Cylinder 3 misfire at low revs

[Whomightyoube]

To me it sound like the pilot circuit is blocked on carb three. To test if the pilot circuit is blocked try starting the bike on choke. Don't touch the throttle just leave the choke on and let the bike idle for a couple of minutes. Check does cylinder three start to get warm when you let it run on choke. The choke circuit is a different circuit to the idle circuit so if it does get warm on the choke but it stays cold off choke it suggest the idle circuit is blocked. You can test if cylinder three is getting warm up by throwing some water on the headers and see if the water dries.

Will try that, next week now as I have a busy weekend. My experience is that the bike dies not start at all on choke although when this trouble started I could only start it on choke.

[His Dudeness]

Brilliant  :thumbup
I knew that it was essential that the diaphragm should be in perfect condition and that  even a pinhole in it would render it useless, but did not understand how it all worked together.
Thanks again Dude for a very helpful post. 
  :woot

yeah if there's a hole in the diaphragm the pressure on both sides of the diaphragm becomes the same so the diaphragm won't deflect up properly so the jet needle won't lift up and the fuel mixture won't flow through the main jet 

[His Dudeness]

Does the diagram and description apply to the FZS 1000 carbs as well?

It's not specific to any bike. It's a diagram and description of a Constant Velocity carb. I've looked at the 400 carb but the 600 and 1000 should be more a less the same or any CV carb should be basically the same. Here's a video that shows it well
https://www.youtube.com/watch?v=IBFq9nHVmks

[Whomightyoube]

I have just started on full choke which was not as easy as starting without any. 3 exhaust got hot but not as quickly as the others. When I took choke off, the engine stopped. I then went for a ride. All four smooth for several miles. Short stop at junction OK. Another slightly longer stop at lights where I let it idle  30 seconds and down to 3 and lumpy at low revs all the way home.


My bike don't like red lights. How does it know ?

[His Dudeness]

I have just started on full choke which was not as easy as starting without any. 3 exhaust got hot but not as quickly as the others. When I took choke off, the engine stopped. I then went for a ride. All four smooth for several miles. Short stop at junction OK. Another slightly longer stop at lights where I let it idle  30 seconds and down to 3 and lumpy at low revs all the way home.


My bike don't like red lights. How does it know ?

That proves it then. It's not getting enough fuel through the idle circuit. It's running on the choke circuit but not on the idle circuit. I think you will have to clean the carbs again. Before you do that have you tried turning up the idle speed screw up a bit? It might just be too low.

[His Dudeness]

In general if you're trying to work out what jet does what you can often follow the casting in the body of the carb. Trace down from the jet to the body of the carb. There's often a raised section in the body of the carb where they've drilled to create a passageway. If you follow that it will show where the jet is going. Once you know where the jet is going you can spray some carb cleaner into the jet and watch does it come out where you are expecting it to. If it doesn't then there's a blockage somewhere

[Dynspud]

Fab stuff Dude.
Thx for all the advice and the pic & explanation of the carb.

[Whomightyoube]

Yeah it can be a right pain getting all the passages clean. The thing to remember is that you have to clean more than just the pilot fuel jet in the bowl. The idle circuit is the pilot air jet, the pilot fuel jet, the transfer ports and the mixture screw port. This diagram shows it well




At idle the butterfly valve is closed so air is drawn in through the pilot air jet (PAJ in the diagram), the air then goes to the holes in the side of the pilot fuel jet (PJ). Fuel is drawn up through the centre of the pilot fuel jet and mixes with the air from the pilot air jet. It then comes out the transfer ports just after the butterfly valve and also comes out the port at the mixture screw. So all of those passages need to be clear. If you just clean the pilot fuel jet in the bowl and don't clean the pilot air jet, transfer ports and idle screw port you can still have problems.

Can you explain your understanding of the idle circuit?


What is the function of the transfer ports?


I did some googling which indicated that the TP come into play when the TBF is just opened so if I am getting no firing when on idle on 3 it must be a blockage in the tube to the MS rather than the TP.


Also, sorry about all the questions, but what is your understanding of the Choke plunger? A choke used to be another butterfly which was closed off to choke air supply to the carburettor to enrich the mixture. One reply above says the choke is a completely separate circuit. If so where does it get the fuel from?

[His Dudeness]

When the bike is idling the throttle is closed so the butterfly valve is closed so very little fuel air mix can flow through the main opening in the centre of the carb so to keep the bike running at idle you need another path for the air fuel mix to flow through, that's the idle circuit. How the idle circuit works is air comes in the pilot air jet, flows to the holes in the side of the pilot fuel jet, at the same time fuel is drawn up through the centre of the pilot fuel jet, the air and fuel mix and the mixture comes out of the transfer ports and out of the idle mixture port. That's about as well as I can explain it in words. If that doesn't make sense have a look at the video below and if might make sense

! No longer available

You're right about the choke. Sometimes it is a second butterfly type valve that closes when you pull the choke lever and that chokes the amount of air and that make the mixture richer. But on Fazers instead of a valve that chokes air it's another fuel circuit that increases the amount of fuel rather than a plate that restricts air.