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MG Midget and Sprite Technical - Midget 1500 - replacing ZS with Weber Downdraft

Hi, all. After 20+ years of making do with my car’s original ZS carb (US spec), I’m considering swapping it out for a Weber downdraft conversion kit. Have any of you done this, and what was your experience with the new rig? I’m looking for better breathing and more precise mixture control. If I actually take this route, I plan to give the head a good cleaning too.

Thanks for any advice,

-:G:-
Gryf Ketcherside

Can you not switch to twin SUs (or single?) as I'd have thought that'd be a better matching to a more standard road-going 1500.

Webers here tend to go on cars that have uprated engines and driven for acceleration more.
Nigel Atkins

25 or so years ago I came across a pair of SUs on a triumph in the local scrapyard, which I got for £5. I sent them to a fellow 'member' in the USA, on the Autox.team.net board that used to be a predominantly USA based group. He was upgrading his local spec 1500 from the Zenith style carbs to get improved performance
GuyW

Gryf
Lots of cars here running the little DG- series Weber
Good little carb. but from what I've experienced with them they are jetted a bit lean out of the box
Std they come with a 145 primary main jet and for smaller cars like a Midget this needs opening up to 150-155
I reckon they jet them for slightly larger engines like around 2.5 - 3 litre but even on a 3litre they,re borderline lean at 145
A 1/16" drill(1.58mm) usually sorts them out nicely
They have got a few little diaphrams about, acc pump ,vent diaphram etc. so care is needed if blowing through things with air pressure, Also touchy on dirty fuel so you need a good fuel filter
Have you given any thought to going for a single 1 3/4" su It'd just about bolt straight to your manifold I'd think
William Revit

The Stromberg is so simple and reliable. It lacks the complexity of twin S.Us but throttle shaft wear is minimal and the mixture adjustment is effective. The Weber does not allow mixture adjustment with the turn of a wrench and is preset according to someone else's assumptions about altitude, accessories, engine condition and how the car is driven. If a modification is in order to keep things interesting, I vote for twin S.Us.

I have a down draft Weber on my 1970 B for heat tolerance. The S.Us are unstable in summer heat with the A/C on. Mikunis were even worse. The Stromberg sat there above the catalyst so it is probably the carb of choice in the heat.

If it ain't broke...

On the Volvo and Range Rover, I found that Strombergs tend to reliably go rich. This is easily corrected by bending the tang on the float valve back to recover the proper setting. The O-ring on the needle adjustment screw was also perishable and lead to dashpot oil loss. Even so, O-ring replacement, periodic float adjustment and diaphragm replacement made for a very reliable setup.
Glenn Mallory

HIF 44?
Oggers

Gyrf. I, too, recommend the SU conversion rather than the Webber DGV series. Whether you wish for the simplicity of a single HS-4 (which I find easier to work on than the more complex HIF series) or a set of twin HS-4s, do not really matter. The Z-S was, quite simply, a cheaply made version of the SU, made using modified 1920s design. (SU tried the flexible diaphragm and could not make it work well so substituted the precision machined pistons for the flexible system which, later, was used in the design of the Z-S carbs.) Thus, the Z-S is an SU of inferior design, made as inexpensively as possible to keep manufacturing costs down. It works, for a while, but does not work as well, nor hold up as long, as an SU HS-4 carb.

As to the Webber, I have run them on three cars for something between 12 and 24 years on each car. These were MGBs and the DGV carbs replaced the far more difficult to tune Z-S carbs and, because it was only a single carb, were allowed by our emissions testing regulations. (Twin SUs, being two carbs, would not meet the regulations even if a car using them passed the physical emissions test.) Thus, the DGV was the carb of choice for local use among the MG community and VW community when they needed a single, reliable replacement carb. It, also, provided relatively good fuel economy, relatively good performance, and was easier to tune than the Z-S. Rebuilds are relatively easy, especially if you have ever rebuilt a US carb. Trouble spots are the diaphragm of the accelerator pump and the power jet, both of which can rupture, leak, and cause excessively rich running.

