Misfire - can anyone help? (long-ish)

I've picked up a bit of a misfire on my SIII with an '86 RR V8 (carb
version) in it. Low RPM is fine, but get her up to 3k or so and there's a
change in the exhaust note (disappears if I ease off - i.e. the misser
re-lights) and a definite feeling through the superstructure of one cylinder
not doing things right. Even then it's a bit "intermittent". This only
seems to happen under load (say second gear and turning up the wick) and if
I just rev her out of gear (no load) she barks sweetly all the way up to
4500+ (with an oh-so-sweet crackling rumble when she falls back to idle) and
no apparent misfire. I have had the very occasional cough from her (sort of
"raaaaa...PAH...aaaarrrrrrrrr") when at moderate throttle in the cruise
(2000-2500 rpm or so) so today I've done the following:

Changed all the filters
Put in new carb diaphragms an made sure the dashpots are filled correctly
Checked the carb balance
Checked, cleaned and re-gapped all the plugs
Checked the ignition timing


Solex (Stromberg) 175 CD carb's, electronic ignition unit (standard RR) and
new electronic Lucas coil. The cap is new, the leads just about new - ditto
rotor arm. There were no apparent splits in the diaphragms. The timing is
spot on (6° BTDC for this engine) and vacuum/mechanical advance seems to be
working ok. The plugs were perfect, a hint of light golden deposit on the
rim and earth electrode of each and all the centre electrodes were clean
with just the faintest hint of brown/grey deposit. Gaps at 30 thou (0.8mm).
She doesn't smoke, there's no blowing from the crankcase (filler cap off,
cover it with the palm of my hand and there's just a hint of suck,
presumably from the flame traps to carbs) and nothing pouring out of the
rear breather. There's no water loss or oil in the water.

The irritating misfire is still there.

So, can anyone please shed any light as to what might be amiss? Perhaps
I've got a lazy valve spring that's letting one of the valves bounce at
higher rpm? But then why the occasional and possibly related cough?

Hope someone can help.
TIA,
Steve

"Steve" <[email protected]> wrote in message
news:[email protected]...
> Hope someone can help.
> TIA,
> Steve

Have you tried a colour tune on it? It sounds like it may be a mixture
problem which wouldn't show on the balancing check.

Lee D

Steve wrote:
> I've picked up a bit of a misfire on my SIII with an '86 RR V8 (carb
> version) in it.

Vacuum leak in the inlet manifold area? Or maybe a spark plug going out
intermittently. I'd try a new set of plugs first, followed by a very
careful look for vacuum leaks.


--
EMB
change two to the number to reply

"Lee_D" <[email protected]> wrote in message
news:%[email protected]...
>
> Have you tried a colour tune on it? It sounds like it may be a mixture
> problem which wouldn't show on the balancing check.
>
> Lee D
>

Not tried a colour tune, no. The colour of the residue on the plugs seems
to suggest that they're all burning correctly (except when the problem
manifests itself) and not very far out on mixture either. I suppose it's
worth a try tough.

Steve

"EMB" <[email protected]> wrote in message
news:[email protected]...
>
> Vacuum leak in the inlet manifold area? Or maybe a spark plug going out
> intermittently. I'd try a new set of plugs first, followed by a very
> careful look for vacuum leaks.
>
>

Plug is possible, although I'd have thought it would tend to show itself at
intermittent times too, not just when I'm making a noise. I guess it could
be vacuum, although how I'd find that I dunno! Watch for disappearing soapy
water??

Steve

"Steve" <[email protected]> wrote in message
news:[email protected]...
> Plug is possible, although I'd have thought it would tend to show itself
> at
> intermittent times too, not just when I'm making a noise. I guess it
> could
> be vacuum, although how I'd find that I dunno! Watch for disappearing
> soapy
> water??
>
> Steve


Bit of rubber hose... one end in yer earrole and the other used to sniff
around the manifolds.... when you hear the sea then you've found it.... if
the sea sound is coming from the viscous fan then try again

Lee D
--

www.lrproject.com

Workshop photos from Landrover repairs
& other such tinkerings.
Home of Percy the Jag powered Landrover

Steve wrote:

> Plug is possible, although I'd have thought it would tend to show itself at
> intermittent times too, not just when I'm making a noise.

Mixture under acceleration is summat harder to ignite so it's the first
time a marginal problem manifests itself.

> I guess it could be vacuum, although how I'd find that I dunno! Watch
> for disappearing soapy water??

Stick a gas analyer up it's tailpipe and squirt something flammable
(even WD40 works) around the potential leak sites. If the readings
change you've found the vacuum leak.

--
EMB
change two to the number to reply

On or around Thu, 30 Sep 2004 21:51:33 +0000 (UTC), "Steve"
<[email protected]> enlightened us thusly:

>"EMB" <[email protected]> wrote in message
>news:[email protected]...
>>
>> Vacuum leak in the inlet manifold area? Or maybe a spark plug going out
>> intermittently. I'd try a new set of plugs first, followed by a very
>> careful look for vacuum leaks.
>>
>>
>
>Plug is possible, although I'd have thought it would tend to show itself at
>intermittent times too, not just when I'm making a noise. I guess it could
>be vacuum, although how I'd find that I dunno! Watch for disappearing soapy
>water??

from experience on V8s, then it's worth changing the plugs unless they're
reasonably new. What leads have you got on it? Magnecor ones really are
the dog's reproductive gear, and worth the extra.

dizzy cap, too is a possibility.

oh and do a compression test.

you probably have one pot which is slightly "off" for some reason, and this
only shows under whatever conditions accentuate the fault.

