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Author Topic: RM125 Rebuild  (Read 19492 times)

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Offline TMKIWI

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RM125 Rebuild
« Reply #60 on: April 19, 2012, 10:58:25 PM »
The DG pipe weighs 1.5kg and the stock pipe weighs 2kg.
The DG muffler is also alot lighter then stock but is on the bike so haven't weighed it.

Question. I have a 34mm carb of the 86' engine. Stock is 32mm.
Any advantage if I run it on the 85' engine. Or more trouble then its worth.
Remember I am rebuilding 2 bikes. The first engine is .25mm oversize and will be going first.
The second engine will be 2mm oversize and I have a good stock exhaust system plus the DG system to play with between the 2 motors. Plus 34mm carb.
Thoughts ?
« Last Edit: December 31, 1969, 04:00:00 PM by ' »
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Offline ford832

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RM125 Rebuild
« Reply #61 on: April 20, 2012, 01:55:13 AM »
Smaller carbs are better on 125's imo,unless you want more top end and even less bottom.Looking good so far TMKIWI.  8)
« Last Edit: December 31, 1969, 04:00:00 PM by ' »
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Offline TMKIWI

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RM125 Rebuild
« Reply #62 on: April 20, 2012, 04:49:27 AM »
So I decided to start on the engine.
I cleaned up the ports a little. just a few casting flaws.
The liner matches up to the ports nicely.
Matched the cylinder to both cases.





Not much was needed but might as well do it right the first time.
The worst parts were the exhaust flange, which overlapped the port on 1 side by about 3mm. Abit of grinding had that sorted.



And the carb intake boot was also about 3mm too small on 1 side. So that was grinded back to match the carb and intake. The motor will flow nice now.

I think it is funny that people throw all these after market bolt on parts onto their motor but forget about the basics first.

Next I need to put the crank in and measure up the head.
That wont happen this weekend now as I am taking the TM out tommorrow. 8)
I can't believe it has been 2 months. :-[
« Last Edit: December 31, 1969, 04:00:00 PM by ' »
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Offline TMKIWI

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RM125 Rebuild
« Reply #63 on: April 20, 2012, 04:55:29 AM »
Quote from: ford832 on April 20, 2012, 08:55:13 AM
Smaller carbs are better on 125's imo,unless you want more top end and even less bottom.Looking good so far TMKIWI.  8)

Cheers ford.

I know ford, Just thought I would ask since I have 3 carbs.
Too many people over carb an engine and stuff up the midrange. Especially cars.
The 86' came out with a bigger carb but the engine was different as well.
No idea if it would be a good or bad thing on the earlier engine.
« Last Edit: December 31, 1969, 04:00:00 PM by ' »
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Offline SachsGS

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RM125 Rebuild
« Reply #64 on: April 20, 2012, 05:34:25 AM »
I know through personal experience those early 80's RM125's have very little (as in "non existent") bottom end so I'd almost be tempted to go the other way - a slightly smaller carb.In 2010 I built a vintage Sachs 125 motocross and as that engine has a decent amount of lowend torque I upped the carb size,the bike became more temperamental but retained most of the lowend with a significant increase in topend. ;D
« Last Edit: December 31, 1969, 04:00:00 PM by ' »
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Offline TMKIWI

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RM125 Rebuild
« Reply #65 on: April 20, 2012, 05:50:38 AM »
What the hell is bottom end on a 125. ;D
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Offline SachsGS

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RM125 Rebuild
« Reply #66 on: April 20, 2012, 04:13:35 PM »
Ride a Husky WR125 - you'll be surprised! ;D
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Offline ford832

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RM125 Rebuild
« Reply #67 on: April 20, 2012, 06:16:37 PM »
My 02 KTM 125SX had remarkably good bottom end for a 125-and a 38mm PWK carb(you read that right) so a lot depends on many other factors than just displacement I guess-it's not normally the case from what I've seen though.If you could find one it may not hurt to do what sachs suggested (did I just say that? :o ) and try a smaller one,then maybe a larger one as well,just out of curiosity-mine mostly-seeing as though it's your money  ;D 
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Offline 2T Institute

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« Reply #68 on: April 20, 2012, 11:53:03 PM »
If the pipe is light it should work good, but leave the step at the exhaust/flange (if it's not to late) a 2-3mm step actualy creates bottom end and helps a bit with top end. On modern engines the step is put there deliberately. Something you can do is grind down where the liner meets the transfer ducts so instead of a small radius you have a large one. The larger the better  ;)
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Offline TMKIWI

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« Reply #69 on: April 21, 2012, 07:10:49 AM »
The step was in the flange not the port.
In other words it was restricting the port as in was a smaller diameter. Only on about 1/3 of circumference.
You are talking about an oversize flange or larger diameter. Are you not.?
« Last Edit: December 31, 1969, 04:00:00 PM by ' »
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Offline 2T Institute

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RM125 Rebuild
« Reply #70 on: April 21, 2012, 10:07:16 AM »
No I'm talking about the where the exhaust flange bolts to the cylinder, it's a deliberate mismatch and is far from a 'restriction' IIRC the RM pipe exits on the left with a angled exhaust duct to the left? Was the step was on the short length into the exhaust duct?
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Offline TMKIWI

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« Reply #71 on: April 21, 2012, 11:09:36 PM »
The exhaust duct is straight out. No angle.
But yes the step was on the short side of the pipe.
I have ground it out on the flange, but I do have 2 more flanges.
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Offline 2T Institute

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« Reply #72 on: April 22, 2012, 02:47:58 AM »
I keep thinking of the old air cooled models. I see the flange was smaller than the exhaust duct outlet diameter????? Have I got that right?
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Offline TMKIWI

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RM125 Rebuild
« Reply #73 on: April 22, 2012, 08:12:15 AM »
Quote from: 2T Institute on April 22, 2012, 09:47:58 AM
I keep thinking of the old air cooled models. I see the flange was smaller than the exhaust duct outlet diameter????? Have I got that right?

Stop confusing me. ;)

The diameter was actually the same but the 3 flange holes were not centre so it overlapped on 1 side.
Not anymore. ;D
Now it has a smooth transition from port to flange.
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Offline 2T Institute

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RM125 Rebuild
« Reply #74 on: April 22, 2012, 10:27:43 AM »
Needs a step there at the meeting of the flange and the cyl, works much much better than a smooth/matching. A Honda RS125 looses 0.5- 1HP when you remove the step.

About half way down.

http://www.bonavolta.ch/hobby/en/moto/rs125r.htm
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