Carburettor Service

[Ji Gantor]

If you own a dirt bike eventually you will have to perform a service on your bikes carburettor.
It is after all the lungs and stomach of your bike. As internal organs go this is pretty important and can dictate whether you win or lose races. If ill the consequences can be very costly.
This topic should place the spot light firmly on this mysterious grey life giving Mix Master.
We over the course of this thread will get right into the arteries with the help of Dr Lozza.
I am not talking about some pussy Key Hole Surgery, we will be grabbing the rib separator and going full blown Open Heart Surgery.
To be politically correct those that may find this subject offensive please log off now.

There are many different manufacturers of carburettors like Mikuni, Bing, Jikov and so on, but I will be concentrating on the most common of all time the Mikuni VM round slide.

Usually I try and upload as much of my postings as I can when I start a new topic but this time I am going to take it very slow as this is a very important issue and will take some time to cover. This will be more in depth than any magazine article you have ever read.

So lets scrub up and place our face masks on ready to probe away without worry.

Ji 

[Ji Gantor]

If you are going to do any kind of an operation you will need the right tools for the job.
Mikuni sells this great little tool kit that will allow you to service all parts of the carburettor while on the bike or on the bench.
Motion Pro make an aluminium ring spanner that is great for changing main jets or draining the bowl while the carburettor is still on the bike.
These tools are not the only ones that will do the job they are just designed for this purpose.


Ji

[Ji Gantor]

Two more tools that make the job a bit easier is a compressed air brush and a magnifying glass.
The air brush will help clean out all the small passageways while the magnifying glass will allow you to read the jet sizes.

Ji

[Ji Gantor]

The more bikes you buy and have to restore the larger the pile of old carburettors.
You can just leave them where they lie or you can strip them down for usable parts.
I clean them up and place them into a box that goes to the track with the tool box.
This way if I or another rider need a part then they are where they are needed.

Ji

[Ji Gantor]

I urge all buyers of bikes when they get them home to drop the bowl.
You can tell a lot about how the bike has been treated by what you find or don't find in the bowl.
The attached image is of a bowl off a bike I purchased at the begining of this year. First thing I did after unloading the bike was to remove the bowl. I am sure glad I did, after all you would not want that rusty crap going through your engine.
If you find it to be clean that is even better, while you have the bowl off check and record the main jet size.
This will help later on when tuning.

Ji

[Ji Gantor]

With the bowl off and the carb on the bench we can remove the floats and their lever arm.
This allows us to remove, inspect and clean the float needle and jet.

Ji


 

[Ji Gantor]

The attached image is hard to see but if we look close it can be seen that the silver sprung loaded end has been worn on an angle.
This means that the needle and jet need to be replaced. As it is the floats can not be correctly set. This would mean that the carb would have been flooding.

Ji

[Ji Gantor]

Next we can remove the main jet, splash guard and needle jet.
The screw top and slide have to be removed for the needle jet to come out.
The needle jet should just push up into the carb, but some times if the carb has been sitting for years it may jam. If it does get a rubber mallet and a brass drift and help it out slowly.

Ji

[Ji Gantor]

With the main jet, splash guard and needle jet removed we can now inspect, clean and record their sizes.
Also with the help of a magnifying glass and a light check the bore on the jets.
Some riders prefer not to buy larger jets but instead use a drill to make the original ones larger. The jets are not drilled to size in the factory they are honed.

Ji

[Ji Gantor]

Next remove the idle adjustment screw and spring.

Ji

[Ji Gantor]

Inspect and clean the idle adjustment screw.
Check to see if it is still able to raise the slide.

Ji

[Ji Gantor]

With the help from an engineering screw driver remove the pilot jet.

Ji

[Ji Gantor]

The pilot jet can now be inspected, cleaned and have its size recorded.
Also take a peek at the bore.

Ji

[Ji Gantor]

Now we can remove the air mixture screw, spring and seal.

Ji

[Ji Gantor]

The air mixture screw can now be inspected and cleaned.
Take a good long look at the pointy end of this screw.
If the screw has been screwed in to tight it will have a groove.
If there is a groove than you will need a new one as the mixture will not be able to be set correctly.

Ji