up-jetting and float level

 Not much theory here these days, due to the season I guess, so if it's
okay I'd like to ask a rather general question.  When people up the size
of carb' jets, is it typical that float level is also raised?


(I realize that other factors such as air flow and exhaust are usually
involved but don't know how to phrase questions about those very
succinctly.)

 
The main jet is deep in the gasoline in the float bowl, but the idle
jet is higher.

If a tuner raises the fuel level in the float bowl, it will be easier
for the idle jet to pick up gasoline, as it will take less vacuum.

The idle mixture may become a little too rich, or "soggy".

 . wrote:

I wonder if this is why my bike burns too much gas. It smokes black on
start up. It also smokes black when I gun it. It's obvious that it is
burning rich. The idle adjustment screws do nothing about it. It's not
the air filters, they are new. It's a Suzuki 1400 intruder 1996 year.
I only get 31 to 33 miles per gallon. I would be happy to put an intake
manifold for both cylinders and a SINGLE carb on it.

 e:


If the idle mixture is too rich and the tuner has attempted to
compensate for slow idling or stalling by turning the idle speed up,
the idle RPM will run away when the engine is hot.

It will get up to 3000~4000 RPM when the engine is started while hot.

That's because the accleration transition ports are uncovered by the
slightly open butterflies and the transition ports are NOT controlled
by the idle mixture screws at all.

 

I had a paragraph of advice here but I decided to strike it since I don't have
my flame suit handy this morning.  There is a world of info available.  You need
to identify the engine speed and throttle range as nearly as possible.
Increasing the size of the main jet before determining exactly where the problem
may be is a common tuning mistake.  The main jet is primarily responsible for
the top end above 3/4 throttle.  If you are having problems at less than 3/4 to
WFO throttle or so, you need to be looking at the lower range settings.  I would
not touch the float level until you zero in on the problem.

Lugnut

Here is a general theory of carburetor operation.

http://www.iwt.com.au/mikunicarb.htm

Here is a Mikuni Tuning Guide

http://www.mikuni.com/fs-tuning_guide.html

Here is the Mikuni Tuning manual for the VM series.  These are manual slide
carbes directly operated by the rider.  Constant velocity carb use a butterfly
throttle plate controlled by the rider.  A vacuum diaphram operates the slide.
Other than that, operation is very similar.

http://www.mikuni.com/pdf/vmmanual.pdf  

Keihn or other carbs are similar.  This ought to get you started