WR426 New top end now oil smells like gas?

I am looking for some advice. I have had a WR426 for the last 10 years, bought in new in 02. Probably has 25K+ miles on it. I recantly rebuilt the motor. Went with the 444 cylinder and piston kit from Kustom Kraft, and had my head rebuilt by Fastheads. New valves, springs, seats, everything. Put it all together and she runs great. Not exactly the boost I expected but it runs great. Very smooth. The jetting is spot on..... I think. Starts easy (2 or 3 kicks), idles smooth, no pops, no bog.

Here is my problem. I changed the oil after two rides. Each 2hrs of single track. The oil was black......????? So i changed it and went for a 2 hr ride today, pulled the drain plug and the new oil with 2 hours of use is pretty dark, not black but dark... and it smells like gas.....?????? I am not sure if the bike is running too hot??? I have over sized radiators with engine ice and I don't get any boiling over or boiling at all. I don't have any oil blowing out..... but some times it smells a little like burnt oil....?

Is the color and smell of the oil typical on a new top end? I cant say I ever noticed the smell before with the 426.... always poured out dark thick odorless oil..... I think. Am I just being over sensitive with the new motor or do I have a problem?

The oil I use is 20W40 Maxima 50/50.

Edited by q1445

I cannot imagine a spot where the oil and gas would mix.

Check the measured quantity of oil you are draining. That may tell the story. Maybe its coolant in the oil, which would mean that the head gasket or water pump seal is no good.

I checked my oil levels today and the oil is a nice rich light golden colour and smells new. Has done at least 10 Hrs of moderate offroad (single track and fire trail) riding since last change. And my bike is certainly not perfectly jetted. Has a bit of popping on decel.

Your oil smelling anything but good after that amount of riding is of concern and the fact that it is black means something is not right. Many people will disagree with me, but IMO running engine ice is not actually helping your motor. It has a much higher boiling point so it doesn't boil over, it does not actually help your engine run cooler. In fact it allows it to run much hotter.

I think you may have a lean condition and your are running the oil very hot and it is deteriorating quickly. Very degraded oil can smell like gas, especially the mineral blends.

Edited by td42ti

Thanks for the imput so far! Let me add a few things. I checked the carb to make sure the float was properly set and it is. Some have said that it is possible for the carb to over flow into the cylinder and seep down to the bottom end. I am convinced that in not the case here. I did read that if it was way too rich it could make the oil smell a little....... I did notice that with the bike on a stand..... at start up, and on decel, when flicking the throttle I got a little bit of smoke. I do live at 7000ft and put the carb jetting back to stock when I did the motor. I leaned it up quite a bit... from a 165 to 158 main and she woke up quite a bit..... a lot more snappy, and no hint of smoke. Still no major popping...... I am kind of hopping this might of fixed it. I will take a couple more ride to test the jetting then dump the oil and replace with new oil. Then ride some more and see if the color and smell returns to normal. Any thoughts????? I sure appreciate the help!


Wouldn't you see the coolant separate from the oil after it sat for a while....? I will double check the levels to make sure.


I agree with you on the engine ice..... I have always felt that way but run it anyways.......??? As far as the lean conditions goes, it is tuff to get a lean condition at 7000ft, and if it was lean I would have expected the bike to run a lot worse when I dropped the main jet size. But I am not an expert at jetting and will have to do some more testing to see if its rich or lean.

I think you may have a lean condition and your are running the oil very hot and it is deteriorating quickly. Very degraded oil can smell like gas, especially the mineral blends.

Like I said, I don't think it is lean but I am not ruling out the possibility that it is getting hot for some reason and burning the oil......... with out an oil or water temp gauge it is tuff to tell other than I am not currently getting any oil or coolant spitting out.....

How's your clutch? If you abuse your clutch, your oil will deteriorate very quickly.


How's your clutch? If you abuse your clutch, your oil will deteriorate very quickly.


Clutch is great. Pulled when I did the rebuild and it is in good shape, plates, discs, and basket. I am not a big clutch guy....... It's either in or out, not much feathering. Still hoping it is just contamination from running too rich. Any other thoughts or suggestions?

Food for thought:

Increasing the cylinder volume (engiine size) with all other things being equal will increase the vacuum signal through the carbureator which could mean that the jets will allow more fuel into the motor.

With the engine size increase you should start with same jets you were using before and then maybe go one size lean to check how the jetting is performing.

Since you are well past your break in I would recommend a cylinder leak test where you apply compressed air to the cylinder to make sure the cylinder is (rings) are holding pressure. If you have any more than 10% leakage you may need to hone and rering and start over.

The only way for the oil to be fuel contaminated with fuel is fuel leaking past the piston and ring. engine running or not.

Don't wait until something bad happens to find out what the problem is save lots of money by working it out now.

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