Mystery YZ250 Problem

[Crazymlh]

I have recently purchased a 98 YZ250. When I first got it and took it for a 10 minute spin it when really well, strong power, crisp throttle & scary wheel stands. I checked the plug after the ride and it was a nice tan colour. Since then I have spent another 15 minutes riding on it, which by the way was on the fuel left in the tank from the previous owner, I topped up the tank with some 32:1 premix a mate had left for me and it still ran really well. I always noticed it was a little smoky so when I needed to fill it up again I changed the mix to 40:1 and it seemed to run really well.
Then my wife, who is learning on an XR200, wanted to go for a ride with me so I had to putt around and I could feel the YZ starting to splutter when I gave it some throttle and could feel the plug starting to foul. By the time I made it home the plug was dead and no spark at all. Replaced the plug with a brand new one of the same type but the bike still runs like crap. It splutters and boggs down then rips hitting band then boggs and so on and is a total pain. The plug is now a black not so wet colour.
Things I have tried so far are replacing all the fuel back to the original 32:1 mix, Cleaning the twin air air filter with the correct air filter oil.
Ever since purchasing it it would not idle with the choke off at any temp and after a chat with my local dealer they said that this was normal and set up pretty well. I have not taken anything apart or messed with the carby as I did not see why when it was running so well then after a fouled plug it started to be a real pain. The only mods to it that I can see are a Pro Circuit pipe and silencer. Any help would be great as I so need to go riding this weekend.

 

[Rcannon]

I own the same bike. Mine really likes Yamalube R mixed at 32:1. If your carb is set up properly, the bike is easily able to follow an XR around afternoon. I have spent many afternoons following my nephew around on his XR 50. A 10 oz flywheel weight makes this bike come alive!

What jets do you have in your carb?

I would first check the reeds in your bike. These can chip or burn over time. This could cause what your describing. Dont change the mixture of your fuel/oil. Using less oil in the gas may vhave added to your problem rather than correcting it. Less oil equals more gas being burned in the engine. It works opposite of how we commonly think about the ratio!

The YZ is very sensative to the needle setting in the carb. Depending on the temperature of the day I may need to move mine 1 notch leaner. This time of year when it is cold, the stock center setting is fine.

Hopefully you have a 168 or 170 main. Either of these should be close enough to get started. The 48 pilot jet is usually decent as well.

Being that you just bought the bike, when was the last top end job?

Keep an eye on your powervalve. Over time the stop on this can and does wear. This lets the powervalve touch the piston. If you remove your pipe and look through the exhaust port you can see the area I am talking about. My used bike had this problem. The slight contact made a very notable polished stripe on the piston. I was lucky to have caught this early!

 

[Crazymlh]

Thanks for the advice. I was aware of the effect the extra fuel in the mix has in making it to "Fuel" rich. I think it was also bad timing changing the mix to 40:1 as we have been having some very humid hot days this week here in Oz which probably made the mix richer. I have a repair manual coming in the mail and have been hesitant in touching the bike until I have a reference. I still don't understand why this has happened as it was running fine with 32:1 and since I fouled a plug and am now using the same 32:1 mix, which is Belray semi synthetic, it still is playing up without me touching anything mechanical and only having at most 35 minutes running time since purchase.

 

[Rcannon]

I know we all look at bike problems like that. Its natural. Still, the bike does not know its new to you. Yes, it might have ran great before, but things change.

I remember reading about the spark plug cap. I have never had trouble with mine, but I hear it can be possible for the resistor to go bad in the cap. Being that you did remove yours........I wonder????

How about your airfilter??? Is it clean and properly oiled?

I worked on a Ladies Derbi 50 a few months ago. (Please dont tell anyone!)

She had ordered a big bore kit for the bike. 500.00 worth of parts. I offered to install them. Before she dropped the bike off I warned her....."Bring it to me with a dirty airfilter and I will send it back untouched. Dont try me on this. I am not cleaning your airfilter." She did.

I should have thought when she complained about the cost of airfilter oil, but I did not. The airfilter WAS clean.

Three days later the bike still will not run over 6000 rpm's. I then started to consider checking HER work.

You got it! She used the entire quart of filter oil. When I squeazed out 95% of the oil, the bike started running. :eek:

 

[Crazymlh]

I have already cleaned the filter. I assume I did it properly by cleaning it with air filter cleaner not kero or fuel. I then washed it out in warm soapy water, let it air dry for a day or so and then used K&N air filter oil to re oil it. I have been thinking about it and I will raise the clip on the needle 1 position to lean it out and see how it runs.

 

[souphmars]

i had the same type of problem on the exact same bike- i did everything, and then 1 day i sat down with the carb, jetted and adjusted all day until i got it running right- i rode it a few times, and then sold it- it rode good until i sold it

 

[jmics19067]

I have already cleaned the filter. I assume I did it properly by cleaning it with air filter cleaner not kero or fuel. I then washed it out in warm soapy water, let it air dry for a day or so and then used K&N air filter oil to re oil it.

K&N air filter oil isn't meant for foam filters don't know if it is a problem or not though

 

[Crazymlh]

Update

I had a close look at the carb and the lock nut on top of the slide cover, where the throttle cable comes down, was quite lose so I tightened it up. I also cleaned all the contacts where the coil bolts to the frame. It ran a little better so I started to wind out the air screw. It is now about 3 full turns out and it runs a lot better. Is this OK? It still boggs at full throttle but the throttle response is a lot better. I wouldn't call it perfect and is still not what is was like on it's first couple of rides but it is better. The plug has browned off a little but is still more black than brown.

