Classic cars forum & vehicle restoration.

[CWhaley1]

Currently trying to diagnose an issue I'm having with my Super Minx. She will idle nicely but struggles to move without over-revving the engine! She will stutter and hesitate a little going down the road too.

Does anybody have any advice on tuning the Zenith 30 IZ Carburettor? - or Zenith carbs in general?

I had to take the carb off a while ago and since I've put it back on the car, the idle control and mixture screws were disturbed. I've re-set the best I can but something must be wrong... The choke, when fully pulled out, also cuts the engine out which makes me think the mixture is too rich but the screw is only out around 3/4 of a turn??

Any pointers from those familiar with these?

[Rick]

Hi,

I found this link

http://zenithcarb.co.uk/zenithseriesiz/?___store=zenith

Is the accelerator pump working correctly? I remember my truck (Stromberg carb) being unable to accelerate with any alacrity, yet idled ok. It turned out that whoever put the carb together during its resto, had omitted to include any of the pump mechanism.

RJ
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[alanb]

Are you sure it's the carb? Many year ago I had a similar problem with my Standard 8, it would start, tick over and rev okay until you put a load on like trying to pull away or even a slight incline once going. After many frustrating hours messing about with the carb, a neighbour who was a mechanic fixed it by replacing the points and condenser and resetting the timing.
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old tourer


Morris 8 two seater

[kevin2306]

I had a similar episode with my Singer, ended up buying a brand new old stock carb which improved matters but didnt cure them, my main issue turned out to be the 1970s lucas electronic ignition breaking down.
I put an accuspark unit on it as a stop gap and its still fitted, runs fantastic with it.
I do have a NOS standard distributor to fit but i am loath to disturb matters now its running so well.

good luck

kev

[CWhaley1]

Thanks all for the replies here, gives me some thought.

It can't be the ignition system as I have fitted new contacts, sparks, condensor, coil, distributor cap and leads - all of these worked fine before. It was after I removed the carb (to re-drill the exhaust manifold) that the problems started.

I have yet to try giving throttle with the engine off to see if any fuel is squirted into the venturi. This did work before though, so I can't see why it would have stopped. Unless my handling of the carb upset something and the float chamber/jet are clogged?

Not looking forward to taking it apart to look! Thanks for the link, Rick - that should help me out. If there's any daylight left when I'm back from work I'll check to see if the pump is working...

[alanb]

Are you sure you don't have an air leak between carb & inlet manifold or manifold & engine? Even the smallest leak will prevent it running properly.
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old tourer


Morris 8 two seater

[CWhaley1]

There was a rather large air leak at the exhaust manifold to downpipe flange, which has now been fixed (needed new studs fitting). That quietened noise down considerably.

I can feel a small rush of air when idling, but I don't know where it's coming from. The inlet and exhaust manifolds are securely bolted together and I've fitted a new gasket (from Speedy's) between the engine and exhaust manifold.

Air leaks are another area to search during my investigations, so I will take a good look at any areas that may be leaking.

[alastairq]

Air leaks downwind of the carburetor will be inward rather than outward.

To check,start the engine [warm up too] then spay something like WD40, or carb cleaner, etc around the carb base flange, and the area where the inlet manifold bolts to the cylinder head.

Any change in engine note will indicate an air leak [which will weaken the mixture]

When assembling the inlet manifold to cylinder head [and carb to manifold] a useful tip I received was to smear seam sealer over the gasket areas.

Take off the air filter.....look down carb mouth, pull throttle open, and you should see a squirt of fuel go down the hole..if not, your accelerator pump isn't working.....either the diaphragm has a small hole in it, either the pump linkage is wrongly adjusted/attached.

o tune [the Zenith is similar to other carbs of this type].....set idle speed using appropriate screw....then set idle mixture,, by steadily undoing idle mixture screw until revs stat to rise a bit....keep steadily unscrewing [take your time]...until revs start to fall again....then turn back to roughly the halfway point....re-adjust idle speed screw, and do a quick check again on the mixture screw [tiny bit either side of your previous setting]...

Worth checking whether the throttle butterfly spindle hasn't worn a hole where the spindle passes through the side of the carb body?

Again, with engine running, a quick spray around the throttle spindle [listening for change in engine note] will give an indication of wear at this point. This will make setting the idle mixture very difficult.

Check your ignition timing too....see if the timing advances properly?

Things like, bobweights in the distributor can stick, or the springs can break/detach....if you have a strobe timing light, this will give an indication if all is well in that department...very easily.

