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MG Midget and Sprite Technical - Fuel Tank Pipe

Is there a differencein the fuel pipe between the 6 gallon tank and the 7 gallon tank NPR9Z.
Moss list two AHA7383 = 1275cc and CPP7148 = 1500cc
Peter King

Hi Peter,
Attached shows the MGOC take on the fuel system.
Item 30 for the 1275.
Item 76 for the 1500.
I reckon it’s more about the type of fuel pump than the size of tank.
1275 - electric pump mounted near the tank.
1500 - mechanical pump mounted on the engine.
Simon


S Holt

Yep, Simon's on it.
Different fittings on the ends.
A series should have another short hose (tank to pump) bundled with the pump wiring, as shown in the breakout diagram.
1500 has a short hose the other end, pump to carbs.
You don't mention what car you have though.

Incidentally, as I understand it the six gallon tank is generally the same as the seven gallon one, except that the six gallon tank has the anti-splash device (air vessel) inside to comply with US pollution regs.
Greybeard

Hi Simon, Greybeard
Thanks for your input. The car is a 1974 MG midget but thats as far as it goes.The reason for me asking this question is that I have just changed the tank from the old 6 gall to 7 gall.I found that I could not draw fuel from the tank to the pumps in the boot, drawing air in from somewhere.
The car set up is twin competition Facets in the boot to a filter king in the engine bay to a 45dcoe.
I've run with Facets for years so I know there ok even though these are new. I have also primed and bench flow tested so there ok. I've found that there are two fuel pipes but I don't what the difference is. I use part of the old pipe with union/nipple. I did used a thread sealant this time but found that it had not seale the threads so reverted back to PTFE tape. The tank now seems to hold vacuum. So the question is do I change the pipe and fittings or stick with the old one.
Peter King

Peter,

You shouldn't meed any thread sealant or PTFE tape to form a seal. The nut pushes on the cone on the end of the fuel pipe and pushes it against the internal pick-up pipe flare and so against the tank boss forming a seal. I found this out as the source of air in the fuel line when I used a compression fitting into the tank boss instead of the standard fitting so the internal pick-up didn't seal to the tank boss and picked up air when fuel was below the tank boss level.
David Billington

Ah, now. Clearly your setup is a bit non-standard so all bets are off!
Did the new tank come with the pickup pipe installed? It's a common occurrence that some kind of tank lining treatment is applied which blocks the strainer on the end of the pickup pipe inside the tank. I wonder if that has happened.
Also the tank should not hold vacuum with the filler cap in place. The cap should "breathe" and a number of BBS-ers (including yrs truly) have had fuel starvation trubble related to blocked cap breathers.
David is spot on (he usually is in my experience) saying there should be no sealant or tape required on the tank outlet. It's a flared seat as he says. It has to be because that's what seats the pickup and locates it in position.
Is it possible that there is no pickup pipe fitted inside the new tank? The fuel hose would still screw onto the boss but unless the tank was near full it would only suck air.
Greybeard

The tank is out of the car and the cap is off when I did the vac test which was to determine if there was an air leak in the tank. What my question was does anyone know why there is two part numbers for fuel line AHA7383 = 1275cc and CPP7148 = 1500cc accroding to Moss the CPP1748 does both chrome bumper and 1500.
As far as using PTFE tape an old school mechanic told me better seal threads where you can (no leaks).
Peter King

Peter
Following this thread out of interest-
At one point there you mentioned if you poked a wire into the hole in the tank all you could feel was the baffle
That, sounds to me like the pickup tube is missing in the tank---Have you made up a new tube for it now--
You can't use a tube out of an old tank in a new tank as the old tube originally has a filter on the end of it and when you pull it out the filter drops off---If used in the new tank you can't refit the filter so the tube ends up too short and won't go anywhere near the bottom of the tank---a custom made longer tube is the only fix if you have a tank without one.
Also, PTFE tape-You shouldn't need it--The seal is between the taper of the flared pickup pipe and the taper on the end of the fuel line, the nut is the means of pushing them together, if there's a leak there it means the tapers aren't sealing and needs tightening.
Using thread tape here is like threadtaping your wheel nuts to stop the hubseal leaking. and if the taper is leaking it will still suck air from inside the tank.
You need to have a good look at both tapered surfaces and check that they're in good condition, very rare to have a problem there----
William Revit

The pickup pipe could be missing or it could be misaligned, pointing off to one side instead of pointing down to the tank bottom.
Now..
I very seldom disagree with Willie because he's a clever mechanic and he's right more often than I am but I should say that it is possible to remove and replace the pickup pipe and strainer. I know because I have done it. It has to be done through the tank level sensor hole with the sender not installed.
I assume that's how it was done in the factory.
It's a PITA - very fiddly indeed, but it's do-able.
The pipe has to go in first, then the strainer can be installed from inside the tank. If your hands are any bigger than mine this will be a challenge.
This also gives the opportunity to verify the orientation of the pickup pipe. When I did it I made a mark with a Sharpie on the flare face to indicate which way was "up" on the pipe.
The flared end of the pipe has to seal both sides. The back side seals against the seat in the tank boss and the front side seals against the "bullnose" on the end of the fuel line which in your case goes to the Facet pumps.
All these tapers are clamped together by the nut on the fuel hose, tightened down on the tank outlet boss.
WRT PTFE tape, it should never be used on parallel threads. It actually encourages the fitting to unscrew as it's just about the slipperiest thing ever. In this assembly it contributes exactly nothing to a seal.
(Although I confess I have used it in the past on stainless steel fasteners exposed to saltwater to minimise thread galling).
Greybeard

I stand corrected Greybeard, I must admit I have given in early in the past trying to fit an original pipe and just made up a longer one--the thinking was to eliminate the filter/screen anyway - one less thing to go wrong.

Peter's early comment about poking the wire in the hole and hitting the baffle sounds a lot like the tube isn't there. Or wasn't at that stage.

Willy
William Revit

I should learn to read more carefully:

>>>What my question was does anyone know why there is two part numbers for fuel line AHA7383 = 1275cc and CPP7148 = 1500cc accroding to Moss the CPP1748 does both chrome bumper and 1500.<<<

Do the early chrome bumper A series cars use an AC Delco mechanical fuel pump like 1500s?
(ISTR that other things with A series engines had them. Minis for example, and I'm fairly sure my old Morris 1000 postie van had one).
If so the 1500 fuel pipe would be wrong.

Somebody hereabouts will know, and I imagine it's in Horlers book. Which I don't have.
Greybeard

This thread was discussed between 27/05/2024 and 29/05/2024

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