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New to the tread, but long time reader. Need 89' YZ250 help


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I picked up a 89 yz250 dirt cheap ( traded rc Boat and not a great one) was a basket case and ebay has been my best friend) rebuilt bottom end and replaced main bearings and everything is good down there, i honed the jug and replaced the rings and ring gap is good and everything looked clean, so i believe my problem is in the carb as the reeds are good as well as the powervalve assembly, no slop. kick start is a little sloppy so was hard to start at first, i pull started it and after that it starts fine when warm. i never adjusted the carb settings as they seemed fine and i mixed the oil just a tad heavy for the first gallon. then gallon 2 i mixed 24:1 and leaned the air to fuel screw on the right side of the carb which gave it a more snappy responce but not overly lean as it still sounds good but my problem is it is just not making the power it should and i really dont know much about bike carbs and jetting and really dont know where to start, i did rebuild it and make sure everything was free and clear but made no adjustments. it is hard to start cold but i do manage to get it started within ten kicks if i dont flood it. I turn the bike on its side at first until a bit of gas dribbles out the overflow then it give it a kick with the choke on some times it starts sometimes it does not. but once warm it starts first kick everytime, is this part of the main bearing, still being a bit stiff? I know it should be making alot more power than it does, it as fast as it shuld be but does not get there like it should, this bike should be lifting the wheel in third but hesitates in first. any ideas and things to look for would be great.­čĹŹ

Jeremy

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Stolen from another forum site: (1989 yz250)

The airscrew is for fine tuning, but it isn't the whole equasion. Your bike came stock with a 60 pilot jet, a 340 main jet, and a #62 needle.

SInce you already cleaned out the carb (my first suspect) and you already checked the compression (my second suspect), then lets turn our attention to jetting. That carb will need to come off again

Please pull the carb and check the float height. Also, please check the float needle and make sure it isn't deteriorated on has grooves worn into it. Also, pull the main jet and see what number it is. Do the same with the pilot jet. Finally, pull the slide out and check to see which needle it is (it's stamped on the needle) and in what groove the clip is presently in. That will tell us if your jetting has been messed with. WHen you put your carb back on, make sure to re-install all the vent lines and make certain that they are clear (best way to ensure that is to try to blow through them).

Once we have made our way through the carb and ensured the jetting is at least 'close' - we can move on to other areas if the problem still persists.

When testing the performance of this bike as we go along, please make sure to use a clean/fresh NGK B8ES spark plug (not the BR8EG/BR8ES), and fresh fuel. Also, when you start the bike, make sure to warm it up until the radiator feels hot and so does the cylinder before you take off riding.

One final note - when the bike is off, make ABSOLUTELY certain to turn the fuel valve (located on the bottom of your fuel tank off) - if it is left on, raw fuel can overflow from the carb into the motor, 'loading it up' and it would cause your bike to run poorly until it is all blown out.

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how do i get notifications when threads im involved in are replied to? ill get pics tomorrow, wish i had pics when i got it, it was a pure mess and all the parts were in Jewel Bags but i was too eager to get i back together and never took any pics of the process.

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The needle valve is a 62 and the e-clip is on the second position from the top. would moving it down lean it out? and the main is 370 and the pilot is a 50. where to go from here i ride in The lower great lakes areas

Edited by riverat1540
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i dropped the needle to the top slot but dont think it made it better, ran air fuel mixture 1.5 turns out. compression is at 140 did not seem to want to go much higher after further kicks. still seems to be lacking snap. it will pull the front wheel up in first but nothing else. the power band does not feel aggressive as i have had a cr80 and a 250exc and this bikes pretty week in bottom and midrange power,so the jets have been changed from stock then, where should i go from here?

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The needle valve is a 62 and the e-clip is on the second position from the top. would moving it down lean it out? and the main is 370 and the pilot is a 50. where to go from here i ride in The lower great lakes areas

here they are, what should the compression be, i only replaced rings the piston was fine cylinder looked clean, honed it and reassembled.

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I am not an expert on this bike, but I think your compression should be around 180-210 psi. Maybe someone who has owned one will know a little better or maybe you just need to let the rings seat and test it again.

Regardless, I suspect your jetting is way off.

I don't know what the altitude is where you live, but I suppose the great lakes area is less than 1000 feet. The pilot jet stock is a 60, you have a 50 in the carb. The bike is probably very lean off the bottom, that is why you can't start it cold.

The main jet stock is a 340, you have a 370 in the carb. The bike is probably very rich on the top, that is why it probably just blubbers when you grab the throttle.

My suggestion, RETURN TO STOCK JETS!

Adjust from there. You seem to have the right needle, you can play around with needle position once you have the stock jets. Oh yeah, get a new air filter and a clymer manual if you haven't done so already.

Hope this helps. Good Luck!

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thats exactly what it is doing and the ring very wel could be seating, ill check it after some more riding, right now i have t o deal with the plastic spring retainer on the kickstart shaft and i also found a busted tooth on the water on the gear that attaches to the power valve in the clutch case. ill get the stock jets and go from there.

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I am in the middle of restoring my 89 yz250 thats been sitting since 2003. First off you have a 88 or older yz250. 89 was the first year for inverted forks. So you really need to find out what year your bike is first, to get the right specs. Your bike also has a steel sleeve. The sleeve WILL wear and 99% of the time they need to be bored out to the next size along with a new piston and rings. You need to find out what size is the bore of your cylinder. Now your bike has 140 lbs of compression. That is way to low. 180lbs is when you should be thinking about new piston and boring the cylinder.

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Good eye It is in fact an 88, i was told by the other owner it was an 89 but after looking at the title it is in fact an 88' still having trouble finding a filter guide, but also explains why the 89 throttle cable i bought is too long. thanks

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Ebay will have just about everything you need. Just type 88 yz250. Look under buy it now and auctions only.

I'm still worried you need to bore out your cylinder. Your not even sure what the bore is. So how did you know what rings to buy? There are different rings for every overbore. Your bike could be .040 over. Who knows.

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im very aware of the oversizes,its a standard bore. the piston is standard size so i bought rings based on that, i honed the cylinder just to take off the glaze but may have left imperfections. my compression being this low makes me think the rings may still need to seat or may need to be bored over as well, how much does the machining cost? i am going to change the jetting back to stock, ride it some more and see if the rings seat in over some time. otherwise ill have it bored out to the next oversize. i found a cage looks like the one i need. short on funds and broke the spring guide for the kickstart inside the case and also found a tooth missing off the powervalve actuater so ill most likely just do the clutch as well as i cant figure out why it drags. may also paint the frame while im bored.

Edited by riverat1540
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agreed, im about 400 deep including my trade value, so think i will rejet it clean it up good and sell it if it runs decent and tell them it needs a top end as i dont think this bike at its age will bring more than 1000 at its best, so if i can make it look nice and run better for not spending much more ,id like to sell it and get something newer perhaps a project as well if the price is right. i might even have the top end done if it will be worth paying for knowing im not planning to keep it.

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LOL after suspension gets revalved and resprung on my 89 I will have about $1700 into it. But I'm also replacing all bearings, powervalve, rear wheel/spokes. New pipe/silencer and powdercoat fame. But I'm also gonna keep it for years to come too soo the money really don't bother me cause I'm gonna get it back out of enjoyment.

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