Winter 2-Stroke Jetting & Ice tyres

[Lowe119]

I just bought my first 2-stroke - a dirtbike that I want to do some ice racing with. I Don't want to blow it up or do more damage than necessary to it, but I don't really know how to figure out what the best jetting is. Do I get bigger jets for winter? Do I get a bigger jet if it is lean or just the opposite?

A friend told me to run it wide open and kill it. Then look at the plug and see if it is white, black, or what ever it is.  Is this how y'all figure it?

PS - any of you know where I can get inexpensive ice tyres? I talked to a guy who races on the ice and makes/sells the 'good ones', but he wants $500 for a pair. He said I could go cheaper, but I wouldn't have near the traction.  :( So any of y'all know where I can get decent tyres for cheap? for my 2003 KDX 200?
OKAY - I just found my answers for tyres on a previous thread
http://www.racemotorcycles.com/cgi-bin/board/YaBB.pl?board=ccs1;action=display;num=1074776043;start=

[Tunersricebowl]

I don't see how you would get enough heat in the engine to burn it up. Make sure you have antifreeze mixed with distilled water in the radiator. If it runs and makes power,take a look at the plug,if it's very light colored you might consider going to larger jets,but really I doubt you will get on the main jet very much,it would probably just need the needle richened up if it's lean at all,whichI doubt.It may be too rich because of the cold engine temp. You may end up taping up the radiator just to build some heat.Ask the guys at the ice track,they will be glad to give you gentle nudges in the right direction.

[tzracer]

Yes you can stick a 2 stroke in cold weather. I had a KX250 that I rode on the ice a few years. We had oval tracks that I could easily top out in 6th gear. Some open class bikes were getting close to 100 mph. You can also stick a bike when it is running at part throttle. Most 250GP bikes that sieze these days are not on the main jet.

In cold weather the air is more dense, you may need larger jets. How much larger is a good question. You start out too large and work your way down. With dirt bikes it is possible that the jetting is already plenty rich.

Running the bike wide open and then killing the engine is called a plug chop. Run bike in a high gear (4th, 5th or 6th), high rpm, full throttle. While holding the throttle open, hit the kill switch and pull in the clutch. This should be done with a new plug. You want to check the color where the insulator meets the metal part that has the threads that screw into the head (most people look at the insulator - the porcelain part - up near the center electrode, this does not give the best reading - the best reading is obtained by pulling the head and looking at the piston, but would be a pain on a cold day). I use a flashlight and a magnifying glass. The plug chop will tell you about the main jet. The other jetting has to changing based upon feedback from the rider.

I learned my jetting from a decade racing motocross, 2 years racing karts, and a decade racing TZs.

What is the coldest weather you have ridden the bike in?
How well do you want the bike to run?
If it were me, I would buy the next 4 larger main jets, the next larger needle jet (changing needle jets is less drastic than changing needle position), and the next larger pilot jet..These choices are based on the bike being jetted properly for warmer weather.

Toss in the largest main jet,the needle jet and pilot jet, set air screw at 1.5 turns to start. Warm up the bike (if you have no thermostat you may want to tape over part of the radiator). Stop, put in new plug, do a plug chop (you may want to practice before putting in the new plug). Read the plug. Color can vary depending upon the oil and gas you use. For ice riding, a lightly colored insulator is will work fine. As far as the other jetting, that requires some experience to get right. I don't recall how much larger I went with my KX. I went larger with the pilot, can't recall if I changed the needle jet.

Mikuni used to sell a product to help with jetting (cardboard thing with a piece that slid). But for it to work properly, you have to get your bike jetted properly at a known temperature for a baseline setting before the tool would work. Proper jetting, like proper suspension settings is not difficult, but it can be time consuming.