Rough running on Zeniths

[pauldvern]

Hi all,
My car has started to run a bit rough while pull from below 3000 rpm and has lost its usual smooth idle. It sound a bit rich and has that hunting sound. I cleaned out the idle mixture screws and dialled them in. A couple of them have ridges and could do with replacing. The oddest thing is that the screw on cylinder 4 seems to do nothing, right from fully closed to nearly hanging out it doesn't effect the idle speed. Any ideas? Do I have a blockage somewhere?

Cheers

Paul

[Gary71]

I'd start with checking the points before doing much else to the carbs


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[pauldvern]

Sorry should have said, timing has been checked and is spot on.


Paul

[roy mawbey]

Paul,

The idle screw must make a difference to the rpm. I presume the other carb does have the effect of raising and lowering the rpm on each of the 2 screws.

The air correction jets numbers 66 and 67 on the attached linked drawing should also be checked. The circuit is blocked if no alteration can be made. But also because that mixture screw with the brass point tapered end might be clean the entry hole of it could be blocked and the bore the taper sits into might need cleaning by air compressor.Might be best with the carb off then manifold.

I presume you are certain the idle jet itself is clean the bore is so small have you tried a stiff bristle from a brush though it.? The drawing gives some help but that mixture screw should affect rpm when turned.

e356registrhttps://porschy.org/article/46

This link looks odd it should work if it doesn't I will try and resend it

Roy

[roy mawbey]

Paul,

The link did not work so I try again.

https://porsche356registry.org/article/46

this looks better, scroll down for the drawing.

Roy

[Gary71]

Ah! Not 911T Zeniths. In which case I'm out


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[mycar]

Paul, co-incidently there's a thread on the Registry concerning a non responsive cylinder number 4.

http://forum.porsche356registry.org/vie ... 1ee#unread

[pauldvern]

Thanks chaps, I will persevere. Do need to drive to Amsterdam on Sunday though!

[mycar]

As Roy says, it does sound like a blocked jet but one thing you might try is an old trick Jb reminded me of a couple of months ago.

Blockages in carburetors may be cleared by simply revving the engine and briefly putting your hand over the air intake of the carb.

[Ashley James]

As Roy says, it does sound like a blocked jet but one thing you might try is an old trick Jb reminded me of a couple of months ago.

Blockages in carburetors may be cleared by simply revving the engine and briefly putting your hand over the air intake of the carb.

I remember back in the sixties with this happening to my £15 Ford Prefect. It was always at 11PM in November and somewhere near Stow on the Wold, so bleak with a light drizzle and cold. The car had no heater. I used to use a book, I'd Rev the engine and slam It down on the top of the carb half a dozen times. It never worked. Torches weren't great then either and there was very little traffic. I've only recently started to recover.

I agree it's a blocked jet and I suspect the best solution is to take the carb off and meticulously clean and reset it to the book.

[roy mawbey]

Paul,

Below the drawing there is a layout of the top of the carb. You will see the defined air correction jets and the idle air correction jet. Check those out. Remember the bottom brass mixture screws allow both fuel and air to combine into a mist that goes down the venturi. No air but only fuel might be the reason its running rich.

The ridges you have on the tapered end will not help but if that engine was perfect until suddenly you noticed it then they should work for the time being but.. change them! They can also get bent with spanners! Been there till I made my special spanner to get the carbs off the manifold.

Also maybe just look over the top of the open carb and push down the throttle levers to see if both accelerator pump jets are pushing fuel through the 2 small diameter pipes. You can see these easily.
The pump jets might have no influence on your problem but might?? prove fuel is filling that cylinder.

Hope you find it Paul for your trip !!

Roy

[jb]

As Roy says, it does sound like a blocked jet but one thing you might try is an old trick Jb reminded me of a couple of months ago.

Blockages in carburetors may be cleared by simply revving the engine and briefly putting your hand over the air intake of the carb.

I remember back in the sixties with this happening to my £15 Ford Prefect. It was always at 11PM in November and somewhere near Stow on the Wold, so bleak with a light drizzle and cold. The car had no heater. I used to use a book, I'd Rev the engine and slam It down on the top of the carb half a dozen times. It never worked. Torches weren't great then either and there was very little traffic. I've only recently started to recover.

I agree it's a blocked jet and I suspect the best solution is to take the carb off and meticulously clean and reset it to the book.

Despite the book not working for Ashley my hand worked well as I do not mind getting my hands dirty. Other than getting a slightly dirty hand I am not sure what you have to lose by trying it. However it may be nonsense but it worked for me on my beetle.

[neilbardsley]

Did you solve the issue? I would be tempted to suggest checking the valves and spark plug/lead to no 4?

[Winston Teague]

I wonder if the inlet port to manifold gasket might be leaking. W

[mycar]

Did you make it to Amsterdam Paul ?