YAMAHA XS250 1982 - why does the motorcycle cut out when I turn the choke on?

K

Kaycee

New member
My motorcycle was having some issues starting so I took it to the mechanic and it was sorted. Great news. I have now noticed however, that if I try riding the bike with choke on, the engine will run, but only if I am reving it. As soon as I idle it cuts out. Admittedly this was on a warm day and choke probably wasn't needed, but I would expect a bike to just rev high if the choke was on without needing to be?

Also, I have been putting unleaded petrol into the motorcycle. But my friend recently asked me if it needed leaded due to it being built in 1982. I wasn't told this when I bought it, but now I am wondering if this is why the engine dies with the choke on?
 
When you use the choke if it is not needed than you are choking it with excess fuel and will not run right. Leaded fuel was to help protect valves in four-stroke engines. I believe yours is a two-stroke.
 
Don't run with the choke on.

The choke actually restricts the air flow into the carburettors thereby effectively enrichening the mixture (ie more gas per volume). This extra fuel is needed to help a cold engine fire. On a warm engine, however, the mixture becomes too rich. Now, if the outside temperature is warm but not extremely hot, the incoming air is dense enough to contain sufficient oxygen to burn the extra fuel, hence the engine will keep running even with the choke on. On a hot day, however, the incoming air is not dense enough to contain sufficient oxygen, lots of fuel remains unburned and the engine has a hard time running. It also means that at least some of this unburned gas will find its way past the piston rings and into the sump. Bad news unless the engine is warmed up enough to boil it off...

Unlike European bikes, all 1980's Japanese four-stroke bikes, like yours, were designed for unleaded gas. No worries.
 
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