MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - think about this?

My car is a Turbo with blow through carb. This means the fuel has to be delivered at about 2/3 psi above boost pressure. This is achieved by using a high pressure injector fuel pump that feeds a regulator that senses boost pressure. The regulator send excess pressure back to the tank and keeps fuel at the carb at a pressure of a couple PSI above boost.

OK hopefully that makes sense so far, well unfortunately the car only runs now for a couple of miles and misfires and cuts out.

The reason for this is lack of fuel prssure, I know this because I have a gauge installed in the carb fuel line and this will have reduced to zero when the engine cuts out.

To resolve this I tried the following.

I left the fuel pump running for about an hour at home circulating fuel hoping to see a deteriation in pressure due to filter blockage etc? no it remained absolutely constant!

Even though there was no direct evidence that the pump was faulty I still bought a new injector fuel pump but still the same!

I can leave the engine ticking over for hours simply stood in the garage without any issues? this would suggest to me that heat and vapourizing is not the problem.

I am considering changing the regulator.

Any thoughts welcomed.



Robert (Bob) Midget Turbo

Could you be losing power to the pump?

When it misfires and cuts out, does it start again straight away?
Dave O'Neill 2

So it goes:
tank->pump->carb bowl->pressure gauge->regulator->tank

?


Trevor Jessie

Maybe the fuel vaporises (cavitation in the pump?) in the low pressure before the pump and the pump can no longer function.

maybe you need two pumps with a reservoir in between.

In trevor's way:

tank->lp pump->reservoir->hp pump->carb bowl->spill to reservoir.

Or simply not enough air let in to the tank, (partly) blocked vent.
Alex G Matla

Bob,

Just to be 100% clear.

This a new problem and that previously you've ran the turbo without a hitch?

If so, either the regulator (rising rate regulator?) is failing or you've removed it and fitted it the wrong way round. Or the pipe that tells the regulator what to do is blocked, worked loose or whatever.
Daniel Thirteen-Twelve

Bob
have you checked the vacuum hose from the side of the carb to the front as your carb is a sealed system, a slight split in the pipe and the car will run fine on tickover but as soon as you raise the revs it will suck air and the car will splutter and die?
Best of luck.
Rob Newt

Just to clear a few queries and many thanks

Car will run fine for a short while and will pressurize fuel to above 20psi on demand no problems

Originally car ran well for many miles and has slowly developed this failling?

Robert (Bob) Midget Turbo

How is your system vented?
Have you replaced the filler cap recently?
David Smith

Bob what carb do you have, is it the Metro/Maestro/Montego turbo type? if so do you have an anti-run-on valve fitted to the outlet on the carb? These can fail giving odd symptoms.
David Smith

Bob
In my mind it appears to be regulator related, but you say it ramps the pressure up ok initially-------hmmmm
I would be checking the fittings and pressure hose that operates the reg. for contamination or even a split in the hose first BUT I am going to throw my hat in the ring and say a pin hole in the diaphram in the reg.
If it had a tiny hole in the diaphram it would work initially and then as the pressure wept through to the other side of the diaphram it would act as though there was no pressure on it at all , same as how a brake booster works or not
Cheers Willy
William Revit

David

Have released the fuel cap (Monza) to no avail sadly and it does not have an anti run on valve fitted, but this is a fuel related issue the pressure is falling off.

Willy

Many of my own thoughts for sure and have decided to do the following test.

I shall block off the return when the problem occurs during the next test this morning. If the regulator is at fault I ought to get full pressure!! if not then it must be fuel delivery?
Robert (Bob) Midget Turbo

I remember the days of blow through carbies before injection. I saw a few with twin twin webers. I think done for max power but the problem was keeping the carby pressurised?

As you've started the idea of the regulator is to maintain constant pressure differential between fuel in carby and inlet manifold. So manifold pressure up full pressure a few pounds higher. I assume you have a regulator with the small vac line from the manifold going to the regulator to vary the pressure. So with engine not running why not connect a hand pump (vac pump will go +ve to I think) with an accurate gauge to the regulator. Turn on fuel pump. Pump it up by hand and see what pressure you have in the fuel line at the carby. If the fuel pressure is not rising with the pressure then problem in the regulator. Unless there is a flow restiction like blocked filter or pickup in tank. Also be worth checking this small vac line for holes.

This approach would have also worked to check the fuel pump flow at high boost.
Greg H

Hi Greg

Yep have done that and the pressure on test in the garage can rise above the pressure required by boost.
On a test run the fuel pressure rises above boost without problems for a few miles until the fuel pressure drops off to zero.

I really need to confirm the regulator health? will let you know the result shortly.
Robert (Bob) Midget Turbo

GregH
Do I know you ?? Where abouts in Tassy are you mate
Cheers Willy
William Revit

Don't think Willy's idea will work, as the "other" side of the diaphragm must be open to atmosphere.

Some temporary plumbing to put a manual restrictor valve in the return line inside the car, could be twiddled when the problem happens. Checks regulator.

Similar plumbing in the feed line with a vertical sight glass (clear filter, glass tubing) would show fuel flow - or bubbles/air - when it dies. Maybe put some balls (plastic beads or marbles) in it to show motion, but make sure they can't get through the outlet! Checks pump/pickup.

