so you shouldnt see the sand casting

with the s being a rare bike and SFA of them finding thier way to the US of A records will be sketchy being an  half year bike.

my records show the S starting at 19800 but exact build numbers i have never been able to find. ED may have a few more clues seeing he has more than his fair share of the nice little things ;D

albrid-3

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Re: RM125S questions for the Guru's
« Reply #3 on: January 11, 2015, 11:08:22 PM »
RM 125 S 1975
Overall Length: 2,040 mm (80.3 in)
Overall Width: 860 mm (33.9 in)
Seat Height: 901 mm (35.5 in)
Ground Clearance: 245 mm (9.6 in)
Wheelbase: 1,360 mm (53.5 in)
Dry Weight: 86 kg (189 lbs)
Engine type: Air-cooled 123 cc single-cylinder 2-stroke piston valve. 26 hp/ 10,500 rpm, 1.79 kg-m/ 8,500 rpm

Offline pokey

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Re: RM125S questions for the Guru's
« Reply #4 on: January 11, 2015, 11:09:22 PM »
http://www.suzukicycles.org/RM-RMX-series/index.html?RM250-RMX250.shtml~isoraami



The first model version of the RM125 had "downpipe", the exhaust pipe placed under the engine. The TM125 was discontinued and instead came the RM125. When tested against the competition in the US market the RM125  was found not powerful enough to compete with the Honda CR, kawasaki KX and Yamaha YZ models and had to be quickly modified before mid year 1975. The model name to the revisited model was RM125S.

To make the bike faster Suzuki did some interesting alterations in the following areas of the engine: they reworked the porting of the cylinder barrel massively, all ports were opened significantly and they appeared to have been finished by hand when viewing the cylinder. The new hand built exhaust pipe for the RM125S was slightly larger in diameter and made of many rolled steel pieces welded together, not stamped with two londitudal folded seams like the M model. The stinger bulb was also welded on at the end of a slim diameter pipe with a stiffener rib above and below it to stop cracking where this pipe met the end of the expansion chamber.

The headpipe also still carried the OEM chrome exhaust collar, but now used extra "L" shaped  pieces placed under the bolt heads, these 'ears' each held one end of the springs that kept the pipe in place to the cylinder head.

The pistons skirt had two holes drilled into it to help cool the thin exhaust port bridge section as the new exhaust port was now quite wide, the head gasket material was also slightly different.

The S models Mikuni carburettor was also quite larger and didn't have the tall top cap as the M model, and the carburettor intake rubber/alloy pipe was also larger and carried an adapter plate. The new carb had increased jetting values and a special needle valve orifice with three extra angular drilled holes to allow fuel to bypass the lower needle valve body into the bigger float bowl.

The newer and bigger CDI unit for the S version was now mounted onto the front of the frame headtube with a bracket instead of in front of the aircleaner case as with the M, and finally the air cleaner case was altered to accomodate a bigger rubber connector boot. To let the owner know they had bought an 'S' model, the fuel tank had a decal affixed right below the filler opening that said RM125"S".

As noted, the power output went up to 26hp, and this was enough to allow it to run with the CR, KX and YZ models that were beating the M model. The S model only lasted for about four months (later in some countries, it was sold as a 1976 model at least in Finland) as the new A model was being tested for '76 and the S therefore never sold in large quantities.

albrid-3

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Re: RM125S questions for the Guru's
« Reply #5 on: January 11, 2015, 11:12:19 PM »
1975 RM125M specifications

General
Length: 2,040 mm (80.3 in)
Handlebar width: 860 mm (33.9 in)
Footpeg height: 350 mm (13.8 in)
Ground clearance: 245 mm (9.6 in)
Wheelbase: 1,360 mm (53.5 in)
Seat height: 901 mm (35.5 in)
Dry weight: 86 kg (190 lbs)
Wet weight: 92.7 kg (204 lbs)
Front brake: 130 mm diameter
Rear brake: 130 mm diameter
Front tire: 3.00-21-4PR
Front tire: 3.00-18-4PR
Frame: Semi-dual cradle

Engine
Horsepower: 23 hp/ 10,500 rpm
Torque: 1,7 kg-m (12.3 ft-lbs)/ 9,500 rpm
Engine type: Two-stroke, air cooled, 6-port scavenging
Bore and stroke: 56 x 50 mm (2.20 x 1.97 in)
Displacement: 123cc (7.5 cu in)
Compression ratio (corrected): 7.4:1
Lubrication: Fuel and oil premix, 20:1 (5%)
Carburetion
   Bore: 28 mm
   Main jet: #180
   Jet needle: 5DP7-3rd
   Needle jet: P-4
   Cutaway: 2.5
   Pilot jet: #50
   Pilot air screw: 1 1/2 turns
   Float level: 25 mm (0.98 in)
Air filtration: Wet polyurethane filter
Exhaust system: Expansion chamber with external silencer
Starting system: Primary kick
Clutch: Wet, multi-plate type
Type of transmission: 5-speed constant mesh
Primary drive: Staright-cut gears
Primary reduction: 3.389 (61/18)
Final drive: #428TM chain, 126 links
Final reduction: 4.286 (60/14) Solid mount rear sprocket
Gear ratios
   Low 2.143 (30/14)
   2nd 1.588 (27/17)
   3rd 1.250 (25/20)
   4th 1.045 (23/22)
   5th 0.913 (21/23)
Ignition: PEI (same as TM125)
Spark plug: NGK B-9EV