As to "drilling out the jet(s)", why? This is a quite common conversion carb (it was even licensed for a version to be produced in the US), and rebuild kits and jets are commonly available. It is rather simple to remove the old jets and install new ones. Simply remove the air cleaner, remove the top cover, use an eye-dropper to remove the small amount of fuel in the appropriate chamber, use a common screwdriver to remove the old jet, install the new jet, reassemble and test.

I prefer the SU system myself. But, I would never try to argue anyone out of replacing a ZS with a Webber DGV. Rather, I would offer to come over and help them to get it set up easily and quickly. Which is not that hard.

Les
Les Bengtson

Thanks for the advice, all. I’ve considered a single larger SU, and that’s certainly a possibility. One of our local Midget 1500 guys did that with his car, so I think I’ll send him an email and see how it worked for him.

-:G:-
Gryf Ketcherside

I take it that the -why drill out the jets is pointed at me Les.
I agree that new correct jets will always be better than drilled, even the factory supplied Weber drill kit isn't fine enough and goes in 0.1mm steps so goes straight from 150 to 160
155 is the most likely size to go for and if you haven't got one the 1/16" drill is as close as you can get unless you have a set of metric number drills
My reference to drilling is/was more of a size guide------but if you haven't got jets on hand-??
William Revit

Willie. Yes, it is a reference to your post, but only marginally. When I have worked in the arctic and in central America we often had to make field expedient repairs and modifications to get exactly what we needed. Here in the US, especially for those of us who live in major cities, factory parts are often easily available. They are, frequently available in a range of sizes (e.g. shoes, carbie jets, etc) and, it is often easier to start off with a selection of sizes and narrow down to the size showing the greatest potential than to keep drilling out pieces. Thus, when available, I recommend a selection of factory jets be used for initial set up of Weber carbs in the same way I recommend using factory SU needles when setting up SU carbs. Yes, you might find you have to modify either, but you can get a close approximation more quickly than you can by drilling jets or spinning and sanding on needles. Hope that clarifies my thinking.

Happy New Year all. Les
Les Bengtson

All good Les, I agree 100% to that

Happy New Year
William Revit

Hmmm. For those of us out in the sticks I would beg to differ. Drilling out jets is a cheap and easy way of finding the correct jet size and thus mix. Once found, you can then obtain the jet. Micro drills are a fiver on fleabay - a selection of jets many times more.
Oggers

I dread to think how much I spent over the years on those silly little brass bits for my DCOE!

But go for it, anything can be made to work. It is, at the end of the day, just a means of measuring out and delivering fuel into an airstream, ZS, SU, DGV, DCOE or EFI. It's just some systems are more accurate and more consistent than others!

The Herald 1300 manifold with an SU, to me, would be the best choice, as it was a factory fit for that engine (sort of!). But it won't ever be a screamer of an engine as the manifold looks quite restrictive.

Good luck,
Malc.


Malcolm

Yes Malcolm, it can be an expensive little habit getting a range of DCOE bits, I've got boxes of the little wallet burners, specially emulsion tubes, they certainly don't give them away, and when you get fiddling with a Ferrarri or something with half a dozen carbs it's scarey how much you can spend very quickly
With these little down drafts I tend to drill and solder and drill until it's right then buy the end size new, they're not as readily available here as DCO bits
I guess if you were doing them all the time it would be worth kitting up but for the odd car or two, it's just not worth the outlay

willy
William Revit

If you have a Ferrari, you should be able to afford a few little carburettor bits ;-)
Malcolm

LOL Malc.
You know the old saying --watch the pennies, the pounds will look after them selves
William Revit

Gryf

What about using the stock UK spec 1500cc set up of twin 1/5inch SUs, either with a stock UK manifold or an aftermarket one? Buy secondhand standard set up of manifold, carbs and linkage together from UK eBay or UK specialist MG breakers such as Andy Jennings or Mechspec? SU carb rebuild kits as well as springs and needles for upgrades should be available in USA, if not direct from manufacturers Burlen here (as well as from other suppliers). A decent set of air filters.

Guess shipping might be costly?

