Flat slide carb on 78 rm125

78rm125
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78rm125
Edited Date/Time 3/27/2015 3:34pm
Im thinking of getting one for my bike. Trying to decide between a 34mm TM or a TMX. Also read that the factory Yamaha 125 from 98 and up had a half moon slide for better throttle response.
Anyone have a recommendation ?
My engine is stock, but getting the ports cleaned up before the season starts.
Thanks ,Chris
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mxrose3
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mxrose3 2/26/2015 5:36pm
2/26/2015 5:36pm
In my past experience with the non-powervalved air cooled 125 motors, you will probably want to stick with the 34mm or less carb. The newer YZ125 carbs are 38mm, which would be a bit big for that engine... Unless you want a top end motor.
78rm125
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78rm125 2/26/2015 5:51pm
2/26/2015 5:51pm
The TMX from mikuni is a 35mm. Also the 2000 YZ125 is a 36mm. Would those be too big ?
Looking for a bit more throttle response.
Thanks ,Chris
mxrose3
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mxrose3 2/26/2015 6:08pm
2/26/2015 6:08pm Edited Date/Time 2/26/2015 6:26pm
I tried putting a 38 Mikuni on my 84 RM125 and it gave it incredible top end, but took away any low end I had. Then I tried a 36. It was better, but still didn't have the low end of the 34. I ended up with a 34. If you can give up a little low end power, give it a shot. You should be able to find a cheap used one on ebay. Any of the flat slide carbs will give better throttle response than the stock round slide.
slomoto
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slomoto 2/26/2015 6:16pm
2/26/2015 6:16pm
I have a 79 RM125 Im thinking about putting a 34mm on. stock is a 32, ?

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mxrose3
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mxrose3 2/26/2015 6:25pm
2/26/2015 6:25pm
slomoto wrote:
I have a 79 RM125 Im thinking about putting a 34mm on. stock is a 32, ?
yes, stock is 32.... both on the 78 and 79 I believe.
MaxPower
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MaxPower 2/26/2015 6:26pm
2/26/2015 6:26pm
Depending on where you race a flatslide may be against a rule since it wasnt available in 1978 like the Vforce reed block was
78rm125
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78rm125 2/27/2015 8:12am
2/27/2015 8:12am
No heavy rules where I race. Thanks for the suggestions guys. I think Ill try finding a 34mm flat slide.
Chris
Mike P.
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Mike P. 2/27/2015 3:37pm
2/27/2015 3:37pm
I own a heavily modified 77 and heavily modified 80 RM125s. i have a 36MM set up by DG for the 77, but this motor was ported and tuned to run that big carb (I only tell you this because it can be done), but realistically, I would stick with a 34MM. If you are going to go with a flat side, then why not just buy a Lechron? They are very easy to jet (basically you only have to tune the metering rod). But, since you are not held by strict rules where you ride, I recommend a Lechron Powerjet flatside carb (34MM). They work great with vintage motors and way easier to jet than a standard carb. The only drawback is they can be a little expensive, but if you want to be able to set the carb up and basically not have to touch it much, then go with the Lechron. I do not recommend the newer 38MM TMX carbs because as someone else already pointed out, they really don't work well on vintage bikes as we have power valves and case reed motor and such.
78rm125
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78rm125 3/26/2015 7:38pm
3/26/2015 7:38pm
I ended up with a 1990 rm 125 34mm Mikuni TM . It has stock a 330 jet in it . Im assuming it will be rich on my stock 1978 motor.
Even if I change the premix ratio from 32-1 down to 20-1, am I still going to be over rich?
I going to run it this weekend ,if the plug is too dark ,should I put a #8 heat range plug down from a #9 ?
Thanks,Chris
AHRMA361 3/26/2015 8:19pm
3/26/2015 8:19pm
78rm125 wrote:
I ended up with a 1990 rm 125 34mm Mikuni TM . It has stock a 330 jet in it . Im assuming it will be...
I ended up with a 1990 rm 125 34mm Mikuni TM . It has stock a 330 jet in it . Im assuming it will be rich on my stock 1978 motor.
Even if I change the premix ratio from 32-1 down to 20-1, am I still going to be over rich?
I going to run it this weekend ,if the plug is too dark ,should I put a #8 heat range plug down from a #9 ?
Thanks,Chris
Going with a heavier oil ratio will lean you out more. More oil, less fuel. Fuel combusts, oil lubricates.

Start with a 300 Main and do some plug chops at WOT. Read the plug and adjust.

You may have more issues with the pilot circuit and needle position/taper than your main depending on your elevation.

Jetting can seem to be black magic, but you can figure it out....
Mike P.
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Mike P. 3/26/2015 8:23pm
3/26/2015 8:23pm
You can go with the #8 (that's all i use on all my RM125's, 77, 80 & 1990). I am going through my jetting whoas with my 1990 RM125 as I put a 38mm Mikuni TMX carb on it. I started it this week and it was way lean on the pilot at 40:1 so I I'll probably leave the premix at 40:1 and just richin up the pilot. Worse comes to worse I'll go back to the stock 34. Mike
78rm125
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78rm125 3/27/2015 2:42pm
3/27/2015 2:42pm
Thanks guys!
neverwas 3/27/2015 3:34pm
3/27/2015 3:34pm
Mike P. wrote:
I own a heavily modified 77 and heavily modified 80 RM125s. i have a 36MM set up by DG for the 77, but this motor was...
I own a heavily modified 77 and heavily modified 80 RM125s. i have a 36MM set up by DG for the 77, but this motor was ported and tuned to run that big carb (I only tell you this because it can be done), but realistically, I would stick with a 34MM. If you are going to go with a flat side, then why not just buy a Lechron? They are very easy to jet (basically you only have to tune the metering rod). But, since you are not held by strict rules where you ride, I recommend a Lechron Powerjet flatside carb (34MM). They work great with vintage motors and way easier to jet than a standard carb. The only drawback is they can be a little expensive, but if you want to be able to set the carb up and basically not have to touch it much, then go with the Lechron. I do not recommend the newer 38MM TMX carbs because as someone else already pointed out, they really don't work well on vintage bikes as we have power valves and case reed motor and such.
This

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