Have you set the points/ dwell recently. Worth checking with a meter, in
case the dissy's at all worn. Shoud be around 25 degrees from memory. Its
also worth replacing the coil if you have a spare to hand you can try. Had
similar problems a while back at high ish revs under load on a SIII, symtoms
seemed to point to fueling, yet it turned out to be the coil.

Andy
"Steve" <[email protected]> wrote in message
news:[email protected]...
> I've picked up a bit of a misfire on my SIII with an '86 RR V8 (carb
> version) in it. Low RPM is fine, but get her up to 3k or so and there's a
> change in the exhaust note (disappears if I ease off - i.e. the misser
> re-lights) and a definite feeling through the superstructure of one
cylinder
> not doing things right. Even then it's a bit "intermittent". This only
> seems to happen under load (say second gear and turning up the wick) and
if
> I just rev her out of gear (no load) she barks sweetly all the way up to
> 4500+ (with an oh-so-sweet crackling rumble when she falls back to idle)
and
> no apparent misfire. I have had the very occasional cough from her (sort
of
> "raaaaa...PAH...aaaarrrrrrrrr") when at moderate throttle in the cruise
> (2000-2500 rpm or so) so today I've done the following:
>
> Changed all the filters
> Put in new carb diaphragms an made sure the dashpots are filled correctly
> Checked the carb balance
> Checked, cleaned and re-gapped all the plugs
> Checked the ignition timing
>
>
> Solex (Stromberg) 175 CD carb's, electronic ignition unit (standard RR)
and
> new electronic Lucas coil. The cap is new, the leads just about new -
ditto
> rotor arm. There were no apparent splits in the diaphragms. The timing
is
> spot on (6° BTDC for this engine) and vacuum/mechanical advance seems to
be
> working ok. The plugs were perfect, a hint of light golden deposit on the
> rim and earth electrode of each and all the centre electrodes were clean
> with just the faintest hint of brown/grey deposit. Gaps at 30 thou
(0.8mm).
> She doesn't smoke, there's no blowing from the crankcase (filler cap off,
> cover it with the palm of my hand and there's just a hint of suck,
> presumably from the flame traps to carbs) and nothing pouring out of the
> rear breather. There's no water loss or oil in the water.
>
> The irritating misfire is still there.
>
> So, can anyone please shed any light as to what might be amiss? Perhaps
> I've got a lazy valve spring that's letting one of the valves bounce at
> higher rpm? But then why the occasional and possibly related cough?
>
> Hope someone can help.
> TIA,
> Steve
>
>

"Andy Warner" <[email protected]> wrote in message
news:[email protected]...
> Have you set the points/ dwell recently. Worth checking with a meter, in
> case the dissy's at all worn. Shoud be around 25 degrees from memory. Its
> also worth replacing the coil if you have a spare to hand you can try. Had
> similar problems a while back at high ish revs under load on a SIII,
symtoms
> seemed to point to fueling, yet it turned out to be the coil.
>
> Andy

No points/dwell - it's electronic ignition. Coil is brand new.

Steve

On or around Fri, 1 Oct 2004 17:54:22 +0000 (UTC), "Steve"
<[email protected]> enlightened us thusly:

>"Andy Warner" <[email protected]> wrote in message
>news:[email protected]...
>> Have you set the points/ dwell recently. Worth checking with a meter, in
>> case the dissy's at all worn. Shoud be around 25 degrees from memory. Its
>> also worth replacing the coil if you have a spare to hand you can try. Had
>> similar problems a while back at high ish revs under load on a SIII,
>symtoms
>> seemed to point to fueling, yet it turned out to be the coil.
>>
>> Andy
>
>No points/dwell - it's electronic ignition. Coil is brand new.

is it the right coil? i.e. one for electronic ignition, not a points-type
one?

The message <[email protected]>
from "Steve" <[email protected]> contains these words:

electronic ignition unit (standard RR) and
> new electronic Lucas coil. The cap is new, the leads just about new - ditto
> rotor arm.

The Rotor Arm did you pull it off? in which case very likely to have
pulled the centrifugal advance to bits, may well have completely
knackered the dizzy by now, service bits are not available and if you
broke or lost the plastic ring under the rotor which holds the whole
gubbins together you may as well forget it.

My other sduggestion was cracked distributor cap, look for a black line
between centre post and one of the output posts, very good a causing
backfires on a damp morning! but you just replaced this you say.

--
If you received this through the miracle of modern technology then all
is well; if not then situation normal.
Chris father of ( also at [email protected] )
www.users.zetnet.co.uk/barnes_firsnorton

> I've picked up a bit of a misfire on my SIII with an '86 RR V8 (carb
> version) in it. Low RPM is fine, but get her up to 3k or so and there's a
> change in the exhaust note (disappears if I ease off - i.e. the misser
> re-lights) and a definite feeling through the superstructure of one
> cylinder
> not doing things right. Even then it's a bit "intermittent". This only
> seems to happen under load (say second gear and turning up the wick) and
> if
> I just rev her out of gear (no load) she barks sweetly all the way up to
> 4500+ (with an oh-so-sweet crackling rumble when she falls back to idle)
> and
> no apparent misfire. I have had the very occasional cough from her (sort
> of
> "raaaaa...PAH...aaaarrrrrrrrr") when at moderate throttle in the cruise
> (2000-2500 rpm or so) so today I've done the following:

Might be worth checking the fuel pump - I had a very similar problem with
SWMBO car - new pump sorted it.

Buzby