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 MGB Technical - Two fuel filters?

I've removed the fuel filter from my car to try and get to the bottom of flooding and starvation problems. I found significant amounts of debris in the filter that was not visible when fitted in the car. Looks like paper thin tiny pieces of black plastic or rubber.Small pieces also found in float chamber. I was considering fitting a second filter between the existing one and the carbs, until I am sure all the debris has been caught. Is this likely to reduce the fuel pressure below a working level?
ph barton

You could maybe try an inline diesel fuel filter. That will be much more thorough and still have a good flow rate owing to its size (about the size of a food can).
R Kelly

Do you have rubber hoses between the filter and the tank? If you do I would replace them.

Clifton
Clifton Gordon

I run a filter before the fuel pump and a second filter in the engine compartment on both MGCs and my MGB.

I've experienced to problems with fuel PSI at the carb.
rick ingram

PH-
No, the extra fuel filter will have no effect upon system pressure. Install a transparent fuel filter in the feed line just prior to the junction that feeds both of the carburettors, and then install a second transparent fuel filter in the feed line that runs from the fuel tank to the fuel pump. Should you see debris in this filter, simply replace it with the one that is before the carburettors and then install a new filter in the line before the carburettors. By using this approach, you can best protect the carburetors and the fuel pump as well. If the transparent filters that you elect to use should happen to have glass housing bodies, these can be easily protected by sliding a short section of transparent thick-wall plastic tubing over them. A petcock-type valve installed inline inside of the boot will simplify replacement in the future, preventing fuel from the carburettors from draining whenever the fuel line is disconnected from the filter.

However, a word of warning is in order. The fuel tank needs to be free of internal rust. If not, instead of preventing any problems, a filter on the inlet side of the SU fuel pump can cause problems, the biggest being that it is an unseen problem. While the SU fuel pump will pass all but large chunks of rust without jamming, if a modern filter is installed between a rusty fuel tank and the fuel pump it will trap any fine rust particles, clogging up rather quickly. When it does, it will cause the pump to stall in a “current on” condition. If left with the power on very long when this condition exists, it will burn out the internal swamping resistor inside coil of the pump. Once the filter is replaced, everything will appear to be normal and the owner will go his way thinking that the problem has been solved. Unbeknownst to him is the fact that the burned out swamping resistor defeats the pump’s arc suppression circuit and that the points will burn out a short time later. The owner then installs a new set of points, only to have them again burn out in a short period of time. This is the reason that a set of replacement contact breaker points will seem to burn out prematurely, so be sure to check the fuel filter on a regular basis. As a precaution, you should seal the inside of the fuel tank against rust and corrosion. Eastwood sells an excellent sealer that you simply slosh around inside the fuel tank and allow to cure. However, be sure to blow out the screen inside the fuel tank with compressed air before the sealant cures, otherwise the fuel pump will not be able to deliver the fuel to the carburettors. Eastwood has a website at http://www.eastwood.com/ .
Steve S.

Like Rick Ingram above , I have also run two filters with no fuel flow problems.
The first before the pump and the second before the carbs.
The second was mounted on the bulkhead near the heater box well away from the exhaust heat.

The plastic "flakes" you are getting may be coming from the either the tank lining or the hoses.
Are you using Ethanol by any chance?
That can play all sorts of havoc with plastics not specifically designed for it.

Pete.
Peter Thomas

A filter should not be installed between the tank and the fuel pump - see my article SU Fuel Pump Facts and Myths at: http://homepages.donobi.net/sufuelpumps/Fuel_Pumps/SU_Fuel_Pumps_Facts_and_Myths.pdf
SU fuel pump are very vulnerable to clogs on the inlet side as described in the above article. Steve has done a good job of quoting what is in my article. Cheers - Dave
David DuBois

PH-
By all means, click on the above hyperlink that Dave Dubois posted and read his article. The information you'll find will give you some pretty good basic information on your SU fuel pump that all MG owners should have.
Steve S.

So nobody would recommend using a diesel fuel filter then?
R Kelly

A diesel filter should work fine as long as it is placed on the outlet side of the pump. Cheers - Dave
David DuBois

I have done as Steve outlined in his original posting above. However, I hasten to add that I use an aftermarket pump of a different design which has always had sufficient capacity and PSI output to handle both fuel filters. If you also use an aftermarket pump, setting up your filters as stated does not have to be superflous.

Even though I have a replacement fuel tank on my GT, a difference between it and the OE unit I removed was that the replacement did NOT have a filter inside the tank at the pickup tube, whereas the unit I removed did! That fact was the reason why I installed a filter between the tank and the pump, even with an aftermarket pump. If you know that your replacement tank does NOT have a filter at the pickup tube inside, a filter may well be worthwhile in keeping the big chunks of debris from passing into your pump.

The second filter I use at the carbs is there for a different reason. I did not replace the OE metal pipe from the OE pump to the carbs and after a short while of use, I discovered that there had accumulated some sort of greyish flakey crud from inside of this metal tube. With fresh gas of our current formulation and with ingredients to help clean injectors, these old deposits came loose and found their way into the float bowls. The second filter did the job and I have not had any more of this crud pass into the carbs. Not everything that ends up in our carbs comes from the tank or is rust or dirt. FWIW
Bob Muenchausen

Thanks for the comments...have been on holiday so unable to reply.
The tank was new when I rebuilt the car a couple of years back and has done very low mileage since. A moprod pointless fuel pump was fitted, plus new copper fuel pipe ans hoses.
The car has had running problems ever since which I now put down to this debris problem. It's a 1980 LE roadster, so I will put a new standard filter in ...plus maybe a temporary one before the carbs and see how it goes.
Will go further if problems remain.
ph barton

A filter on the inlet side of an after market pump is not a problem since most after market pumps use a bypass valve to set the outlet pressure and will continue to run (although not pump fuel) even if a complete clog occurs. On the other hand, the Moprod, which is a clone of the SU pump is just as susceptible to having the coil overheated as the SU pumps are. Worse, the Moprod, Having a plastic case and coil housing will incur more damage than the SU pump do. As I state in my article linked above, there is no real reason to put a filter on the inlet side of the SU fuel pump as they will pass all but the largest particles. The proper approach to debris in the fuel is to correct the source of it. Cheers - Dave
David DuBois

Since you have said the tank was new and you replaced the lines and have low mileage it doesnt make since where the debris is coming from. But with that said, I had a midget with a similar problem, although mine was rust from an old tank. It would run great then about once a year start starving for fuel. I cleaned my tank which did not help, what I did was kept a fuel filter in the trunk and just changed it when it did finally bog down. You may eventually filter out all the trash. (not the best method, but it worked for me)
Bob

PH-
I'd recommend that you disconnect the fuel lines and shoot carburetor cleaner through them. You'll be surprised at what comes out of the fuel lines of a 28+ year old car! It'll probably save you the cost of a few fuel filters, plus the hassle of changing them in the future.
Steve S.

This thread was discussed between 08/03/2008 and 29/03/2008

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.