Two spark plugs on one carburetor, one shows lean, one rich! Why?

Hi Folks,

I have a 67 2+2, with triple SU’s. Of course, each carb feeds two cylinders. How can one of the plugs
show lean and the other, rich?

I’m going to go back and do the tuning procedure over from the beginning!

I’m having trouble wrapping my head around this! Is there simple answer I don’t see?


Air leak at the manifold of the affected cylinder?

Any misfiring or uneven idle?

If your carbs are not perfectly balanced for air flow then one cylinder may be getting a bit of a richer mixture from one carb and that would make it run richer than the other cylinder

ONE carb–feeds air /fuel mix to two cylinders–so same and only one carb:

agree with Les look for an air leak between carb and cylinder-or an obstruction in the intake path, -but are you sure it is fuel rich–or is it oil getting by rings on one.

It’s not as simple as you think. There’s a balance passage that links the carbs. So the flow can cross the obvious boundaries. What’s going on with the other plugs?


Check the compression on both cylinders

Actualty, I’d probably check compression on all the cylinders. My guess is that the sooty plug will correspond with low compression.

Bob Frisby
E-type S2 FHC
Boise, Idaho

Thank you for all the replies!! I hope to have time this weekend to check out your suggestions. I will reply as soon as I find something.


I had time today to compression test all 6 cylinders. The engine is a replacement from an xj6 and is 8:1 compression. All 6 cylinders tested between 138 and 142.

I haven’t checked for vacuum leaks, but will do so as soon as I can.

Also, is it possible a problem with the distributor, Lucas 22D, could cause a difference between plugs on the same carburetor? Difference in strength of spark? The 3 carbs are SU HD8.


can you run a temporary spark lead to the sooty plug? Also, clean the questionable plugs, or put in new.

There are a million and one reasons why they can be different. For example, if the two inlet runner lengths are different, then the air pressure in the longer runner must be lower and the airspeed will be higher, all other things being equal. Those two columns of air don’t move at constant speeds - the inlet valve opens, the air column starts moving in, it then gets chopped when the inlet valve shuts. Cylinder filling and exhaust emptying isn’t a constant across all cylinders at all revs.

Next factor with carburettors is that atomised fuel is always condensing onto the inlet runner surfaces and always evaporating off of them. This isn’t a constant either as it’ll vary with rpm and with temperature.

We haven’t even got to the cylinders yet or considered whether spark plugs are identically performing…

kind regards

Gary ….have you given it a nice long run? Sometimes the only thing it needs is an “Italian tune up”

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Oh no, buckle -up, here we go again.

Removed carb which was feeding the two pistons in question(one lean, one rich). Re-centered needle, cleaned and looked for anything off and reinstalled.

Redid tune as outlined in manual. Looked for vacuum leaks by spraying carb starter around likely areas.

Still getting piston one lean, and piston two rich. I used a Gunson Colortune and it did show piston one running lean, and piston two running rich. With those pistons being fed by on carburetor, I don’t see how it is possible!! I guess if there were a vacuum leak around the manifold where the number one piston is showing lean, that could explain it. But I’m not finding a leak. I also, took the car for a long run previously.

As Marek says, there are a million reasons why this could occur! I’m not sure I’ll ever find the answer.
The other plugs are all fine with the expected color, light tan.


I assume that you have done a compression and leak down test?

I did a compression test and all cylinders tested between 138 and 142. The engine is a replacement from an XJ6 and is an 8:1 compression. I haven’t done a leak down test.

Valve clearances all within spec?

Hi Gary…swap the plugs and leads around…it may be a plug problem/lead problem…Steve

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Haven’t checked valves, but I’ll put it on the list!

Good idea! Thank you, Steve!