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.

[yuckhil]

I removed the jets and slide assemblies.  Lots of blackish fuel residue on the white plastic slide holders.  Some fuel residue even on the air inlets before the butterfly valves.  This bike is my daily commute vehicle, I ride mostly at low throttle 5,000 rpm.

Jet needles look like new.  Needle jets look good except for a little corrosion at the entrance of #1 (LHS).  I poked the back of a 2.6 mm drill bit through each one, and I see no significant wear.  A 3.2 mm bit goes smoothly in each one, and stops 8 mm from the end.

Main jets #1 and #3 (clock-wise from top RHS) have a visible mark.  I scraped it with a toothpick, and it seems just a discolouration, not really a worn groove.  Is this what you described?

How would I clean out the pilot air passages - just flush the 2 air jets below the throttle body with carb cleaner spray and then compressed air, or any other ports as well?

Let me know if I can post any more information or pictures to help diagnose the problem.  Please let me know if you have any other ideas, thanks!

[greg737]

Good post, great pictures. 

The truth of the situation is that all of the soft brass parts of a carb are considered (by the manufacturer) to be "consumable parts".  It's a mindset that you have to embrace (and I realize doing this isn't easy, mostly because we want to believe our eyes and our eyes tell us "hey, this part looks fine!").

"Back in the day" of carbs, during a service or rebuild on a set of carbs that were no longer working well, it was just normal procedure to get the appropriate rebuild kit and replace all of those parts (the brass parts, springs, and anything made out of rubber such as gaskets and O-rings).

I'm not saying that any of your carb's brass parts are worn out, in those pictures your Main Jets and Emulsion Tubes look pretty good to me and they could all be within-spec and good-to-go.  It's very possible that your Main Jets and Emulsion Tubes are not the real source of your problem.

So, because of this, I have to ask: What did you find when you disassembled the Pilot Jet system from your carbs?

The Pilot Jet system on these Mikuni BST ("Slingshot") carbs can be a problem.  The Pilot Needle Jet Screw itself is tiny, and the fuel supply passageway through the body casting of the carb that feeds the Pilot Needle Jet Screw is the longest and most narrow passageway there is in the entire carb. 

And... to make things even worse (and more prone to problems of a long-term, age-related nature) the BST Pilot Jet system has a very, very small rubber O-ring on the base of the Pilot Needle Jet Screw that is absolutely vital to proper operation.  This tiny rubber O-ring has to provide an air-tight seal at the base of the Pilot Needle Jet Screw (where it screws into the carb body).  If the base of the Pilot Needle Jet is not sealed air-tight it will not be able to make use of the vacuum pull that the carbs generate during cranking and low-RPM operation.  This vacuum pull is critical because it (the vacuum effect) draws fuel up into and through the passageway from the carb bowl and through the Pilot Jet orifice.  If there's no vacuum pull (or not enough vacuum pull) the Pilot Jet system won't move any fuel to the engine.

This would explain why you have to "crack the throttle" when starting the bike.

Obviously, I recognize that I'm now offering up my second theory of why your Bandit is so hard to start.  But I'm just trying to help.

When you try to start the engine in colder-than-normal conditions the fuel delivery requirements are different.  And if the Pilot Jet system isn't functioning due to either being clogged up or unable to pull fuel out of the Float Bowl, you will have to deliver the necessary amount of fuel by "cracking the throttle".  But on a cold day your normal "crack" probably won't be enough to start the engine right up.  So you probably end up "cracking" it too much which causes a flooded condition in one or more (or all) of the cylinders. 

[greg737]

[greg737]

Of course it's also true that anything else that might reduce the amount of vacuum pull on the carb's "Slow System" will cause the Pilot Jet to move less fuel.

A perfect example is the "cracking the throttle" thing.  I can easily imagine how this problem would begin and then get worse. 

Imagine that the "Slow System" begins to get clogged up, so the rider finds that it's now necessary to "crack the throttle" to get the bike to start.  This gets the bike started but it really takes the Pilot Jet system out of use, because the rider is using the Main Jet to get the bike started.

And if the Pilot Jet system is partly or completely clogged the rider will probably have to open up the "Throttle Stop Screw" (the long, flexible finger-adjustable thing that most riders think of as the "idle adjuster") a bit more than normal to open up the throttle plates because he's idling the bike mainly (or completely) on the Main Jet system.  This abnormally opened-up setting of the throttle plate stop screw would also have the effect of reducing the amount of vacuum pull exerted on the "Slow System".

[greg737]

You should definitely get some lengths of copper wire (because it's really soft and won't hurt the carb) and try your best to clean out the "Slow System".  Poke around through all of the orifices in the system: the Pilot System Air Jet, the Pilot Fuel Jet, the Pilot Jet Needle hole, the Main Bore Bypass Ports.  Poke around with the copper wire, then shoot some carburetor cleaner through the passages.  Then do some more poking, then more carb cleaner.

Then replace all the rubber parts: gaskets and O-rings.

After looking more closely at your pictures I have to say it appears that your brass parts are in really good condition.  Just a bit dirty from time.  So just a good cleaning should be all you need for those parts.

I'm guessing that the gasoline you've been using in the bike doesn't have any ethanol added? 

[yuckhil]

Thanks for all this info, and the picture.  I am still trying to process the possibilities, and post an intelligent response.  Yes, I did have to set the throttle stop further in, and I also had to turn the pilot screws out more than the recommended 1.5 turns.  Sorry, I have been living with this problem so long, I forgot some of the details.  Yes, luckily regular unleaded 91 octane fuel is widely available here.  I never use ethanol blends or premium fuel.

This week, all o-rings looked in good condition, esp. the ones at the emulsion tubes, which were very squashed when I first rebuilt the carbs 6 years ago.  I have not removed the pilots screws in ages.  What damage would I look for - the tip?  Since I have had to turn them out almost one extra turn (2-2.5 turns each), does that imply that mechanism is likely working ok, and the problem starts before the slow operation mixture reaches the pilot screws?

I will try to inspect and clean out the pilot air passages as you suggested.  Thanks again very much for your assistance.