[xk-engine] no power and backfire through carbs

XK-Engine
1

I remain totally paralyzed in trying to repair my ailing
Series 1 XJ6. I have 420G intake and XJ6 Series 3 ignition.

As posted before, most recently in Saloons, my engine idles
perfectly, but loses power and coughs through the carbs on
acceleration or when encountering a slight grade (both
around 2000 RPM). I can’t drive the car except at 20 MPH on
flat ground. I discovered that I can get these symptoms by
revving against the torque converter. I had my wife do this
yesterday, and I observed the coughing for the first time.
It is truly awesome: fireballs out of all three carbs–each
impressive and all three about the same. A generous spit
from the hisser landed on the wing and burned for a second
or two.

As posted before, I’ve tried everything I can think of to
find out what’s wrong. The low power problem has come on
gradually, but one day reached the ‘‘backfire’’ stage with
extremely low power. The ignition advance is as Paul S.
specified–about 10 deg at idle and about 20 at 1500 RPM,
about 25 at 2000 (didn’t try to go higher).

The problem, however, is unaffected by radical changes in
ignition timing and unaffected by radical changes in
mixture. In other words, I can’t seem to get the problem
slightly better by changing these parameters.

Cam timing is spot on and has been since I’ve owned the car.
Fuel delivery is within specs. Compression is even and fair
(90,000 miles). Valve clearances in spec. I’ve swapped out
every ignition component, sequentially, with no effect.
Connecting the ignition directly to the battery had no
effect. I’ve done every conceivable vacuum leak check–but
all at idle speed.

Questions: Does the equal cough up among all three carbs
give a clue? Could it be a single intake valve? Any vacuum
leaks that occur only under load? Thanks!–
Bob Wilkinson, 73 XJ6
Saint Louis, MO, United States
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2

In reply to a message from Robert Wilkinson sent Sat 22 Feb 2014:

Bob
You probably already did this, so please excuse my simple
response. Have you checked the oil level in your dampers?
Are the carb needles centered, and at the right height?
Joel–
ex jag, '66 E-type S1 4.2, '56 XK140dhc, '97 XJ-6
Denison, TX, United States
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3

In reply to a message from ex jag sent Sat 22 Feb 2014:

Joel,

Thanks. Yes, I’ve been through all three carbs. I kept
thinking I had missed something in one of them, but the
cough is through all three equally.

Needles centered, scribed lines aligned with where they
enter the pistons. 20W damper oil. All click nicely when
dropped. Drop down time within specs on all three
pistons/bells. All pistons rise the same amount as RPM
increase. Springs same free length and same length when
compressed with a weight. Floats correct, float needles
seem OK, inlet screens and passages clear. Sorry, just
recalling some of my futile tests over the last month. :-)–
The original message included these comments:

response. Have you checked the oil level in your dampers?
Are the carb needles centered, and at the right height?

–
Bob Wilkinson, 73 XJ6
Saint Louis, MO, United States
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4

In reply to a message from Robert Wilkinson sent Sat 22 Feb 2014:

Here are some possible culprits, among others:
1.) Intake valves not closing, valves hanging on seat edges,
2.) Clogged exhaust line,
3.) Distributor advance intermittent (isolate vacuum
advance, then check static and mechanical advance, then
check with vacuum added, using timing light looking for
intermittent large timing jitter under problem conditions),
4.) Cross talk on spark plug leads (check in dark garage
under problem conditions).

Always be prepared for fire.

Engine compression can be good with cold engine and then
have issues as the valves warm up under load.