Any MGBs shipped to USA with twin 1.5 inch SUs as a source of carbs only?

Got anyone near you who knows about SU carbs? 1500 gurus on here will tell you whether a rolling road will be needed (depends how modified you go I guess).

Decent exhaust?

Cheers
Mike


M Wood

I was think of going to UK set up but I've no idea how it'd fit in with the added USA paraphernalia on the car and how many parts would need to be changed, added or removed.
Nigel Atkins

Good point Nigel, I guess it depends whether Gryf's car has anti-Smog components and whether they are required to remain.

I note my typing error above - twin 1 1/2 inch SUs on 1500 Midgets in UK IIRC (not 1/5 as I clumsily typed).

Cheers
Mike
M Wood

Thats why I said a couple of days ago about supplying a set of UK spec 1500 carbs to someone in the USA. He was very pleased with them though I cannot remember which State he was in so he might not have had a problem removing any anti- smog stuff.
GuyW

Gryf

UK 1500 Midget twin SU set up: https://www.ebay.co.uk/itm/Triumph-Spitfire-MG-Midget-1500-Twin-SU-HS4-Carburettors/363229404415?hash=item549228dcff:g:84QAAOSwxatf4LPT

Andy Jennings MG spares has a set for sale: https://www.mg-cars.org.uk/andyjennings/used_mg_parts_midget_carburation_fuel.htm

Also try Mechspec: http://www.mechspec.co.uk/popularusedparts.html

Perhaps another one of our BBS 1500 owners has a set of carbs, manilfold and linkage under their bench?

Cheers
Mike
M Wood

Gryf,
I was watching the following video and thought whilst it's a different engine it'd still give you an idea of what's involved. HTH.

Triumph TR6 SU Carburetor Conversion - https://www.youtube.com/watch?v=5M7m2WoMKIE
Nigel Atkins

This is all great stuff... thanks again! The carb conversion idea is on hold, as SWMBO balked at the price. But all the advice is gratefully received, and I’m intrigued with the “convert to UK spec” approach.

-:G:-
Gryf Ketcherside

You told her how much it cost? That is a rookie mistake! :-D
Malcolm

SWMBO was aghast at how much my PM competition half shafts were when she saw the credit card bill.

What I didn't tell her was that was the cost of one, I paid for the other one in cash!

I thought everyone had a slush fund for such eventualities. :)
C Mee

Gryf

It might be a cost saving to get a secondhand manifold and linkage from the UK and source some twin SU HS4 carbs stateside (MGB or were they Stromberg in N America?) and refurbish and rebuild to 1500 Midget spec? But try the specialist wrecking yards I suggested - Andy Jennings and Mechspec - as you might be pleasantly surprised at what the cost of a secondhand manifold, carbs and linkage from an old 1500 Midget might be.

There should be enough info in the UK factory parts manual, UK Haynes Manual and Burlen website on needles etc to convert carbs from another vehicle.

I appreciate the challenges of car up-grade budgeting and procurement approval processes shared by a number of posters!

Cheers
Mike
M Wood

I am agast at the price of one PM competition half shafts, and I am supposed to be into that kind of thing! ha ha.

… and you still get the duff, cut splines at the diff end. Oops.

Sorry, we have drifted...

Malc.
Malcolm

Malc

Sounds like a great project for Chevalier Classics to design and get produced new super-dooper half-shafts.

Cheers
Mike
M Wood

It's my favourite bit of the car... but you can't see them :-(

Malcolm

Growlerised?
GuyW

Some nice old threads on the subject of half shafts with many of the above folk:

http://www.mg-cars.net/mg-midget-sprite-technical-bbs/growlerizing-halfshafts-shot-peen-vs-nitriding-2016051810012927528.htm

http://www.mg-cars.net/mg-midget-sprite-technical-bbs/help-broken-half-shaft-2016051811295611984.htm

http://www.mg-cars.info/mg-midget-sprite-technical-bbs/twisted-or-what-2015012414191429128.htm

Cheers
Mike

PS I do like a good thread drift!



M Wood

This thread was discussed between 27/12/2020 and 27/01/2021

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