Dies immediately after starting when cold/wet

[yuckhil]

Hello everybody, I hope somebody has advice for me.  Bike always starts strongly first crank without choke.  No problem once the engine is warm.

When it is cold and/or raining, even if parked under cover, it immediately tends to die within 3-5 seconds.  If it dies then it will not fire AT ALL, just cranks over and over.  Leave it for 30 minutes, then it starts instantly.  If I manage to get moving, it runs very rough (sounds and feels like on 2-3 cylinders) and exhausts a LOT of vapour for about 1 km, then rides normal after that.  One day I rode into a rainy patch, and it became rough until I passed through the rain patch.

Bike is fully stock.  New plugs, battery, air filter.  Plugs, jets, oil looked clean at the last change.  Float valves were leaking; new float valves reduced the severity of the problem, but it still dies.

Any ideas?  Thanks.

[greg737]

I believe your problem is carbs that are running too rich.  If a carbureted engine will start from cold without needing any choke it means the carbs are running too rich.

The choke is a necessary item on a carb (by design).  On the Bandit's Mukuni carbs using the choke during starting provides both fuel and air (not just fuel) to properly balance the air-fuel ratio for a cold startup.  A set of carbs that are running too rich will only provide the extra fuel (not additional air) which means the air-fuel ratio is very likely to go too-rich during the startup.

This sets up a situation where you (the rider) might develop the habit of "cracking the throttle" a bit because you're very familiar with the bike and you know that you need to admit some extra air during a cold startup. 

This method of starting a carbureted bike creates a highly variable condition that could easily go "over-rich" and fail to start.

Because "Cracking the throttle" opens the throttle plates just a bit it has the effect of slightly reducing the vacuum pull experienced by the Main Jet, which reduces the amount of fuel that will be sucked up through it.  If the Main Jets are worn to the point where they are flowing too much fuel this slight reduction in vacuum might have the effect of lowering the amount of fuel delivered to a point where the air-fuel ratio is correct for cold startup.

If (by chance) you happen to "crack the throttle" just the right amount the bike will start right up.  If you "crack the throttle" too much you might have to crank longer because the air-fuel ratio will be on the too-lean side for a while.  But if you "crack the throttle" too little you could easily end up with one or more of the cylinders being fueled into an over-rich condition, which means a "flooded" condition (an air-fuel ratio that is too rich to burn).

So when the bike fails to start and then refuses to start, it's because one or more of the cylinders are flooded-out with fuel.  This is why waiting 30 minutes is the solution, because waiting 30 minutes allows all that extra fuel to evaporate away.

One of the things that causes Mikuni carbs to run too rich is when the soft brass metal of the main jets has been worn by the needles.  The wear from the needles causes the original round hole of the main jets to become oval and larger, which increases the amount of fuel the main jet can flow, causing the too-rich running condition.

[yuckhil]

Wow, thanks for the detailed reply.  Yes, I do open the throttle slightly when starting, always have done for 56,000 km.

I will check the needle jets, thanks.

[greg737]

Let me know what you find, I'm interested to hear about it.