Trying to understand turbo fueling process

[Sussex Pete]

As I'm still suffering with fuelling issues, I posted on the Maestro Turbo FB page looking for some help and guidance. Lee and Dan came back with some suggestions, but what I'm really struggling with is an understanding of how the Turbo fuel system works. Using the diagram below, taken from a Haynes manual, I can see that the fuel is pumped directly from the tank to one side of the vapour separator. Once that fills the fuel is sent to the float chamber through the top outlet. Once that fills, any excess fuel is returned to the tank via the third vapour separator outlet and the pressure regulator to the spill return pipe and the fuel tank. However once the turbo starts to apply boost pressure, this is seen on the pipe from the plenum chamber and applies the pressure to the top of the diaphragm in the pressure regulator, which in turn is applying pressure on the line into the vapour seperator that the fuel normally returns along. Now there is an increae in pressure to force the fuel into the float chamber and limit the fuel being returned to the tank. Am I correct in that description so far?

Thats then brings me to me problem with the mixture going excessively lean as soon as the turbo starts to apply boost. Could it be that the fuel pump is just not good enough to overcome the boost pressure, so I end up in just sending a dribble of petrol and boosted air to the float chamber? As far as I am aware the tank is good, and there are no problems with the gauze or swirl pot, the pressure regulator is brand new as is the carb balance pipe, the pump is Ok and pumps at a reasonable rate (when using it to drain the tank), the vent valve is working when igntion is switched on and off, although I will test with the valve being replaced by a bolt. With the vapour separator being non-serviceable, should I look to change it, even though it looks OK, in case something is wrong internally? Other than these, I'm at a loss to understand why it only goes lean with the turbo on boost. The car drives fine as long as I don't exceed 2000-2500 rpm, and if I do, then letting the engine return to idle speed for a few seconds, brings everything back to running fine. The mixture is about 4%CO at idle at the moment.

[G4GLM]

I would advise investigating - Carb fault, spark plugs (reduce the gap) and vacuum lines.

[Sussex Pete]

I would advise investigating - Carb fault, spark plugs (reduce the gap) and vacuum lines.

OK. Thanks for the input Gavin. Considering the carb was fully overhauled by Southern Carbs during the rebuild, what sort of carb fault should I look for. Again spark plugs were new for the rebuild, and why would reducing the gap improve things when on boost, as opposed to all the time? Same applies to vacuum lines. These were all checked during rebuild and why would a vacuum pipe problem only manifest itself on boost? I may be sounding a bit thick here, but because this fault is reproducible just under those conditions (off boost, all OK, on boost excessively lean, let engine idle for a few seconds all ok again), I am assuming that whatever the fault is, its going to be boost pressure related

[Doctordiesel77]

check that pipe in top of pressure regulator is applying boost pressure to the regulator, don't see why it shouldn't be, but if it isn't then fuel pressure will not be increased on boost...If it is ok, then you need to check fuel pressure,.. If the vent valve was faulty I would expect petrol to be leaking out of it on boost.. I think I've got a service bulletin on all of this somewhere, I intend to upload the ones I've got sometime, but my attention span ran out of steam after I scanned the first few

[G4GLM]

OK. Thanks for the input Gavin. Considering the carb was fully overhauled by Southern Carbs during the rebuild, what sort of carb fault should I look for. Again spark plugs were new for the rebuild, and why would reducing the gap improve things when on boost, as opposed to all the time? Same applies to vacuum lines. These were all checked during rebuild and why would a vacuum pipe problem only manifest itself on boost? I may be sounding a bit thick here, but because this fault is reproducible just under those conditions (off boost, all OK, on boost excessively lean, let engine idle for a few seconds all ok again), I am assuming that whatever the fault is, its going to be boost pressure related

i once had a XR2 with a pace turbo conversion, the carb had to be reworked to run with the turbo. I had it rebuilt using standard gaskets and it didn’t like boost, had it re done with updated items (haven’t a clue how they were updated but they were specified by pace) and it ran like a dream.
Plug gaps are ultra critical on boost, too large a gap and the boost will literally blow out the spark. It will run fine on little or no boost. If you reduce the gap the spark stands more boost. It was a trick on the old Lotus Turbos back in the day!
Both those scenarios could cause what you have described.

[Doctordiesel77]

i once had a XR2 with a pace turbo conversion, the carb had to be reworked to run with the turbo. I had it rebuilt using standard gaskets and it didn’t like boost, had it re done with updated items (haven’t a clue how they were updated but they were specified by pace) and it ran like a dream.
Plug gaps are ultra critical on boost, too large a gap and the boost will literally blow out the spark. It will run fine on little or no boost. If you reduce the gap the spark stands more boost. It was a trick on the old Lotus Turbos back in the day!
Both those scenarios could cause what you have described.

Interesting and I guess that if the coil was not 100% it could result in serious head scratching too...

[Sussex Pete]

i once had a XR2 with a pace turbo conversion, the carb had to be reworked to run with the turbo. I had it rebuilt using standard gaskets and it didn’t like boost, had it re done with updated items (haven’t a clue how they were updated but they were specified by pace) and it ran like a dream.
Plug gaps are ultra critical on boost, too large a gap and the boost will literally blow out the spark. It will run fine on little or no boost. If you reduce the gap the spark stands more boost. It was a trick on the old Lotus Turbos back in the day!
Both those scenarios could cause what you have described.

