Zienith single carb problems.

[Ofcalipka]

I started this post on the MG forum as I wasn't sure if my car qualified as a Spridget and they have suggested that I post here for help as well.

Aloha
https://www.britishcarforum.com/bcforum/ubbthreads.php/topics/566286/1

 

[Midget78]

Ah yes, the good ol Zenith Stromberg carb. When I first purchased my 78 this was the first headache I had the pleasure of getting. I had problems with the water choke sticking (which is common and most guys rig up a manual choke) and the idle was all over the place. I purchased the rebuild kit from Moss but this I found really doesnt rebuild it properly. You also need a manual to know how the thing goes together cause Moss wont give you instructions. In the end I put on the Weber Conversion. Cost wasnt to bad and no more trouble. I will suggest checking out University Motors - John Twist videos on the Zenith Carb. It may help you or better yet just go to his web site and ask the question using his tech lunch phone hour. You can talk to him directtly and he will feed you all kinds of information free of charge.

 

[Ofcalipka]

I posted over there that I am thinking of replacing the meetering needle and gross jet. Moss lists 4 different needles how do I know which one to select? Is there markings on the old one somewhere I'm not seeing. I am trying to put her together as stock as possible and I just don't have the money for the Webber conversion right now.

 

[Morris]

I read a little bit of your post on the MG forum. It seems that your primary issue is that the ZS is running rich. I am sorry to inform you that the ZS carb will probably always run a bit rich. That is one of their many "charming" characteristics. Unless there is conspicuous wear on your needle, that is an unlikely culprit. Here are some things to check:

1) Float. I don't have personal experience with grosse jets, but there seems to be very mixed emotions about them. In my opinion, it would be woth giving the float and valve seat thorough going over. I have frequently seen varnish and trash get in the valve seat thus allowing the bowl to overrun.

2) Choke. The 76 should have a manual choke I believe. Still, the mechanism can get bunged up such that it will not allow the choke to fully disengage. It may be worth temporarily removing the choke and the choke mount with a piece of tape. This will allow you to eliminate the choke as a culprit.

3) The dapshot diaphragm. It's very susceptible to damage from cleaners and solvents. It's definitely worth giving a through check.

4) I am not sure about the earlier cars, but there is an air bypass adjustment on the later carb. Ensure that your air bypass is clean and open.

5) Timing and ignition. Many, many carb problems turn out to be ignition problems. Give everything a thorough going over and make sure your timing is set properly. People will often advance or retard their timing to attempt to "tune out" issues with old worn out carbs. I spent many, many hours messing with my carb only to find that my "carb issue" was that the previous owner had advanced the spark to an astonishing degree.

It is possible to make the car run decently on the stock carb. Good luck.

 

[Ofcalipka]

Everything is new in the ignition except the distributor itself and the coil which I checked twice and found good. The manual choke was bypassed and made no difference so I don't believe it is a problem at the choke. The diaphram is new and in good condition so it is not that either. The only thing I have left to replace on this carburator is the float which is in good shape and the gross jet and meetering needle. The car will run in it's current state but it is so rich that it is fouling the plugs and is only just barely able to achieve freeway speeds of 55MPH and that is downhill or on the flats uphill I top out at 40MPH on a good day.

 

[dklawson]

Forgive me for not reading all the pages of the parallel thread running through in the MG forum. However, I did skim several posts and then I searched for a couple of keywords.

Early on in your thread you discussed the air filter and what went on with engine speed and such when the filter was off. Above you state that you can only achieve 40 or so MPH under load. I don't remember definitive statements in the other thread about what filter you are running. Is there any possibility that your air filter is very, very plugged? I've seen similar, power limit issues on a car with a plugged K&N filter. When properly cleaned the car which previously could only reach 45 MPH was suddenly able to reach highway speeds. The plugged filter was also responsible for terrible fuel consumption which improved when the filter was cleaned.

With all the other work you've tried at this point, have you tried tuning and short test drives with the air filter removed?

 

[Ofcalipka]

Brand new filter on there right now. So I do not think this could be the problem unless the brand new filter from Moss is defective.

