1972 cb350 idle issues

mwhyte wrote:

>Is it as simple as adjusting the idle here, or am I missing a more complex
>problem?

http://www.bikebandit.com/partsbandit/showschematic.asp?dept_id=2316300

The idle mixture screw is shown on the side of the carburetor as 8, 8, and 3.

Turning it counterclockwise about 1/4 to 1/2 a turn should make the engine
start idling on both cylinder when cold.

If that doesn't help, remove the screw after first tightening it until it
lightly seats, while counting the turns.

Then spray some aerosol carb cleaner in the hole and reinstall the screw the
same number of turns out from lightly seated that you counted.

If that doesn't clean the idle circuit out, try putting about thee ounces of
Berryman B12 Choke and Carburetor Cleaner in a full tank of gasoline and
going for a slow ride to clean the gum and varnish out of the carbs.

Another possibility is that the floats are set too low, or the float valves
are gummed up and stuck shut.

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Albrecht via MotorcycleKB.com wrote:
> mwhyte wrote:
>
>
>>Is it as simple as adjusting the idle here, or am I missing a more complex
>>problem?
>
>
> http://www.bikebandit.com/partsbandit/showschematic.asp?dept_id=2316300
>
> The idle mixture screw is shown on the side of the carburetor as 8, 8, and 3.
>
> Turning it counterclockwise about 1/4 to 1/2 a turn should make the engine
> start idling on both cylinder when cold.
>
> If that doesn't help, remove the screw after first tightening it until it
> lightly seats, while counting the turns.
>
> Then spray some aerosol carb cleaner in the hole and reinstall the screw the
> same number of turns out from lightly seated that you counted.
>
> If that doesn't clean the idle circuit out, try putting about thee ounces of
> Berryman B12 Choke and Carburetor Cleaner in a full tank of gasoline and
> going for a slow ride to clean the gum and varnish out of the carbs.
>
> Another possibility is that the floats are set too low, or the float valves
> are gummed up and stuck shut.
>

I'm curious whether synchronizing the butterflies/vacuum would have any
effect (assuming the carbs can be balanced on this bike).
 >> Stay informed about: 1972 cb350 idle issues 

paul c wrote:

>I'm curious whether synchronizing the butterflies/vacuum would have any
>effect (assuming the carbs can be balanced on this bike).

Mechanic$ in $tealer$hip$ often recommend carb $ynchronization ju$t before
Chri$tma$ and April 15th.

Seriously, you cannot synchronize dirty carbs, and CV carbs with shafts
joining the two butterflies don't go out of synchronization as easily as the
older slide valve carbs with multiple throttle cables.

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 >> Stay informed about: 1972 cb350 idle issues 

Albrecht wrote:
>http://www.bikebandit.com/partsbandit/showschematic.asp?dept_id=2316300
>
>The idle mixture screw is shown on the side of the carburetor as 8, 8, and 3.
>
>Turning it counterclockwise about 1/4 to 1/2 a turn should make the engine
>start idling on both cylinder when cold.
>

Thanks a ton for the tips; I'll give it a go and see what happens.

>Another possibility is that the floats are set too low, or the float valves
>are gummed up and stuck shut.

I just got the bike back from the mechanic yesterday after a carb rebuild ...
I hate to think this is possible, but I guess it could be ...
 >> Stay informed about: 1972 cb350 idle issues 

"mwhyte" <u35079@uwe> writes:

> I had a problem with stalling and the left piston not firing until the engine
> heated up significantly. So I did the obvious: Rebuilt the carbs -- even
> replaced both diaphragms -- replaced the air filters, replaced the points,
> set the timing (all by my mechanic, of course).

Depending on your mechanic this may or may not be a good thing.

> Both pistons are firing now, but unless the choke is open (minimum halfway)
> the bike will stall when idling (ie. sitting at a light). Also stalls
> sometimes on downshift.
>
> Is it as simple as adjusting the idle here, or am I missing a more complex
> problem? Any help would be appreciated. Thanks much, MW

What's the idle speed set at? Also, the following potential problem
areas come to mind:

- Air leaks
- Sticking float needle valve, although that would show itself pretty
much all the time.
 >> Stay informed about: 1972 cb350 idle issues 

"mwhyte via MotorcycleKB.com" <u35079@uwe> wrote

> bike seems to idle fine as long as the choke is open; but it shouldn't
need
> the choke open after a certain point to run properly, I wouldn't think ...
>

First, I think what you are trying to say is that you need the choke
(partially) CLOSED.
Normal running position is choke off.....open.
For starting, choke is on......closed.

