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MG TD TF 1500 - Iridium spark plug carbon pattern

I installed some new NGK iridium plugs and then went for a 140 mile run. I’m using a relatively rich GS needle, but the engine is running well. The plugs have this odd carbon burn on one side of the plug (see photos). I’m wondering if an experienced plug reader would have an explanation.

Thanks,

Jim




J Barry

Have you tried regular ole Champion N5C plugs?

Isn’t that all these old clunkers need, under $2/plug?

That is all I use.

Peter
P G Gilvarry

I think you have the wrong length plug installed. Look at the blackened portion of the plugs threads and that ortion extends into the chamber too far. There are two different cylinder heads. One has 1/2" reach and the other 3/4" reach. By the way it looks you have a short reach head and you have installed long reach plugs.

What is the casting number on head?
Sand Sanders
Sanders

Merson TD List says his car is a MKII 16480, Engine 16468. Should take 1/5" reach. Bud
Bud Krueger

My observation --
The plug does look too long as Sandy picked up on
Yes, it's on the rich side
You'll get a burn mark like that on the plug if the plug is too hot for the application combined with a rich mixture
But why iridium plugs, and why that monster plug gap
What is the rest of the ignition system made up of.
Also that plug hasn't been tight enough, it hasn't squished the sealing washer at all
What's the number on the plug---
William Revit

Use the recommended ES needle, 7/16" float level Jet set .030" below the bridge for '1 1/4" carbs. Iridium NGK> because they are the BEST!! Rock auto has the best US price, set gap @ .030" when using a sport coil. Oh and a CSI distributor.
Len Fanelli
Len Fanelli

Thanks everyone. Carbs are 1.5”, GJ needle was too lean at speed, GS is a big improvement. 3/4” plugs were in there for years before the rebuild, but yes they are wrong for the head. Shoots, always pays to check. Iridiums were recommended by Len Fanelli a while back (thank you Len) and I believe in performance parts where applicable.

Jim
J Barry

interested to know the numbers on them plugs--
I agree that iridium plugs are good and stay good over a long distance but a bit of an overkill in a T type in my eyes unless you're racing it
That plug gap in the pic looks more like .050"ish to me and the end of the electrode has burnt down already so not so good value wise already, there's usually a fine little stepped end on there that's missing/gone
I'm suspecting the heat range of the plug is way too hot
willy
William Revit

Let's see 0.5" = 1/2" reach. (1/5" is a shortcut?).
mea culpa -- Bud
Bud Krueger

Also- be aware that there were or are some fake NGK iridium plugs about
Luckily the one in the pic is genuine, but they're out there so be aware
You can tell by the shape of the base of the plug, the genuine ones are rounded and have nicely finished threads, the others are dead square at the buttom and rough finished as are the threads

It's all good Bud, i hadn't noticed till you said
William Revit

Plugs are NGK 6637 (BPR6EIX). Gap is between .030 and .025. Iridium electrodes are tiny. Will try the shorter plugs.
J Barry

Cheers Jim, the heatrange of that plug is ok so that wrecks my hot theory
The only thing i can think of is that the plug itself 'might' be getting hot because of undertensioning but they'd have to be fairly loose to do that --NGK spec-14 mm =25-30N·m (16-22ft/lb)
Iridium plugs usually take ages to colour up on the pocelein for some reason
I've pulled them from modern cars with 50,000klm on them and the porcelein looks brand new with no colour or a little burnt spot like yours
Usually you have to rely on the colour of the base of the plug which needs to be darkish grey but not damp, you should be able to lightly wipe the end of the plug on your hand /skin and not leave marks on your skin- If it's a bit dampish and leaves a mark it's too rich-----This is after a full throttle run
The colour should of course be lighter if checking after cruising, but no lighter than a mid grey
I notice you are using resistor plugs, does this mean you are using electronic ignition-----
willy
William Revit

NGK 4085 BPR6HIX Iridium IX Spark Plug > 1/2" reach, 5K resistance. Jim @ what speed / RPM / load did you find GJ was too lean? How did you determine that is was too lean? Wide band 02 meter? Also what carb springs are you using red or blue? For the 1 1/2" carbs jet height below the bridge >.040". (For mixture strength @ sea level)
Iridium spark plugs feature a fine wire center electrode that is designed to conduct electrical energy better and increase firing efficiency > Champion spark plug company.
$ 23 for power!
Len


Len Fanelli

I use an oxygen meter and found the ratio to be around 15-16 at 3,000 to 4,000 rpm with the GJ needle. The GS runs about 12 to 13, which is close to perfect for max power. The car runs noticeably better and cooler. I’m thinking of dropping the needle a mm or so. I look forward to getting the right plugs!

