Carb / idle problem on a Honda CB350

I have a '72 Honda CB 350 twin. It sat for 10 years before I bought it
so the tank was quite rusted and gumming things up. I cleaned and
coated the tank, replaced the fuel lines, and rebuilt the carbs. It
runs now but the idle sometimes fluctuates and the right cylinder is
running rich (it's smoking and blackening the plug). I have the mixture
screw screwed in as far as it will go but it's still rich. I set the
idle and fuel mixture using a tachometer as the clymer manual
suggested. The idle is set at 1200 but sometime is will speed up to
3000 or drop down low so the engine dies.

Any ideas as to why one side is rich and the idle is irregular? I've
tried everything I can think of.

Thanks!

 

Are you sure whether the idle mixture screw is actually a fuel/air
MIXTURE screw, or is it an idle AIR screw on that model? I can't find
an online parts diagram for CB-350's.

There are different ways of doing the idle mixture VS idle air on a
constant vacuum carburetor.

The idle MIXTURE circuit sucks air through the pilot air jet and
gasoline through the idle jet. The gas and air MIX, and the idle
mixture screw *actually* adjusts the flow of idle mixture, so turning
it clockwise LEANS the mixture. Idle mixture screws would be on the
downstream side of the throttle butterfly. They would be on top of the
carb or underneath.

OTOH, if you have an idle AIR screw, it's upstream of the throttle
butterflies and it adjusts the amount of AIR mixing with the gasoline.
So turning the idle AIR screw clockwise richens the idle mixture.

An idle AIR screw would be on the SIDE of the carburetor, near the
inlet bell.

Then there is the issue of transition ports. Transition ports will be
just downstream of the throttle butterfly. If the main outlet port of a
carburetor with an idle mixture screw
is plugged up, the engine will idle too slow and the owner will turn
the idle speed up.

This uncovers the transition ports and the engine will idle much too
fast when hot and stall when cold.

Sounds like plugged a idle jet and idle passages and the adjustable
port is plugged and the idle speed is set too high, uncovering the
transition ports.

Don't worry if you have a hard time figuring that one out. It took me
about 30 years to realize exactly what was going on with transition
ports.

The other alternative to idle mixture or idle air problems is a stuck
float valve on the one carburetor. That would raise the fuel level in
the float bowl making it too easy for the engine vacuum to lift excess
fuelinto the idle circuit.

 

I'm not sure whether the screw is air or fuel. It is referred to in the
manual as the "idle mixture screw". I posted the exploded view from the
manual here if it helps: http://invisiblefire.com/bike/carb.gif

I believe turning it clockwise makes it lean because it runs poorer and
smokes more as I unscrew it.

I put in new float valves when I rebuilt them. Could it stick even if
it's brand new?

Thanks

 

Good man! Can you post more pages from the manual so we can read what
it says about adjusting the idle mixture? Many owners can benefit from
whatever text and pictures to can post.

Looking at the idle mixture screw (# 9), I notice that it is horizontal
and that there is no tiny o-ring associated with it. Idle MIXTURE
screws need this o-ring because there is liquid flowing past them. Idle
AIR screws don't have the o-ring because all they can do is leak a
little extra AIR around the threads. This tiny gap usually gets sealed
by
evaporated gasoline that turns to varnish.

My best guess is that # 9 is an idle AIR screw and that turning it
clockwise richens the idle mixture by restricting air flow.

Maybe turning the screw counterclockwise is allowing more air into the
idle mixture and the excess idle mixture will burn out after a few
minutes?

Also, remember what I said about the transition ports. They wouldn't
show on that carb drawing because the ports are just holes. If you
richen up the idle mixture too much, and the engine idle speed slows
down, the natural tendancy is to turn the idle speed back up, not
realizing that the extra butterfly opening is causing the engine to
suck extra mixture through the transition ports.

The transition ports flow extra mixture for acceleration as the
throttle is just cracked open.

This extra mixture isn't needed for idling, so the idle RPM runs away,
or the spark plugs foul.

If there is gum and varnish on the float valve, it will stick. Float
valves are often square or even triangular, in order to reduce the area
of contact with the cylindrical sides of the float valve seat and to
enhance fuel flow.

A little bit of varnish from evaporated gasoline will cause the float
valves to be "glued" in place, and gasoline can't flow past a stuck
valve (or too much gasoline flows when the valve is stuck in the open
position).