Carb questions

[Jez 356]

Dear all

I have been having problems with my recently rebuilt Zenith carbs, in that they seem to allow the fuel to evaporate very easily. Obviously in hot weather and with on long journeys the carbs can dry out in about 10 minutes!! I have installed an electric fuel pump to prime the carbs but it still takes alot of work from the battery to get the motor running. It has been on a rolling road and all seems fine with jets and tune.

My questions:
Do other 356 owners experience this with Zeniths or other carbs that Porsche used?
Would it be a good idea to upgrade to Solex carbs?
I never had this sort of problem with my VWs so would it be an idea to use Weber or Dellorto carbs?

Any views would be appreciated

All the best

Regards Jeremy

[40 scout]

Seems a bit drastic to change the carbs - mine uses Zeniths and is fine when used regularly.

If left for a week or so I have to crank the engine over a few times, leave for 10 mins or so then try again and it fires right up.

I've heard the problem relates to fuel evapouration but I don't think yours will be evapourating that much in 10 minutes.

I suppose the obvious one to check is float levels - then fuel delivery maybe.

Is there a primary circuit on Zeniths that could be blocked??

Mine runs great on the Zeniths - I'm sure yours could be fettled to run well without the extra expense.

Good luck

Steve

[roy mawbey]

Jeremy as Steve says, the Zeniths should work okay even in hot weather and with un-leaded fuel. It is true if the engine is really hot if you stand behind the car after its stopped for about 10 minutes you can hear the petrol boiling. This didn't happen as far as I remember with the old 4 star.

It is also true the fuel evaporates over a period of time so that leave the car over a week before starting and its a little more cranking somtimes although actually whilst the level in the fuel bowl might drop the accelerator pump should still ( or mine does ) skwirt fuel through the nozzles into the venturi's. Then normally in the time it has some fuel it turns over enough to keep it going with the foot pedal. ( I also still have the mechanical pump.)

Have you checked as Steve says the fuel level in the bowls and do the accelerator pump nozzles deliver the 0,2 - 0,3cc's of fuel when the throttle lever is depressed fully to the stop twice?

When the engine is cold you do still depress the accelerator pedal about 3 times to the floor slowly to prime the system before starting? ( never do this when hot although some advise holding the accelerator to floor when you crank the engine till it fires.)

Are you sure its not another problem like faulty ignition items plugs, points, condenser, ignition leads, that are not operating correctly when hot? or a fuel pump just not making the correct delivery.

My Zeniths like Steve's work fine but it has taken me years to fully understand them. If the car runs fine when driven it can't be a big problem to sort. ( how do you know the bowls really are dry after 10 minutes when its very hot???) Without taking off the tops how do you determine that? Have you tried removing the air cleaners looking over the top of each carb to see if the accelerator pump pushes fuel through the nozzles as explained above?


I would not change my Zeniths for other types.

Let us know what you find. ( have you the books? ) If not look on the www.356registry.com pages and look under the heading troubleshooting
then scroll down to tuning Zenith 32NDIX carbs. This might help you.

Roy

[40 scout]

Good idea Roy - for the low cost / hassle factor - I'd change the condensor just to rule it out.

Steve

[58herbie]

sounds like the carbs are only running on idle side of things. a good strip and clean seems the order of the day..

If you have fitted a electric fuel pump are you running on 12v??

if so i could mail you what you would need to make up a small relay and push button circuit.to prime and act as a fuel cut off.

1) use push button to prime the carbs before starting.
2) the relay only energizes to supply power to the fuel pump, when the generator warning light goes out. thus giving you a fuel cut off system should anything happen. the last thing you want is to squirt neat fuel everywhere.. with the engine not running. If you leave on it or can not switch the ignition off.

Jon

[Jez 356]

Thanks alot for all the input

The car runs very well once up and running so I have a feeling that the float chamber levels is something that might be in need of review.

The condensor could also be an area to check.

At least these are less costly than changing carbs!!

I suspect checking the float chamber levels is a bit specialist so I had better get saving the pennies!!

Many thanks for all your input it really is appreciated. Now I know that the Zeniths should not be so troublesome I can at least working on getting them running more effectively!

All the best

Regards Jeremy