Zenith carbs running any thoughts

[silverboydreamer]

I am running a T6 on zenith carbs and after rebuilding both the car and the carbs, I have over the last six months getting to know the car, one situation that has shown itself has to be the carbs.

When accelerating the car pulls well but when on half throttle or in crawling traffic the engine seems to hunt ie, the running gets lumpy, tick over is good at 750 ish rpm when warm and I have setup idle mixture as per the book.
As the mixture is fixed and not able to be altered I am wondering Is this normal or do I have to keep in a lower gear and so rev higher, does this behaviour show something is worn or not adjusted correctly?, has anyone else experience this?
Or should I bite the bullet and fit Webber’s?.

[Nine One One]

This any good?

https://porsche356registry.org/article/47

[Ashley James]

Mine did that and I removed them, dismantled them, cleaned them in my cheap Chinese ultra sonic tank and the car ran perfectly.

The general view is that the Zeniths are the easiest and best behaved of all the carburettors that can be fitted to a 356, so stick with it, they’ll be fine.

Those carbs or very very similar ones were on Cadillacs and early post war Rolls-Royce as well, they’re excellent, so I think there’s dirt somewhere.

It does make a difference if you have a vacuum gauge to balance them.

[mycar]

Here's the info for tuning Zeniths...

https://porsche356registry.org/article/46

.. but I do wonder if the symptoms you describe are cause by the distributor rather than the carbs.

[Ashley James]

Here's the info for tuning Zeniths...

https://porsche356registry.org/article/46

.. but I do wonder if the symptoms you describe are cause by the distributor rather than the carbs.

He’s right, never any harm in replacing plug leads at least, but usually a flat spot in some part of the Rev range is a carburettor issue.

If I’m understanding what you’ve said as you meant it, might just be that they’re not well synchronised. I have a vacuum gauge for mine for that reason.

[silverboydreamer]

Hi, thanks for your comments, for your info I have stripped and ultrasonicly cleaned carbs, fitted new gaskets etc including accelerator pump, fitted inline fuel filter and ensured no air leakage when fitting carbs to engine, have synced the carbs to each other and ensured the timing is bang on. Fitted new plugs and leads.
As I say it ticks over steadily and pulls well with no flat spots on acceleration, after raising the revs the engine falls back to tick over with no hesitation or drama, picks up well but on half throttle starts to hunt at around 2k revs.

[Winston Teague]

Rolling Road, there's a chap called Malcomb in Bromsgrove/Stoke Prior who set up my car (Webers and Malloty dizzy). great value for the improvement, just wish i could remember his business name! Il'l find out today and post. W

[POR68]

Ok what jets (idle and main) are you running? Have you checked the sizes with a pin gauge? Zenith carbs are simple devices but there are things you must get right such as the float levels by using a level gauge (preMat is my choice) and the make sure that the butterfly valves are properly working. There are dozens of useful threads on the 356 Reg, especially from Ron La Dow. Should rev freely through its range. As for balance, I always use two flowmeters - one on each carb at the same time

However, the majority of carb problems are electrical. You have checked the timing, the distributor condition, plugs and leads, but also the integrity of the pesky condenser (a known trouble item, especially the new ones)?

[Mike Smith]

Could be a number of things but first make sure that you have the correct jets for the engine that you are running

`Life`s too short for Weber`s (on a 356)`

[POR68]

How did you go SilverBoy? What idle and main jets were in there?

[dcraigmyle]

I always found this site by Charlie White really usefull.

http://derwhites356literature.com/New35 ... Carbs.html

David

[POR68]

We may never know ...