Carburettor Service

[pmc57]

next installment please, I'm enjoying the journey

[Ji Gantor]

The screw caps come in all sizes.
The extended tops reflect an extended slide.
Yamaha YZs have extended screw caps, I am not sure which other bikes are similar.
If your bike in the show room had an extended screw cap and some one along the way removed the carb and replaced it with another type you will struggle to get an original. Mikuni does not sell extended screw caps.

Ji

[Ji Gantor]

Over the past 30 years these old warhorse carburettors have needed servicing but no one has.
This screw cap should have an O ring but it was not there when I purchased the bike.
No O ring, dust and all sorts of crap will be going into your engine.
Check it.

Ji

[Ji Gantor]

This screw cap should have a rubber sealing washer.

Ji

[Ji Gantor]

Slides are very important.
If your carb had an extended screw cap then more than likely your slide will also be longer.
These two slides have the same outside diameter.
Slides have a cutaway. A cutaway is the curved removed section on the bottom facing the airfilter.
This removal allows air to mix with the rich idle fuel mix.
The cutaway size helps with idling so stay with what your manual suggests until a tuner proves otherwise.

Ji

[Ji Gantor]

The slides have different ways of attaching the needle and throttle cable.

Ji

[Curly3]

You did well to squeeze in a comment PMC, he's on a roll.
Great help for us Knuckleheads, well done Ji.

[Ji Gantor]

Jet needles smooth out the fuel flow from 1/4 to 3/4 throttle.
These needles really effect how the engine powers up.
As can be seen they come in all lengths and diameters for different engine types.
On the top you can see the grooves and the circlip.
If you move the circlip up a groove you lean the mix.
Clean, inspect for damage, record size and circlip location.

Ji

[Lozza]

Hmmm I generaly  say moving the circlip 'up' richens the mixture and 'down' leaning as that's what your doing to the needle(in relation to the needle jet). The fuel is actualy metered by the annulus the difference between the diameter of the needle jet and the needle.
I have recently discovered the relationship between Mikuni flow rated jets an Keihin/Delorto size jets to be infact linear. In Mikuni's case a 200 to a 210 main is a 5% increase in fuel flow, a Keihin 160 to a 165 is still 5% increase in area of the bore thus fuel flow, although the numbering does not indicate this to be the case

[JohnnyO]

Lozza moving the clip up lowers the needle into the needle jet further up the taper and leans the mixture and vice versa.

[Lozza]

Read what I posted again

[yz490rules]

Thanks for this post.  Tuning and finding problems in these are difficult.

[Ji Gantor]

Hi YZ490,
No problems.
There are a few more things I can share when I get time.
Stay tuned !

Ji

[mainline]

The extended tops reflect an extended slide.
Yamaha YZs have extended screw caps, I am not sure which other bikes are similar.
If your bike in the show room had an extended screw cap and some one along the way removed the carb and replaced it with another type you will struggle to get an original. Mikuni does not sell extended screw caps.

Just wondering what the ramifications are (if any) of not using an extended slide (and cap) on a bike that originally had one?

[colmoody]

Top step of the podium for Ji Gantor on this topic. GOLD.