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MG MG Y Type - H2 Carb throttle disc.

Just replacing throttle shaft, bushes, butterfly disc etc to get restored YA running a bit sweeter; while I wait for parts as usual, could members clarify for me : just how much gap or "daylight" is acceptable around the throttle body wall when the throttle disc is fully closed? I have a good condition spare disc which, when installed and held up to the light, shows a crack of daylight around the 2 o'clock mark. I'm tempted to do a trial fit for the fun of it, as parts will be 2 weeks. Am I wasting my time?
Thanks all.
John.


J P Hall

Hi John
probably needs rotating around a tiddle
In a perfect world there should be 0 daylight---that is the aim
Have you tried holding the shaft towards the closed position and tapping the ends of the shaft and tapping the butterfly to get it centred up--if it won't centre up try pulling the butterfly out and doing a flipover top to bottom --you never know it might just fit better
You need the butterfly screws in it, done up lightly while you're checking to get it all sitting properly

willy
William Revit

Good thinking as always, Willy - I'll have a play this afternoon. John.
J P Hall

After a fun few hours (I'm slow) this weekend, I ended up borrowing a better H2 carb body from the sleeping TC, fitting all the known-good parts from the YA assembly, adding the best throttle shaft and butterfly I could find, and following the usual routine of "up a bit, down a bit" on the jet adjuster and the throttle screw - until finally, using the side of the screwdriver trick to lift the piston, got the desired result. Another hour or more checking and improving the ignition timing side of things, and ... she's running sweet as a nut.
One question: I noticed that before the above fine-tuning,the electronic SU fuel pump was ticking more than once a second at idle; now, at idle and running, it only ticks once every couple of seconds. Is there a known "ideal" rate of pulse for the LP electronic SU in the xpag setup? Just curious, as this is all learning-curve for me.
Thanks all.
John.
J P Hall

Slight amendment: the pump speeds up a bit, as I would expect, as revs increase - but still the question remains - is there a generally accepted pulse rate at idle, and perhaps another rate at say 3000 revs?
John.
J P Hall

I shouldn't think there is any hard and fast answer to your question about pump speed John. I have had satisfactory use out of pumps which vary wildly in tick rates from one every 10 seconds to one a second with the engine at rest. if I had to make an educated guess I would go for an average of 5-6 seconds. I wouldn't think that being electronic would have any effect though as the rate will be determined by the diaphragm and valves rather than the points. Other than that any slight leak anywhere would also affect things but I would expect other problems under such circumstances. When the engine is running I would expect the fuel consumption of the engine to override other effects so I wouldn't like to guess what that would be.
Ian
ian thomson

John
Question
Is your pump mounted as per original with the outlet to the engine bay on the top of the pump--
William Revit

Most of the problem with airflow management through an older SU arises from worn shaft bushes, which allow air in from the sides. (The other main culprit is failed gasket at the carb/manifold joint). This usually leads to weak mixture at idle and uneven running. If you've replaced the bushes that's a good sign there's no or very little unwanted leakage. I wouldn't worry too much about a perfect fit between the butterfly disc and the throttle body - if the engine is to run at all there will have to be a gap to allow mixture through, so even at idle the butterfly is partially open.
Tim Griggs

Yes Willy - an earlier photo, but pretty much by the book, I think.
John.

J P Hall

Tim, I hear you - I have a new spindle and disc arriving soon, so enjoyed the practise putting together "known good" older parts. It'll be nice to button it up with new bits, and walk away from the carb side of things. Also, it's all great learning and practise for when I get stuck into the ex-hillclimber TC!
John.
J P Hall

Looks right but interesting -my Y has it's pump down underneath at the back ---What's the part no on your pump
William Revit

William - it sounds like you have a later SU pump set-up. Minis and MGBs, among others, have the pump near the tank, configured for a short suck/long push to the carb. This gets over the perennial problem for so many Nuffield cars, which we know to our cost: fuel vaporisation occurs with the long suck/short push set up with the pump at about the highest point on the system, particularly when the underbonnet temperature rises. But SU pumps don't really like being out in the fresh air below the car, being bombarded with water and road dirt, so it's swings and roundabouts I guess... :-)
Tim Griggs

