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MG MG Y Type - YB singleCarb

All of the factory information I find states that the YB was fitted with a single SU H2 carb, p/n AUC456. I believe the H2 indicates the carb had a 1 1/4" diameter intake bore.

The carb on YB0952 is definitely a Type H, but both it and the intake/exhaust manifold have a 1 1/2" bore. The carb body is marked AUC4070; the float bowl is a Type T2 and is marked 3495. The bolt affixing the float bowl to the carb body is a later type using rubber grommets rather than metal and fiber washers.

I realize that previous owners may have modified things over the last 70 years. However, a single H2 carb seems too small for the XPAG. A single H4 seems more appropriate.

Can anyone shed light on this mystery? Do I have a later MGA carb? Is a F.I. needle still correct with this carb?

Rocky
Rodney C "Rocky" von Dullen

Rocky--Hi
Just a question --is that carb body definitely AUC4070 or is that a typo---is it AUC7040 ----
Cheers
willy
William Revit

Willy,

Good catch! It is indeed AUC 7040. My dyslexia must have kicked in. I will be having cataract surgery next month. Hope that helps as well.

I guess my real question is whether YB's have a 1 1/2"
carb as standard. The manifold and air cleaner pipe also have 1 1/2" bores and certainly look original.

Rocky
Rodney C "Rocky" von Dullen

If you are in doubt about the carb size on YB, just search the Technical Data section on the IMGYTR website.

Safety Fast

Tony Slattery
The Classic Workshop
A L SLATTERY

Thanks, Tony.

Lawrence's LTBY's pg 77 states "The Y and YB saloons were fitted throughout production with a single SU Type H.2. 1 1/4 inch semi-downdraught carburetor to specification AUC 456." This is in agreement with all the other MG and SU factory information I can uncover.

Would someone be kind enough to check their car, measure the carb bore and reply to this post?

Rocky
Rodney C "Rocky" von Dullen

Rocky
Like you, I've looked everywhere and all I can find is H2's
But an H4 would have to be better suited you'd think
Your car being --952 is an early YB being about the 700'th built so you'd think if the H4 was original on your car all YB's would have them---maybe
Just to try and id your carby---If we're lucky your carb will be stamped
If you have a little search on the carb.-On the edge of the mounting flange up around the side near the top mounting bolt there should be a stamped single letter and number or maybe two--Sometimes the stamping is very light and sometimes the overenthusiastic polishers have rubbed them off--but hopefully they'll be there
See how you go

willy

keep an eye out for them 'weather' balloons
William Revit

Willy,

"N10" is stamped into the mounting flange of the carb body as you describe. Is this a date code?

Rocky
Rodney C "Rocky" von Dullen

Rocky
Yep, it's a date code
Your car--YB0952--was built on 5 Dec 1952
The H4 on your car is dated-N10---Oct 1956
so not original, but I wouldn't worry too much about that
The code for 1952 is H and the carbs were usually dated a month or two before the cars were built as they were built by SU then sent to stock at MG, so original for your car would be stamped H10(Oct) or H11(Nov), maybe H12 but i doubt it being built so early in Dec

willy
William Revit

Hi Rocky

H4 is an after market mod and was used on the Magnette - an altogether much heavier car and in twin carb set up. As mentioned before all Y Y/T and YB only were ever fitted with H2s. 1-1/4” is actually perfectly adequate and my Y has easily performed well on it!

What is the diameter of your inlet manifold hole? Post a picture of your manifold set up as it sounds well modified.
Paul Barrow

Willy,
Thanks for explain[ng the date code on SU carbs. Is information on this system published somewhere? I have seen articles on how to interpret the codes on Lucas electrical components and Triplex window glass.
On page 77 of LTBY’s, Lawrence states “The [carb] body was 11.3 cm long (flange to flange). Sometimes bodies 12.3 cm long are seen on “Y” Types – however these are not original, and are possibly from Morris 10.” The carb on my car has a body 5 Ό” (13.3 cm) long, flange gasket face to flange gasket face. Different yet again?
I assume the differences in body length can be accommodated by varying the length of the rubber coupling between the alloy intake pipe and air cleaner body.