Thanks for the clarification. Southern carbs were very good, as not only did they refit their last remaining autochoke unit, as the one that was on my carb was beyond repair, but they also fitted the clamp ring for the dashpot damper screw, which was also missing, so they were aware that it was for the turbo engine, and not just an NA one. On that basis, I'm assuming that if upgraded gaskets were required, they would have been fitted. It certainly cost me enough!!

BTW the coil is brand new as well. Changed when I was trying to sort the lack of spark problem earlier this year!

[G4GLM]

Just pop the plugs out and try a smaller gap, don’t be afraid to go down quite a lot you can always adjust later. Worth a try for free!

[E_T_V]

The turbo carb has turbo specific gaskets - I can't remember the difference but there is one I think - to control the pressure within the carb - I think it has an extra hole (or one blanked off I can't recall).

Anyhow the pressure regulator works to keep fuel pressure at the outlet above the pressure in the carb (by 4psi) This is why the pressure regulator is connected to the turbo to provide the pressure reference. If this pipe leaks then fuel pressure will be inadequate under boost and it'll run lean or run out of fuel. If you install a fuel pressure gauge you should see fuel pressure at the carb stay 4psi above the boost pressure at the turbo.

The vent valve allows air to vent above the fuel level inside the float bowl to prevent vapour lock when the engine is turned off when hot. The valve needs to be sealed when the engine is running as otherwise the pressure inside the carburettor is too low and this messes up the fuelling - often blowing fuel out of the vent pipe (rather than into the engine where it should go!).

The vapour separator is just a swirl pot to remove air bubbles from the fuel. In theory it shouldn't be needed. You can bypass it to prove whether it is faulty or not.

The bit to remember is that there are two sorts of carb applications for turbo engines.
Suck through - which is essentially a normal carb installed before a turbo so the turbo compresses the air/fuel mix before putting it into the engine (this is unusual and not the sort we have).
or Blow through - which is a carburetor which is pressurised by the turbo and so must have a fuel pressure that can cope with this. For this type to work it must seal properly and have the right pressure of air inside it (boost pressure) to allow the carb to fuel normally. Our type is a blow through one and so it is fussy about making sure things are sealed correctly.

[E_T_V]

Narrowing the plug gap won't hurt - but it won't cause the engine to go lean under boost - quite the opposite - you'll get unburnt fuel out of the exhaust.

[G Force]

Hello Pete
You might have already realised but the diagram is incorrect. The fuel pressure regulator has 4 connections not 3 as in the diagram & the vapour separator is not supplied directly by the pump it is fed regulated fuel pressure from the pressure regulator.
It is the plenum chamber to carb gaskets that are turbo specific, the turbo type gasket has extra holes as the float chamber needs to see air pressure from the turbo. See images The first image shows a set of A series gaskets but you can see it has the N/A carb elbow gasket.Note the extra holes on the plenum gasket in 2nd image.

[Sussex Pete]

Thanks Gary. When I first read your post, I thought you may have spotted my problem. BUT, I think you may be confusing the set up for the Mini and Metro Turbo set up, that uses an FPR012 regulator with the four connections. The regulator for the Maestro/Montego Turbo is an FPR013/ADU9217. This has just three outlets. I was also a bit nonplussed that AR would have misprinted the diagram in their official workshop manual for the Maestro and that it would be then copied by Haynes for their Montego manual. Just for the record I have attached a screen grab from the EPC for the fuel pipework for the Turbo setup. There you can see that the pipes do relate to the diagram and, I hasten to add, to what I've got under my bonnet!

To clarify Pipe 1 is fuel feed to vapour separator; Pipe 5 is vapour separator to regulator; Pipe 8 is regulator to plenum chamber; Pipe 12 is vapour seperator to carb and Pipe 21 is regulator to spill return

Having written all that, the Mini/Metro system does look simpler in that turbo pressue is being applied directly to the fuel supply and its that I was scratching my head a bit with, as the Maestro uses turbo pressure indirectly, via a pipe that seems to have two roles, pressuring fuel in the vapour separator and returning unused fuel to the tank.

[Doctordiesel77]

Diagram looks right to me...the regulators I've seen are just normal 2 port, like an ordinary hydraulic system, with a third port over the diaphragm ie on the spring side) connected to manifold pressure, thus increasing fuel pressure on boost. The entire fuel system should be maintained at manifold pressure +4psi, apart from the return pipe from rgulator to tank, which should be at zero or thereabouts

[G Force]

, I think you may be confusing the set up for the Mini and Metro Turbo set up, that uses an FPR012 regulator with the four connections. The regulator for the Maestro/Montego Turbo is an FPR013/ADU9217. This has just three outlets.

Oops yes! I have. Me bad apologies, Pete.

[E_T_V]

I'm glad I wasn't too far wrong with the turbo specific carb gaskets. (well plenum to carb - near enough!). Worth checking you have the right number of holes in yours.