Second, the most common air leak is at the rubber boots between the carbs
and the intake manifold, that is, the side of the carb toward the engine.

Third, two of us have asked you nicely now what the idle speed is and you
have not responded.
SO WHAT IS THE IDLE SPEED......or don't you have a tach?

And lastly, given your (apparent) mechanical ability, you probably should
NOT be replacing the head (or other) gaskets on the engine unless you KNOW
they are a problem. Likely they are not.
 >> Stay informed about: 1972 cb350 idle issues 

>
>What's the idle speed set at? Also, the following potential problem
>areas come to mind:
>
>- Air leaks
>- Sticking float needle valve, although that would show itself pretty
> much all the time.

When you say air leaks, where would be the most typical sites? In the carb,
or the engine (ie. head gaskets) themselves?

I'm considering getting a gasket set and just replacing all of them; I'm sure
they haven't been touched in 35 years ...

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Timo Geusch wrote:

>- Sticking float needle valve, although that would show itself pretty
> much all the time.

Also, Timo, if the float needle valve is sticking, what is the behaviour? The
bike seems to idle fine as long as the choke is open; but it shouldn't need
the choke open after a certain point to run properly, I wouldn't think ...

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"mwhyte via MotorcycleKB.com" <u35079@uwe> writes:

>>
>>What's the idle speed set at? Also, the following potential problem
>>areas come to mind:
>>
>>- Air leaks
>>- Sticking float needle valve, although that would show itself pretty
>> much all the time.
>
> When you say air leaks, where would be the most typical sites? In the carb,
> or the engine (ie. head gaskets) themselves?

Usually the rubbers between the carb and the inlet stubs on the
engine. They'd be the only ones to have that kind of effect of leaning
the mixture.

> I'm considering getting a gasket set and just replacing all of them; I'm sure
> they haven't been touched in 35 years ...

Don't bother, it won't help in this case anyway...
 >> Stay informed about: 1972 cb350 idle issues 

>Third, two of us have asked you nicely now what the idle speed is and you
>have not responded.
>SO WHAT IS THE IDLE SPEED......or don't you have a tach?
>

Sorry, missed that. When the choke is open (off), it's hard to tell, because
it almost immediately stalls out and dies, so I can't tell. WIth the choke
closed, even slightly, it hovers around 1000.

>And lastly, given your (apparent) mechanical ability, you probably should
>NOT be replacing the head (or other) gaskets on the engine unless you KNOW
>they are a problem. Likely they are not.

Good to know. I wouldn't attempt it myself in any case ... thanks for all the
tips ...

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 >> Stay informed about: 1972 cb350 idle issues 

"mwhyte via MotorcycleKB.com" <u35079@uwe> wrote

> Sorry, missed that. When the choke is open (off), it's hard to tell,
because
> it almost immediately stalls out and dies, so I can't tell. WIth the choke
> closed, even slightly, it hovers around 1000.
>

OK, so one of the things that happens when the choke is operated (closed) is
that it pulls the throttle open a bit, up off of the idle adjustment
throttle stop. When you return the choke to the normal position, the
throttle returns to the adjustment stop. The obvious answer to your problem
(although it may not be right) is that you simply need to adjust the idle
adjustment throttle stop screw for a higher idle speed...........to maintain
that ~ 1000 without the choke.
 >> Stay informed about: 1972 cb350 idle issues 

Ken Abrams wrote:
The obvious answer to your problem
>(although it may not be right) is that you simply need to adjust the idle
>adjustment throttle stop screw for a higher idle speed...........to maintain
>that ~ 1000 without the choke.

OK, so here's an update from my fiddling that might provide further insight:

This morning, I twiddled the idle mixture screws on both carbs a bit, abotu a
half turn counter clockwise on each.

Then, I turned up the idle adjustment throttle stop screw, in hopes of upping
the idle speed directly at the throttle.

The result: A semi-consistent idle around 1100 rpms, BUT the bike was
seriously sluggish -- ie. had no power, and took forever to get to higher
rpms in pretty much any gear. Seriously, it had less than half the torque it
usually does.

I thought I might have a piston out, but both tailpipes were puffing out
exhaust, so that doesn't seem to be the case (I also put my hand on the
engine on both sides, and could feel the pistons firing at idle.)

So, no idea what the deal is now. Very mysterious. Anyone who has any ideas,
I'd be grateful to hear them. Many thanks, MW

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