[xk] burbles misses ONLY at even throttle WHY

1953 xk120…all stock parts…starts right up, idles just fine
with no misses or burble at idle, good power on acceleration,
fine wide open, no misses but underway at even throttle and
only at even throttle burbles a lot and minor misses…back to
throttle, even just a little and all us good. Some burble on
back off decel…
so what gets me is…instant start up, fine idle, v good under
power…ONLY at even throttle does this burble miss occur?
where do I start to look? for the cause and cure: Please chine
in…
thanks,
Nick–
Nick53XK120S
Spokane WA, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

In reply to a message from Nick53XK120S sent Mon 14 Sep 2015:

Nick;
Check to see if your vacuum advance diaphragm is torn or
leaking.–
Roger, '54 120 (SE) DHC, BRG, S678300
Tamarac, South Fla., United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

In reply to a message from Nick53XK120S sent Mon 14 Sep 2015:

Nick
What is your static advance, and what plugs are you using?
Joel–
ex jag, '66 E-type S1 4.2, '56 XK140dhc, '97 XJ-6
Denison, TX, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

In reply to a message from ex jag sent Fri 18 Sep 2015:

7 deg BTDC (2.5 flywheel teeth), NGK BP5ES…sometimes BP6ES
gapped at .025, points at .016. plug wires are (seem)
good…as do ‘‘seem’’ rotor and cap…I may try a diff rotor…easy
enough as I have heard of some rotor issues…with rivet or
with black ones with carbon tracking…simple enough to try.
Lucas sports coil (the gold one).
Nick–
The original message included these comments:

What is your static advance, and what plugs are you using?


Nick53XK120S
Spokane WA, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

In reply to a message from Nick53XK120S sent Fri 18 Sep 2015:

Nick;
Do a Compression Check to INSURE all cylinders are
close… also check your valve gaps…
Try changing your timing to 10 degrees BTDC AND you have
the Points & Plugs gaps backwards… 0.025’’ on the Plugs,
0.016’’ on the Points!
As an FYI, the NGK ‘‘5’s’’ are the hotter of the two you
list… Good for in-town… Run the '6’s for long high-speed
runs (60MPH+). (I found that the Champion N5C’s worked
better in MY 120 DHC back when I last drove it-- circa
'67-'75… YMMV… of course today’s gasoline is different…)
On ‘‘rotor/Dist Cap Tracking Issues’’… simple tests…
First, run engine at night in a dark garage with bonnet
raised… see if you have ‘‘fireflies’’ going off around the
coil & plug wires (these are called ‘‘ignition leaks’’)…
Second, pull the Dist Cap and actually LOOK for tiny black
lines. like earthworm trails, on the inside of the cap…
That is Carbon Tracking, usually the result of a cracked cap…–
The original message included these comments:

7 deg BTDC (2.5 flywheel teeth), NGK BP5ES…sometimes BP6ES
gapped at .025, points at .016. plug wires are (seem)
enough as I have heard of some rotor issues…with rivet or
with black ones with carbon tracking…simple enough to try.


Charles #677556
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

In reply to a message from Nick53XK120S sent Fri 18 Sep 2015:

It�s very likely your distributor is not giving you enough
dwell with a points setting of .016�. Although a points
setting of .014 - .016 is specified in the later XK120
Handbook, this setting contradicts the XK120 / Mark VII
Service Manual and the LUCAS Technical Service Manual for
Coil Ignition.

The LUCAS Technical Service Manual for Coil Ignition
explains that the revised setting of .014 - .016 only
applies to distributors with the high-lift distributor cam
used on the newer DM type distributors, as supplied to the
XK150. The LUCAS Technical Service Manual further states
that the DM type distributor�s new high-lift cam provides
increased cam dwell � �that is a longer closed period� for
the points when using a point gap of .014 - .016.�

If you have the original DVX6A distributor as supplied to
the XK120 and XK140, the LUCAS Technical Service Manual
explains that these distributors were supplied with an
�asymmetric� distributor cam. The recommendation is allow
the points to bed-down to .010-.012 to �obtain full
performance with all Lucas ignition coils.� The XK120 /
Mark VII Service Manual specifies 38 degrees of dwell with a
points setting of .010 - .012.

I totally rebuilt my DVX6A distributor using all NOS Lucas
parts and set the points at .015�. Then I had the
distributor tested on a Sun Distributor Machine to see if
centrifugal advance, vacuum advance, and dwell would test at
the specifications listed in the XK120 / Mark VII Service
Manual. Centrifugal advance and vacuum advance followed the
exact curves specified in the Service Manual, however, the
distributor would only provide 32 degrees of dwell at a
point setting of .015�. After learning about �high lift�
vs. �asymmetric� distributor cams, I reset the points to
.011�, and now the distributor delivers the specified 38
degrees of dwell.

One of the advantages of using a Petronix replacement unit
for points and condenser is that the Petronix unit should
deliver exactly 38 degrees of dwell. Because many XK120 and
XK140 owners have opened their points to .014 - .016, this
may explain why so many notice an immediate increase in
performance after retrofitting a Petronix unit.–
The original message included these comments:

7 deg BTDC (2.5 flywheel teeth), NGK BP5ES…sometimes BP6ES
gapped at .025, points at .016. plug wires are (seem)


Mike Balch
Iowa, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

1 Like

In reply to a message from Mike Balch sent Tue 22 Sep 2015:

thanks all…great info here, I will be trying to sort the
even throttle burble…great info…and BTW if anyone has a
spare DVX6A 40199 distributor…contact me direct private
reply…
thanks
Nick–
Nick53XK120S
Spokane WA, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

In reply to a message from Nick53XK120S sent Tue 22 Sep 2015:

Hi Nick,

Please keep us updated on this. I would be interested on
the final outcome.
Thanks,
Bob–
'53 XK120 DHC
Memphis, TN, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

installed a rebuilt distributor with a rebuilt vac unit…(Rob at British Vac Unit) and the burble is no more: … rebuilder says slightly diff advance: no advance below 1100, 36 D max advance, and slight diff vac specs.to 7-14-8, with .pts set at .016, and recommends to now time at 10-12 D Before TDC…which I did at 10…best the engine has ever run…then after that (so as to not introduce too many variables at one time) I added new spark plug wires a.7mm no suppression (virtually no resistance). and plugs…the usual BP5ES at .025 (could be to 028 even 030) .so it was not carbs.it was .likely since burble was at even throttle… the vac unit. With the no suppression plug wires…If I can manuever close enough at the stop lite to the vette next to me…I can fry his electronic modules.
Nick