Cold weather dirt bike riding does not require putting the bike away. It requires accepting that three things have changed: the air going into the engine, the carburettor’s willingness to keep working, and how long your hands stay useful. Motocross Action make the point that the best way to keep a bike in good condition over winter is simply to ride it. Here is what to change first.
Cold air is denser, and dense air runs lean #
This is the part riders get backwards. ProX explain that air density is governed by temperature and pressure, and that as temperatures decrease, air density increases. Denser air means more air entering the engine for the same amount of fuel — a leaner mixture than the one your carburettor was set up for.
Eric Gorr’s carburetor tuning guide states the same relationship from the other end: when air temperature increases, air density becomes lower, and that makes the air-fuel mixture richer. Run it backwards and a cold morning leans you out.
So you richen the jetting to compensate. The guidance Dirt Bike Magazine published, courtesy of Enduro Engineering, is specific for two-strokes: usually a couple of sizes up on the main jet along with richening the needle one clip position is enough. Check the jetting is good before you ride it hard, and if you are not sure, err on the rich side.
Worth being clear that this is a carburettor problem. ProX note that while carburettors meter fuel precisely, most of them cannot account for changes in air conditions at all — they do what they were set to do regardless of the weather. They also point out that OEM owner’s and service manuals usually carry jetting charts giving guidance by temperature and elevation, which is the first place to look before guessing.
It helps to know what you are actually turning. ProX describe the carburettor as a set of circuits, each covering a different part of the engine’s speed and load range, metered by small brass jets and by a needle whose height is set by its clip position. The circuits are not fully independent, so a change to one has some effect on another — which is why changing three things at once teaches you nothing. Humidity counts alongside temperature and elevation, and if you have fitted an aftermarket exhaust, ProX advise starting from the setting the component supplier recommends rather than your old numbers.
Our jetting calculator gives you a starting point for temperature and altitude, and for two-strokes the premix chart covers the oil side of the same mixture.
How to tell it is too lean #
You do not need a dyno. Eric Gorr’s guide lists the symptoms of lean jetting as an engine that runs sluggish, a pinging sound, and bogging. On a cold morning those are easy to mistake for an engine that is simply not warm yet, so give it a fair warm-up before you start changing jets — and note that Gorr also attributes pinging to crankcase air leaks, so a lean symptom is not automatically a jetting problem.
Carb icing is the failure that strands you #
This one is specific to winter and rarely mentioned. Per Enduro Engineering, snow thrown up and around a two-stroke carburettor can eventually freeze it, and the consequences are fuel starvation or a stuck throttle. Neither is something you want halfway through a loop in the cold.
Their fix is low-tech: cover the sides of the carburettor. A set of plastic roost deflectors zip-tied into place does the job.
Snow gets into the airbox too #
If you are riding in deep snow, stop and check the airbox periodically. Enduro Engineering warn that two-strokes will sometimes suck in enough snow to plug and freeze the filter. Four-strokes, they note, are not as prone to it.
A filter that is already dirty has less margin before it blocks, so going into winter with a freshly serviced one is sensible — the routine is in our air filter maintenance guide.
Traction on frozen ground #
Two routes, per the same guide. Buy tyres made for snow — they recommend studded tyres — or screw ice screws into the tyres you already have. For the screw route they suggest roughly three bags for a rear tyre and one to two for a front, and note that screw length matters depending on whether you are running tube protection. Their own preference is the studded tyres.
Your hands decide when the ride ends #
Everything else is academic once you cannot work the controls. Enduro Engineering call grip heaters a must, and the reasoning is not comfort: you will not be able to hang onto the bike or operate the throttle, clutch lever and front brake lever when your fingers are numb.
The cheap companion trick is to wrap the clutch and brake levers with cloth athletic tape, which insulates your fingers from cold metal. Bare alloy levers pull heat out of your fingertips for the whole ride.
Layering, not just more gear #
Their clothing advice starts underneath rather than on top: a good set of polypropylene full-length thermal underwear, with your normal riding jersey over it. For feet, fleece-lined Gore-Tex socks — the fleece keeps them warm while the outer shell keeps them dry.
Wet is what actually ends cold rides, not cold on its own, which is the same lesson that applies to autumn mud. If the ground is wet rather than frozen, our guide to riding motocross in mud covers the bike side of that.
Or put it away properly #
If the honest answer is that you are not going to ride again until spring, then do the other job properly rather than half-doing both. Leaving a bike sitting with untreated fuel and a flat battery costs more than a winter of not riding. That routine — fuel, oils, battery, tyres off the ground — is in our winter storage guide.
What does not work is the middle option: parking it in October with a full tank of ordinary pump fuel and a vague intention to ride if the weather turns nice.
Sources #
