The 'E' Type Forum

1965 series 1 4.2 When tuning my SU carbs, to get a satisfactory mixture,I end up with .065 ,055 and .047 jet depths. Should I leave them as they are , or average them out.
Thanks Keith

Up to you. From my experience yours are not unusually different.

You could average them, then re-tune if you wanted to. Sometimes getting the middle carby right is a little tricky and this might bring down the error a bit.

Thank you , will concentrate on the middle carby

Hmmmm, am I the only one screaming No No No, do not average the carb settings. Think about it for a minute. If I adjust the carb so that the mixture being burned in cylinders one and two is optimal, why would I change it just because the carb feeding cylinders three and four needed the jet at a different height? How can the jet height on the middle carb have any effect on the mixture needed by cylinders one and two, or five and six?

Once the carbs are set, I leave them alone until either the next service interval (3000 miles) or there is evidence that the mixture has changed.

That's not what I'm saying Chris.

What I do is measure them, average them out and then retune them. Then check them again.

I find this makes it easier to get the middle carb right. I'm using a CO meter so measuring the output of the 3 carbs into the two exhaust pipes requires mental gymnastics that is sometimes beyond my meagre brain.

I agree that each needle may need a slightly different setting from the others, but with the float levels equal and assuming everything else is OK with the engine all 3 should be within cooee of each other.

The range that the OP has is around what I usually end up with but I like to do it a couple of times to try and iron out errors.

chrisfell wrote: ↑

Fri Sep 06, 2019 5:30 pm

Hmmmm, am I the only one screaming No No No, do not average the carb settings. Think about it for a minute. If I adjust the carb so that the mixture being burned in cylinders one and two is optimal, why would I change it just because the carb feeding cylinders three and four needed the jet at a different height? How can the jet height on the middle carb have any effect on the mixture needed by cylinders one and two, or five and six?

The simple answer to your question ( in your case of a 4.2 litre car ) is that each carburetter does not merely feed its nearest cylinders. Because of the large size of the balance passages in the 4.2 manifold, each carburetter's setting also affects, albeit to a lesser extent, the mixture strength in the other 4 cylinders. This is why averaging out is very effective, and is also why the middle carburetter is the one which can have the greatest influence because it is near not only to its "own" 2 cylinders, but also to the adjacent cylinders on each side

Thank you for your reply’s , plugs 1 and 6 are fine, plugs 3and4 are mildly sooty , car is going will try and fine tune middle carb . Thank you Keith