Careful reading of a vacuum gauge attached to the engine
under the conditions where the problem occurs can help
distinguish exhaust and valve seating issues. Nowadays one
can hook up the vacuum gauge, put a mobile phone video
camera aimed at gauge while driving under the problem
conditions and then review the video.–
Roger McWilliams
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5

Hi Robert.
The fact you mention about the gradual loss of power and the investigations
you have carried out seem to say that the inlet/ignition timing is operating
as best as it can.
Rogers suggestion #2 of a clogged exhaust may be worth checking out.
Try dropping the down pipes off and see if that makes a difference (you may
want to check the nieghbours are out when performing this trial :slight_smile: )
Regards Robin O’Connor
'92 XJ40 4.0 Ltr
'66 'S’Type MoD
'95 XJ6 4.2 LWB
Auckland NZ

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6

In reply to a message from Robin and Maureen O’Connor sent Sat 22 Feb 2014:

Attach a vacuum gauge and check the readings at 2000 rpm. if
the reading starts to fall off drastically at that rpm then
the burnt gases cannot escape ‘exhaust problem’.

Other than that I would be looking at the plug
wires/distributor cap/ for arcing out or firing order–
sidsnot
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7

In reply to a message from Roger McWilliams sent Sat 22 Feb 2014:

Thanks, Roger.

I’m certain that the advance is rock stable (I’ve removed
the vacuum advance for the time being). I’ve also looked at
the plug leads in total darkness (nothing) and have
separated them by removing all of the guides. But the dark
test was at idle–not under load.

I will retest the compression with the engine hot. I have a
vacuum gauge in the car. I will pay more attention to it.
It is undamped. Vacuum drops with throttle opening. With a
spitback, the gauge movement is something else–I keep
thinking the needle will break off.

The clogged exhaust is something I’d not thought of…will
investigate. There is certainly at least some coming out
both sides.

Car (inside too) smells of petrol, if that’s a clue. I know
its set a bit on the rich side, but this smell is new.–
The original message included these comments:

Engine compression can be good with cold engine and then
have issues as the valves warm up under load.
Careful reading of a vacuum gauge attached to the engine
under the conditions where the problem occurs can help

–
Bob Wilkinson, 73 XJ6
Saint Louis, MO, United States
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8

In reply to a message from sidsnot sent Sat 22 Feb 2014:

Thanks! I have a vacuum gauge in the car and will check.
Spent today trying to get a substitute car ready for
Monday’s commute to work. Hoping it’s an exhaust problem, as
that one never occurred to me.

Double checked firing order; replaced wires, cap, rotor,
dizzy, coil, ignition amp without change in the problem.–
The original message included these comments:

Attach a vacuum gauge and check the readings at 2000 rpm. if
the reading starts to fall off drastically at that rpm then
the burnt gases cannot escape ‘exhaust problem’.

–
Bob Wilkinson, 73 XJ6
Saint Louis, MO, United States
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9

In reply to a message from Robert Wilkinson sent Sat 22 Feb 2014:

Check around on the internet to see various ways
of ‘‘reading’’ vacuum gauges. An example with several video
displays of different conditions is at
http://www.secondchancegarage.com/public/186.cfm .

Be careful to think about where you take the vacuum reading
and how that enters into the vacuum reading interpretation.
And note that vacuum conditions can change as the engine
goes from cold to warm to hot and also from idle and higher
throttle and under load, changing throttle, and so on.

Good luck.–
Roger McWilliams
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10

In reply to a message from Robert Wilkinson sent Sat 22 Feb 2014:

Bob
Did you check the diaphragms in the carbs? The E10 fuel has
caused the elasticity in some of the older ones to diminish
and crack. Also, check your coil wire if you haven’t
already.
Joel–
ex jag, '66 E-type S1 4.2, '56 XK140dhc, '97 XJ-6
Denison, TX, United States
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11

In reply to a message from ex jag sent Sun 23 Feb 2014:

Thanks Joel. Yes, I’ve replaced the coil secondary wire, and
connected the primary directly to the battery–no joy. But
I’m still hoping it’s something simple like this.

I’ve not visually inspected the jet diaphragms by taking the
carbs apart. But Joe Curto (thanks as always for about $1000
of your time, Joe) assures me that if one were cracked I
wouldn’t miss the fuel leak. The jets move properly with the
mixture screw, and I rebuilt the carbs with stainless jet
springs recently.–
The original message included these comments:

Did you check the diaphragms in the carbs? The E10 fuel has
caused the elasticity in some of the older ones to diminish
and crack. Also, check your coil wire if you haven’t
already.