 

[tony barnhill]

I'm constantly defending the ZS carb - it is installed on numerous other cars, MGB, TR8, Volvo just to name a few, and it performs perfectly on them. So, what's different about the carb on your car.

Well, the DPO screwed up several things that you need to put back to original before you can start figuring out what exactly is wrong.

Yes, you need the proper heat shield - not just a piece of metal between the carb & exhaust manifold.

Yes, you need all the emissions hooked up correctly or removed <span style="text-decoration: underline">correctly</span>.

Yes, you need everything hooked up to the air filter and ZS correctly (i.e., choke, preheat apparatus, etc.)

One of the easiest things to screw up on a carb rebuild is the float setting - yes, the float has a setting....do it wrong & the float will allow flooding which is part of your problem IMO.

Another part of your problem, IMO, is metering needle setting - without the proper tool & understanding how the needle is set, everybody screws it up. And most everybody screws up the needle & seat adjustment. You installed a new one that came with your rebuild kit, I hope! If you didn't go back & throw that Grose jet away & install the proper one.

IMO, you're working with multiple DPO problems in the fuel & ignition circuits.

 

[Ofcalipka]

Now the preheat apparatus. I do not have that. Was this something on all the MG's with the ZS or just the California car. I do not have the California car, thankfully, and I don't plan on building one. Now I have the proper adjusting tool for the ZS and I'm waiting for a colortune I bought on eBay to come in. I used some fairly thick sheetmetal and the drawing of the heat shield as well as knowledge from when I helped my friend with his 40's dodge that had a Zeinith carb to fabricate a new heat shield as I can't find a real one listed for sale anywhere. I ordered the 44A metering needle and the gross jet from Moss but I'm still waiting for it to come in.

 

[bthompson]

All the 1500 MG's had 'em. Most have probably been removed, either because the PO's thought they were "restricting airflow" too much<span style="color: #FF0000">*</span>...
or when the foam strip around the air door crumbles away, it bangs around constantly and drives you NUTS.

<span style="color: #FF0000">* </span>My personal theory about missing snorkles, which has no basis in anything but my overactive imagination: When Zeniths get old, the throttle shaft wears. This causes a vacuum leak in the carb, which drives it rich. The PO, (whose previous experience is Quadrajets,) knows that when his Skylark ran rich, he took off the air cleaner, so it sucked more air and ran leaner. So he took off his Midget's snorkel. Not knowing anything about variable venturi carbs, he didn't know Zeniths <span style="font-style: italic">need </span>a little air restriction to function efficiently. So, instead of improving things, he inadvertently drives it even <span style="font-style: italic">richer</span>.
Then he starts tinkering.
And <span style="font-style: italic">that's </span>why no one's Zenith works anymore. :wink:

 

[Ofcalipka]

Okay that sounds about right. Now, does anyone have one they pulled off with all its associated bits and pieces that they are not planning on using and would care to part with? I will pay something reasonable for it or maybe allow use of an MG Midget for a day next time you’re in Hawaii. You'll have to pay for your own petrol though.

 

[tony barnhill]

Now the preheat apparatus. I do not have that. Was this something on all the MG's with the ZS or just the California car. I do not have the California car, thankfully, and I don't plan on building one. Now I have the proper adjusting tool for the ZS and I'm waiting for a colortune I bought on eBay to come in. I used some fairly thick sheetmetal and the drawing of the heat shield as well as knowledge from when I helped my friend with his 40's dodge that had a Zeinith carb to fabricate a new heat shield as I can't find a real one listed for sale anywhere. I ordered the 44A metering needle and the gross jet from Moss but I'm still waiting for it to come in.

When the Gross jet gets there, see if you can make it skip across the Pacific back towards Moss. That's all its good for....order the correct jet.

A metal heat shield doesn't shield anything....as heat is transferred from the exhaust to it, it also heats up...heat shield is designed to keep heat away from gas so the gas doesn't boil before ignition....now that you've made the new shield, go to NAPA & get their heat shield material...it looks like asbestos but isn't and comes in different sizes by the feet....& pop rivet it to the exhaust side of the metal heat shield.....it won't be 'correct' but it'll do the job.

As for the preheat parts, look at page 23 of your Moss catalog...items 87 & 88 & the unnumbered items that connect to them....there's also another part that connects between the unnumbered parts & to the front of the exhaust manifold. The only way to find those items is used.

