KX 85 runs rich in cold weather

[NEW ROOST]

My sons KX 85 is fouling plugs in cold weather.

To my understanding, as the temperature drops the mixture burned becomes leaner. Therefore, the fuel flow needs to be increased. A simple correction factor chart shows me that the Main Jet is to be made larger when the tempeture drops below a certain degree at a specific altitude.

This all sounds great, though as the weather gets colder my son's bike runs rich, (air screw over 2 1/2 turns out) fouling the very expensive spark plug. Not only do I need to fix this problem, I really want to understand it. I do not understand how it is running rich. I don't understand why the bike is so far off from the correction factor chart.

The bike is completely stock with stock jets. The bike has 11 hours on it with a routine ring change at 10 hours. The timing is set to the factory setting and the plug is getting proper fire. The filter is clean and the muffler has new packing. The bike has good compression. We are using the factory recommended NGK R6252k-105 spark plug. We are running Klotz R50 synthetic with 93 octane pump gas (Shell winter mix).

My son is an average 11 year old rider that is starting to slip the clutch. I believe that he is running hard enough to clean the plug, though not constant on the pipe.

I have received allot of suggestions from other track dads. I have yet to rejet, change oil brand, reduce oil mixture ratios, run hotter plugs, run race fuel- ect. I have run chop tests on the plug, though the plug fouls shortly after riding.

Again, I would like to understand this problem and I would like to remedy it. What does make sense to me is that using a smaller jet is opposite theory of what a correction chart suggests.

The local Kawasaki dealer service guy stated to me that I should pay attention to the plug itself rather than a chart. That made good sense to me even though I don't fully understand why the problem exists. I don't think he did either.

I would like to add that in the past, I had heard the saying that more oil is leaner?. Could this be true? Maybe I should go larger on the main jet?

Thanks for reading this thread. Any help is greatly appreciated.

 

[oldguy]

The general rule of thumb is as the temp drops you need to put in larger jets to get more fuel into the denser air. Do not toss the oil ratio into the confusion (We run our oil/fuel at 24 to 1 year round)
Did you foul plugs in the summer?
How much has the temp dropped?

My son just started racing my RM 144 on the ice and I went up 2 sizes on the pilot and 2 on the main for a temp drop from summer (average in the high 70s) to about 26*F. Below 20* we go up one more jet size on the main

 

[NEW ROOST]

The bike performed well this summer. though the plug always stayed black on the ceramic. The air screw factory setting is 1 3/8 turn out. We were only running about 1 turn out in a 70 degree temperature. Today the temperature is in the 20's and again we fouled a plug. The fouling of the plug really started happening when the outside temperatures dropped below 50*f.

 

[_JOE_]

I'd try a hotter plug before leaning the jetting. In the cold temps the engine will run quite a bit cooler which could prevent the plug from cleaning off. If he's not wringing it out for long periods you should be fine to up the heat range.

 

[whenfoxforks-ruled]

I would perform a leak down test, and specifically check the right crank seal. If its gooey black on the plug, look for a coolant leak. Run the premix from 32 to 20 to 1, and jet the engine. Vintage Bob