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

 

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."

 

I've taken the air screws all the way in and set them each in the middle
of the 3/4 - 1 1/2 turn range as specified in the 750K manual. This
seems to have smoothed out idling a little and it isn't nearly so smoky.
It still lopes at slow speeds.

It doesn't really idle at all with the choke on, as it'll stall out quickly.

I noticed when I pulled the carburetors the throttle slides do not fully
close; there's about a 3mm gap at the bottom, as set by the previous
owner. Should the gap be there?

The RPM is far steadier now. However, I noticed plugs 1 and 4 have
turned black, and 2 and 3 only mildly tan. Do cylinders run rich at
idle (thus blackening the plugs) or should they stay clean? I get spark
on every wire.

I don't have tools for these tests. My guess is, somebody with enough
knowledge to be dangerous fiddled with the bike and I'm going back
through (with the help of r.m.t) setting it all back to rights.

That reminds me though; sometimes when starting the bike I can hear
hissing somewhere inside the left part of the engine, below cylinder 1.
It doesn't sound like it's coming from the head, and it stops when the
engine starts.

Very likely any gasket material is going to be as old as the engine. If
I do use carb cleaner or gas additives, how likely am I to damage
gaskets, rubber seals, or the non-metal floats?

Thanks, guys; this is helping very much.

Sergeant Tibbs