I agree with Alastair. My XK120 does the same thing. It happens after the
starting carb kicks off but before the engine builds up a little warmth. It
only does it with a moderate or heavier foot on the accelerator pedal and is
worst right after the starting carb kicks off. It improves as the engine
temp rises. To avoid embarrassment, I have learned to drive like there’s “an
egg under my foot” until the car is suitably warm.
That said, the problem was much worse after I “adjusted” the carbs. I
adjusted and checked the plugs many times until I achieved that optimum
gray-brown they show in the pictures. There was only one problem – that was
proper adjustment at idle.
I have learned that the secret to tuning SU carbs, short of experimenting
with needles, is to take the car to a flat stretch of road where the you can
hold the rpms at your normal cruising speed. On the XK120, that is 2500-3000
(or 55-65 mph in 4th gear). After you’ve held the speed for fifteen seconds
or more, depress the clutch and immediately turn off the ignition. If you
need turn signals, throw it out of gear and turn the ignition back on. Coast
to a stop someplace safe. (It’s best to plan ahead for that part.) Don’t
forget to compensate for lack of power brakes, if equipped.
Pull the plugs and check for the proper color. Pull #2 & #5 first. If they
are way off, don’t bother to pull any more. Adjust the carbs as needed:
clockwise leans, anti-clockwise richens. Repeat, and pull two different
plugs, one from the front three and one from the back. When you get close,
check all the plugs. When they all look good, balance the carbs.
I did this after setting my carbs perfectly at idle. The plugs came out bone
white! Perfect idle meant grossly lean at speed. After a number of runs, I
had the plugs coming out a light gray, which may be just a tad lean. At
idle, however, the plugs come out black. That tells me I need to do some
needle work, but at least I know I’m not going to burn a piston.
“Mark 1” Mark Stephenson
1952 XK120 Roadster #673129 (w/XK140 engine and C head)
1958 3.4 Litre Saloon / 1984 XJ6 4.2L / 1985 XJ6 VDP
Jaguar Club of Central Arizona (USA) – www.cableone.net/jcca----- Original Message -----
From: “Steve” firstname.lastname@example.org
Sent: Thursday, December 09, 1999 8:22 PM
Subject: [Saloon-lovers] Back Firing thru the Carbs during Warm-up (Chch,
Hi Jag Lovers,
I am new to the group but have been a saloon lover for many years. I own a
1962 Mark 2 that started out a 2.4L but was converted to a 3.4L a few years
ago when I had the drive train completely re-built.
Since New Zealand converted from Leaded to Unleaded petrol during my
rebuild the engine has been run-in on 91 unleaded. Recently after about 5000
miles I have converted over to 96. This has provided more power, less
pinging but a slightly rougher idle. More seriously, I find that when the
engine reaches about 50 deg C, upon acceleration the engine back fires
through the carbs.
I have taken my classic to several Jag mechanics who have tuned and
re-tuned it. Some are now blaming the inferior quality of our higher octane
petrol (one might say the introduction of unleaded petrol in NZ did not go
I should add that with earlier fears of damaging valve seats I had a
device fitted in series with the fuel line called a “Fuelstar”. This has
been fitted close to the carbs and is claimed to add further protection to
the valve seats and boost the fuel’s octane rating so that 91 fuel can be
used without pinging (I can vouch for the latter but not the former).
If anyone has any suggestions on what I should look for to prevent this
backfiring through the carb’s I’d be very appreciative. The engine runs like
a clock once hot (75 deg C), but the warm-up period can be a bit of a
Thanks for your time.
Steve W. (1962 Mk2, 3.4L)
P.S. I think the responses to this news group have been great! My only
criticism is that more women saloon lovers should be contributing! Why
should Jag saloons be a mostly male domain?