 

[Rcannon]

Three out on the airscrew is quite a bit. I always change my pilot jet if I am anythign over two out. The stock bike runs well with a 48 pilot and 1 1/2 turns. It woudl be a good idea to clean out your float bowl and verify your pilot jet size.

In my mind, your bike sounds like the reeds are split or burned. I would get a set of boyesen replacement reeds. They are 30.00 american dolors or so. I have owned a V force reed (versions 2 and 3) as well as a rad valve. Neither of the aftermarket reeds are worth the money. The jetting with the aftermarket stuff is a nightmare as well.

 

[BradFrost]

I think the Cannon might be onto something there with the reeds. Maybe the pfffting around gummed them up. Crack those carby boots off and have a squiz...at least clean them if ya can't replace them.

 

[keithb7]

Have you taken the bowl off the carb and given it a real good cleaning? Is there possibly some gunge in the carb? How about in the bottom of the tank near the drain petcock? Is the screen in there clean? Boggin out on top end sounds like a main jet problem. How about float level? Everything ok there?
I'd be yankin the carb off to get a look at the reeds, then at the same time giving the carb a good cleaning. Keith

 

[Crazymlh]

Update

All the advice is good. I will be getting on the tools when I get my manual tomorrow. One thing I did do is went for a 10 minute ride then pulled the plug and had a good smell. It smelt like transmission oil. Having never smelt a plug before is that just the smell of a "too rich" plug from the premix oil or does the pre mix oil have a different smell? I spose what I'm getting at is could it be a crankcase seal gone and tranny oil mixing in and making it oil rich? Although I have been keeping an eye on the smoke from the exhaust and it doesn't seem to smell any different or notice any increase in the amount of smoke or the colour being any different. There is no great spoonge coming out the exhaust either. What do you think am I just being paranoid??

 

[Rcannon]

Have you noticed the tranny oil level drop?

Nevermind. I remember when the oil check bolt is on these. Buried behind the brake pedal....great.

Maybe fill it, go for a long ride, then drain and measure.

I hope your just being paranoid!

 

[Crazymlh]

I will be getting some more tranny oil tomorrow. I will measure how much comes out and compare to the 750ml I put in it not so long ago. The tank is clean and petcock is not blocked in any way. There must be some crap in the carby or something blocking it up a bit but I will be taking a look at the bore and piston/rings tomorrow and will measure everything and clean everything. I will check the reeds too.

 

[Crazymlh]

Problem Solved

I received my workshop manual and promptly realised that the stock Mikuni carb had been replaced with a Keihin PWM. Never the less I removed the carb and flipped it over and removed the float bowl. The first think I noticed was that the main jet was just hanging on to the last part of the thread. It was so loose that when I bumped it trying to screw it back in it fell out!!! No wonder it was running like a pig.
So I screwed it back in place and re adjusted the air screw to stock position and put it all back together. First kick and she started. Took it for a run today and after a slight adjustment of the air screw my beast is back!! It's amazing how such a simple problem has you thinking the worst. Anyway thankyou all for your input and advice.

 

[Rooster]

Great news! Don't you just hate it when you assume something is horribly wrong, only to find out there was a loose jet?

 

[Rcannon]

The manual is wrong!!!

You bought a Climer, right????

Did you notice the recommended jetting is EXACTLY what we are talking about? Seriously, I jumped to the same thoughts when I got my YZ. Your original carb is the Kehin!

The only mikuni used on the YZ's of that era was on the WR 250. Still, it was the old round slide Mikuni.

The Climer manual has many small errors in it. If at all possible, look on that egay auction site and find the real Yamaha version for the 98 yz 250.

There is also a missprinnt about some small fork bolt being torqued to 108 lf/ft.

Your bike is just fine and no one changed the carb!

 

[jrg]

i was thinking the same thing, i used to have a 98 yz and it had a keihin, one of the faster bikes ive own too :yeehaw:

 

[Crazymlh]

Well there you go. The manual is a Clymer and thanks for the heads up about the torque error. It's hard to believe they got it wrong about the type of carby, it really makes you think what else might be wrong in the manual!!.I just love this bike it just pulls and pulls through the power. Long live the YZ250 two smoke. :aj:

 

[Rcannon]

That manual thing was something. I cannot imagone how they missed that! I have noticed enough errors that I ended up buyign the original Yamaha manual. Used, from the auction site, should not be over 20.00.

Be sure and check your powervalve. http://ericgorr.com/techarticles/mxbooktun97.html

There is an excellent article on the powervalve problem that eric wrote. It is worth reading. It might save you buyign a piston!

 

[Crazymlh]

It's been a while but I spotted this old thread I started a while back and thought I would finish it off with what turned out to be the real problem.

Turns out it was the CDI. It had a vortex CDI on it and remember thinking "Oh It can't be the Vortex CDI ...they are too good to fail arn't they....?" But shure enough I actually tried a stock CDI when going through the fault finding process but only kicked the bike over by hand to see If the spark looked any different. Well after messing around with the jetting and what not I finally had the $hits well and trully and while sittiing on the bike trying to get it to rev I hit the CDI with my hand and ...... RING A DING DING DING then boog boog boog. Every time I hit it with my hand it would rev perfectly. So back into the shed swaped out the CDI with a 99 YZ250 CDI and have never had a problem since.