So what does the 'Tee Piece Trick' actually do?
Turbo Mini Forums

Back to basics.

A nat asp carb has the air sucked through it by the vacuum created from the engine.
As the air is dragged over the 'bridge' in the throat of the carb, it creates a venturi effect, effectivly generating a suction over a localised area.
Situated at this point is the jet - a long tube of which the lower end is immersed in the petrol in the float chamber underneath the jet, hence as the air is pulled over the bridge, so it drags fuel up the jet and into the airstream.
We do have a needle and piston to contend with aswell, but this is best not covered just now (keep things simple!)

So, now we start 'blowing' air into the carb under forced induction...
Now picture the same layout in the throat of the carb. As the air is being forced over the bridge, it STILL acts as a venturi and attempts to drag fuel up the jet.
Unfortunately however, we have 'pressure' in the throat of the carb that is trying to escape out of whatever orifice it possible can! Hence, the fuel is being pushed back down the jet, with greater force as tjhe boost increases!

The solution is to increase the air pressure in the float chamber to the same as the pressure in the throat (although in reality, we actually want to have a higher pressure in the fuel chamber). What this does is basically 'cancel out' the pressure difference between the throat and fuel chamber, so the carb then acts just like it's in nat-asp operation.
As i said however, we want a higher 'ambient' pressure in the fuel bowl than the 'ambient' pressure in the throat - it makes sense if you think about it - the pressure in a nat asp float bowl is typically 0 psi, whereas the pressure in the throat is around -3, maybe -4psi; i.e. a vacuum (not to be confused with manifold vacuum!!!). Of course, this means the pressure in the fuel chamber is around 4psi 'greater' than that in the throat in absolute terms...


So where does this 'increased' pressure come from???
Well that's why we have a restrictor in the plenum. If we restrict (at the mouth of the carb) the volume of air being forced into it, so it will have a higher pressure on the 'turbo' side of the restrictor. As you've probably guessed, this is where we take the 'high pressure' air signal from which we use to pressurise the fuel chamber!
The standard MG turbo SU and plenum have a hole for this very job on the 'flywheel side' of the throat hole, above the main bolt hole. This passes the 'high pressure' air in the plenum, straigh into the carb, where it travels along the internal passages to come out at the top of the fuel chamber.

So what goes wrong?
Well 99% of the time, it's that a 'nat-asp' (non-turbo) gasket has been fitted between the plenum and carburettor. This means the essential hole to send the high pressure air to the fuel chamber is blocked. It is an easy fix however - just remove the plenum, and if your hole is not there, just cut it out with a scalpel or similar.

Other times, it is the carb vent valve (the pipe 3/4" forwards of of the 'fuel in' pipe) that is open to atmosphere, hence allowing the impotant air to escape, meaning fuel will be pushed down the jet, rather than dragged up it and into the engine! The easy fix here is to ensure the pipe is well plugged, ideally with a short length of 6mm high pressure fuel hose, and a shouldered (non-threaded) bolt or piece of bar in the other end. I've seen plain bolts used before - again, this will allow air to escape down the thread, reducing the effectivness, so do it properly!

Last one is that the rubber balance pipe (the one that loops from the flywheel side of the carb to the front of the carb) is split / fractured. The original shiny rubber pipe is notorious for this. If you still have it on your carb replace it before this happens!!! Even more reason, is that when it does split, it's usually a 'hairline' fracture, that is often impossible to see under normal inspection. So you've been advised - if it's shiny, replace it!


Finally, I can personally testify for the fact that the above does not always apply, and that desite the carb being very clean, something is seemingly blocking the internal passage.
It is on this occasion, that we have to use the 'tee piece trick'.
This is simply a 6mm tee piece that is fitted into the pipe running from the flywheel side of the plenum to the top connection on the regulator. The outlet of the tee piece is then connected up to the pipe just in front of the fuel inlet connection on the flywheel side of the carburettor (yes, the one i advised you to block off just a few paragraphs ago)!
Why do we do this? Simply because it is taking the 'high pressure' signal from the plenum, and sending it direct to the fuel chamber - basically bypassing the internal porting in the carb.
This rectifies the problem on every example i've used it on.
However, DON'T just go straight for this approach - you must inspect all the other points mentioned above first - for example, if the balance pipe is split, the 'tee-piece' will NOT solve your problems.

MOD: Moved to Knowledge Section.

Edited by AlexB on 16th Jul, 2004.

good concise explanation dave,

it would be great if bits and pieces like this could be captured in some sort of FAQ/Knowledge section.

Moved.

Cheers alex, the website is getting more and more comprehensive!

So the Tee piece is not related to the piston ring damage then?.

Maybe the fooked, poorly maintained metro enging that was dragged out of a nearly dead metro then planted into a mini and expected to give thousands ao miles of trouble free motoring decide to give up the ghost as a result?

There is NO WAY, I repeat, NO WAY that this mod is responsible for those engines dying, other than that the engines would have lasted for many months / years covering no miles, sat in the workshop gathering dust with a fuelling problem, compared to a goosed engine that is asked to start working hard again...

Edited by TurboDave16V on 18th Jul, 2004.

Well said Dave ....

WWW.TURBO-MINI.COM

after seeing daves car at avon believe every word,good sound knowledge and advice,
thanks again

Is there any danger of petrol coming out of the vent pipe from the carb or is this purely a air vent?

if the float valve gets stuck then fuel can come out of this pipe IIRC.

are there pictures that can make me understand easyer

anyone can tel the path address to any picture of 'Tee Piece Trick' ?

thank u

Edited by supermini on 28th Dec, 2004.

************** EASY INSTRUCTIONS **************

Fit the Tee-piece into the pipe that runs from [1] to the top of the regulator. Connect [4] to the tee-piece.

Job done.

Edited by TurboDave16V on 5th Jul, 2013.

I'm having problems wih mine engine.
Does someone have a picture of the tee piece trick?

there is nothing wrong with a A-series that a turbo can't fix.

www.turbomini.nl

A bit late in responding, but here goes;

8 years after, tee piece solve my problems



now i´m wondering, if is better to use a non return valve between the t-piece and the carb... to stop the fuel to go to the plenun when the carb don´t have pressure. What do you think?

Pedro Silva

http://miniciados.blogspot.pt/

connect no7 to tee piece.

Medusa + injection = too much torque for the dyno ..https://youtu.be/qg5o0_tJxYM