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MG Midget and Sprite Technical - Carbs adjustment

Good afternoon,

I am sure some of you where waiting/expecting this... So I don't want to disappoint you :)

I am lately playing with my carbs (1275cc - twin SU HS2 - H6 needles - everything fairly standard) and of course, I am not happy with my set-up.... I did started with what I think is the "starting" set-up: 3/4 open idle screws, 2 turn down of jet after being flush with the bridge.

When the car is warm, like this, I have a high idle (around 1 500rpm) and my second carb (close to gearbox) is making much higher "hissss" than the first.

I have now the idle screw maybe only 1/4 open on both, I didn't move the jet position, my pistons are only around 1mm height, and I am still at 1000rpm...

=> Do you know what is the positions on yours ? Should I compare with your settings, or it doesn't make sense ?
=> Should I move the jet up maybe ? I tried a little, didn't look like big improvment....
=> Do you trust the "lift the piston" method to adjust the jet ?

Thanks !
(Plugs are alright, valves are adjusted, timing too, etc..... :D)
CH Hamon

first thing to do is check for an air leak. My recollection is that the starting point for the jet position is 0.060" (1.6 mm) down from the bridge. I measure with a Vernier depth guage.
To balance the air flow of the carbs you have to loosen the arms on the throttle shaft (the ones with the tang that goes into the forked ends), then balance the hiss (tube in ear) from each carb using the idle screws. I do this at 1500 rpm. You can stop the engine now whilst you set up the linkage. There has to be a clearance between the fork and the tang so I wedge open the linkage slightly and then set the tang on the bottom of the fork and do up the nuts. Take the wedge out, then you have to set the idle also in balance with the screws.

There's plenty of help on the internet but they can make it sound over complicated, but google it anyway.
Paul Hollingworth

Try this:-
https://www.mgexp.com/phile/1/492119/SU_idiot_guide.PDF
Paul Hollingworth

Cedric,
one thing to check, that could easily be missed by even the very best and good looking of us, is that you have the gasket between the air filter and carb the correct way up and have not closed off the air relief hole with the gasket.

Always check the basics first otherwise you could be wasting more of your (and whoever you might be with's) time.

accelerator pedal and pedal linkages

cable tightness and free movement both ends

free movement of carb linkages and cams

free movement of choke cable and cams

there is a small gap to fast-idle/choke cams from the adjusting screws

dashpots full of correct oil

free movement and drop of lifting pins

you have the correct tools.
Nigel Atkins

Did you slacken off the interconnecting link before setting the idle screws?
Chris at Octarine Services

Hi,

Thanks for inputs ! I did struggle with everything you mentionned at my first set-up, but I do have a better understanding now and took all this in consideration for this second set-up :)
I do get the logic behind the parts and the chronological order now (not so long time ago :D), but of course "my engine is not doing what it is suppose to do"....

I have to admit that I didn't know that the butterfly is almost closed at idle, and both pistons are only about a mm height or so
=> Does it sound normal to you ?

Now I have set-up the car with idle between 900 and 1 000rpm, as the engine is really unstable under (books say 800rpm...) and she feels happy like this...
I finally set-up the jet height using the method "engine rpm is not increasing if i move them more downards, and does decrease if I move them upwards".

I am pretty happy because my pistons move in synchro, and have the same height at 3 000rpm.

I did also purchased a synchrometer to check the flow of the two carbs, but not sure yet how to use it, as I am happy with my pistons being synchronised...
CH Hamon

Nigel
What fantastic tips, who ever has holes blocked on their carb gaskets !!!
Listen to Nigel,Cedric
Scott Coe

Haha they were wrongly put when I went to Aldon Rolling Road few weeks ago... The shame I felt there comes back to me everytime I am installing them now: can't forget to put them wrong now :P
How are your carbs now ? :)
CH Hamon

I think I did exactly the same thing !! Someone called Nigel noticed it, he helped me with the fast tickover
Scott Coe

Cedric

If you have been to Aldon's rolling road, why are you 'tinkering' with the carbs? Did they not set them up?
Dave O'Neill 2

It would have helped more if I'd have noticed a lot sooner, especially as I've blocked a hole before on mine when rushing to fit a stub stack - but fiddling with carbs is very distracting and makes you forget logical sequence (as if I could remember that in the first place).
Nigel Atkins

Hi Dave,

Nope, they didn't. The Aldon event was a "group" event: 30-45min each for £40 if my memory serve me well, including BHP curves before/after.
Depending on the state of your car, they are doing different things: mine had the air cleaner gasket wrongly put, the carbs slightly imbalanced and the timing at 44deg. Time to correct this, before and after BHP curves, then your time is gone... If I would go again, I guess they would touch the carbs (or find something else that is not in order...)

I "tinkered" with the carbs because I wanted to understand how it works, clean/service them and try H6 needles :)

The car will go to a good tuner at one point, but I want to have it working properly before, to use most of the time for tuning, not servicing/fixing...

Maybe I am doing it wrong ? No feedback on my set-up, comments regarding piston height,...?

Also, I did a nice sunny drive yesterday. I found that by 0*C, if I have to stop at a roundabout, the car would stall. But, when I am in traffic and temperature around 5*C, then my idling speed is at 950rpm ish.... Any advice ?
CH Hamon

I lost the post I previously put up, basically I think your height might be OK based on JT saying IIRC 1/16" but I would not be able to measure 1mm let alone see if that is what it is.

What book are you looking at that says 800, not that it matters as your car is not new and as factory standard so if the car ticks over happily at 900-1,000 then stay with that for now at least. Which leads on to are you sure that whatever instrument you are using to see these figures is accurate at these figures (or any others).

If your car was fully warmed you should have no difference at 0c or 5c on idling, there could be lots of reasons why your engine cut out. I am unclear as to if Aldon set up your timing. I would suggest you check your timing if not then check and if required adjust you carbs for mixture and balance and again road test the car.

Are you doing the carb checks with the engine fully warmed and kept warm during tuning and testing and fully revving out between each adjustment and check or reading. Same with road test, getting the car fully warmed up, very short and/or quick runs may not achieve this (unless the car is fully pre-warmed).
Nigel Atkins

This thread was discussed between 18/01/2020 and 20/01/2020

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