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Most likely it's the battery, which loses power in the cold and doesn't supply enough current to start the engine 🔋 It could also be that the oil is too thick in low temperatures and doesn't flow properly, or that the spark plugs are worn out and aren't generating enough spark. If it's been years since you had it serviced, I'd check the battery first (make sure it has a good charge) and then the spark plugs, which are cheap to replace. Fuel also behaves differently in the cold, so make sure you're using good quality gasoline.
I had exactly this problem with a bike I left sitting during the winter. Turns out the culprit wasn't just the battery (though that suffers too), but the gas had gone bad and the spark plug was shot. Cold freezes the moisture that builds up in the tank, and if you haven't used it for months on top of that, the gas degrades. When you try to start it, the engine tries to turn but there's not enough spark and the fuel is kind of gummed up.
First thing I'd do is check the battery with a multimeter; if it reads below 12 volts, that's a big part of the problem right there. But before you replace it, drain the old gas and fill it with fresh fuel, change the spark plug if you haven't touched it in a while, and make sure the battery terminals are clean and tight with no white corrosion around them.
If all that checks out and it still won't start, then yeah look at the oil: in winter it needs to be thinner (check the viscosity specified in your manual). One trick that works is warming it up a bit before you try starting, or stick the bike in a warm garage the night before if you can. If you keep having problems, think about taking it in for a checkup because there might be something more serious going on with the ignition system or carburetor.
How long has the bike been sitting between starts? That can give you clues about whether it's just the cold or something more maintenance-related.
What nobody's mentioned yet is that if the bike's been sitting for days or weeks in winter without being used, the spark plug can get dirtier than normal from incomplete combustion in the cold. The engine tries to start but can't get that clean spark it needs. Plus, if you have a carburetor (not fuel injection), the cold itself makes the fuel flow worse to the cylinder.
My advice: before you think about replacing parts, try leaving the engine in contact position for a few seconds so the battery can kind of "warm up," then try starting. If it still fails, warm up the bike a bit with a blanket or leave it somewhere less cold for an hour. Sounds weird but it works pretty well. If it still won't start after that, then yeah, you need to check the battery or spark plugs more thoroughly.
lobo77 asker
The bike has been sitting idle for a week or two between starting attempts in winter.
Cold contracts the gases in the combustion chamber and makes ignition harder, so besides checking the battery and oil, make sure the spark plugs are in good condition because they fail first in low temperatures compared to other times.
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