Greetings, I’m having a rich condition on my rear carb. (I have 3
SUs) I have installed new main jets and new needles (UM) and gone
through them with a Colortune several times but can never get the
rear carb lean enough to burn a blue flame. As it nears the lean
color I run out of adjustment and the engine runs rough. The air
bypass screws are all even, out about 1/8 turn from their stop.
And, yes, I have made sure the choke rod isn’t hanging up. When
the choke pull is pushed forward to the dash, the choke is off. I
have also checked for vacumn leaks by spraying starting fluid
around, but found none. Any ideas out there or similar conditions?–
70 XKE, 52 XK120fhc
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from Stu H sent Mon 8 Dec 2014:
Well, Stu, I have JUST gone through all of this (including the
Colortune) with the SUs on my '70. I noticed that you did not
mention the float adjustment. Peering into your rear float bowl
you might find that the float itself has gone the way of the
Titanic, i.e., it has gone to the bottom allowing petrol to
flood into the throat of the carburetor.
If your floats are original, I’d recommend replacing and
adjusting them all as a matter of course.
I might add that I enjoyed the Colortune experience. I saw a
nice blue color at all six cylinders, but an air-fuel meter
revealed that I was running seriously rich, so now I depend on
that more accurate device.–
The original message included these comments:
Greetings, I’m having a rich condition on my rear carb. (I have 3
SUs) I have installed new main jets and new needles (UM) and gone
through them with a Colortune several times but can never get the
rear carb lean enough to burn a blue flame. As it nears the lean
color I run out of adjustment and the engine runs rough. The air
bypass screws are all even, out about 1/8 turn from their stop.
And, yes, I have made sure the choke rod isn’t hanging up. When
the choke pull is pushed forward to the dash, the choke is off. I
have also checked for vacumn leaks by spraying starting fluid
around, but found none. Any ideas out there or similar conditions?
–
Randall Carlson '70 FHC, '71 MGB roadster, '97 Ply Voyager
Fairfield, California, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from Rancarl sent Mon 8 Dec 2014:
I’m sorry, I was just going to mention my float level when your
answer appeared. I have them set at the correct 7/16’’ but now
after searching the forums I see some are running float level
lowed. The original brass floats are still good, still swimming as
Nobby Clark would say. Grose jets are fitted, now wondering about
returning to Viton?–
The original message included these comments:
mention the float adjustment. Peering into your rear float bowl
you might find that the float itself has gone the way of the
Titanic, i.e., it has gone to the bottom allowing petrol to
flood into the throat of the carburetor.
In reply to a message from Rancarl sent Mon 8 Dec 2014:
An addendumb to my previous message: I should have mentioned
that the needle in the rear float bowl could be the culprit
as well. Sometimes it is unable to close due to a piece of
grit. Sometimes it’s just shot.
An interesting test is to to turn on the ignition without
starting the car. Does the SU pump (if you have one)
continue to click? Is raw gas streaming into your carb throat?–
Randall Carlson '70 FHC, '71 MGB roadster, '97 Ply Voyager
Fairfield, California, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from Rancarl sent Mon 8 Dec 2014:
I don’t think that’s the problem as the SU pump thumps a couple of
times while filling the bowls, then quits as per usual. I’ll check
for raw gas though. Thanks.–
70 XKE, 52 XK120fhc
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from Stu H sent Mon 8 Dec 2014:
Check that the needle is correctly positioned in the
piston. The groove in the side should just disappear into
it when inserted, then tighten. Kind of a stupid procedure.
Jerry–
Jerry Mouton '64 FHC 889791 ‘MIK Jaguar’
Palo Alto, California, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from Stu H sent Mon 8 Dec 2014:
Check to make sure the piston drops freely all the way down onto
the bridge and that the jet is properly centred.–
The original message included these comments:
Greetings, I’m having a rich condition on my rear carb. (I have 3
In reply to a message from Stu H sent Mon 8 Dec 2014:
Standard setting for the air bypass screw is 2 turns isn’t it? And
you’re are at 1/8 turn. That tells us you are getting too much air
somewhere else. First make sure the choke fast idle screw isn’t
holding the butterfly open.
I set my needle’s height the same with a caliper to the thousanth.
I set my jet height at identical ‘‘drop’’ height as well when
starting the tune up. Set each carbs mix screw to standard setting.
Measure/average/ adjust to identical ‘‘drop’’ First make sure the
fast idle screw isn’t holding the butterfly open. It seems to get
me in the ballpark quickly
Are the clamps inter-connecting the carb shafts all disconnected
while tuning?
If no air leak, grab each carb’s throttle shaft, and see if you can
move the shaft axially. You could spray carb cleaner at the shafts
and listen for an engine note change. If you detect play your shaft
bushings are perished. Renew. See Mr. Curto for parts.
Also, your butterflies may not be closing fully. Carbs off, can you
see light around the perimeter? If so, loosen the 2 peened or split
screws holding them to the shaft. Center the discs in the throat.
Best if you have new screws to resplit as the old split part could
break off. Don’t touch’em if you don’t feel you can improve
the ‘‘fit’’.
good hunting~!
Dave–
1969 BRG OTS
Skaneateles, NY, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from David Ahlers sent Mon 8 Dec 2014:
Must admit I didn’t know to disconnect the carb shafts, I’ll try
that. I also suspect the fast idle screws are in fact holding
throttles open a bit. The shafts feel tight, and no leaks detected
when I spray around with carb cleaner. Thanks for the help!–
The original message included these comments:
Measure/average/ adjust to identical ‘‘drop’’ First make sure the
fast idle screw isn’t holding the butterfly open. It seems to get
me in the ballpark quickly
Are the clamps inter-connecting the carb shafts all disconnected
while tuning?
In reply to a message from Stu H sent Tue 9 Dec 2014:
Stu,
A thin feeler gauge or piece of paper should easily slip
under the fast idle screw when the choke is off.
Gee, once the carbs are balanced I would not disconnect the
carb shafts until the next decade when I needed to balance
them again…?
Jerry–
The original message included these comments:
Must admit I didn’t know to disconnect the carb shafts, I’ll try
that. I also suspect the fast idle screws are in fact holding
throttles open a bit. The shafts feel tight, and no leaks detected
when I spray around with carb cleaner. Thanks for the help!
–
Jerry Mouton '64 FHC 889791 ‘MIK Jaguar’
Palo Alto, California, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from mouton sent Tue 9 Dec 2014:
Jerry - didn’t Ray L. write up a SU tuning tutorial? Where can Stu
find it?. He should step by step it before resorting to any drastic
measures.
D–
1969 BRG OTS
Skaneateles, NY, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from David Ahlers sent Tue 9 Dec 2014:
Dave,
Yes he did – I believe it’s on here somewhere but I’m not
sure how to get to it. I agree about step by stepping it.
Stu, you could follow the instructions in the service
manual, and ask for any clarification…
Jerry–
The original message included these comments:
Jerry - didn’t Ray L. write up a SU tuning tutorial? Where can Stu
find it?. He should step by step it before resorting to any drastic
measures.
–
Jerry Mouton '64 FHC 889791 ‘MIK Jaguar’
Palo Alto, California, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from Craig_G sent Wed 10 Dec 2014:
Thanks, Craig. Rays method is just a bit different from
the Bentley book, and I’ll bet it works better. I’ll try
it out on Saturday.–
70 XKE, 52 XK120fhc
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–