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90 rm 125.... 35mm or 38mm carb?

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I just rebuilt my old rm 125 ( 1990 ) and rode it twice and have been noticing that during low rpms the bike boggs pretty good and need some room and feathering to get the rpms up then watch out, it runs great..... thought it was a carb issue and seen one on ebay so I purchased it and turns out its a 38mm and mine is a 35mm... would it be worth fitting it on my bike or should i just deal with mine?

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No, going to a 38mm (most 250s in the era ran a 36mm to 38mm) will not help your problem. It will have zero low and mid range and then (if you can even get the jetting close) will hit every inch of power and rip into the top end range. If you want to increase throttle response and a little more low to mid range power, you need to go down in size of the carb. In other words if your running a 35mm, then a 34mm will increase the low and mid, but you will loose a bit of top end power.

In the 90s, 125s just did not have much low end range at all. You can mill the bottom of the cylinder to change the port timing or deck the head to bump up compression to get a little more, but its still not going to jump the low end power up to what your looking for. The bikes are designed to run in the high/mid to top end range of the power. You just need to learn to ride it on the pipe and keep the rpms up to make power and have fun.

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Thanks for the info Padgett. Would you know what bikes came with a 34mm carb or is that something aftermarket?

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Theres a few 34mm carbs on ebay but they dont say what they are from. Theys some flat slides like mine but dont know if they would fit or work.

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Actually to me it sounds like a normal 125. You abuse the clutch, ring it's neck, grab another gear and repeat. No one rides a 125 for the awesome bottom end. FWIW KTM has been using 38mm or 39mm carbs on it's 125's for the past 10 years (or more), so if top end power is more important then go for the 38. Or just spend an afternoon with a can of carb cleaner and a toothbrush.

Personally, I would just clean out the stocker and make sure that the needle and air screw is set where it should be.

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yeah, its just probly time to spend some $$ and get a 250, thanks for all your help......... P.S. I have a nice 38mm carb for sale

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yeah, its just probly time to spend some $$ and get a 250, thanks for all your help......... P.S. I have a nice 38mm carb for sale

Exactly what kind is it? Pic's please, or just PM me with the details.

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Thanks for the info Padgett. Would you know what bikes came with a 34mm carb or is that something aftermarket?

87 rm125's ran 34's i have a 34 with a stuck choke that i am going to fix up and I have a 35 on my 88 (which has no powervalve :bonk: )

Just stick with the 35, and get the jetting close :smirk:

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never really pissed with jetting a carb, but im sure one of my buddys can help me out with that....thanks

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never really pissed with jetting a carb, but im sure one of my buddys can help me out with that....thanks

well there my lay your problem, head over to the sticky in the jetting sub-forum and have a quick read through that, and see if you can make heads or tails of whats going on.

My 88 acts similar to what you are talking about, but thats because it has no power valves.

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