Vintage Dirt Bike Q & A

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PostPosted: Fri Jul 13, 2007 2:15 pm 
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Joined: Fri Jul 13, 2007 9:08 am
Posts: 10
I have a 1982 Suzuki RM250Z I've just restored. It starts and runs great, but craps out from 1/2 to WOT.
http://www.bikepics.com/pictures/947743/
http://www.youtube.com/watch?v=GbkZN2Md6xQ

It has the stock jetting and air box, no holes drilled. I ordered all new pilot jet, needle, needle jet, and 2 sizes of main jet smaller than the stock 290. That's the only thing I can think to do is to replace everything and try a couple sizes smaller main jet.

Moving the needle clip all the way lean doesn't solve the problem. I'm not crazy about running it with the air filter off, too dusty in my yard. But I've tried the other end of the spectrum by turning on choke once its warmed up and it gets worse. So it seems rich 1/2 to WOT.

Symptoms are that it sputters and does kind of a back-fire when you get up in the throttle past 1/2 way. I have some dents in the pipe and there may be a small hole on the seam, would this cause so much trouble?

It starts, idles and revs fine when not moving. Choke is working correctly. I got this bike in pieces though so I've never seen this carb run on a bike.

I read somewhere that a worn needle valve will cause the problem I'm having, so hoping new parts will fix it.

Any help would be appreciated, I haven't worked on 2-strokes for more than 20 years so I'm a little rusty but pretty solid under the rust. :D

Thanks,
Marc


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PostPosted: Tue Sep 23, 2008 7:57 am 
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Joined: Fri Jul 13, 2007 9:08 am
Posts: 10
Wow, no help here for a year, WTF. Good thing I don't give up. I got the bike running perfect now. New carb, new Jemco pipe, new racing coil, new new new. The new Mikuni came from Vintage Suzuki. I actually have an idle now because the newer version of these flat sides come with it! Be warned if you buy a new carb from VS, the jets and needle it comes with are nowhere close to what you need even though it's being sold for a specific bike? I ended up at 330main, 65pilot, 6FJ40 needle clip on second from bottom, air screw 1 turn out.

I'm nearly done with my second 1982 RM250 now. Come see me building it on my shop cam here: http://www.youngerdryas.com/1982_suzuki_rm250/


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PostPosted: Sun Nov 30, 2008 2:59 pm 
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Joined: Sun Nov 30, 2008 12:10 pm
Posts: 1010
Location: Connecticut, USA
Marc,

Thanks for the jetting info, much appreciated. I tried a 4.5 throttle valve on mine, but I went back to the stock 4.0. Right now I'm running a 280 main and 55 pilot. I got mine to idle by taking all the slack out of the cable and putting a little tension on it; the idle changes a bit as I turn the bars, but so far so good.

My bike has a strange sort of dead spot/slight hesitation in the low midrange that doesn't seem related to jetting. I'm no MIT grad, but I think it might be some kind of intake tract resonance. I'm going to try using one Boyesen dual reed and one stock reed and see what that does (if anything), and then I've got something I'm dying to try; the complete boost bottle setup from the 80's YZ's, along with an eBay manifold I can butcher to make it work. Maybe one of these days I'll find the time to actually do it. :)

I also think the reed block square rubber gasket arrangement has a tendency to leak, so after I do the reeds I'll Permatex it back together.

I've been riding over 30 years and have had some really fast and pipey bikes, but this bike still scares me every time I ride it.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric


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