[CWhaley1]

That's really useful and in-depth, so thank you for writing that up it certainly gives me lots to think about. Your guide on tuning the carb is really useful too - although I have made adjustments to the mixture and idle speed screws as well as replacing them with new ones from Zenith, I think it would be good to start again once I've checked everywhere else.

So next steps are to get the carb cleaner out and see if there are any leaks sucking air in. I'd hope not and that I've simply set the adjustments wrongly, but it gives me somewhere to check. I can get hold of the replacement gaskets and using some seam sealer is a good idea to get an air-tight seal. I don't like the instant gaskets I've been advised to use before as they're a mess and seep into everything.

I do have a strobe timing light, but I don't believe the timing is off - it worked nicely before I touched the carb, so why would it have adjusted since then?

Thanks again for the advice.

[Bitumen Boy]

I wonder now... when you had the carb off, did you disconnect everything and place the carb right way up on a bench, shelf or similar while carrying out work on the manifold - or did you just remove "enough" and move it to one side, maybe on an angle and leaving stuff like cables and vacuum advance tube connected? I ask because I once did the latter and had very similar running problems once it was all back together. After several weeks of head-scratching and part substitution, I found that the vacuum unit on the diz was half full of stale petrol, which could only have got there from the carb's float chamber via the vacuum tube. Once the vacuum unit was removed, the old petrol shaken out and refitted it ran well without any further problems.

[CWhaley1]

Hello - yes I disconnected everything from the unit (choke cable, throttle, vacuum advance, etc.) and took it inside. Interestingly, when I got the car it didn't have the vacuum advance tube so I salvaged one from my old Minx and put that on.

I have to admit, while the carb was out, it was tipped onto its side which drained it of all the fuel inside. When put back in the car, it was effectively dry inside meaning the float chamber would have to have been refilled.

Would this have created any problems or would it have refilled fuel to the right level when connected back up?

[ajlelectronics]

I would recommend NOT using sealants on the gaskets. Either fit dry or if you must use something, a smear of grease rubbed in is all that should be used. It is vitally important to check flatness of the mating surfaces before re-assembly too. The brake cleaner test is a good one to disclose leaks and pay attention as already pointed out, to the throttle spindle area. If everything checks out, then you have an ignition problem, despite your replacements.
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[kevin2306]

id agree on the possible electrical issue, if you have used new replacements as opposed to new old stock items may be worth putting the old stuff back on and seeing if there is any improvement or deterioration.
The quality of some of the points and rotors are quite suspect (esp the ebay stuff).
its a bit of a trial and error but good luck

Kev

[CWhaley1]

Thanks for the advice and I do take on board what you say about using replacement parts on the ignition.

I only bought Lucas parts though and once fitted and correctly set, the car has always started with them. These issues have only started since re-fitting the carb. As I've probably been running rich when trying to get the car moving, I'll check the plugs as there's a fair chance these are caked in black.

So I have a few jobs to do this weekend (investigate pump on carb, the throttle spindle and any potential air leaks). If I do each area section by section, I'll gradually be able to eliminate each area and hopefully find the cause of my poorly Super Minx!

You've all given me plenty to think about so fingers crossed the issue will be resolved. I'll post a few pics of my progress should I get anywhere.

[CWhaley1]

Thought I'd update on this. Six years have passed since I posted about my Super Minx and I have learnt a lot about mechanical and electrical maintenance/repairs on these mid-'60s Rootes cars!

I sold this Super Minx in 2017, actually tracked it down again in 2021, bought it back in the March but sold on again after doing a bit of welding. I've now settled for a Mark V Singer Gazelle which I'm very happy with and have owned since Spring 2021.

The Zenith carb on the Super Minx needed a complete overhaul. I used an ultrasonic bath to clean it, changed all the jets and gaskets and refitted. It ran much better but was still a little ropey. I later discovered that the choke flap was closing when the choke cable was pushed fully home. The linkage wasn't quiet set up right and it took me a while to work out. All resolved in the end and I eventually sold it in November 2021 to a chap from Cheshire who had one of these as his first car. He drove it from my house to his house (probably about 90 miles and he said it was fantastic! Still has it I believe.

My Gazelle came with a Zenith 30VN which looked tired, seemed to leak fuel (float bowl was always empty after a day or two) and didn't ever seem great on fuel. I cleaned it and changed jets, but noticed the throttle spindle was sticking.

Took the plunge and decided to replace with a brand new Weber 34ICH. Wow the difference is incredible! Doesn't leak fuel, is better on fuel, has a much more responsible throttle response and is generally more 'punchy' to drive. I did keep the Zenith as it's original to the car, but have no intention of fitting it back! The jets on the Weber are .50 I believe, and this seems fine.