–
Bob Wilkinson, 73 XJ6
Saint Louis, MO, United States
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12

In reply to a message from Robert Wilkinson sent Sun 23 Feb 2014:

Bob
Our club had a tech session where we adjusted everyone’s
A/F mixture. We found that two of the cars had cracked
diaphragms with no fuel leakage, JAT. The coil wire I was
referring to was the high tension lead from the coil to the
distributor. I had a carbon cored wire that broke off at
the distributor. The car still started, and ran fine until
I accelerated. Apparently the torque of the engine caused a
momentary disruption, which in turn resulted in backfiring
and loss of power.
Fortunately, these are simple engines, requiring only
compression, spark and fuel to run. So, it has to be one of
those three:-) Good luck.
Joel–
ex jag, '66 E-type S1 4.2, '56 XK140dhc, '97 XJ-6
Denison, TX, United States
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13

In reply to a message from ex jag sent Sun 23 Feb 2014:

I’ve been reticent to give an update because until today
I’ve made little progress. I’ve had little time to work on
the Jaguar due to my day job and the need to keep my other
pitiful car functional as a daily driver after years of neglect.

Thanks very much to all who have offered suggestions, both
on- and off-list. I tried every one of them. Collectively,
they have made a big difference. I think replacing the
dizzy (whose reluctor gap was way too close) and replacing
the relatively new but sooty plugs made the most difference.
To complicate things, opening the exhaust seemed to make
things worse. Meanwhile, the vacuum (thanks for that
suggestion)seemed to keep getting lower. Turned out that
just before removing the exhaust I didn’t tighten the dizzy
pinchbolt enough and timing became retarded. Now that that
is fixed (or partially…my advance timing light died), I
will retry relieving exhaust pressure. The left tailpipe
seems a bit anemic compared to the right.

Still not out of the woods, but I did want to thank this
list for the guidance provided. Until today, I was sure I
would be removing the head at the very least.–
The original message included these comments:

Fortunately, these are simple engines, requiring only
compression, spark and fuel to run. So, it has to be one of
those three:-) Good luck.

–
Bob Wilkinson, 73 XJ6
Saint Louis, MO, United States
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14

Hi Robert.
The actual timing is controlled by the computer, not the dizzy.
It is important to have the dizzy position accurate and there will be
members who can advise a better way than I am going to.
Mark the position of #1 plug lead on the body of the dizzy.
I would then pull the plug from #1 cylinder and rotate the engine untill it
is at top dead centre.
At this point the rotor arm should be pointing either at the mark for #1 of
the engine or towards the far left corner of the engine bay.
The correct position is towards the RH corner.
If it is not close to that position then you need to remove and re-position
the dizzy body.
If things are not aligned I believe the spark cannot jump the gap when
things get hectic.
Regards Robin O’Connor
'92 XJ40 4.0 Ltr
'66 'S’Type MoD
'95 XJ6 4.2 LWB
Auckland NZ----- Original Message -----
From: “Robert Wilkinson” wilk@wustl.edu
To: xk-engine@jag-lovers.org
Sent: Friday, March 28, 2014 10:36 AM
Subject: Re: [xk-engine] no power and backfire through carbs

In reply to a message from ex jag sent Sun 23 Feb 2014:

I’ve been reticent to give an update because until today
I’ve made little progress. I’ve had little time to work on
the Jaguar due to my day job and the need to keep my other
pitiful car functional as a daily driver after years of neglect.

Thanks very much to all who have offered suggestions, both
on- and off-list. I tried every one of them. Collectively,
they have made a big difference. I think replacing the
dizzy (whose reluctor gap was way too close) and replacing
the relatively new but sooty plugs made the most difference.
To complicate things, opening the exhaust seemed to make
things worse. Meanwhile, the vacuum (thanks for that
suggestion)seemed to keep getting lower. Turned out that
just before removing the exhaust I didn’t tighten the dizzy
pinchbolt enough and timing became retarded. Now that that
is fixed (or partially…my advance timing light died), I
will retry relieving exhaust pressure. The left tailpipe
seems a bit anemic compared to the right.

