1985 rm250 General help

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3/12/2018 11:31 AM

Hey folks, new here but I've been reading a bit. I just bought an 85 rm250 project. The bike runs but barely. The PO said it had the wrong carb on it. So first things first what is the correct carb? The one on it now says dekni made in Japan. Number on it is dd300g03. It looks like it has an electric choke on it and it's just kinda plugged into the intake with some hose clamps.
Second is the compression. I haven't tested it but the kick starter is super easy to push. I could do it with my arm really but it doesn't fire until I've nearly worn myself out kicking it. Should it be this easy to kick over?
Third is the exhaust. It just kinda hangs there. Fitting into the exhaust port it's loose like nothing else. If it weren't zip ties to the frame it would just fall out. I assume the exhaust should fit nice and tight so I'm thinking it must be something not original to the bike.
My goal on this bike is just some mild trail riding and the price was 400 bucks so I couldn't pass it up.....i don't want to have more than about 1000 more into it. Any thoughts on where I should start and what to do? I'm good mechanically but not at all familiar with old dirt bikes. Photo


3/12/2018 12:11 PM

Looks like an 83 model.


3/12/2018 3:53 PM

Really? Cause of course it didn't come with a title. I got the number off the engine. J102-113495. I really don't know anything about it.


3/12/2018 3:54 PM

The vin tag on the frame isn't there, painted over where it would be


3/12/2018 4:47 PM

Yeah the bodywork put it at either 82 or 83 . Yours looks like it has aluminum struts and I'm not sure if the 82 did, the 83 for sure did. The 84 was a redesign and the 85 had a front disc brake.


3/13/2018 3:07 PM

So do you know what type of carb I should be running?


3/13/2018 5:22 PM

I think it came with a 38mm flat side mikuni.


3/13/2018 6:17 PM

Thumb throttle, nuff said.


3/13/2018 6:25 PM

you have a 83 RM250. The yellow forks with the goofy front brake cable is the give away.
Fix the dent in that pipe. I have a PSI on my KX. Runs great. you need the correct tank. How do people create such messed up contraptions?


3/13/2018 7:27 PM
Edited Date/Time: 3/13/2018 7:38 PM

Yep - what MaxPower said. Its an 83, with what looks like an 89' or 90' fuel tank. Get the correct tank, and side panels, fork boots, and a silencer and that thing will look worlds better. Also, as far as no compression - that sounds like something wrong with the top end. Either worn piston/rings/cylinder, or something scarred the cylinder. You'll have to remove the cylinder to be able to tell.
As far as the pipe goes - there should be a mount towards the top of it that bolts into the pipe and frame. Springs should hold it to the cylinder also. The silencer (which is not there) holds the rear of the pipe in place.
Also, I just noticed that it doesn't have the airbox or air filter, which means the prior owner probably sucked dirt in the carb which pretty much destroyed the top end at minimum.

Here's what its supposed to look like:


3/14/2018 1:42 AM

a 400.00 vmx bike isn't always a deal the low price may make you think it is. The 3000.00 ready to race bike probably is. Most of the time you won't get your money back and definitely you wont get your effort returned on a nice restoration.
The good thing is the Floater 250 is a good machine for vintage racing


3/14/2018 6:36 PM

Progress update. I took the top end off and measured the piston. 70.69mm. The cylinder looks pretty good with no scratches. Rod has a slight side to side movement but it's very minimal. No up and down movement. Not sure what size piston to get though. I'll measure the cylinder and add cylinder to piston clearance to figure size of new piston I guess. Gas tank was just zip tied on, I'll get a proper one for 83. Also I'll pick up a mikuni 38mm flat slide and air cleaner. Gotta change out that thumb throttle too. I'll keep you all posted. Thanks!


3/14/2018 8:47 PM

Glad to hear the cylinder is good. Get the correct piston/rings, and it should have better compression. Stock bore is 70mm. 2nd oversize is 70.5, 3rd should be 70.75. Sounds like it is on the 3rd oversize, but measure the cylinder to be sure.


3/15/2018 6:29 AM

MaxPower wrote:

you have a 83 RM250. The yellow forks with the goofy front brake cable is the give away.
Fix the dent in that pipe. I have a ...more

Unfortunately this bike has the wrong pipe on it (maybe '84/'85?). The PSI is a nice period performance part.
The 1982/1983 RM250 pipe crosses over and connects to the muffler on the left hand side of the bike. See mxrose3 bike pictured above.

I 100% agree with you on the cost of buying vs. building. Luckily I enjoy the process of restoring these old bikes. I don't mind putting in the effort. It's part of the appeal.
I see the price '82 RM's are selling for and wonder why I didn't buy a good runner. It would have been cheaper than my build!