William, I think the plug color is basically okay even though it is on the rich side. I think the odd pattern might be due to the excess length. I’ve also had a tough time getting the float bowl set right, so there might be some additional richness from that.

Interesting that you posted a picture of the MGB intake, Len. I’ve been trying to figure out how to use them with a good paper filter. If you have a solution, please let me know.
J Barry

Other stuff: red springs; resistor plugs were recommended by Jeff Schlemmer (plus I listen to the radio).
J Barry

Jim
Sounds like your mixture is pretty spot on , these engines like a strong mixture around that 12.5-12:1 mark on full hook, anything less and they do get hot as you say and if in competition get hotter and hotter till something gives in
The reason i asked about the resistor plugs was more to do with your ignition system than radio interference
If you're running electronic ignition then yes you need a resistance there somewhere to protect the electronics--either resistor plugs or leads or both
BUT
If you're running a std points/coil system there is a trap, you can run resistor plugs ok, or you can run resistor leads ok but not both together--The std system isn't good enough to drive through both and will cause a weak spark at high revs which 'can' cause misfiring and will result on the exhaust analyser as lean, so a little tricky to diagnose the missfire itself as either electrical or mixture
willy

The iridium plugs will get them little random burn marks on them eventually, it's just a bit weird yours got them so early
William Revit

Just for comparison and no other reason, here are a couple of plugs that came from a Ford 4cyl that have a book service life of 80000Klm --they don't get that far and 40000 is there limit usually
These were pulled at 50000, and as you can see the centre needle part of the electrode has completely gone--It's quite typical of these particular cars to develop a misfire caused by the gap getting too wide and the spark takes the easy route down the outside of the plug instead
What I wanted you to see was the colour of the insulator, basically uncoloured apart from the burn marks and this at 50000Klm so don't expect any quick colouring---This car is injected with an O2 sensor so most of it's running would be at around 14.7:1 mixture which is quite lean compared to the colour of the burn marks meaning that plug reading has to be done from the colour of the base of the plug
The second pic is a Chev race plug
On methanol this is typical of a plug after a race leaving no clues of mixture, the only way to check these at the track is to pull the exhaust and check the colour of the back of the exhaust valves, A good set of race pipes will have No1 header pipe removeable specially for this reason





William Revit

You missed a bullet many a piston has been holed by inadvertently fitting long reach plugs in an engine designed for short reach.
G Evans

Jim are you you using 100% gasoline or 10% Ethanol?
My engine on 10% ethanol GJ needles > 12.8-13.0 WOT 400-5000 RPM.
Len

Len Fanelli

Hmm
Bit light on for 10% ethanol
If you are using E10 pump gasoline, an A/F ratio of 12:1 will deliver best power.

For peak power with straight pump gasoline (no ethanol in the mix), the A/F ratio can go as rich as 12.5:1. If you are using E10 pump gasoline, an A/F ratio of 12:1 will deliver peak power.

willy
William Revit

The NGK BPR6HIX are available by special order only for some reason, so I'm back to standard plugs.

Len, I use standard E10 gas. I dropped the GS needle by a mm, and still run in the 12 to 13 range, but more on the 13 side. The GS has a nice smooth curve. My engine is a .080 overbore (1328 cc), and that might be a reason why the GJ ran lean. I finally got the idle mixture leaned out and I'm really happy with how the car is running.

I use an AEM fuel ratio gauge, and there is no way I could get the reading to two decimals because the the gauge needle is constantly moving.

William, thanks for the tip on resistor plugs and wires. The mix seems good where it is. I can move the needle back if I decide to go a little richer.

Jim
J Barry

This thread was discussed between 15/11/2020 and 23/11/2020

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