Tim
Yeah it's interesting -my pump and pipework are all in their original positions and my car is a YA so the position must have changed somewhere during YA production before my car-----something I haven't looked into really--it just surprised me to see the pic of John's pump up the front, I hadn't really thought about it and thought they were all down the back--interesting

What's your chassis number John, I think I might need to know when they changed just out of interest
William Revit

Willy - a new electronic SU, AUA25, so probably doesn't help. I thought they used Low Pressure pumps at the front though, and possibly HP if pushing from the back like the MGB?
My chassis is Y3348 (the 8 is upside down), 1949, but I don't know the exact build date. Must find that out one day!
John.
J P Hall

Hmmmm---Yes well, you have the correct pump for up the front (low pressure) and it's mounted correctly---I was just checking because i struck an MGA once that the pump ticked a lot on and it turned out to be the pump was mounted upside down leaving an air pocket in the pump- it came right on it's own though, probably flushed the air through, before we got round to flipping it over---it's still going--upside down
Interesting with the chassis number, my car is earlier than yours and the pump's down the back---weird----and I'd say it was original as it had been parked up since 1959--might just have to go and have a look at the bracket just to see but when I had the pump off painting the chassis everything looked right with the bracket and pipework-------weird
William Revit

We've digressed a bit here as often happens, Willy - but strewth, "parked up since 1959",I presume by a previous owner, not your good self??
John.
J P Hall

Got him from a deceased estate John, in hindsight I should have just left it there.
William Revit

Ha! - now I don't feel so bad about taking Y3348 off the road in about 1998 for something like a leaking welch plug, and basically taking 24 years (but mainly the last 5) to finish what ended up being a nut-and-bolt restoration.
Back to the topic: I'm now nervous about undoing the carb innards just to put in new throttle shaft and butterfly disc, so sweet is she running at the moment. BUT - a proper road test and some hill climbing at the weekend will no doubt tip me one way or the other.
John.
J P Hall

All good--now, what's this TC hillclimber you have--I could save you a heap of work, just sell it to me.
William Revit

It's certainly an interesting car, Willy - I'm dying to start playing with it, including figuring out whether to leave it as is, all metal framework, no wood, '60s hillclimber mods etc.,or bring it back to some sort of "standard" - although I can't see myself shelling out all those $$$ for a wooden frame. I just want to drive the b##### thing! You need to come up to north Qld for a holiday.
John.
Sorry - we digressed again.
J P Hall

Hello John. Another thing to watch for on the early Y and TC carbs is when they have been tightened too tight the casting will warp out of round. Then you can never get them to idle right unlwss you sleeve the body back to size and round. I have seen them with over .030 difference at 90 degrees, it lets a lot of air thru and you cant get the idle down.

Butch
R Taras

Butch - sheesh, with your experience that's interesting, so thanks for the heads-up. John.
J P Hall

Hi Willy,
I can assure you that the position of the fuel pump on the Y-Series never changed during production as far as years of research have determined. The only real movement was when the battery box was moved from the offset to centre position.

Review of all the parts manuals lists only one fuel pump mounting hardware listing (including fuel lines).

Lot's of owners modified these cars in the early years - just because you found it like that a long time ago, doesn't mean it left the factory like that.

Cheers

Tony
A L SLATTERY

Have to agree with Tony there William R. The Y only ever had the pump up on the battery box, never at the rear. Your "mod" is a relatively new one and not original. Sorry for the disappointment.

Paul
Paul Barrow

Tony/Paul -- I stand corrected.
To be honest I never even thought about it and just thought that that was how it was supposed to be.
I've had a look at it and although it looks as if it's always been there, it does look probably not original.
Because it's a HP pump it's staying put.
Interesting though that the ends of the fuel lines look like factory made sweated on items, Whoever did it did a cracker job of it and pre 1959---------------not me.
Cheers
willy
William Revit

This thread was discussed between 26/05/2023 and 15/06/2023

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