Still not out of the woods, but I did want to thank this
list for the guidance provided. Until today, I was sure I
would be removing the head at the very least.

The original message included these comments:

Fortunately, these are simple engines, requiring only
compression, spark and fuel to run. So, it has to be one of
those three:-) Good luck.

–
Bob Wilkinson, 73 XJ6
Saint Louis, MO, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
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15

DOH !! sorry ignore that load of rubbish I just posted, I was thinking of
the AJ6 engine for the '40
Regards Robin O’Connor
'92 XJ40 4.0 Ltr
'66 'S’Type MoD
'95 XJ6 4.2 LWB
Auckland NZ

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16

In reply to a message from Robin and Maureen O’Connor sent Thu 27 Mar 2014:

Sadly, I am still right where I started…backfire through
carbs under even slight load (accelerating or going uphill).

I have completely rebuilt the rear carb, have the front carb
on my bench and will do the center carb as well.
So far I can find nothing wrong with them. Then, on the
advice of a local guru, I will observe fuel pressure whilst
driving the car. Just about at the end of my rope!

Will post again when the above completed.–
Bob Wilkinson, 73 XJ6
Saint Louis, MO, United States
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17

In reply to a message from Robert Wilkinson sent Fri 18 Apr 2014:

Bob,

Guessing you have an S1 with carbs? If EFI, no experience
but if carbs then check the fuel filter bowl. Had a friend
with similar problem - no power up hill - so basically when
under load. Piece of gasket had let go and was blocking the
fuel flow but only under high flow/demand. It was not able
to be seen in the glass bowl - think it may have still been
partially attached. If it is any consolation the initial
assumption was the carbs…
Observing the fuel pressure while driving would seem to me
to involve a lot of fuel line running into the car. And the
risk of something letting go and spraying gas everywhere
would not be something I would do if I could avoid it.

Fuel flow is more important and easier to measure. You can
check the fuel pressure but the pressure is usually quite
low if carbed - in the 2.5 to 3.5 psi range and if you lose
pressure then you will still have to check the fuel flow
anyway. And the fuel flow is more likely to be causing the
problem.

So if nothing obvious in the fuel filter then check the fuel
flow from the fuel filter to the carbs. With the electric
fuel pump I think you should be around 1 liter/1 quart per
minute - may be more - can’t remember… May be in the
archives somewhere. If the flow down then blow the filter
housing out with some compressed air. While you have the
filter apart then also check the flow from the fuel tank to
the filter and that should tell you something as well. The
input form the tank should equal the output to the carbs and
be in the 1 quart per minute range. If the input is low I
would guess around or less than 1 pint per minute then there
is a problem back to the tank. If the input is OK and the
output low then the fuel filter/bowl is the problem.
Start with pulling the bowl apart and having a look.

Regards

Keith–
The original message included these comments:

Sadly, I am still right where I started…backfire through
carbs under even slight load (accelerating or going uphill).
I have completely rebuilt the rear carb, have the front carb
on my bench and will do the center carb as well.
So far I can find nothing wrong with them. Then, on the
advice of a local guru, I will observe fuel pressure whilst
driving the car. Just about at the end of my rope!
Will post again when the above completed.

–
Keith Bertenshaw
Rockaway, NJ, United States
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18

In reply to a message from Robert Wilkinson sent Fri 18 Apr 2014:

Another update on my backfire saga and a couple of questions
S1 XJ6 with Mark X intake):

I completed rebuilding all three carbs. I also plumbed a
fuel pressure gauge visible through the windscreen whilst
driving. Said pressure is about 3 PSI at tickover or while
driving at low speed, but drops with acceleration to about
1.5 PSI and, accelerating more, drops to near zero when the
backfire occurs.

This happebns regardless of which fuel pump/tank is switched in.

