Zenith Carburettor

Refurbing the Zenith carb. 1972 series 3 2.25 109. I've a Bearmach kit. Pump jet in the emulsion block are slightly different. Whilst existing one's hole is all cleaned up, the head has been damaged and sticks in the barrel of the emulsion block in which it's screws in. The replacement looks to have a narrower calibre jet albeit not by much. What effect will it have on the vehicle if I installed the new one? Loss of power I'm guessing. See pics- old jet on the left. Should I just stick with the original?
Secondly I've found a odd looking screw with a split end. I can not place it anywhere on the carb. It's similar to the screw that hold the choke cable bracket to the top housing of the carb but as you can see it's not as I've put that screw in its place. See pic.
Thirdly, idle mixture screw. Clockwise turn makes the mixture richer and anti CW leaner? Is that the right way round? Can't get my head around which direction. Thanks.

It's been a while since I had a Zenith on the bench (my last Series had a Weber 34ICH), but the jets should have a number stamped on the top. Moving one size up or down from standard probably isn't going to make a big difference but best to use the standard sizes, at least to start with.
The screw with the slot looks like the ones used to secure the throttle butterfly or the choke flap. The idea being that you splay the ends to stop them unscrewing and dropping into the engine...
The idle screw is tapered so turning it anti-clockwise makes the idle richer and clockwise makes it leaner.

ExMil109 said:

It's been a while since I had a Zenith on the bench (my last Series had a Weber 34ICH), but the jets should have a number stamped on the top. Moving one size up or down from standard probably isn't going to make a big difference but best to use the standard sizes, at least to start with.
The screw with the slot looks like the ones used to secure the throttle butterfly or the choke flap. The idea being that you splay the ends to stop them unscrewing and dropping into the engine...
The idle screw is tapered so turning it anti-clockwise makes the idle richer and clockwise makes it leaner.

Click to expand...

Many thanks! I decided to keep the original jet. It was patent and it screwed back in with some care. The slotted screw looks nothing like the brass peening screws that keep the butterfly on the throttle/choke shaft. I wondering if the screw just crept in there from junk I have in the garage.
I've bolted the carb back on. I was running too lean but today I turned the mix screw anticlockwise a 1/4 turn to rich the mixture. It made a big difference. Thanks.

I've found this page really helpful - the procedure at the bottom of the page has worked a treat on two different vehicles.

https://www.glencoyne.co.uk/carbs.htm