Best spark plugs for a 120 with 123 ignition

Hi tger I installed a 123 ignition system in my 1959 xk120 about 2yrs 500 miles ago… Its starting to miss badly over 3000rom when hot… I pulled the plugs and they look like the are burning out… They are champion n12yc… Any advice???
Thanks Scott

One is to make sure your not running too lean. You might also try an N11YC. It is a cooler plug. My 2 cents

Hi…a few numbers mixed up in the year/model…but assume it is a 120 of some year…anyway…I just dealt with this on the E Forum…so a copy here follows…but first…not quite enuf info to diagnose that it is only a plug issue…is the miss under power only…or at cruise, You say when hot…so if you are under power over 3,000 and not hot yet…no miss? There could be multiple issues all at the same time, please share a little more info, …but…you say the plug seems burned…so.I shall proceed with plug info. The…letter N is the size 14mm and reach (3/4), and 13/16 hex, N is correct. 12 is the heat, Y a projected tip, C is copper. Champion are the higher the number the hotter with the range from 4 coldest to extreme 16 hottest, thus 12 is hotter than a 9. It is always better to err on the cold side…too cold may foul with black or wet on the plug. …too hot will show white or lite gray and can burn a piston. A clean dark tan or brownish is just fine. (note OLD champion numbers are different : that is a N5 is an OLD is not a lot colder than today’s N9 or 12…) The C is for copper. Common for the 3.4 and 3.8 are N9YC , N11YC, N12YC. Champion are more narrow in the range of each plug…NGK has a wider range per plug…and the heat number is opposite…higher number is colder…for the 3.4 L applications the NGK BP5ES a little hotter, BP6ES is most common, …Note that the Champion N12 crosses to the NGK 4 and 5., I know of no one that uses that hot an NGK) . I have felt the N12YC to be on the too hot end…but it is quite commonly used. For sustained hi speed in a well tuned engine…the colder BP7ES. So is the miss severe…or one cyl? SO…I would replace all…with N11YC or N9YC…or better…the NGK BP6ES and test…I would also want to verify timing, and that the centrif advances do move, and the vac advance work s. Do not ever over tighten…finger tight when seated and …then just a little more…1/3 of a turn at most if new…less if re-used as the crush washer is already crushed. Then let us know…if not cured…the troubleshoot list will be longer…Nick

Hi guys thanks for your very helpful responses. I’m just starting to look at the problem so will keep you posted… I think I’ll change to the cooler plugs. That’s a very helpful breakdown. The plugs look like they are running a bit too lean. I’ll keep ye posted!
Regards,
Scott

So all. Good news. I changed to the n9ys plugs and charged the battery as I’m running an 123 system which needs 14volts. I’m still running the dynamo. Took it for a good spin today and it ran beautifully!! Thanks again for your help.
Regards Scott

It needs 14 volts?a fully charged battery only puts out 12.6? And I know that the charging sytem obviously puts out more, but to get the car started it’s working on that 12.6 or less

John
NGK plugs…stock gap.
NO BALLAST coil …1.0 or 3.0 ohm
YOUR SPARK PLUG WIRES SHOULD BE CONVERTED FROM COPPER CORE TO SOLID SPIRAL.
This is why your plugs are melting.
Your car is running ok now because the plugs are new but they will crap out too.
Go to stock gap ora little wider but the key here is resistance from the wires…
Good luck
gtjoey1314

Joey, the spiral core is similar resistance as solid core. The spiral simply Is there to reduce EMI. switching to a cooler plug or perhaps richening the mixture if he is lean, have a bigger effect

now…to see if over time a miss develops…and try to find if just one cyl plug seems to be the one…Also…as to wires…resistance and EMI is NOT needed or wanted in this system. No resistance wire can be found…easy test with ohmmeter. New cars require VERY hi resistance.so much that a 1.5V continuity test lite will NOT even lite on a 2 ft section. Test for zero resistance with an ohmmeter. Be sure no suppressor on coil or distributor. I have stock dizzy/pts…sport coil…use .025 to .028. plug and .016 points.

Just dont use copper core wires you will get resistance and spikes.
Ive been using fibercore centers with zero issues.
gtjoey1314

I do not believe that to be true about copper core. It is true that For modern cars it should not be used because of the LOW resistance and the EMI and RFI.,Carbon wrapped hi resistance and almost no EMI/RFI is what modern cars must use. (ther is not an issue with copper core of resistance as there is zero to little when it is not wrapped, and not spikes) also copper low resistance is not for for older cars with electronic ignition or other electronic modules installed. From Summit … “Copper core is the choice…for older cars with no computers, standard points distributors and non electronic carburetors. If you’ve got a vintage hot rod or race car with a carburetor and old-school ignition setup (magneto or points-style), you’d benefit from the ultra-LOW resistance of solid core (copper) wires. In this scenario, the added noise from this style of ignition wire won’t be a factor unless you have an MSD ignition box or like to listen to the radio a lot!” So for the non electronic XK…use zero resistance copper core. Do not use carbon.

2 Likes

Agreed: on Stone Age ignition systems, copper wire works juuuust fine.

Nick are you running a modern set up?
I’m talking about modern 123 not points and confessors he says he has a 123
Even the worse Petronix calls for no copper leads it creates a spike to the module
But what ever you like
Here’s the telltale signs
Car starts fine then after it’s parked and got it won’t restart or back fire
Or
Buck and backfire when driving or pops the unit and dead on the road
The 123 is wonderful
I have them in three different cars and they are one and done!
Good luck
Gtjoey13-4
Ps champions are not champions any longer they are cheap Mexico sweatshop labor junk out of Mexico
Please go ngk and call it a day

Hi guys so I’m a bit confused are you talking about the core wire if the plugs themselves or the ignition leads? I’m using silicon ignition leads…are these the ngk plugs you recommend NGK BP6ES??
Best regards Scott

J-L: good point.: .I did refer to both…but mainly stated that for no computer, standard points,distributors to use the no resistance copper core…My intent was to clarify your post…so that it did not appear that ALL 120 14 150 could use hi resistance wire that is needed for modules. People look these posts up…years later.

Years later
Get your battery pack ready!:grinning:

John
Silicone leads refer to the sheath, not the conductor. Conductors are Generally either carbon, copper, or copper wrapped in SS.

John , yes now your cooking with gas…those plugs are great, along with the 123, get your timing dialed in and your doing great.
gtjoey1314

OK that’s great I’ll suss them out… Yes they are silicon leads with carbon core… Thanks for all the help