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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

Reply to
Steve
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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

Reply to
Lee_D

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.

Reply to
EMB

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

Reply to
Steve

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

Reply to
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

Reply to
Lee_D

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

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.

Reply to
EMB

On or around Thu, 30 Sep 2004 21:51:33 +0000 (UTC), "Steve" enlightened us thusly:

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.

Reply to
Austin Shackles

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.

Reply to
Andy Warner

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

Steve

Reply to
Steve

On or around Fri, 1 Oct 2004 17:54:22 +0000 (UTC), "Steve" enlightened us thusly:

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

Reply to
Austin Shackles

The message from "Steve" contains these words:

electronic ignition unit (standard RR) and

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.

Reply to
Warwick Barnes

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

Buzby

Reply to
Buzby

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