Carb Adjustment - slide guide

> Are they Mikuni CV carbs? I never heard of a Mikuni CV carb that had a
> removable "slide guide".

Yes. Mikuni BDST 38-B67

> Is it possible to install the slide backwards? Most CV carbs have
> assymetrical slides to prevent this from happening, but slides will
> have a bevel shape *cutaway* on the intake side tom enhance off idle
> acceleration.

Impossible. I could send photos if you wanted to see better what I'm
trying to explain

> Do you have a link to online Ducati parts fiches?

No.

> It sounds like the vacuum slide isn't lifting, so the carb never goes
> on the main jet.
> However, Mikuni CV carbs have such problems with dirty idle jets and
> idle passages, that also keeps the engine from responding to the small
> throttle openings that uncover the acceleration transition ports under
> the lower edge of the butterflies.
>
> Other possibilities include incorrect plugged up air jet, wrong float
> height, stuck float valve, disconnected vacuum hoses, dirt in the fuel
> supply, etc.

I'll check the above mentioned.

> Any time you clean a carburetor with aerosol carb spray the neoprene
> rubber parts will swell up a little bit, but they return to their
> original size when they dry out.

The diaphragm actually shrunk.


> You can use a little grease or vaseline to make the lip of the
> diaphragm stick in the channel if you want.

I'll try a little vaseline.

> Some Mikuni CV carbs had a little rubber o-ring under the vacuum cap.
> Home mechanics wouldn't know about the o-ring and they would lose it
> when they disassembled the carbs for cleaning. The o-ring sealed a
> passage in the pilot air circuit.

I know the o-ring your explaining because I did
lose it and replaced it with a new one.

Thanks for your input.

Cheers!
Greg
 >> Stay informed about: Carb Adjustment - slide guide 

On Jul 14, 6:30?am, Greg <greghasti....DeleteThis@sentex.ca> wrote:

> Yes. Mikuni BDST 38-B67

http://www.desmodromico.com/sito2/carburati.htm

Italian is OK, I can deal with Italian.

Semi-downdraft carbs have some extra parts inside the float bowl
to make them work like side-draft carbs.

#3 is the main jet and pilot jet holder

#6 is the main jet

#8 is the idle jet

#11 (on the second drawing) is the pilot air jet

#16 has the float bowl pivot pin and the float valve

#13 is an air cut off device that shuts off and air passage when the
throttle is closed. This reduces farting and backfiring as the
motorbike slows down.

#17 is the anti-tamper plug that conceals the idle mixture screw.

Idle circuit cleanliness is absolutely essential with CV carbs.

Engines equipped with CV carbs start hard and don't run worth a damn
if the idle ports and passages are all plugged up.

If you decide to remove the idle mixture screw for thorough cleaning
of the idle mixture ports, turn the idle mixture screw all the way
clockwise until it just seats.

Count the number of full turns and fractions of turns before removing
the screw, spring, tiny washer and tiny o-ring. Write the number of
turns down and keep the two screws separate.

Squirting aerosol carb cleaner (like Berryman B-12) through all the
ports and passages will solve many problems with CV carbs.

What you squirt through the idle jet must come out through the single
idle mixture port that is controlled by the idle mixture screw, and it
must come out the three acceleration transition ports that are
controlled by the bottom edge of the throttle butterfly.

The aerosol cleaner must also come out the pilot air jet in the
carburetor mouth. In order to get a good flow of carb cleaner out of
every port in the idle mixture circuit, I will cover two ports with my
fingers, and squirt aerosol cleaner into one port until it flows
easily out the remaining port.

Then you can reinstall the idle mixture screw parts in the correct
order and turn them all the way in, then back them out according to
the numbers you wrote down and you should be back to the factory
settings, unless some amateur mechanic has tweaked with the screws.

Turning the idle mixture screw counterclockwise richens the idle
mixture. Idle mixture is very important between fully closed and 1/8th
throttle. That's when you get your all-important off idle
acceleration.

Amateur tuners get into trouble by expecting the idle RPM to continue
to increase as they turn the idle mixture screw counterclockwise. They
get to the point where the idle mixture is so rich the idle RPM
actually slows down and they compensate by turning the master idle
knob up.

This uncovers the three transition ports under the throttle butterfly
and the engine idle will hang up at high RPM when they blip the
throttle. This effect is more pronounced on inline four cylinder
engines and won't be as noticeable on a V-twin.

However, too rich is still too rich. It wastes fuel and soots the
spark plugs.

These carbs are set up with really large jets. They must waste a lot
of fuel keeping the engine cool.The main jets are huge, #140.

With a huge idle jet (42,5) the idle mixture screw setting specified
is 3 turns for the horizontal cylinder and 3-1/4th turn for the
vertical cylinder. The needle clip is in the 4th slot.

Be sure you have the spring seat and the two washers shown in the
second drawing assembled in the correct order.

Some amateur mechanics manage to get both washers on top on the needle
clip and the needle sits so low the carburetor comes on the main jet
very late.

Also, hold the rubber diaphragm up to the light and look for tears and
holes. The carburetor cannot come on the main jet if the diaphragm
leaks.

If you have a torn diaphragm, do not fear the greed of Bologna.

There is a company in England that makes diaphragms a whole lot
cheaper.
 >> Stay informed about: Carb Adjustment - slide guide