Just a quick question! The last couple of times I’ve taken the car out it has developed a (sometimes quite severe) misfire after say, half an hour’s town running at low revs. When I can get onto the open road and get the revs up to 3000 or so for a minute or two, it clears itself. Obviously, the plugs are fouling for one reason or another. Looking at the plugs today, I could see no signs of oil fouling (engine recently rebuilt) but the plugs were very sooty. I have yet to fit my new cometic cam cover gaskets, so to mop up the oil pooling in the plug wells I’ve been putting a rolled-up wad of kitchen paper abound the plug bases to soak up excess oil. Could this be causing any issues with the plugs misfiring, I’m wondering?
I leaned up the front carb a bit, which made no difference. I’ve yet to check the mixtures properly, but thought in the meantime I’d lower the float level to 1/2" rather than 7/16" on the measuring bar.
Thoughts, anyone?
Certainly oily dirty paper can conduct electricity, as can dirt on the outside of the distributor cap and plugs.
Sooty plugs sounds like running too rich.
Float level can be checked by removing the pistons and looking down the jets; the meniscus of fuel should be down in there about 1/8" or so, not at the top. I’ve twice had a case of partially sunk floats making the level too high.
Thanks, Rob. I’d better get those Cometic gaskets on pronto! I have the “unsinkable” Nitrile floats, but will check the fuel levels as you suggest.
Chris, one thing to check is where the plug wires enter the distributor cap. My car developed a pronounced low rpm miss that ended at about.2000 rpm and above.
It turned out to be engine oil seepage that got onto the plug wires and, over time, worked its way down the wires and saturated where the plug wire ends contacted the terminals of the distributor cap. I cleaned the wires and cap with solvent and everything was good again.
the oil near and around plug insulators or wire connections won’t help…can cause problems…but you say it loads up in town, clears up with a bit of a run at 3,000 or so…(Italian Tune Up !!)…and sooty plugs on inspection…so it seems the loading up, richness is the issue…But yeah…fix both. What heat range are the plugs…(plug number), what gap do you set.? is it standard stock points distributor or electronic no points? Which could make a difference in what plug gap you set…Nick
Mike. That makes you Sherlock Holmes of Jaguarland. Amazing
Mike, I’ve had the distributor out and rebuilt very recently (made a big difference!) so I know all is clean and dry at the distributor end.
Nick, I’m running the stock points distributor (recently rebuilt and set up by the Distributor Doctor) - higher lift distributor cam so gap set to .015", plugs are NGK BP5ES set at .022". Copper core leads.
I think the main problem must be an overly rich mixture…
agree…too rich, do try .025 to .028 on your plugs, with a rebuilt dizzy. That is not so wide as to overload the dizzy but wide enough to help with the carbon fouling at the tip. I run points…have used 028 on those same plugs for years. Nick
Thanks, Nick - I’ll open them up to .025" to start with. I didn’t know if I could go wider than the manual suggested. I’ve always used .025 - .030" on other cars…
I know that 022 was the manual spec…but it was also determined on poweer and road tests on 1950 British fuel of about 80 octane…and Champion NA 8 and later N8B plugs not as good as your NGKs…sure start with 025 if you like…just sayin on my XK120…similar I think…with as orig but rebuilt points distributor, Lucas Sports coil and same spark plugs…I have been at 028. I do think a try at 025 will help once the richness is toned down…but always err a bit on the rich side…plug soot is manageable. too lean as you know…is not good on valves and pistons. Nick.