[xj] Stumbling on takeoff, 1978 XJ6 ser.2

XJ
1

Have a stumbling problem on the 78 after driving a
while.

Its warmed up every time it does that.
Its hesitating before picking up speed, it might
do this about 3 times, then its OK.
Adjusted mixture screw to lean, with no effect.
Adjusted mixture screw to richer, with no effect.
Adjusted .002’’ throttle plate to .003’’ with no effect.
I have a new fuel pump.

Have new fuel filters.
New spark plugs.
New ignition coil.
Replaced ‘‘O’’ ring at mixture adj. screw.

I had this problem for some time now.

Any advise from anybody, what I’m overlooking?

                             Walter--

Walter Schuster 78XJ6 FI Ser.II, 2002 xtype 3.0
Albuquerque/New Mexico, United States
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2

Walter,

How about a distributor problem? What is the timing set at?

BTW, what kind of new plugs did you install and what did you set the gap
to?

Regards,

Paul M. Novak

1990 Series III V12 Vanden Plas
1990 XJ-S Classic Collection convertible
1987 XJ6 Vanden Plas
1984 XJ6 Vanden Plas
1969 E-Type FHC
1957 MK VIII Saloon
Ramona, CA
P.M.Novak7@gmail.com-----Original Message-----
From: owner-xj@jag-lovers.org [mailto:owner-xj@jag-lovers.org] On Behalf Of
W. Schuster
Sent: Tuesday, November 01, 2016 1:39 PM
To: xj@jag-lovers.org
Subject: [xj] Stumbling on takeoff, 1978 XJ6 ser.2

Have a stumbling problem on the 78 after driving a while.

Its warmed up every time it does that.
Its hesitating before picking up speed, it might do this about 3 times, then
its OK.
Adjusted mixture screw to lean, with no effect.
Adjusted mixture screw to richer, with no effect.
Adjusted .002’’ throttle plate to .003’’ with no effect.
I have a new fuel pump.

Have new fuel filters.
New spark plugs.
New ignition coil.
Replaced ‘‘O’’ ring at mixture adj. screw.

I had this problem for some time now.

Any advise from anybody, what I’m overlooking?

                             Walter

–
Walter Schuster 78XJ6 FI Ser.II, 2002 xtype 3.0 Albuquerque/New Mexico,
United States --Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

===================================================
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3

Have a stumbling problem on the 78 after driving a
while.

Its warmed up every time it does that.
Its hesitating before picking up speed, it might
do this about 3 times, then its OK.

‘Warmed up’ means temp gauge showing normal engine temps, Walter - or
what…? ‘…after driving a while’ - the engine may still be in the warm-up
phase…

Does it misfire, or is the throttle response just slow - when this happens?
And it happens at cruising speed, or if the throttle has briefly been to
idle/low?

As you have readjusted the throttle gap, with no effect on the stumble; this
should also increase the idle rpms. But as it had no effect on the stumble;
reset gap to specs…

If you have the O2 sensor; disconnect sensor and test drive. Somewhat
convoluted; as the O2 sensor gets hot and the Lambda start playing, it may
cause a change in fuelling that the engine does not like at that engine
temp. As the engine reaches proper temp - it is satisfied with the mixture,
and responds normally. The point being that with any suspected fuel
problems, the O2 sensor should be routinely disconnected to ensure the
Lambda is not involved. The engine should run perfectly with the O2 sensor
disconnected - or basic settings are incorrect…

I assume you had the problem before you changed the items mentioned - and
the changes made no difference? Paul’s ide about the spark plug gaps should
be pursued - the old ones may have a wrong gap as well…?

Watching the tacho when the stumble occurs; does the readings fluctuate
outside the engine rpms…? Indicating an ignition issue…

Frank
xj6 85 Sov Europe (UK/NZ)

Adjusted mixture screw to lean, with no effect.
Adjusted mixture screw to richer, with no effect.
Adjusted .002’’ throttle plate to .003’’ with no effect.
I have a new fuel pump.

Have new fuel filters.
New spark plugs.
New ignition coil.
Replaced ‘‘O’’ ring at mixture adj. screw.

I had this problem for some time now.

Any advise from anybody, what I’m overlooking?-----Original Message-----
From: W. Schuster
Sent: Wednesday, November 02, 2016 9:38 AM
To: xj@jag-lovers.org
Subject: [xj] Stumbling on takeoff, 1978 XJ6 ser.2

===================================================
The archives and FAQ will answer many queries on the XJ series…
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4

Change your coil and try it out.

Fazal
Melbourne, AustraliaOn 2/11/2016 6:19 PM, Frank wrote:

-----Original Message----- From: W. Schuster
Sent: Wednesday, November 02, 2016 9:38 AM
To: xj@jag-lovers.org
Subject: [xj] Stumbling on takeoff, 1978 XJ6 ser.2

Have a stumbling problem on the 78 after driving a
while.

