250r plug fouling question

[whiteman350x]

Here is what I have: 85 ATC 250r.

This things fouls plugs badly in just a few miles. The fouling is black and oily. Does not smell like fuel smells like burnt oil. Tried Yamalube 2r, and PJ1 synthetic. 20:1 then 32:1 then 40:1 still fouling. New piston w/ bore job. 142 main in stock carb. Ct midrange pipe & silencer. Needle clip 2 from top. New uni air filter. All air box tubes are clean. Do I need to change the main or the needle clip or something else? Tried 3 diff. heat range plugs. Using 93 oct. gas. Please help, going broke buying plugs.

isn't that alittle bit lean, my 250r ran a 170, alittle porting, old paul turner exhaust, stock everything else

[tecatecrazy]

have you checked your float height. I had the same problem with my tecate , just had the float set to high. Before I changed the float height it would backfire when I kicked it over and about breake my foot every time. just a suggestion.--Joe 8)

[TimSr]

Hey Whiteman, Im impressed, youve covered all the basics in your post so that some good information can be given! Sounds like everything is setup right, and looking at carburetion is the next correct place to go. The last question I would have is, when you go a few miles and it fouls the plug, are you running the throttle 3/4 to wide open, or is it more of a half throttle cruise? If your running it in the upper end of the spectrum, and its fouling, you need to drop the main jet size. Stock for that is 145 or 150, so you're near there now. Try a 140. If its fouling at half throttle cruises, but does okay so long as you keep it in the high rev's, you need to raise the clip and drop the needle. I prefer mine to run very lean in the midrange Anyways because this is where 2 strokes always load up.

One other factor in carburetion is winter weather, depending on where you are. In theory, cold air is denser causing you to run lean, however a cold engine will not burn fuel as efficiently, and may run rich. My Tri-Z, which ended up with the thermostat removed, has difficulty reaching proper operating temp in winter, and runs rich. The liquid cooled motors take a lot longer to warm up, and burn complete. If youre covering a few miles of trail riding, this shouldnt be a problem, but if youre crusing at half throttle at 50mph down a dirt road, your engine will have difficulty heating up properly.

Also, get a JC Whitney catalog, and order one of those $8 electric plug cleaners that hooks to a car battery. They work great, and will pay for themselves wiithin the first few plugs you clean.

if u ride to slow it could foulthe plugs i had a roblem with my 200s like that but it was because i forgot to turn off the gas when i put it away.

[ATCRYDER]

WHiteman,

Youve got quite a collection of machines there! Congratulations and welcome to the board....stay a while. :Beerchug

ROB

[whiteman350x]

Thanks for the info everyone. Tim Sr. the bike fouls plugs regardless of the weather. Down in Alabama it does not get really cold. Usually we ride w/ temps. above 70 degrees. We rode yesterday at 40 degrees though and that did not make any diff. Do not usually run slow for long distances. Ride open fields mixed with woods. But even on the sandbars on the river where we keep 'em reved out all the time, it fouls.
Thanks again for the help.

If you havent done so already, repack your silencer..Might be building to much back pressure..And if you smell "burnt oil"..like 90w?.. You might be getting gearbox oil in the combustion chamber :(

[whiteman350x]

The silencer is brand new. So is the pipe. I have replaced the crank seals. The gear box is not losing oil.
Thanks

[TimSr]

I guess winter weather is not a factor in Alabama!

Sounds like you need to start with the main jet if its fouling both at the top, and in the middle. The needle shouldnt be monkeyed with until you have the main jet dialed in. DK Catalog has the Keihin 5 jet kit for $19.99 sizes 130, 132, 135, 138, 140. Id bank on the answer to your problems being amongst that selection.

[Kilborg]

I agree with Timsr on this. If your main jets do not affect the fouling, train your transmission. Smell of premix any? I have a feeling your crank seals have gone bad. After your riding for about 15 minutes and not babying it, it will suck in a ton of transmission lube, causing the bike to foul. Some other signs of this (other then the tranny oil) could be excess splooge coming from the exhaust, losses of power after you have been riding about 10 minutes, and Tranny oil and Fuel mysteriously going somewhere.

[whiteman350x]

Sounds like I need to work on the main. Trans does not lose oil. Have replaced the crank seals already. This had no effect on the fouling.
Thanks for the info.

[TimSr]

If a crank seal leaks, it will smoke like a freight train, and get worse as it gets hotter.

[whiteman350x]

Does not smoke worse as it gets hotter. Just smokes bad when you really gas it.

[Bigdaddyjlove]

I feel I should chime in on this. I am Whiteman's sometime mechanical assistant and oftentime test-pilot.

The bike is STRONG. Good mid-range (for a two-stroke) amazing top end rush. This way to the fourth gear wheelie! :-D It smokes when cold but this lessens as it warms, although it never completely goes away. Always a burst of smoke on sudden throttle. It only seems to foul on part throttle openings.

Thanks for the help to this point. You guys have given much food for thought and some places to start experimenting.

[sandrooster]

whiteman,
Have you rebuilt your carb?

I had a similar problem for a few years. Turned out to be the float valve just needed to be replaced.

Assuming you checked for proper seal of the reed spacers, carb, and air intake boot clamps. I would startup your bike and then turn off the fuel selector. Blip the throttle to see response. If throttle response is good, Its the float valve that needs replacing. Do this for awhile to see if the spark plug fouls. You will have to periodically open fuel selector to fill float bowls when the engine starts to sputter.

If not, I recommend a compression test to check for possible seal leaks before you replace any crank seals.