See, unlike others, I only turn to the Zenith carb as a last resort.

 

[dklawson]

When the Gross jet gets there, see if you can make it skip across the Pacific back towards Moss. That's all its good for....order the correct jet.

I agree completely. I have removed Grose Jets from the LBCs where I mistakenly installed them. Some people love them but every set I have had caused me problems.

A metal heat shield doesn't shield anything....as heat is transferred from the exhaust to it, it also heats up...

That's true to a point. The metal shield will conduct some heat, but its real job is to be a barrier to radiant heat. In that capacity it does work. An insulated surface on the metal heat shield will be even better but a metal heat shield will help.

 

[tony barnhill]

When the Gross jet gets there, see if you can make it skip across the Pacific back towards Moss. That's all its good for....order the correct jet.

I agree completely. I have removed Grose Jets from the LBCs where I mistakenly installed them. Some people love them but every set I have had caused me problems.

A metal heat shield doesn't shield anything....as heat is transferred from the exhaust to it, it also heats up...

That's true to a point. The metal shield will conduct some heat, but its real job is to be a barrier to radiant heat. In that capacity it does work. An insulated surface on the metal heat shield will be even better but a metal heat shield will help.

To a point. The heat shield of a 1500 Midget wraps under the carb to protect the bottom fuel bowl from exhaust heat. So, heat from exhaust transferred to metal heat shield and then to fuel bowl. Why did BL design a heat shield for that car completely from a non-metallic material.

 

[Ofcalipka]

Whilst I agree that the heat shield would benefit slightly from a non metal material, but for the amount of space that this item can possibly cover it seems that it is more meant for if the car is sitting at idle for long periods and less likely to even matter as long as the vehicle is moving at all. For the 4" X 4" that it covers it really can't stop any heat at all from anywhere on the engine even with several inches of asbestos mounted to it. It also seems to me that the heat shield really only helps displace the heat rising directly onto the bottom of the carb which this metal one would seem to do. I have seen, like I posted earlier, on older Dodge powerwagons that use the Zeinith carb a simple stamped piece of sheetmetal nothing nonmetalic and it worked fine. Now so far this has not been my problem with my car. It is so rich that it is fouling the plugs and just runs really rough when it does run at all. This is why I wanted to try replacing the jet and needle. It is the only part of the carb I can't account for being in proper order as everything else is either new or clean.

 

[tony barnhill]

the heat shield really only helps displace the heat rising directly onto the bottom of the carb which this metal one would seem to do.

True, the exhaust is directly under the carb; and the heat shield is the only thing between the exhaust and the carb's fuel bowl/float chamber. Exhaust heat rises to the metal heat shield which heats up & that heat then rises to the fuel bowl which, at best, is an inch or so from the metal heat shield with no room for the heat to dissipate. Maybe it isn't enough to make a big difference but it could be part of the overall carb problems....I'd tackle it last though - but I'd tackle it.

I think there was more space in that old truck's engine compartment & between the exhaust and the heat shield and the heat shield & the carb for the heat to dissipate.

 

[bigjones]

FWIW, when I first started driving my 1975 Midget (10 years ago) it suffered from vapor lock. For example, after a drive and stopping off at the gas station, it would be very reluctant to start up and run again. It would be fine when eventually it got going.

Apart from being very annoying, it was also a safety issue. The original heat shield is NLA so I "temporarily" installed a flat piece of aluminium under the carb (screwed the aluminium sheet to the fender). This stopped the vapor lock from every occurring again.

I suppose my point is that, whilst not ideal, a simple metal heat shield solved the problem.

Cheers,
Adrian

 

[JPSmit]

I don't want to divert this conversation from the real issue but I see no evidence that my car ever had a heat shield - would that make sense?

 

[mk2sprite]

Here is your carb in pretty decent looking shape, with the choke installed which yours ?????? ebay item 330321559858

 

[tony barnhill]

I don't want to divert this conversation from the real issue but I see no evidence that my car ever had a heat shield - would that make sense?

No..it had one but they're fiber & disappear quickly with DPO's who don't know how to handle them.