91 RM125 Awoken from a Long and I mean Long retirement

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9/8/2018 4:24 AM

So we rebuilt my friend and my own cause we split it in middle school, 91 RM125, man this thing needed a top end ?. So we have a Yamaha demo day today at Former Gotham Mt, and have a extra rider coming not for demo day but to ride who didn’t own a bike. So we got this one ready at beginning of week knowing he’s coming. Took the bike apart changed what needed to be changed, cleaned the carb, and all we do for Our own newer two smokes. Bike kicks and goes threw the gears and all. But the little fucker won’t hold a idle, it will ride but it’s a bitch to find that idle spot. My question is will this harm the bike and what would most likely be the problem? Like is the carb vacuuming itself? Or a air leak at the clamps? How would I diagnose these problems? Mostly can I ride the bike today? Would it harm the top end much? Literally need it for the day and that’s about it will disassemble again if I really have to.
Thank you and Ride Hard, have fun!!!


9/8/2018 4:47 AM
Edited Date/Time: 9/8/2018 4:50 AM

Easiest way to see if it's an air leak is to get it to idle (at any speed, usually a real high rpm) and spray some wd40 around the clamps and fittings. If the idle drops youve got an air leak right there.

I had a bike that wouldn't idle but ran great at 1/4 throttle on, it ended up being a stator going bad. Not enough juice at low rpms for the plug.

Good luck

Lastly ... i have to think it easily could be a crank seal after sitting that long.

Again best wishes for a fun day.


9/8/2018 4:57 AM

Ok thank you. Yeah def something but if it’s good for the day and all I have to do is check for a air leak while I’m chillen then cool. Will do and will let you know my findings. Just run it? I mean I ran the bike for like 15 min in the streets it was fine just wouldn’t hold idle much. It did then it die next time I had to get off the gas and wait at traffic.


9/8/2018 6:42 AM

Might be electrical.
But I've found when a bike is put away, especially if the tank hasn't been drained, and the fuel in the carb hasn't been
cleared out by running it until dies, the gas left in there turns to a kinda gum at first. Then hardens to varnish.
Some of those passageways, especially the low idle ones, are real small. It's almost impossible to unclog them.

I mess with Vintage Honda 750s mostly now a days.
4 carbs per bike.
That all should be delivering the same amount of fuel at the same time, at all throttle positions.
Try getting that to happen when they have been sitting in a guy's garage, half full of gunk, for 30 years.

Look down the barrel of the carb, with the slide out. If there are any real small holes in the side, chances are idle passage is clogged.

Small guitar strings are good to try and pick that crap out, along with 100psi compressed air. Starting fluid is good for cutting that stuff. Some guys soak them for days in acetone to loosen the stuff up. I've heard of guys boiling them in a pot of water for a few hours to clear idle passages. And this one, you gotta be real careful, Tidy Boil toilet bowl cleaner, mixed with water, soak the carb in it for 10, 15 minutes. Too long and the carb will dissolve altogether.laughing

Whatever, if there are small passages going from the inside of the slide|barrel, which end up somewhere in the float bowl area, you gotta get that stuff out.to get clean, adjustable idle.

Good luck dude.

9/8/2018 10:19 AM

Reclean the carburetor.


9/8/2018 10:36 AM
Edited Date/Time: 9/8/2018 10:48 AM

I'd recommend soaking the carb in Berryman's B12 carb cleaner for a day. Been using it since the 80's with good results. I'd disassemble the carb & soak it, also try not to put any part of the carb that has any seals or remove them.
As mentioned above start with fresh premix & flush the tank / fuel lines as needed.



Speak softly and carry a big stick.

9/8/2018 11:59 AM

H4L wrote:

I'd recommend soaking the carb in Berryman's B12 carb cleaner for a day. Been using it since the 80's with good results. I'd ...more

+1...Berryman's B12 is the way to go...


Many thanks to everyone helping me out this GNCC season: SRT Offroad, Acerbis, FCR Suspension, O'Neal Racing, Evans Waterless Coolants, Rekluse, Twin Air, Braking Brakes, Carbsport

Profile image credit Ken Hill Photography

9/8/2018 1:03 PM

Adjust the idle screw to where it will idle(even if fast), then turn air screw full in 1/4 turn at a time (in is less air) till it starts to blubber. If that doesn't help, back the air screw back to 1 1/2 out and pull the choke to half choke. If it runs, the pilot jet is plugged. Simple.

To check for air leaks,
Instead of spraying WD 40 all over and making a mess, use brake cleaner. Same method, same result, no mess.


My post is my opinion. If you don't agree with it, I'm OK with it.

9/8/2018 2:46 PM

Well this fucking moron as my friend cause this is where it was at for the past 13 years since it’s been kicked over decided 13 years ago let’s fucking drill holes into the boot and that entire area. After trial and error I found out that and I have suckin coming from the basket so, rebuild kit prob won’t harm it. Now for the bitch part starting this fucker. Finally after 10 min it started. Bike ran fine actually tired to idle after riding a few times it held so odd. Anyways gonna break her apart from carb back, since all else checked out. I’ll let you guys know my findings