carburetor tuning

I have a 1976 CB750F with a Keihin CR-Special. When I first bought
the carb and installed it on the bike it ran good even though the
mixture was pretty rich. The bike sat in outside storage for a couple
of years and gained some oxidation. I took the bike apart, including
the carbs, to clean up the corrosion. Now I can't even get it to idle
right. When I took the carbs apart to clean them up I replaced the
slow jets so that it would run leaner. I don't know what I did, but I
have replaced the jets and tried all combinations of slow jets and
needle jets. The best I get is idling at 1200 with the air screw
completely closed which of course makes the mixture very rich.
Because it is idling rich I put smaller slow jets to lean out the
mixture. The smaller the jets the higher the idle, especially if the
airscrew is open. I have to have the airscrew completely closed to
even get close to a normal idle. turning the airscrew even a half a
turn counterclockwise (leaner) makes the idle run away up to 4000 to
5000 rpm. But if I put larger jets which richens the mixture, the
idle slows down, but I still have to keep the airscrew closed or the
idle runs up to 4000 rpm. Does anyone have any ideas? What am I
missing?

Please help!
tjbabar

 

You're full of shit, Jack. Item #44 on the Keihin CR Special parts
list is Keihin P/N
1014-004-20 "air screw".

 

Yes, the air screw on a slide valve carburetor does regulate AIR FLOW.
That's why turning the air screw CLOCKWISE makes the mixture richer.

OP says that the idle stop screw isn't even touching the throttle
quadrant...

 

Sticky float valves may be the problem. As the fuel evaporated out of
the bowls during the two years the bike was stored, the float valves
dropped down, then they became glued in place by the gum in the
evaporated gas.

When the motorcycle was returned to serivce, the float valves couldn't
move to shut off the flow of gasoline, so the fuel level in the bowls
was too high.

It takes less vacuum to lift fuel out of the float bowls when the
level is too high, so the engine runs rich.

 

Thanks for all the great advice. I did take the carb apart and clean
all the passages. I did set the jets to the original. And I realize
that the jets didn't just magically change sizes while it sat in
storage. The only thing I haven't done is check the float level.
Thank you for that suggestion. That is the only useful one I have
read so far. The throttle linkage is not stuck or compromised in any
way. The choke is clean and working properly. The reason I suspect
the mixture is rich is because the spark plugs are dark black with
carbon build up. I have read elsewhere that float level can cause the
idle to run away. Hopefully that is it.

Thanks again all.

 

Are the carbs 29mm, 31mm, or 33mm?

If you bought the carbs used, you might try contacting Sudco
International
to find out what jet needle and needle jets (nozzles) the carbs were
originally fitted with.

The difference in diameter between the needle and the nozzle when the
slides are completely closed is supposed to be just a few thousandths
of an inch so the needle shuts off flow through the nozzle at idle.

But, if you have smaller diameter needle (or larger diameter nozzles)
the carbs will always drool gasoline through the nozzle.

Drag racers wouldn't mind this, in fact, having a rich mixture just
off idle is good for low ET's at the drag strip.

But a street rider or canyon rider would be better off with a leaner
mixture at idle so the motorcycle will slow down when the throttle is
fully closed.

 

Not sure about your carbs, but the ones I've worked
on, if you don't get the butterfly back together exactly
right, you get ari passing by it and a resulting fast
idle. Did you have the butterflys apart ? If so, the plate
may not be sealing as well as it should. Orientation
of the plate may make a difference here too.