Q- What is the pump/tank setup? I've had this on regular cars with floating crap blocking the pump pickup, took us two years to find, as it always worked after a bit of sitting. The lady's kids had dropped a roll of caps (remember those?) down the Spitfire's convenient rear deck mounted filler. They would float around until they got sucked up against the intake, and block it, creating a vacuum lock in the fuel line to the pump. Then after 20 minutes the vacuum would bleed off through the paper, the caps floated away, and the car would be fine until the next time. It finally died at a traffic light right in front of out shop, so we got to check it while it was actually dead.

My Mazdas have a reservoir that the pump sits in, about a quart or two. This has a tubular feed that starts at the left front corner and goes around the reservoir and enters at the right rear corner. The idea being that cornering doesn't starve the pump. A blockage in that tube would produce the same thing you have, at least as long as the fuel level isn't above the top of the reservoir. The return dumps into the reservoir, so the thing would idle a long time even if the tube were totally blocked, and if the tube were partially blocked it would only die at heavy loads.
I assume that any FI pump setup would have a similar arrangement.

FRM
FR Millmore

Bob,

There is an easy get out here, you know you want to, go 'K'! All the power without the problems. (I'm fishing for a bite here :))

Steve
Steve H K-ser

William. I'm in Hobart. I don't think you would know me nor have seen my car as it's been off the road for a long time. '69 MKIII midget GAN4 VIN:922

Getting it painted now. Light blue and intend to make it like new again. It's a good time to do it as parts are cheap from the UK.

Greg
Greg H

Cheers
William Revit

I had a Scimitar once that did something similar. Re-built the carb (x2), replaced the filter, etc... I became a little fed-up of 10-15 mins into the journey losing power. And it's a reasonable heavy car to push off the road.

It would always start/run fine after 30 mins or so; and idle in the drive for England.

Turns out there was gelatenous globes in the fuel filter; they'd all find their way to the gauze and block flow after running for a while.

The globes were old fuel that had been evaporated over many cycles of heat/cool; they were around 1/8 inch in diameter; you could squeeze them between your fingers and they'd change shape. Never seen anything like it before, nor since.

The fix for me was <1 min - remove the globes and the problem was gone.

I'd check your fuel system under the bonnet for somehting similar...

A
Anthony Cutler

Some great comments and observations, thanks very much. Just need to update a couple of things.

Steve H LOL Steve not sure I could run a K series I think it too needs a high pressure from the fuel pump!

By the way if this engine comes out the only thing that will go back in will be a Rolls-Royce Pegasus or a turbo diesel! LOL

I don't think it is the fuel regulator, managed to check for fuel return when the problem occured and NO FUEL was returning suggesting this issue is fuel delivery.

Thinking along the same lines as Ant I routed a piece of pipe from the pump inlet to the tank via the fuel filler. Took the car for a drive and got the same result :( then the pump failed completely? pushed car home thinking not sure what? decided to change pump for my other one but when removed I tested pump on battery and it worked fine? Second pump operated in normal set up performed exactly the same as the other pump

I think I will set up a test in the garage where by I will apply about 15 psi to the fuel regulator and try it for a while hopefully it will fail and I can test in the confort of the garage and not breaking down in my lane?
Robert (Bob) Midget Turbo

Is it possible that there is an electrical problem (connection) that becomes significant with heat/vibration?
Trevor Jessie

Bob,are you using standard metro fuel pump and regulator?If so where is the fuel pump mounted? Finally what size fuel lines do you have?Does sound like fuel pressure not a regulator problem.
steve cowling

another possibility is some sort of muck or foreign substance in the tank that is aggregating around the pickup pipe, unlikely I know - but you've been through all the likely things !
David Smith

Bob
I've been working on a Ford Focus today with your problem -- It would run for about 4-5 mins. and then loose power
It ended up being a loose (burnt) push on spade connector on the pump
The pump in a Focus is inside the tank and it amazes me how things can sizzle up so much in there without going bang. I know it relies on no oxygen in the tank to make a fire but when you see terminals burnt like this it still scares me

If your pump is mounted externaly, I don't suppose it could be sucking the supply hose from the tank flat---- depending on what material the hose is made from ???????
Willy
William Revit

Electrical?

I wondered that too but this is extremely consistant and when I have got low pressure the pump is still screaming?

David I am begining to think this is a blockage in the tank.

Steve yep basically a metro fuel pump and regulator mounted at the original Midget position and supplied by the original 1/4 fuel pipe. Rightly or wrongly this vehicle ran perfectly for a good number of months
Robert (Bob) Midget Turbo

Hi Bob,the metro pump hates sucking fuel,so look at the line from tank to pump.(full tank would help!!)
The 'standard' lines used are 8mm to the regulator and 10mm return.(must have less restriction than the feed)If say the suction was a bit marginal,then a small problem could lead to progressive starvation.To overcome this issue I put a Facet fed swirl pot in the boot and ran the metro pump from that.
steve cowling

OK Steve I take on board your comments.

I tried to feed the pump on a test via the fuel filler. If you are correct (And why not?) then I would have been relying upon suction to operate the fuel flow, so perhaps that is why that particular test was flawed?
Bob Turbo Midget England

This thread was discussed between 26/02/2011 and 03/03/2011

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.