Suspension
Front suspension: Telescopic fork, hydraulic damper
Inner fork tube poutside diameter: 35 mm
Front fork travel: 190 mm (7.48 in)
Front caster (from horizontal): 61°
Fork trail: 123 mm (7.48 in)
Rear suspension: Swing arm, hydraulic damper 5-way adjustable spring
Shock absorbers: Gas/oil type, forward mount, "lay-down" position
Rear wheel travel: 198 mm (7.80 in)

Capacities
Fuel tank: 5.28 liter (1.4 US Gal)
Transmission oil: 550cc (1.16 US Gal)
Front fork oil: 210cc per leg

Offline sleepy

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Re: RM125S questions for the Guru's
« Reply #6 on: January 11, 2015, 11:24:54 PM »
I had an S when I first went into Seniors and it was a rocket.

Also got a parts book that cover those models and they only list as being different the following.
Piston, carb, air filter, pipe and a bracket to mount the CDI in a different place.
Part number for the barrel is the same.

From my memory there was no hand porting to be seen inside the barrel on my bike.
Most obvious change was the pipe, it looked hand built rather than a pipe looking like a TM125 pipe.

Offline pokey

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Re: RM125S questions for the Guru's
« Reply #7 on: January 11, 2015, 11:32:13 PM »
Cycleguide 1975

This is a test that really highlights the pre 75 class with the new breed of mx bikes and a new era then follows. I rode my first real mx season on a 1975 RM125S and it was coool – big sand woops no longer held any fear, full throttle and just hit em hard!!! The RM125 tested here is the US version RM125M, the faster RM125S came to Australia



For Suzuki, the RM 125 is a giant leap forward. At long last, some of the racing technology that helped Roger DeCoster and Joel Robert garner a fistful of World Championships has found its way to the production line. But Suzuki’s ultra-conservatism, a trait that has characterized most of their motorcycles for some time, prevented them from winning this comparison. The stock RM 125 finished second solely because it didn’t have enough power to keep up with the YZ. But Suzuki is offering a factory kit for the RM, a $200 bolt-on affair consisting of a different pipe, a 30mm carburetor, a cylinder with more radical porting, and a piston with a higher intake cutaway. They put a kit on an RM and allowed us to try it during the comparison. Our conclusion: The RM would have won the test if the kit had been standard equipment. The trick parts make the Suzuki just about as fast as the YZ, and with its slight handling edge, would have made it The Japanese 125 motocrosser. It’s nice to know the kit is available, but it’s also hard to understand why they didn’t build it like that in the first place. The kit doesn’t make the RM any harder to ride, and adds about 10 or IS percent on the top end and mid range without taking much away from the bottom end. If you buy an RM, get the kit. It’s worth the money.

Offline Lozza

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Re: RM125S questions for the Guru's
« Reply #8 on: January 11, 2015, 11:36:38 PM »
biggest difference is the TM has a mere 2 large transfers and the S 6 smaller transfers.
Jesus only loves two strokes

Offline pokey

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Re: RM125S questions for the Guru's
« Reply #9 on: January 12, 2015, 12:03:58 AM »
RM125 port map from suzuki service bulletin

May 1 1975


Offline oldmxracer

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Re: RM125S questions for the Guru's
« Reply #10 on: January 12, 2015, 02:37:14 AM »
I lived 1/2 mile from the local Zuki dealer back then and don't remember an "S" model here in the States. All they did was put the kit (pipe, cylinder and carb?) on for guys who wanted to hop em up. But that's been a LONG time ago, eh?.?.?.

Offline saint

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Re: RM125S questions for the Guru's
« Reply #11 on: January 12, 2015, 08:53:11 AM »

Offline Doc

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Re: RM125S questions for the Guru's
« Reply #12 on: January 12, 2015, 09:47:25 AM »
Thanks for the info ppl.

I should be able to 'physically' confirm port changes with that port map thanks Pokey (providing my S barrel is stock :P)  There doesn't appear to be any radical port changes when looking but a mm here and a mm there isn't overly obvious to my uncalibrated eyes ???

Found this on Gumtree last week....................
Check out the trick pipe.

http://www.gumtree.com.au/s-ad/williamstown/motorcycles/super-rare-vintage-1975-suzuki-rm125s-rms125-suit-vmx/1066755332

Saint, that pipe is what could be called an UpsideDownpipe. The idea has never once occurred to me in the past 40 years ;D

« Last Edit: January 12, 2015, 09:49:28 AM by Doc »

Offline pokey

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Re: RM125S questions for the Guru's
« Reply #13 on: January 12, 2015, 10:07:52 AM »
A while back Dodgee was building an M so he may know or have the port specs for that to compare it with.

From what i get from all the info above is there was only ever one barrel but suzuki hand ported them and in some markets sold it as a hop up kit. Australia who never got the hop up kits for any models actually scored the complete bike with hop up kit fitted and it was called the RM125S

Offline 80-85 husky

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Re: RM125S questions for the Guru's
« Reply #14 on: January 12, 2015, 10:24:26 AM »
I was led to believe that the M models suffered from poor quality control in the factory in the bore to piston clearance area resulting in numerous seizures. the "s" was rushed out to cover these issues and the motor got a work over as well.....not sure how true this yarn is. the us mag story of "why did suzuki suddenly supply a much better barrel" seems a bit strange but hop up kits were a huge market in the US so I could see Suzuki working that market.