Its warmed up every time it does that.
Its hesitating before picking up speed, it might
do this about 3 times, then its OK.

===================================================
The archives and FAQ will answer many queries on the XJ series…
FAQs: http://www.jag-lovers.org/xjlovers/xjfaq/index.html
Archives: Jag-lovers Forums - Jag-lovers

To remove yourself from this list, go to Jag-lovers Forums - Jag-lovers.

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5

In reply to a message from W. Schuster sent Tue 1 Nov 2016:

Maybe perished rubber bushings in the throttle shaft linkage,
allowing one butterfly to open before the other?

Rob–
Robert Laughton www.leatheriquecanada.com 1999 Super 8
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6

In reply to a message from Robert Laughton sent Thu 3 Nov 2016:

The plugs need a higher voltage to spark when you accelerate
than during normal running. It’s not unusual for low plug
voltage caused by an ignition fault to appear the same as a
fuel mixture issue. the mixture should, of course, be richer.
A cheap and cheerful but very effective way of checking the
ignition circuit is to use a bit of kit rather like a pair of
insulated scissors, with the spark jumping across from one arm
to the other. The distance the spark jumps (altered by opening
the ‘‘scissors’’) indicates how much voltage is available. From
memory something like 30kv is about right.–
Dave Collishaw '79 Daimler Sovereign '56 BSA A10
Cambridgeshire, United Kingdom
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7

In reply to a message from W. Schuster sent Tue 1 Nov 2016:

Walter

This is less likely but try running the engine with the hood
up in the dark and look for shorts (sparks) between the
ignition wires and ground.

David–
David Womby 78 XJ6L 5speed
Mount Dora, Florida, United States
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8

In reply to a message from Frank sent Wed 2 Nov 2016:

Attach a timing light to any cylinder, tape it to the windscreen
wiper arm so you can view the continuity of the spark as you drive
then just put bonnet on safety to prevent crushing the lead.Drive
the car till the flat spot occurs and eliminate the temporary loss
of spark, that is if it is an electrical problem.If the light
disappears change the sensor to all plug leads one by one and
check again.You can also do the same with the H T stream from the
coil. Its a commonsense test, but also a bit of fun.–
The original message included these comments:

Subject: [xj] Stumbling on takeoff, 1978 XJ6 ser.2
Have a stumbling problem on the 78 after driving a
Its warmed up every time it does that.
Its hesitating before picking up speed, it might
do this about 3 times, then its OK.

‘Warmed up’ means temp gauge showing normal engine temps, Walter - or
what…? ‘…after driving a while’ - the engine may still be in the warm-up

–
nifty 1
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php –

===================================================
The archives and FAQ will answer many queries on the XJ series…
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Archives: Jag-lovers Forums - Jag-lovers

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9

I had some advise from the other jag forum site a member had the same big
problem with the engine just quitting and the restarts again.

They looked into the air flow meter box, took the cover off, and found a lever
problem inside.

I’m still waiting for pictures from them and description of the fix.

                                                                       Walter
10

Ok, I had the same issue. it was the vacuum retard wasn’t working… well it was actually the distributor wobbling in the bushes. replaced my distributor - problem solved.

Since your referring to mixture screws, I’m assuming this is a carburated kitty did you check the diaphram on the slides, looks for cracks in the rubber. air will bleed through the cracks and cause the slide not to rise. This can be done by pulling off the air cleaner, and stick your finder in the venturi and raise the slide, then let it fall. if it falls fast, there’s a leak. If it falls slowly all is good. FYI, don’t push hard, it should take little to know effort to move it up about 1/4 inch. When you look at the slides, look for wear groove or marks, the surface on the slides should be consistent in color. aka know wear scratches. check the oil level in the slides is use 30w motor oil (clean haha), Check the needle and seats for wear. The needle sides get worn and aren’t round any more the side(s) get flat which allows extra fuel seep into the venturi. checking the seats are kinda tricky business. There’s a small set screw that hold the seat in place. It breaks really easy so be gentle. with the screw driver. I cleaned out with carb cleaner and if you have access to Harbor Freight, pick up a cheap ultra-sonic cleaner and drop the bottom in for a couple of minutes to loosen up the gas goo build up.

To check for air leaks, take a can of carb cleaner with the engine running, squirt cleaner around all of the intake gaskets and linkage joints into the carb body. If there’s a leak, the engine idle will change. If you haven’t already done this, do it before all of the other stuff mentioned above.

I’ve experienced the same issue and made the same adjustments to no avail. my problem was a combination of all of the above.

Hope This is of some use, or sorry, if I’m totally off base.

Cheers! Mark

11

You know, upon reading all of this, you should at least have the courtesy to post what your findings and results.

People take and take within forums, but how many give back?
It helps others and is good character…

Regards