Adjusting two Zenith CD-2 carbs

Am rebuilding 1968 4.2 l engine, which now has good compression pressure on all cylinders and correct timing.

It starts quickly and idles smoothly but too fast at 1700 rpm. Manifold vacuum good at 43 mm. Air fuel ratio (measured with Innovate meter which I have calibrated and trust) is too lean at about 18/1.

My exhaust header temperatures suggest more fuel burning in front carb than rear. But thr front slow idle screw is fully unscrewed and not touching the throttle.

When I atttempt to slow the engine by unscrewing the rear slow idle adjuster, the rpm goes down but engine performance getts worse: rougher, lower vacuum, and leaner at about 20 afr.

So it seems I am geeting too much fuel from front carb.

Apologies for long story but invite your suggestions on what I guess are my options for front carb: lowering adjustible needle down or lowering fuel level in bowl?

Or should I do something else?

No replies in several days so let me add info and rephase my question.

First, I was dumb but finally found that high rpm was simply due to over tight throttle linkage. Fixed that and now the engine runs slowerr and smooth with 42 mm intake vacuum.

But both a very high air fuel meter reading and “piston lift tests” on both carbs indicate fuel mix is very lean.

The idle mix screw is already adjusted to max rich so what can I do to get a richer mixture.

Will raising fuel level in the carb bowl help? I really need good advice.


I just recently went through a similar experience with the strombergs on my 1970 model. We adjusted the floats, checked the valves, took off the carbs and changed the jet setting. We also tried moving the needle up or down. It just wouldn’t run right. I bought and installed adjustable jets which made an immediate difference - the engine runs great now. I should have done this while overhauling the carbs, but I thought it couldn’t be that hard to set them up. I guess now, better late than never. Sorry I have no real advice other than go get adjustable jets, it’s well worth it.


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I like that better than the adjustable needles: how much, and from whom?

Willing to try that. Do you have to change body of carb or can the fixed jets come out??

Joe Curto, about $75 per carb, IIRC.

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Gracias: tis running well enough for this year… winter’s project.

The old jets are pressed out, the existing body is fine to fit the new parts.

I ordered at a german carb service near Hamburg. Jens Peter Adam - he makes them himself. Really nice stuff - main argument being they helped me smooth out the rough. Cost was around 170€ for two.

Since it was me who put Martins in and did the initial adjustment: they’re great. Well made, cheap for what you get, easy and logical to adjust. The only hard part is pressing the old jets out, and that isn’t that hard. Easy with a press, actually.

Nothing is changed or damaged in the process.