carburetor tuning
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carburetor tuning tjbabar 05-31-2008
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Posted by tjbabar on May 31, 2008, 6:57 pm
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

Posted by Jack Hunt on May 31, 2008, 8:09 pm

>Does anyone have any ideas? What am I
>missing?

Well first off, there is no such thing as an air screw.

--
Jack

Posted by . on May 31, 2008, 9:31 pm

> Well first off, there is no such thing as an air screw.

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



Posted by Jack Hunt on June 1, 2008, 12:13 am

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

And it does what when you turn it? Does it regulate air flow? No, it regulates
fuel mixture by restricting the gas flow. Just because some idiot calls it an
air screw doesn't make it so. If there is an "air screw" on a carb, it's the
idle speed screw because that's the only screw on a carb that has anything to do
with air flow.

For the original question, put all the jets back to stock and clean the internal
passages in the idle circuit. If the jets were the right size once, they'll
always be the right size unless the intake or exhaust has been messed with and
has changed the airflow.

The idle is messed up because the idle circuit is partially plugged and the only
way to get it to run is to ramp up the idle stop screw until it's partially
drawing on the main gas jets.

--
Jack

Posted by . on June 1, 2008, 9:09 am

> And it does what when you turn it? =EF=BF=BDDoes it regulate air flow? =EF=
=BF=BD

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.
>
> The idle is messed up because the idle circuit is partially plugged and th=
e only
> way to get it to run is to ramp up the idle stop screw until it's partiall=
y
> drawing on the main gas jets.

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


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