Zenith Carby tuning questions

[Ozdunc]

Howdy chaps, been a while.

After 4 years of running well, old Bill decided to start stuttering on small pedal inputs. If I had thought for a bit I'd have checked the plug leads, but I didn't and I saw that the butterfly spindle on the old Zenith was leaking fuel so I decide to swap out for a new Zenith I've had lying around.

Now I've faced the main joint between the top and bottom of the carby, the O ring is good and in the right position and the float height is correct at 33m.
I've adjusted the tappets, changed the plugs and leads. Timing is around 6deg BTDC and vacuum gauge is rock solid at 20mmHg when warm

Issue is this:
Idles beautifully at around 500, and starts no choke. Pulls hard on full throttle but stumbles low down as it was with the old carby. If I drive with the choke out the low end is great but as you'd expect it it starts to strangle at the 3000+

Volume mixture is out about 3 1/2 turns already. At 1.5 turns out (the recommended starting point) it barely runs. I adjusted the volume screw out until the vacuum gauge stopped fluctuating and it stopped 'chuffing' out the exhaust.

From there I'd do a run, it still hesitates, so I'd add a 1/4 on the volume screw and repeat. I've gone too far and dialed it back a 1/2 but I'm not getting a sweet spot to eliminate this hesitation.

Spraying carby cleaner around the base of the carby doesn't do anything, so I think there's no leaks between the carby and the manifold.
Old plugs were light gray, so I think it had been running a bit lean, but that might also be lots of short journeys.

What am I doing wrong?

[whitehillbilly64]

Another little test could be spray around inlet manifold and engine block, to eliminate gasket leakage.

whitehillbilly

[chazza]

If it idles well at 500 rpm, then forget about air leaks on the manifold.

Sounds to me as if you need a bigger idle jet, as it is running lean at lower revs.

There are technical articles somewhere on this site that explains how the carburettor works; however; whether you can buy bigger jets I don't know,

Cheers Charlie

[Bigbjorn]

What Zenith are we discussing? A Stromberg CD or a downdraft single choke fixed jet Zenith?
How come such retarded ignition timing? I would be looking at 8-10 degrees advance in 91 Octane.

[Ozdunc]

Cheers WhitehillBilly.

Is wasn't the inlet manifold, but looking for that possible leak made me find that the bolt connecting the throttle to the accelerator pump had gone walkabout.

Hi Charlie, I'm thinking either the idle jet is too small, or the accelerator pump jet is too small. But mucking around today my butt dyno says its sitting a little rich at idle. I will keep trying for a little while and if that doesn't work contact Zenith.

Bjorn - its a 36IV. Its running 98 as it doesn't do a heap of miles and it runs nicer. I generally set the timing using a vacuum gauge getting the highest inHg then knocking back 1-2 inches to prevent pinging. Doing it that way allows for any slack in the timing chain. Its just showing 6 deg when I sticking a timing light on it

Cheers for your input fellas

[Ozdunc]

Well after 2-3weeks of mucking about, I stuck the old Zenith back on.
Why? Cause every time I played with the newer one and I thought I had it sorted, the next day it would run funny, generally bogging at small throttle openings. And it always seemed as if it was less powerful/ less responsive.

Maybe the whole carb isn't jetted correctly for the LandRover?

So I bunged the old one back on, and huzzah runs like a top again. And the leak only seemed to happen when I was mucking around with the throttle and the engine not running, so maybe I flooded it somehow and it seeped out the butterfly spindle.
And the initial stutter has been corrected by new leads and plugs.
:)

[JDNSW]

The major issue with these seems to be the O-ring sealing between the top and bottom sections. After being away for a month I found my 2a was running rough, too rich at idle.

Fixed in this case by tightening the screws holding the top on by 1/8 turn.

[Ozdunc]

Thanks John, the o ring is in good condition. I got a bit of time over the weekend and stripped the carby down, just to see if I could find anything that might be causing this running issue.

Facing the 2 halves of the carby and the emulsion block, its became apparent that the faces weren't truely flat and there was some scoring on the faces, so I've flattened them and reassembled. Apparently warped faces can give the symptoms I've been experiencing.

I'll try and refit it some time in the new year, it'll be interesting to see it makes a difference.

[whitehillbilly64]

Pop the o ring in hot water before reassemble.
Helps with fitting and tightening process.

whitehillbilly

[JDNSW]

The warped top means less than full pressure on the O-ring and quite likely uneven pressure.