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carburetor tuning
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
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
Re: carburetor tuning
On Sat, 31 May 2008 15:57:04 -0700 (PDT), tjbabar <tjbabar@yahoo,com > wrote:
>Does anyone have any ideas? What am I
>missing?
Well first off, there is no such thing as an air screw.
--
Jack
On Sat, 31 May 2008 15:57:04 -0700 (PDT), tjbabar <tjbabar@yahoo,com > wrote:
>Does anyone have any ideas? What am I
>missing?
Well first off, there is no such thing as an air screw.
--
Jack
Re: carburetor tuning
On May 31, 5:09=EF=BF=BDpm, Jack Hunt <jhun...@tds,net > wrote:
> 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".
On May 31, 5:09=EF=BF=BDpm, Jack Hunt <jhun...@tds,net > wrote:
> 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".
Re: carburetor tuning
On Sat, 31 May 2008 18:31:49 -0700 (PDT), "." <RhiannonX@gmail,com > wrote:
>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
On Sat, 31 May 2008 18:31:49 -0700 (PDT), "." <RhiannonX@gmail,com > wrote:
>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
Re: carburetor tuning
On May 31, 9:13=EF=BF=BDpm, Jack Hunt <jhun...@tds,net > wrote:
> 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...
On May 31, 9:13=EF=BF=BDpm, Jack Hunt <jhun...@tds,net > wrote:
> 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...
Re: carburetor tuning
On Sun, 1 Jun 2008 06:09:00 -0700 (PDT), "." <RhiannonX@gmail,com > wrote:
>OP says that the idle stop screw isn't even touching the throttle
>quadrant...
If a carb was running right and it sat unused for a couple of years, and now
it's not running right, what's wrong? Did the jets magically change sizes? No.
Does the engine suddenly require some other air/fuel ratio? No. Did some
mysterious screw turn itself several turns in the wrong direction? No.
Is something plugged up? Yes. Will changing jet sizes fix a plugged passage
somewhere else? Get real. First return the engine and carb to the original
state, THEN fiddle with the jet sizes if you must.
Pull the carb, put the jets back to stock size, and clean all the internal
passages. Don't just wipe out the bowl. Used compressed air or carb cleaner in
a spray can with a flexible spout and make sure you have flow through every
passage. Put that "air screw" back where it was before you started messing with
it. When you get everything really clean, it'll run right. Until you get
everything clean, it won't run right no matter what you do.
Check the float level, and check the float for a hole. If it's a hollow float,
hold it near your ear and shake it to see if it has fuel inside, which will
cause the fuel level in the float bowl to be too high and it'll run rich. Check
the "choke" circuit to make sure the enricher valve is closing completely.
If all else fails, make sure the throttle cable isn't too tight and is holding
the throttle slightly open and that you're not compensating for that by closing
off the idle circuit to get the idle speed down.
--
Jack
On Sun, 1 Jun 2008 06:09:00 -0700 (PDT), "." <RhiannonX@gmail,com > wrote:
>OP says that the idle stop screw isn't even touching the throttle
>quadrant...
If a carb was running right and it sat unused for a couple of years, and now
it's not running right, what's wrong? Did the jets magically change sizes? No.
Does the engine suddenly require some other air/fuel ratio? No. Did some
mysterious screw turn itself several turns in the wrong direction? No.
Is something plugged up? Yes. Will changing jet sizes fix a plugged passage
somewhere else? Get real. First return the engine and carb to the original
state, THEN fiddle with the jet sizes if you must.
Pull the carb, put the jets back to stock size, and clean all the internal
passages. Don't just wipe out the bowl. Used compressed air or carb cleaner in
a spray can with a flexible spout and make sure you have flow through every
passage. Put that "air screw" back where it was before you started messing with
it. When you get everything really clean, it'll run right. Until you get
everything clean, it won't run right no matter what you do.
Check the float level, and check the float for a hole. If it's a hollow float,
hold it near your ear and shake it to see if it has fuel inside, which will
cause the fuel level in the float bowl to be too high and it'll run rich. Check
the "choke" circuit to make sure the enricher valve is closing completely.
If all else fails, make sure the throttle cable isn't too tight and is holding
the throttle slightly open and that you're not compensating for that by closing
off the idle circuit to get the idle speed down.
--
Jack
Re: carburetor tuning
On Jun 1, 6:38=EF=BF=BDam, Jack Hunt <jhun...@tds,net > wrote:
> Check the float level, and check the float for a hole. =EF=BF=BDIf it's a =
hollow float,
> hold it near your ear and shake it to see if it has fuel inside, which wil=
l
> cause the fuel level in the float bowl to be too high and it'll run rich. =
=EF=BF=BDCheck
> the "choke" circuit to make sure the enricher valve is closing completely.=
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.
