As some of you know from the XK forum, I pulled my
original motor 2years ago and installed a rebuild XK140C
motor I purchased from John Meering (RIP).
Originally I just threw on the H6 carbs from my original
SE spec 120 motor. With the original 60 year old needles
it was pretty much the opposite of this (14.5 at WOT and
12.5 to 1 at idle). Plugs would foul up around town, PITA.
Because I have installed a spare/new motor plus a wideband
AF sensor, I’ve started tuning the carbs by trying diif
needes. looking for feedback/ideas on tuning.
Without modification but with new RE needles/jets here is
where I am.
IDLE: 14.5 to 1
Light throttle cruise (around town , up to 60mph): 13.4 to
13.7 depending on rpm. Above 3500rpm it drops to around
13:1.
Medium throttle (3000-4000 rth gear highway cruising):
13.1 to about 13.5 to 1
Wide open throttle (accel and up very steep long hills in
4th gear): 13:1 below 2000rpm gradually richening up to
12.4 to 1 above 4000rp, redline is 5500rpm).
What would you guys focus on changing first and what
target AF ratios would you go for? Should I go much leaner
at idle?
Is 13.5 too rich when I’m cruising down the highway at
85mph?
John–
John
Boston, MA, United States
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In reply to a message from John sent Mon 30 May 2016:
John,
14.7:1 is chemically ideal, but not optimum for anything other
than the life of a catalytic converter. It is also not ideal
for emissions, it depends on what you ‘‘designate’’ a
‘‘pollutant’’.
Best power under WOT occurs between 12.8-13.5 AFR. Best
economy occurs around 15.5-16.5 AFR under light load cruise
with +50 BTDC ignition advance. Smoothest idle is around 13.5
AFR, most XK’s struggle at idle leaner than 14:1. These ideals
are easy to achieve with programmable EFI, very hard to get
with SU needles.
My advice is to start with AFR under load at WOT. I would
target 13 AFR, 12.4 is definitely too rich for a N/A road car.
This is the only place that being too lean is a risk. Too lean
at idle or cruise is not a big issue, detonation does not occur
under light or no load. I have run XK’s at +15:1 AFR at cruise
with +50 BTDC ignition timing, (60 mph, 16% throttle, 18
inches vacuum) and the engines look perfect inside. After you
get the correct high load mixture, the trick will be to have a
lean cruise in the 14’s. Idle really is irrelevant (unless you
are running a stationary engine). You just want a stable idle
under all temperatures which is hard to do with SU’s. Have you
measured AFR with the starting carb on ? It’s no wonder these
engines wore out quickly with such sloppy fueling. The reason
modern engines go +200,000 miles is proper fueling and nothing
can touch a closed loop feedback EFI system.
Have you plotted out the ignition timing curve, with and
without vacuum advance ? Don’t forget the ignition timing, it
is half the equation for getting the optimum tune on a SI
engine.
Paul–
The original message included these comments:
As some of you know from the XK forum, I pulled my
original motor 2years ago and installed a rebuild XK140C
motor I purchased from John Meering (RIP).
Originally I just threw on the H6 carbs from my original
In reply to a message from PS sent Wed 1 Jun 2016:
Paul
Ok sounds like I should just crank in the jet screw a 1/2
turn and let it go leaner at idle. I’ll shoot for 14:1 at
light throttle cruise. As long as idle is stable what
does it matter if it is too lean at 700rpm (maybe a little
more heat generated)? Right?–
John
Boston, MA, United States
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In reply to a message from PS sent Wed 1 Jun 2016:
I have not measured AFR with the staring carb on. I’m
afraid it will foul the O2 sensor. I warm it up, wait 5
min for the exhaust to cool slightly then remove the bung
plug and install the sensor.
I have not checked timing other than at idle and at
2500rpm to verify its around the mid 30’s (advance). I
have toyed with the idea of getting a fully programmable
123 distributor but it does seem like a little overkill
for a 120. Oh and I disassembled the distributo enough to
clean the advance weights (lots of old gooey thick oil).
So I believe the mech and vac advance are working.–
John
Boston, MA, United States
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In reply to a message from John sent Wed 1 Jun 2016:
Update: I leaned it out till I say 14.5:1 at idle.
As you said, the idle quality is not the best, it fails
the carb lifting pin test (stalls when pin lifted). Also,
car won’t hold an idle till well warmed up, perhaps as
much as 10min of running required before car will idle at
the warn set point of 700rpm. But the car drives great,
the plugs aren’t loading up so it pulls smoothly in town
from 800rpm even in 4th gear.
So, I need to tweak the fat part of the needle slightly.
Anyone know how far from the big end I should go to affect
idle-1000rpm? And as far as how much to remove, I’m just
going to start with how much open area there is now and
thin it to added a % of area equal to the desired
additional fuel percentage. 400 grit wet and dry glued to
the edge of a 2mm wide piece of abs plastic, and spin it
with the needle chucked up in a hand drill. I’ll mark the
area with a permanent marker (2 colors) so I know where to
sand and where not to.
In reply to a message from John sent Sat 11 Jun 2016:
A few Jay friends got together this weekend be wa took the
ch Fe to check everyone’s AFR. There were three xk120’s
and a mk2. Everyone’s idle was around 12:1. They all idled
b auto fully smoothly. The Mk2 has a true dual exhaust and
th front was 13:1 and the rear half was about 11:1.
Given how well these other cars run, it seems meaningless
to try to achieve an idle AFR any here close to 14:1,
particularly when this lean setting results in a weak
idle.–
John
Boston, MA, United States
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In reply to a message from PS sent Wed 1 Jun 2016:
Agree with what Paul said…kinda…since we don’t have
cats…or emission worries…disregard 14.7, and hope you are
not obsessed with measuring fuel mileage on your xk, so when
you boil that down…for an XK…it is hard to get SU carbs
to idle ‘‘perfectly’’…but you can get quite close, and in
really…1. - doesn’t matter much at idle unless you like
to idle a lot, …and 2. idle will and should probably be a
bit rich…to get what you want in sporty driving…as Paul
said about 13 to 13.5 It may ‘‘burble’’ rich at idle then
smooth out in acceleration. AFR interesting but not
essential: Test drive–adjust–and as always…set plug gaps
(proper heat range for your car and driving), set
timing…and adjust timing via test drive. It is like a
recipe for a good martini…the right quality ingredients in
the right amounts. The old saying…put your martini glass
on top of the engine…if it stays still…all is good. Not
shaken, not stirred. To clarify timing…or perhaps muddle
it…remember factory orig timing was for old 80 octane fuel,
old back in the day narrow range spark plugs, and mechanical
advance in the Lucas Jaguar distributors varied over time
and car state of tune. A good goal now is 34-36 deg BTDC at
2500-3100 rpm with mech all in, but to get this and not
exceed 10-12 static you may have to modify the mech advance
in the dizzy. Adding the vac advance of maybe 8-12…and you
get 42-46 at cruise. I think 50 is a bit too much. Earliest
XKs will be a little less if stock distributor.
Nick–
Nick53XK120S
Spokane WA, United States
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Does anyone have any exhaust gas temperatures to go with these A/F figures? I put an EGT ga. in my 420. At times with a light throttle at highway speeds I have seen 1400 degrees. After rebuilding the carbs it was about 1250. With a plane we just look for peak EGT and then just enrich to lower temp about 50 degrees. Has anyone used an EGT ga on these?