I then removed the fuel filter housing and blew air (without
restriction) through the hard line from the engine
compartment to the boot. I replace the rubber line in the
boot and checked the short metal line in the boot. All
appeared fine. I put it back together sans filter element
(which had looked fine). Identical problem–pressure drops
as the engine demands more fuel flow.

I’ve concluded that both of my pumps are faulty (for good
measure I removed the tank ‘‘sock’’ filter from one side–no
difference). The first thing I did when this problem first
appeared was to test the deadhead pressure of both
pumps–perfectly within specs. I also tested the flow rate
(time to deliver a certain volume) with the container
sitting atop the engine. This too was right in the middle
of specifications. Sadly, these early tests caused me to
eliminate fuel supply as a cause of the problem.

Question: has anyone seen an SU fuel pump that delivers the
correct deadhead pressure (at zero flow) AND the correct
unrestricted flow rate (zero pressure) yet fails miserably
to generate flow under actual operation?

Related question: My hisser has always leaked from its fuel
inlet banjo fitting. When I had things apart, I paid close
attention to all surfaces, dressing them with fine
wet-or-dry sandpaper on a granite surface plate, and
reassembling with five new seals from Joe Curto (a fibre
seal and an aluminum one for the banjo fitting onto the jet
chamber, and three fibre washers for the banjo fitting that
passes through the stamped steel support at the hisser. I
have it tight as I dare, but it still weeps fuel, apparently
from the hisser banjo–mostly below the steel support but a
bit above it as well. Should I use some sort of sealant? I
have absolutely no trouble with the fuel inlet banjos on the
HDSs themselves.

Thanks!–
The original message included these comments:

So far I can find nothing wrong with them. Then, on the
advice of a local guru, I will observe fuel pressure whilst
driving the car. Just about at the end of my rope!
Will post again when the above completed.

–
Bob Wilkinson, 73 XJ6
Saint Louis, MO, United States
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19

Yes, the fuel pump in my XK120 had the same characteristic. The static
pressure was fine, the flow was fine but it wouldn’t let me drive the car.
As a temporary test I removed the flexible fuel line to the carbs and
installed an electric pump from J C Whitney with clip leads connecting it to
the power terminal of the ignition coil and ground. It worked fine so it
stayed that way for several years. I rebuilt the pump in my MK2 in 1985 and
it has worked flawlessly ever since, but despite repeated attempts I was
never able to get the XK120 pump to work properly.

Mike Eck
New Jersey, USA

'51 XK120 OTS, '62 3.8 MK2 MOD, '72 SIII E-Type 2+2

Question: has anyone seen an SU fuel pump that delivers the
correct deadhead pressure (at zero flow) AND the correct
unrestricted flow rate (zero pressure) yet fails miserably
to generate flow under actual operation?

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20

In reply to a message from Robert Wilkinson sent Sat 10 May 2014:

used permatex non-hardening on the ASC equipped HD8 that I
repaired, that we discussed on another thread.

It doesnt leak fuel (in a standing test only)

When first reassembled without permatex, it did weep a
little at the big banjo on the hisser, my other cars have
leaked there too, never had much trouble with other banjos
either.

As you are up against it, you could try this, it worked for
me once. (much easier than what you have done so far, and
your next steps)

buy a universal 3psi fuel pump, plumb safely to a barbed 5
litre fuel container, mount under bonnet, test drive up
steep hill.

A pass indicates pumps, tanks or lines etc.
A fail means something else is at fault.

In my case it was the fuel pump (on an 8 tonne truck)

Still have the pump, and would take it in my boot if my Jags
were single pump models–
The original message included these comments:

Question: has anyone seen an SU fuel pump that delivers the
correct deadhead pressure (at zero flow) AND the correct
unrestricted flow rate (zero pressure) yet fails miserably
to generate flow under actual operation?
Related question: My hisser has always leaked from its fuel
inlet banjo fitting. When I had things apart, I paid close
have it tight as I dare, but it still weeps fuel, apparently
from the hisser banjo–mostly below the steel support but a
bit above it as well. Should I use some sort of sealant? I
have absolutely no trouble with the fuel inlet banjos on the

–
Tony
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