On Jun 1, 6:38=EF=BF=BDam, Jack Hunt <jhun...@tds,net > wrote:
> Check the float level, and check the float for a hole. =EF=BF=BDIf it's a =
hollow float,
> hold it near your ear and shake it to see if it has fuel inside, which wil=
l
> cause the fuel level in the float bowl to be too high and it'll run rich. =
=EF=BF=BDCheck
> the "choke" circuit to make sure the enricher valve is closing completely.=
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.
Re: carburetor tuning
On Sun, 1 Jun 2008 08:19:27 -0700 (PDT), "." <RhiannonX@gmail,com > wrote:
>It takes less vacuum to lift fuel out of the float bowls when the
>level is too high, so the engine runs rich.
Just so we're all singing from the same book here, can the OP tell us what
criteria he used to determine it was idling rich? Sometimes the greatest
maintenance adventures begin with a misdiagnosis. ;-)
--
Jack
On Sun, 1 Jun 2008 08:19:27 -0700 (PDT), "." <RhiannonX@gmail,com > wrote:
>It takes less vacuum to lift fuel out of the float bowls when the
>level is too high, so the engine runs rich.
Just so we're all singing from the same book here, can the OP tell us what
criteria he used to determine it was idling rich? Sometimes the greatest
maintenance adventures begin with a misdiagnosis. ;-)
--
Jack
Re: carburetor tuning
On Jun 1, 1:17 pm, Jack Hunt <jhun...@tds,net > wrote:
> On Sun, 1 Jun 2008 08:19:27 -0700 (PDT), "." <Rhiann...@gmail,com > wrote:
> >It takes less vacuum to lift fuel out of the float bowls when the
> >level is too high, so the engine runs rich.
>
> Just so we're all singing from the same book here, can the OP tell us what
> criteria he used to determine it was idling rich? Sometimes the greatest
> maintenance adventures begin with a misdiagnosis. ;-)
>
> --
> Jack
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.
On Jun 1, 1:17 pm, Jack Hunt <jhun...@tds,net > wrote:
> On Sun, 1 Jun 2008 08:19:27 -0700 (PDT), "." <Rhiann...@gmail,com > wrote:
> >It takes less vacuum to lift fuel out of the float bowls when the
> >level is too high, so the engine runs rich.
>
> Just so we're all singing from the same book here, can the OP tell us what
> criteria he used to determine it was idling rich? Sometimes the greatest
> maintenance adventures begin with a misdiagnosis. ;-)
>
> --
> Jack
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.
Re: carburetor tuning
On Jun 1, 8:47=EF=BF=BDpm, tjbabar <tjba...@yahoo,com > wrote:
> The reason I suspect the mixture is rich is because the spark plugs are da=
rk > black with carbon build up. =EF=BF=BDI have read elsewhere that float l=
evel can cause > the idle to run away. =EF=BF=BDHopefully that is it.
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.
On Jun 1, 8:47=EF=BF=BDpm, tjbabar <tjba...@yahoo,com > wrote:
> The reason I suspect the mixture is rich is because the spark plugs are da=
rk > black with carbon build up. =EF=BF=BDI have read elsewhere that float l=
evel can cause > the idle to run away. =EF=BF=BDHopefully that is it.
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.
Re: carburetor tuning
On Jun 1, 8:47 pm, tjbabar <tjba...@yahoo,com > wrote:
> On Jun 1, 1:17 pm, Jack Hunt <jhun...@tds,net > wrote:
>
> > On Sun, 1 Jun 2008 08:19:27 -0700 (PDT), "." <Rhiann...@gmail,com > wrote:
> > >It takes less vacuum to lift fuel out of the float bowls when the
> > >level is too high, so the engine runs rich.
>
> > Just so we're all singing from the same book here, can the OP tell us what
> > criteria he used to determine it was idling rich? Sometimes the greatest
> > maintenance adventures begin with a misdiagnosis. ;-)
>
> > --
> > Jack
>
> 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.
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.
On Jun 1, 8:47 pm, tjbabar <tjba...@yahoo,com > wrote:
> On Jun 1, 1:17 pm, Jack Hunt <jhun...@tds,net > wrote:
>
> > On Sun, 1 Jun 2008 08:19:27 -0700 (PDT), "." <Rhiann...@gmail,com > wrote:
> > >It takes less vacuum to lift fuel out of the float bowls when the
> > >level is too high, so the engine runs rich.
>
> > Just so we're all singing from the same book here, can the OP tell us what
> > criteria he used to determine it was idling rich? Sometimes the greatest
> > maintenance adventures begin with a misdiagnosis. ;-)
>
> > --
> > Jack
>
> 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.
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.
