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1989 yz250wr set up help please

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Hi all,

This is my first post here, but I have been using the site for a while now and would just like to say to you all what a great site you guys have here.

Ok so on to the topic of my post....

I have recently purchased a 1989 yz250wr for £400, things that are not stock are

  • pro circuit pipe
  • prox conrod kit
  • 1mm oversize piston
  • twin air filter

I have a running issue with this bike and that is when you get it started up and turn the throttle 1/2 open or more the engine begins to bog down, drop below 1/2 and it runs ok but doesnt want to idle and exhaust fumes smell quite heavily of unburnt fuel.

My research would suggest jets are too large and I have been having trouble tracking down what size the stock jets should be (riding in uk , not much above sea level and mostly trail riding).

The compression seems good and silencer has new packing fitted, running 32:1 premix, air screw is set at 2.5 turns out from seated which again suggests needing a smaller pilot jet.

Carb is thoroughly clean along with the tank, fuel tap and filters, needles etc look to be in good condition.

Any help or tips you guys can send my way would be great (or anything I am doing wrong please correct me lol).

I have attached a pic of her, she needs tidying up, dodgy paint job has to go and not keen on the graphics - but nice project for this winter :)

thanks again guys and keep up the good work here.

my bike.jpg

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Based on an online parts fiche, the 89 YZ250WR shipped with a 340 main, 60 pilot.  Check to see how worn the slide and emulsion tube is also, my 92's VM36 was sol tired that no amount of jetting would clean it up.

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Hi, thanks for the reply.

When looking at the slide there are some wear marks showing and I have read that these mikuni flat slides are a pain in the ass.

When funds allow will swap it out for a pwk.

Would like to try and get her running as best I can for now, I will check the float height is correct (18mm i believe)and see if I can find what size jets I have in at the moment now I know what stock jets should be.

Thanks again :)

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the picture you posted looks like a 1990 to me. I have an 1989 wr 250 the plastic is much different.  I live in California at just a little above sea level.  I run pre mix of race gas and motul oil at 50:1.  I had starting issues when I first bought the bike used last year.  I put in new crank seals, new boyseen  reeds. bike still was hard to get running even when hot.  the carb is a TMS mikuni.   The jetting was stock but I now have it running great with a 25pilot (it had a 50), air crew is at 1 3/4 out ,  stock needle jet(R-1) Jet needle (6DJ8-60)clip in third position , 4.0 slide ,  and a 330 main jet.  Since I went to the smaller pilot the bike starts within 3-4 kicks cold and 1-3 when warmed up.   The plug is light tan , and there is no goo coming out of the silencer anymore.   Just wanted to share my info. I hope it can help. 

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Hi thanks for your help also.

I know the plastics are wrong as the rear side plastic only matches up to one of the frame mounts and the radiator scoops mounts only line up with the tank mounts and not with the radiator mounts.

I only say its an 89 as that's what Yamaha themselves told me from the engine number as frame number is missing.

Definitely starting to get some good info on jets etc from you all , it will be a great help.

One thing do you guys think what I am describing sounds like over fuelling?

Thanks once again :)

Edited by Holberg

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Ok so a bit of an update, checked the float height , float needle valve and those are to spec.

Main jet is 370 which I assume is far too large ?

Pilot looks to be a 55 from the markings stamped into it, although they were difficult to make out.

Throttle needle is Oem spec in third clip position.

So going to get some new jets, would it be an idea to say go down to a 340 main as per Oem spec or would you guys suggest that would be too much to drop in one go?

Had her running for a few minutes today, but plug is wet through and still bogging down and stalling at anything over 1/2 throttle

Cylinder is 1mm oversize and running a pro circuit works pipe on standard uk pump fuel and 32:1 rock oil per mix.

Thanks all.

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It sure sounds like you are on the page as far as jetting goes.   Have you looked to see if you are using up transmission fluid?  You could have a bad crank seal on the clutch side.  If the seals are good then try a 340 main if you have one, and see if that helps.   You should also check the reeds as well.   Good luck!

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Thanks guys, really like the bike she feels like she will be a great trail bike when I get her running right, I used to own a kdx 200 till it was stolen and I loved that bike too.

As far as this one goes I have done all the crank seals, reeds appear to be good, so hopefully when I get some smaller jets we should be good to go :)

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well after some swapping and chopping with jets am still no nearer to getting her running properly.

so over last couple of days have broken her down for a rebuild over this winter, have stripped the monoshock ready for rebuild to include uprated spring to suit my weight, replacing all swing arm bearings as some of them had collapsed, frame needs stripping and powder coating (back to white I think), on the look out for a keihin pwk to replace the mikuni, front sprocket on order, front tyre needs replacing (which I have ready and waiting) and forks need a rebuild.

Been busy polishing up the swing arm, silencer and clutch cover - which I will lacquer to keep the shine

All in all a fair list to be getting on with but come spring I hope to have myself a nice trail machine.

Thanks to all the input from you guys.

will take some pics of the rebuild as I go along and post them up for you.

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