OK - Found my overfuelling problem! Fuel Pipe diameter related!
Turbo Mini Forums

I've ran 8mm feed and return for the past 6 years, even when fuel injected.
Over the past 2 years since reverting to a carb, my mid-range fuelling has been all over the place...
This really came to a head recently, when it would stall from over, OR underfuelling, depending on what it felt like that second...

The carb was stripped a whole load of times, and fuell pressure checked at idle. It seemd odd, but the lowest pressure i could get was 3.75psi, even with the screw right out. Hindsight is great, and instead of stripping the carb (for the 4th time), i should have concentrated on the return pipe.

Basically, i put a Tee-piece and gauge into the return just 2" from the regulator. I had removed the filler cap to remove 'pressure' in the tank from the result. The pressure shot up to 14+psi before i switched off the pump! Pulling the pipe off, it was a REAL struggle to blow down this 8mm return. Checking it over, the steel pipe was undamaged along its length, and free of bends/kinks. I removed the rubber hose from the end of the steel pipe to the tank, and magically, it was slightly easier to blow down. Still not enough me thinks though... Of course, it is clear this pipe has obviously collapsed internally, but that is not the whole issue.

After giving it a lot of thought, my analogy is that when running such low pressures at the carb (3-4psi) slight variance in back-pressure in the return line will make a significant difference - more so than if we had a 3 bar fuel rail, like on a fuel injection system... Whist it can be argued that there will always be less flow in the return line, any slight restiction in the return, including it having to go down, then uphill to the top of the tank, will back up and upset the spring relief valve in the regulator...
Bear in mind, i'm running a (relativly low flow admittedly) high pressure EFI pump, this is even more sensitive.

For you guys who are running 8mm supply and return, i recomend you use a restrictor in the pipe between pump and regulator, such that you are assuring the flow through the inlet is always LESS than the potential flow through the return (with a little in reserve). I reckon a simple piece of metal, 10mm long and 8.5mm OD, with a 5mm hole through it will be sufficient if just 'pushed' down the rubber hose just before the regulator.

For me, I'm going to leave my nice 8mm supply in place, but fit a 10mm return pipe to the tank. I'm only doing this as i can't rule out the return to fuel injection or Nitrous in the future, and restrictors in the supply line are not what i need!

Hope this helps you guys!
Maybe one for the FAQ after it's done its course?

dave, the pipe sizes that you have talked about (8 & 10mm) are these the inside diameters or outside diameters?

Inside i think matey.

I think i will not fit my 8mm return, but a 10mm one too then, but still the 8mm feed.

Previous Engine: 1040cc Morespeed engine, 1275 turbo head T2 Turbo Mirage Manifolds Megajolt
Previous engine: STD metro turbo, megajolt
Drives: 399bananahp (flywheel) Honda Civic Vtec b18c4 T28 Turbo 1968 Mini

Megajolt maps to download: http://www.jamesfawcett.co.uk/cms/index.ph...&gid=3&Itemid=3

What did the std metro turbo use?

alex

AlexF

OD - sorry, shuda said...

Metro as std was 1/4" OD supply and 5/16" return...

wel let us know the outcome of the fittin of the 10mm return :)

turbo 16v k-series 11.9@118.9 :)

Denis O'Brien.

Dave anychance of letting me know what the ID is that your going to be running?

I was thinking of getting some stainless tubing so I want to make sure I get the right gauge, then just use small lengths of rubber hose each end.

Cheers

Would that be the same stainless tubing im thinking of running dan?

You could run the same stainless feed and return but then use smaller rubber hose feed to regulator compared to the return.
If that makes sense, i.e. just restrict the feed as dave suggests.

Have to see what sizes are "available" at work mate!

yeah both the same size seems to make sense, and let the rubber hose at the end do the restricting

can't remember the part number for piping, there's a CT drawing with some sizes on, i'll have to mention it to rick he'll probably know!

Im going to look at possibly running them inside although its very tough stuff so maybe just "P" clip it underneath for now.

Hi,
has anyone with a "big" return line ever tried how far the fuel pressure comes down when the screw on the regulator is completly turned out?
I am asking because my minimum fuel pressure is also 3,75psi and wonder if this is maybe the minimum setting which is built into the regulator? (But i am also using same size 1/4 inch ID fuel lines (supply + return)
Harry

Bimmer Twinky headed and turboed A-Series:
http://www.minifreunde.at/harry/projects.htm

Harry - here is the problem!!!

That regulator should easilly drop to just 1psi when screwed all the way out - just open it up and see for yourself!

Thanks Dave!

I will install a 8mm ID fuel pipe from the regulator back to the tank and see if there is a difference!
Does the initial fuel pressure setting have an influence on the AFR? I mean is there a noticeable difference on AFR if you have 3 or 5 psi static setting?? (Of course when the return line is big enough!)
Maybe I will try to fit a restriction of 4 or 5 mm in front of the regulator as you suggested. (Would be a lot less work!)

Bimmer Twinky headed and turboed A-Series:
http://www.minifreunde.at/harry/projects.htm

News: I installed a 8mm ID return fuel pipe today and presto: I could turn the fuel pressure down to just 1,5psi!
What static fuel pressure are you adjusting at idle (no boost)?? I have 3,75psi at the moment. Too low?? What do you think?

Bimmer Twinky headed and turboed A-Series:
http://www.minifreunde.at/harry/projects.htm

I set mine at 3psi with the engine running.

Bear in mind your needle will now almost certainly be wrong Harry!!!!

I am using a bca at the moment; didn't drive the car since the fuel pipe modification, but my widebandlambda will tell me.
Does the static fuel pressure setting have any influence on the AFR; i mean will you experience a difference if you are using 3 or 5psi with the same needle??

Bimmer Twinky headed and turboed A-Series:
http://www.minifreunde.at/harry/projects.htm

Dave, is it supposed to be 3psi running? I always thought it should be 3psi when the ignition was on. Fuck, maybe this is the reason why my engine ran LEAN on the RR-day??

Peter I am using an MPi tank, and the standard MPi fuel lines. I have commissioned the fuel system and it all controls great, tee'd pressure gauges into carburettor feed and return to tank, carb feed controls correctly with boost pressure (provided by my air-line) and the return feed stays at low pressure, indicating that the return to the tank is not limiting things.

Dave, I asked you a question up there :)

I am using both a 10mm flow and retrn overkill maybe but i am restricting it down from the regulator by using 8mm fuel hose. Runnig 4psi of pressure off boost, seems to work well. That is with an mpi tank.