Re: CBR600 F2 idles a bit roughly

On Mar 21, 9:26?pm, "Phil, Non-Squid" <REMOVEphilME_...@xxxxxxxxxxx>
wrote:

But it was like that at any lowish RPM. ?Not sure at which point
the other jets and passages come into play.

The idle circuits are sensitive to fuel level, a high level will make
the idle feel "soggy" and a low level will make the idle erratic
because engine vacuum can't lift the fuel from the float bowl.

I suspect that your float valves might be a little sticky from gum and
trash on the edges of the valves. You might try cleaning the float
valves by draining the carbs and squirting a 50/50 mixture of B-12
into the bowls and let it sit for half an hour before starting the
engine. It will run on B-12.

The other possibility is that whoever owned your bike
before had the carbs apart and set the float height wrong, resulting
in low RPM hesitation.

The adjustment of floats is confusing. First off, the adjstment is
done with the carbs upside down on the bench, and the measurement is
made from the carburetor body to the bottom of the float, which is
on top when the carbs are upside down.

Newbies get confused by the dimension measured from the gasket
surface, they think that they should measure from the float bowl
*gasket* to the bottom of the float, and if they adjust the float to
the nominal setting of say, 14mm, the float will be about 1mm too low
in the float bowl because the gasket is about 1mm thick.

When you look at the float height range of adjustment it might say 14
mm plus or minus 1 mm, that means that 13mm is a high fuel level in
the float bowl because the fuel is shut off later, and 15mm is a low
level, because the fuel is shut off sooner.

So far as the function of the various jets and passages is concerned,
your engine is drawing gasoline through the idle jet from fully closed
throttle until the throttle is wide open. The further the throttle is
opened, the less vacuum there is to pull fuel through the idle jets.

With the throttle butterflies fully closed, the engine has to run on
whatever gasoline it gets through the idle jets and whatever gasoline
drools past the jet needle.

As you open the throttle, the butterflies uncover a pattern of three
transition holes that allow more fuel to flow into lesser vacuum.

Once the engine RPM gets high enough to pull the
vacuum slides open halfway, the tapered part of the jet needle begins
to play a greater role in fuel flow.

The area between the jet needle and the brass tube it goes up and down
in rapidly becomes larger.

When the needle gets about 80% to 90% of the way out of the hole, the
restriction imposed by the difference in areas between the needle and
the brass tube becomes less than the restriction of the
hole in the main jet.

The jet needle/needle jet variable orifice are in series with the main
jet. This is not readily apparent to many amateur tuners who advise
riders with carburetor problems to change their main jets or raise the
jet needle clip by one or two slots or put washers under the head of a
non-adjustable needle.

The mainjet orifice must have a *smaller* area than the largest area
obtained by pulling the needle out of the hole as far as available
vacuum will pull it.

Otherwise, what's a fixed main jet for?

The engine cannot run on the main jet until the throttle butterflies
are almost wide open, but you will hear advice about "clogged main
jets" from ignorant advisers who know almost nothing about constant
vacuum carbs.

Or, they will advise newbies to buy $$$$ worth of expensive main jets,
claiming that the jet installed is probably too small. These advisors
know nothing of the main jet sizing scheme, or how the numbers work.

The advisors may be thinking about how older slide valve carburetors
were tuned and even think that the various circuits operate over
narrow ranges, which was never true even then.

But the simple truth about the CV carb is that it is running on the
idle jets about 80% of the time, and the vacuum slide never gets high
enough for the carb to get onto the tapered part of the needle, let
alone the main jet.

Get to know and love your idle circuits and the float level.


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