Tuning Zeniths

[handleyb]

I'm in the process of tuning the Zeniths on my 1970 911 T. I balanced both carbs so that they were equal in their air intake. After making the mixture adjustments, I noted that the air intake was affected on a few of the stacks. Should the air balance be readjusted or left to the original settings. I've read most of the threads and see no mention of altering the original balance adjustment. Thanks

[1QuickS]

Set idle air screws first (drop links disconnected) then idle mixtures. Once mixtures are set it is time to check idle air screws; again and then idle mixtures...again. Then adjust side-to-side using cylinders #1 and #4 as reference cylinders.

If the idle air flow is off then check cross bar to see that it isn't mechanically tweeked and that your drop links are the same length. Adjust side-to-side using the clearance in the cross bar mounting bracket (clearance in the holes with the studs on the manifolds) and be sure to have a helper hold throttles open using throttle pedal or you adjusting throttle using front-to-rear link attached to cross bar; if you put your hand on the cross bar you WILL affect adjustment...

[handleyb]

Paul,

So when I'm finished with the carbs, air intake on all stacks should be equal as per first adjustment ? adjust the air and mixture settings as many times as needed ?

[1QuickS]

Air flow through carbs at idle is function of throttle plates closing consistently each time and the amount of air you let "bleed" into the bores to balance for minor imperfections in component sealing, valve clearances, engine characteristics, etc.

Therefore: air flow through individual barrels should be unaffected by side-to-side balancing assuming there isn't an issue with drop links and cross bar that prevent throttle stop screws from seating as they did when idle air and idle speed was set.

It will take a couple of iterations to get idle air, idle mixture and idle speed set since making "Lean Best" mixture adjustments will affect engine speed which necessitates readjusting air screws and idle speed stop screws. All that adjustment affects air flow which affects mixture...Two iterations will usually get you close enough.

Another issue is wearing of throttle plates and throttle shaft/bearing interfaces; the more wear-related clearances you have - the more opportunity you have for air flow to be erratic. It is this issue that got me involved with my carbs and eventually with my carb service business.

[gearheadgreg]

Thanks for contributing, Paul. Valuable info for zenith owner/operators!

[handleyb]

Thanks for the clarification. I thought that once the air screws were set at initial balancing, they were to be left alone.