How to get richer carb fuel/air mix?

My 68 series 1.5 engine is running well with two Zenith carbs but with very different air/fuel mix. As background, it has good compression on all cylinders and good manifold vacuum. The two clean carbs have equal air flow rate.

But while the front carb has correct air fuel mix (as shown with air valve lift test), the rear carb is running very lean. This is shown by lift test which almost stalls the engine and a much higher temperature on the rear exhaust headers.
I have done everything I know to get a richer mix on rear carb, including:

  • correct fixed needle in right position
  • adjustible jets from Joe Curto are in full down position
  • carb float level is as high as possible
  • and the “idle trim screw” is full in ( both carbs)
    All this with choke off and engine idyling at about 900-1000rpm.

I have tried raising the “fixed” needles about 1/8 inch higher but this doesnt help

Can anyone suggest what I can do to get richer mix in rear carb?

Have you checked that the rear carb doesn’t hav an air leak? Try spraying engine start to see if the revs rise.

I was just typing the same thing.

Before you try to enrich the rear carb it would be sensible to work out why it’s lean.

What model of ZS carbs are installed? Do either (or both) have by-pass valves? Do you have a smoke tester for checking vacuum leaks in the tubing, etc.? Well worth making one (DIY YouTube) IMHO.



Are the secondaries in place and operating as designed or do you have them jammed open?

Carbs are CD-2
Have done lots of testing for air leaks -smoke, starter fluid, etc. - and dont believe any are signifivant.
By pass valves are fully closed
Secondary imternals all removed

As to why it is lean, I wish I.knew. Very strange that the two carbs are performing so differently but I cant find the reason although there is obviously something different somewhere

Something is very wrong, but properly removed secondaries are not the issue.

Have you ascertained both rubber diaphragms are intact?

I would agree except removing them gives you the potential for an air leak where the shaft went through. As others have said here, check carefully for a manifold air leak.

1 Like

That is true.

That would seem to be the goal and what makes most of us pleased! Your issue seems to be the results of the air valve lift test. I believe many here do not put a lot of faith in that particular test. Do you have other signs of richness, such as sooty spark plugs?

Thinking you may not be able to reply to my request a few minutes ago without my emaii: THX! Bob Annapolis

For Tomdd55, as I wrote, the second indication of lean mix is much higher surface temperature of rear exhaust header. The diffference with front header is very large. Sooty plugs not an issue as my problem is too lean but anyway, plugs are clean

Dont understand post from BigBert

George, sorry, as I was reading “richer” I kept thinking it was too rich. I usually reread things twice, I need to go for three.

I would suggest you take off the vacuum tank hose and plug the hole of the hose (after chedking the hose is not faulty).
If there is an air leak somewhere in the vacuum system, this will make your rear carb behave as it should!



The issue with not having the secondaries in place and operating as designed is that at idle, the secondaries force the air/fuel mixture through a contorted path before reaching the cylinders. That path provides carb balancing between them and removing them or jamming them open removes said balancing. I found this to be the case on my own ‘68 after spending ages trying to chase down a very similar problem which you describe on your car.

If you still have the secondaries in place but set permanently open, I suggest you try resetting them and see how the idle mixture setup performs.

Jaguar designers knew what they were doing.


Maybe I’m alone, but I’m a little confused as to whether we have two folks here with a similar issue (original poster @GeorgeCA and post #6 responder @PhillipW), or one person using two separate login IDs. If it’s two separate people, we shouldn’t assume that one person’s answer is necessarily representative of both problems. Confused…

I hadn’t picked up on that but I think you are correct, do we have the same person or two listers with the same problem?

MY friend Phillip and I are restoring the same e type together. We both individually seek help from JLF. He made the mistake of posting info on a subject I started. No conspiricy.
Apologies for confusion.