Zenith HZ 30 Carb or Parts urgently needed for 1929 car

[ian Howell]

Nigel: -

Thanks for that. One problem solved!

Next. The engine seems to be overfuelling, as the exaust pipe and plugs are as black as your hat (well my hat anyway).

A main jet with a smaller orifice would seem to be a worthwhile 'trial' at least, but what if it doesn't work?

A thought occurs. If I run a piece of VERY thin wire through the jet orifice, this would restrict it a bit - similar to using a smaller jet. Possibly enough to at least see if it makes any difference. If so, then maybe invest in a smaller jet. If not, remove the wire and try something else.

Anyone ever tried this sort of thing?

[Andrew Henshall]

And the top of my 30 HZ is also Mazak, as final confirmation that yours is correct Ian.

[Tony Fearn]

Anyone ever tried this sort of thing?

Sounds a bit like one of George's bodges.
Tony.

[george garside]

if the bit of thin wire is a loose fit in the jet so it bounces around a bit it could even add a bit of 'turbulance' to the proeedings!.

However before rsing around with jet sizes Ian are you absolutely sure that the carb has all correct jet sizes and choke tube size for a 1929/30 Jowett. Also that the air screw is not damaged and that the float level is correct and the needle valve working well and no air leeks on throttle spindle, If its over rich I would suspect all or any of these

george

[ian Howell]

George: -

Many thanks for your useful contributions.

One at a time :-

The jet sizes are correct according to both Jowett and contemporary Zenith data.

The air screw does not APPEAR to be damaged. Would a couple of minor scratches qualify as 'damage'?

The float level IS possibly suspect. I have already made a replacement for the collet that is operated by the levers and is soldered to the needle valve, as the previous (non-original) arrangement of two washers soldered to a spacer had been worn out by the levers! There is a 'notch' in the needle which I took as being for location purposes and I used that to give me a position for the new collet. I did also try altering the float level by slight alterations in the positions of the collet on the needle - simple, as it is soldered in place - but things only seemed worse as far as performance was concerned. Even slight movement stopped the action of the levers on the needle, jamming the valve either open or closed.

Also the levers have slight 'flats' on both top and bottom surfaces that bear on the float. Were these there when the carb was new or do they indicate wear due to the float 'rattling', and reversal of the levers at some time to overcome previous wear? There are no signs of wear on the float surfaces.

The needle valve does not appear to leak, as there is no 'flooding' whether the engine is running or not.

There MAY be air leaks on the throttle spindle but the brass spindle appears to be a reasonable fit in the bronze body. I will have a more careful look, but if this is a problem the cure could be tricky/expensive.

Back to the 'wire' idea, maybe fine nylon fishing line might be better as it should not 'chafe' the jet if left in place for long.

[NigelPlant]

At Last I think I have the car running properly, after the slow running tube/device was replaced, it was slightly better, and with some working adjustment I was able to move on to other things.

There just wasn't enough petrol getting through, so the conclusion was that the main jet was too small, that is now bigger (filed until desired result) and it appears to be running much better.

[ian Howell]

PS to my previous post (Much later!): -

Wouldn't air leaks around the spindle make the mixture weaker rather than stronger?

[Keith Clements]

Probably, but the idle metering hole is close to the butterfly and it might be upset by air coming from somewhere it should not. The main jet metering should not be upset. Also to compensate for this weaker mixture the jets may have been screwed in which would have a proportionate affect at other revs.