taking down my 1980 Rm125t for the winter, was going to take off the cylinder and inspect the top end and put reeds in it. Its on the original bore, this is my first vintage bike so im just really not sure what to look for when inspecting the cylinder and when to know that it needs to be bored to the next size. Also would i clean this up like a nikasil with some scothbrite?
400 then 600 grit wet sandpaper on the bore, check ring end gap and piston clearance to spec as well as overall visual inspection. Chamfer the transfer ports and shine the exhaust port with a craytex bit.
DH, I would recommend measuring piston clearance and ring gap. That will tell you what you might need to replace. I would guess rings minimum, but could need a piston as well. 125's need these items more frequently due to the RPM's you run them. The reeds, being 45 years old, would be a good investment. That is an super clean looking bike! Someone has taken excellent care of it!
Nice bike! Pull ignition cover, grab rotor/ flywheel, check for movement, up/down/in/out… if play replace mains & seals… the old Rm”s are known for the bearing pockets coming loose within the cases…if replacing mains inspect the pockets. perform a leakdown test to make sure no air leaks….
I restored the bike last winter, but it was a very low hour farmers bike when I got it. at that time it got mains and seals and a new piston. I raced it twice this year so decided to take the cylinder off again and replace the aluminum reed and see how the piston looked. This is my first bike with a cast iron bore was mostly unsure of checking the sleeve for wear and how to finish it. But I think you guys got me going in the right direction. Thanks
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That bike has low hours. I wouldn't be too concerned over wear. If that bike starts running horrible out of no where,it's your stator.
I think I’ve read something about the stators being not great on these bikes. I only race this bike 2-3 times a year so hopefully don’t have any issues. Only certain parts seem to be a problem getting.
Hi, Ive had a 78,79 and 80 rm125s for 40+ years racing them. If you are traveling to a race any distance,,it pays to have a rewound stator along. Ive used custom rewind. The varnish on the stators breaks down with heat and age,,leading to misfires when they get hot. Imagine traveling to that 1 race and all the expenses and buildup to have the bike conk out on last lap while doing well? Ask me how I know?!? Its not worth not having a new one in it and spare. I have a good rewound one on bike and spare in van.
the intake boots fail with age,,check it for cracks around the clamp. The cranking compression should be 140+. If less,time to freshen. A DG head one will be 160+ using the thin DG head gasket. Also,,,dont fall into the 50-1 premix trap. Run 20-1 or 24-1. Stock jetting is with 20-1 and the oil removes heat from bearings as well as seals the rings. These have relatively small main bearings and trying to pinch on the oil is harder on mains and rod bearing. I dont care what the oil bottle says...use 20/24-1.Also if you are riding it limited,,,I always run 110 sunuco. It stays fresh a long time- like over a year between starts and will light right up. If you have any pump gas,,even a mix in tank,it will go bad and need a carb cleaing and not run as consistant. Pay close inspection on the swingarm at pivot bearings. They crack there from stress and a loose chaing grinds through the guide and kills it too.
I ran 32:1 this past year and it seemed happy the piston and cylinder looked really good. I’m not even sure what stock jetting is I just have it set to what I had read online somewhere. I’ve replaced both intake boots. Do you just find old stators and have them rewound? I’ve replaced the chain slider and made a new guide. One thing I’ve never been sure with this bike is how tight or loose to make the chain.
The oil is not just for cylinder and piston. Bearing life. I learned what Im sharing, from racing and going right to Eric Gorrs house and picking his brain when getting mine freshened.
I take a old one and send it to custom rewind in Alabama. Take pictures of it before it leaves so you can confirm the coil wires and orientation are same when it comes back. I learned the hard way when one came back 180* out. I dont care if a ebay one is 'like new' or its new(chinese),,or NOS...nothing beats a fresh rewound one,done in USA. Remember the coating can break down from age sitting in a suzuki wharehouse and short.
I use foxs on mine too.Get a big friend to scrunch down the bike so the swingarm/axle/pivot bolt are in line. Then set it so small amount of up/down play. Then when your buddy gets off lift it up and youll have your loose setting going forward. I have so any old bikes I hate to guess a number
Thanks man. So you’re running 20:1 with stock jetting? I believe I have a 250 main in it will have to see what pilot I have written down.
Funny you say that. Back in the day my bros were the local fast guys in our town and this kid shows up with one of these 1980 RMs. My brother had a 79, which was basically the same bike. The chain on this kid's bike was as tight as you could make it while it was on the stand and we told him he was gonna cause some damage. He blew us off cause he was a know it all d-bag, and he hits the first jump and it pulls the countershaft right out of the motor. One of those deals I'll never forget.
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