Paul, I have attached some photos as you requested. The bore of the inlet of my carb measures a bit over 1 3/8”. The bore of the alloy air cleaner pipe matches it. The bore of my Intake manifold measures exactly 1 ½” – the same as the exit bore of my carb. This carb is noticeably larger overall than the H2 carbs on my TC, which have an inlet bores of exactly 1 Ό”.
All the bores are perfectly round and appear original. I do not think they have been modified. Am I mistaken assuming that my carb is not a H2 with 1 Ό” bore.

Rocky





Rodney C "Rocky" von Dullen

Additional Photos

Rocky




Rodney C "Rocky" von Dullen

Rocky
4074 is definately an H4

I have H4's here that measure up exactly the same as yours and the same 5 1/4" length---as you say there are a few different lengths but generally speaking, most of the older su's are the longer type



William Revit

Check out the Technical Data section on the website - most of the answers to many questions already exist there :).

https://www.mg-cars.org.uk/imgytr/techdata.shtml#SU_Carburettors

This covers original equipment.

Without stripping my Y down and measuring it I cannot compare but SU data is quite clear - H2s only to Y/YBs ... and Y/Ts.

Possibly Tony Slattery may have one he can measure across the OUTLET side of the carburettor. That is where the important measurement is, not the air intake side.
Paul Barrow

Paul,
I have reviewed the Technical Data section of your website, that of many other websites, and all the books/articles I have on Y Types and SU carbs. All agree on a single H2 carb being fitted as originally to this car.

It finally dawned on me to examine early factory photos of Y engine bays: all the carbs pictured have suction chambers affixed with two screws located 180 degrees apart, no diagonal casting at the top to allow for internal breathing of the damper chamber and round damper caps.

My carb has a suction chamber with three screws located asymmetrically, the diagonal casting at the top and a hexagonal damper cap.

I must agree with Willy that my carb is a later, non-original H4. Hopefully Tony or some other members can measure the diameter of their intake manifold bore to confirm or refute my single finding of 1 1/2".

Now for my next question, my carb is fitted with a B.I. needle. This needle has a VERY similar profile to the F.I. needle quoted as standard with the H2 carb. Which is optimal, assuming a lower gas velocity passing through the larger carb venturi?

Rocky
Rodney C "Rocky" von Dullen

I just measured the inlet hole on a standard HOME market and the inlet hole on a standard EXPORT market air intakes (I have both here off cars at the moment) and both are 1 1/2" diameter holes Rocky.

Your photos seem to show that the face of your inlet has been machined and the side holes elongated so I am going to guess that you have a non-standard set up there. Quite why anyone would go to those extremes I am unsure as I do not think fitting an H4 versus an H2 would give any appreciable performance upgrade!

Paul
Paul Barrow

Here are some pictures from my YB.




Paul Barrow

Rocky
Interesting needle choice that BI, Someone has decided that it needed just a tiddle more fuel on light throttle and that BI will give that compared to the std. FI
Probably needed it as you say with the lower air speed of the larger carb. BUT I don't like the way it runs off leaner than the FI on full throttle

It's a bit of a minefield suggesting needles without testing the car, specially not knowing if it might have been set up on a dyno as it is but that leaning off at the top end worries me a bit

I'd suggest you check out a CB needle, -It's got the slightly richer bottom end like your BI but retains the same mixture as the FI up high without leaning off like the BI does

Another thing to check seeing it's been fiddled with is to make sure it's still got a Red (4 1/2 oz) damper spring

As a little observation, If all these inlet manifolds have the 1 1/2" hole std.even with the smaller H2 1 1/4" carb, then there's an issue with the std setup.
It's been proven on dyno runs that a step from the base size of the carb to a larger hole in the manifold causes a slight power loss---maybe all them std cars with 1 1/4" carbs should have their manifolds sleeved back and blended out into the manifold

Cheers
willy





CB red spring
William Revit

Willy, your knowledge and experience should be put in a bottle, given an appropriate name (I'll leave that to the advertising gurus), and sold on the MG market. Just sayin'.
Willy's Speckled Sparkles ...
John.
J P Hall

John
I had to look up on the .net how to respond to that---Apparently I have to give you a hug------lol
Cheers
willy
William Revit

It'll be right, Willy - we'll settle for a beer when Joanne and I get down to Tasmania, which won't be 'til next year. This year March and April in Victoria and South Australia (motorhome with Y behind); next year Tassie. For overseas MG friends, Willy's state of Tasmania is 3,500 kms away - or London to Moscow and back.
John.
J P Hall

This thread was discussed between 28/01/2023 and 14/02/2023

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