XK120 cause of misfire

what would the main or common cause of misfire on a XK120? the car currently has 21,000 original miles. Thank you all!

depends a bit on what the ā€œmissā€ sounds likeā€¦and whenā€¦but quick first to come to mind is a spark plugā€¦and easy to fixā€¦Sometimes if only one plugā€¦you can while engine is running, idlingā€¦remove one plug wire at a timeā€¦if the engine gets worseā€¦that plug is ok, do the nextā€¦when you remove the bad one it will stay the sameā€¦sometimes this works to find oneā€¦otherwise replace one at a timeā€¦and testā€¦OR just replace all six and see if a cure. Sometimes inspection of a removed plug will show a fouled oneā€¦black or crusty, maybe to the point where the space in the gap is filledā€¦that will be the bad oneā€¦Can be other things.but a quick 6 new plugs inā€¦gapped correctly will at least eliminate spark plugā€¦the most common cause. Do not overtightenā€¦for new plugsā€¦spin to firm finger tight, then 3/4 of one turn moreā€¦if old plugs put back inā€¦finger tite and 1/4 of a turn more. Not all that tight, you donā€™t want to stretch/ damage threads in the cyl head. see other recent thread on thisā€¦suggested plugs are NGK BP5ES gapped at 025 or -028. start with 025. BPR5ES is the replacement now availableā€¦the BP5ES discontinued but can be found on the internetā€¦I like the low resistanceā€¦donā€™t need 5,000 ohms per plugā€¦on a XK120 . Nick

Thank you Nick! the misfire mainly happens when on 1st or 2nd great I hear a pop. While idling seems smooth.

When accelerating, off throttle, when holding speed, at a certain rpm? Intake or exhaust?

Are the dampers filled with oil?
See what the plugs look like. Most ā€žfuelā€œ problems are caused by ignition so start there.

When accelerating mainly not sure at certain rpm? Sounds like from exhaust. Sorry, not sure what the dampers are or how to check them.
Iā€™m in the process of finding a local mechanic. Since I got the car not too long ago, I needed someone to check it and maybe do full tuneup and make sure its road ready. So far, no luck finding mechanic. The one recommended here on the forum ā€œBritish motorsportā€ are too busy to take any new cars at this time :smiley:

They are the thumb screws on top of the carburettors, unscrew them and check if they have any oil in the bore, people use a range of different oils in them from 30w engine oil down to 3-in-1 sewing machine oil.

1 Like

Thank you so much Robin!

Without oil the pistons will bounce when the car hits a bump and the engine will surge

so Simonā€¦the carb dampersā€¦remove the hex brass plug on the top of the carbsā€¦one each carb topā€¦and just fill to the narrow neck with light oilā€¦replace the plunger cap but just beyond tightā€¦donā€™t overdo. The rod plunger will push down with a little resistance from the oil. Then test driveā€¦if still a missā€¦Iā€™d just replace 6 spark plugs as I mentioned earlierā€¦very easyā€¦test driveā€¦if ok greatā€¦if still a ā€œpopā€ā€¦then a check of the timing to be sure it is OKā€¦that one may be beyond your skillsā€¦but it is easy really just to look and seeā€¦report backā€¦(checking it is easyā€¦adjusting it if needed may require someone that has done it.

Hoping the carb damper (sometimes called ā€œdash potā€) helpsā€¦and next 6 new plugsā€¦as saidā€¦BP5ES at .025ā€¦super easy.
Nick

We should explain that as Simon is new to this subject.
This is the dashpot/damper/piston under this hex head.
The original spec was SAE 30 engine oil, and you can use the modern substitute SAE10W30.
Moss and others sell special dashpot oil, but I havenā€™t figured out what is special about it.
Other folks such as racers experiment with thinner viscosity oils.
You donā€™t need much, like a teaspoonful. It goes in here.
PICT0008a
Donā€™t over-tighten the hex head afterwards, just a light twist of the wrist, it doesnā€™t need to be very tight.

Thanks so much Rob for the detailed explanation and picture. Now I know exactly what to do. Thanks

In fact, some use ATF, some use engine oil, some use sewing machine oil, they all get away with it.
Donā€™t overfill, just to the top of the innermost tube or slightly below, and finger tight is fine.

What the damper does is that it enriches the mixture when you initially step on the throttle. It dampens the upwards movement of the piston which controls the mixture and also the ā€švacuumā€˜ that sucks the fuel into the air.
This enrichment prevents a lean stumble during initial acceleration.

If both dampers are filled with oil that wasnā€™t your problem but itā€™s worth looking at. On the garage, I wouldnā€™t touch a well running car and if yours is doing well apart from the stumble just keep an eye on all the fresh fluids and enjoy it. Second opinion on the running well is always helpful.
Good luck Simon.

David

1 Like

Thank you David! Learning so much from here :slight_smile: