1975 Honda CB750K5 uneven idle, stalls below 1000 rpm

Hello again,

This is the same bike as my previous question; it idles fairly steady at
about 1500 rpm but won't go lower without chugging and after a few
seconds it will stall if it gets under 1000 rpm.

I've removed and cleaned the carburetors. The jets, valves, slides,
ports, and screws are all as clean as I can get them without invoking
deities. I've replaced the excessively old spark plugs (the previous
owner had at least two different types on it) and confirmed solid
regular spark on each plug, seemingly good contact on the points, and
good clean new gas flow to the carburetors.

If I'm really careful I can check the manifold temperature; after a few
minutes of running, pipes 1 and 3 feel somewhat colder than 2 and 4.

I don't need the choke to start it, and it starts /almost/ immediately
when cold. When it's warmer, though still well below operating
temperature, it starts and runs immediately. After a minute or two the
idle speed rises by at least 400 rpm though I'm using the aftermarket
cruise control lever to fix the throttle position. It fluctuates with
audible difference by about 50-100 rpm in short intermittent bursts when
it's been running for more than a few minutes.

The air filter isn't present right now, since mice chewed part of it and
replacements need time to arrive.

Revving the engine is smooth, though if the engine speed is jumpy just
before opening the throttle, it'll backfire in very light pops. While
idling, even while jumpy, the mufflers have a firm steady popping
pressure (but this may be masked since cylinders 1-2 and 3-4 share
aftermarket mufflers).

What are the likely culprits, and which should I inspect first? It has
been sitting for over two years and I'm wondering if riding it around
for a while after I get my motorcycle endorsement would straighten out
this problem.

Thanks again for everyone's help!

Sergeant Tibbs
--
I can neither confirm nor deny the existence of this signature.

On May 24, 11:48 pm, Sergeant Tibbs <gsaprae....DeleteThis@sbcglobal.net> wrote:
> Hello again,
>
> This is the same bike as my previous question; it idles fairly steady at
> about 1500 rpm but won't go lower without chugging and after a few
> seconds it will stall if it gets under 1000 rpm.
>
> I've removed and cleaned the carburetors. The jets, valves, slides,
> ports, and screws are all as clean as I can get them without invoking
> deities. I've replaced the excessively old spark plugs (the previous
> owner had at least two different types on it) and confirmed solid
> regular spark on each plug, seemingly good contact on the points, and
> good clean new gas flow to the carburetors.
>
> If I'm really careful I can check the manifold temperature; after a few
> minutes of running, pipes 1 and 3 feel somewhat colder than 2 and 4.
>
> I don't need the choke to start it, and it starts /almost/ immediately
> when cold. When it's warmer, though still well below operating
> temperature, it starts and runs immediately. After a minute or two the
> idle speed rises by at least 400 rpm though I'm using the aftermarket
> cruise control lever to fix the throttle position. It fluctuates with
> audible difference by about 50-100 rpm in short intermittent bursts when
> it's been running for more than a few minutes.
>
> The air filter isn't present right now, since mice chewed part of it and
> replacements need time to arrive.
>
> Revving the engine is smooth, though if the engine speed is jumpy just
> before opening the throttle, it'll backfire in very light pops. While
> idling, even while jumpy, the mufflers have a firm steady popping
> pressure (but this may be masked since cylinders 1-2 and 3-4 share
> aftermarket mufflers).
>
> What are the likely culprits, and which should I inspect first? It has
> been sitting for over two years and I'm wondering if riding it around
> for a while after I get my motorcycle endorsement would straighten out
> this problem.
>
> Thanks again for everyone's help!
>
> Sergeant Tibbs
> --
> I can neither confirm nor deny the existence of this signature.

My first nomination goes to an intake manifold leak somewhere between
the carb and the engine.
If the mice chewed the filter, they may have also done a number on any
plastic manifolding.
Also Check the lines to those #%$^%@^%^^ vacuum petcocks if
present.
(A cracked one did something similar to this to a friends yamaha 650
once.)
A small leak will not be sensible at higher rev.s but lean you out at
low rev.
It would also be the cause of setting the mix a bit too rich, (that
'popping` you mention, and
the fact that you don't need choke to start point this way).
Failing this it's all carb symptoms so thats where to look.

Best of luck,
MadDog

P.S.It definately takes second place, but just for jolleys, check the
ignition for intermittant
operation of one coil.

" They tell me that if a worm is cut in half,
both halves continue to live.
The half of the worm in the apple seems to live.
but I have never checked on the other half."