FUEL PUMP

#1
I have just rebuilt the fuel pump on my 3500 P6 Automatic, and it works well but the
pressure is very high (new valves as well) and the carbs overflow quickly. The return
pipe is not blocked and join the reserve fuel pipe in the gauge. The pressure in
the return pipe is higher than the pressure needle and seat can absorb but
due to the weight of fuel in the tank, the fuel can't go back in the tank...
Is there anything I don't understand ?
Thank you for your help.
 

cobraboy

Well-Known Member
#2
I cannot believe Rover relied on the return pipe in order to bleed off excess pressure, I think it is there to keep a flow of fuel past the carbs to mitigate vapour lock.
With this in mind I think you need to use a pressure gauge to establish what pressure you have at idle. If you discover you have in excess of 2.75 psi then I would fit a regulator to keep the pressure under 3 psi.
Perhaps consider new seats and Viton tipped needle valves.
 

Julian

New Member
#3
There is a fine gauze filter on the return / reserve point inside the tank, it could be possible that this is clogged up resulting in increased pressure in the return pipe. The return and reserve pipe join together just before the tank, so In theory to prove this failure mode, if you go onto reserve, it should stop the over fuelling or even starve the engine of fuel a bit. I took my filter out the other day whilst my tank was out and found it very clogged up plus the pipe was semi blocked up as well that goes into the tank.
 

SydneyRoverP6B

Well-Known Member
Staff member
#4
The standard fuel pump valves will not increase pressure beyond the design value. Carburettors will overflow when either the float level is incorrect, i.e., set less than the required 45 thou, or one or both floats have a fuel leak within. Have you checked your floats Julian?

Ron.
 
#5
The Pumps are designed with a split linkage between the drive cam and diaphragm. As the pressure in the pump rises the stroke shortens to reduce the flow to the carbs. Sometimes the linkages seize up giving full stroke all the time and no pressure regulation.
its very rare for the needle and seat in the carbs to allow overflow due to high pressure, particularly with the return line fitted. So look at the carbs first and check the return line is clear as above.
M
 
#6
Carbs on my 3500S suddenly started overflowing, after changing the water pump! Eventually traced to carb. float. Visually there was nothing wrong with the float, and no leaks, but after talking to SU specialist, I was told that old floats (brown-yellow colour) were probably the cause. Bought new floats (white plastic) and problem cured! Might be your problem. I have Facet Electric pump with in line filter and pressure regulator, and everything fine until I had to change the water pump.
 
#7
First of all, thanks to everybody for your helpful comments.
Last week, it was very hot in Switzerland and my P6 stopped before
arriving to the top of a mountain. The fuel couldn't arrive to the carburettors. Many times before that day , when I stopped the car for about 15 minutes, it was easy to start the car again but very difficult (for about 5 minutes) to accelerate correctly (vapour lock symptoms even with cold weather). I had rebuilt the carburettors with vitton needles and new floats correctly adjusted. I suspected a problem with the fuel pump.
I then rebuilt the pump with new valves and seals and just after that, the problems of overflow appeared.
It really looks like the pressure is now too high. I am going to check this very soon . I don't think Rover fitted a pressure regulator on
their cars but I will do it if necessary,however I would prefer to keep the original configuration and find why I have this problem.
I would also like to apology for all the mistakes in my english text.
I will let you know anyway.
 

SydneyRoverP6B

Well-Known Member
Staff member
#8
First of all, thanks to everybody for your helpful comments.
Last week, it was very hot in Switzerland and my P6 stopped before
arriving to the top of a mountain. The fuel couldn't arrive to the carburettors. Many times before that day , when I stopped the car for about 15 minutes, it was easy to start the car again but very difficult (for about 5 minutes) to accelerate correctly (vapour lock symptoms even with cold weather). I had rebuilt the carburettors with vitton needles and new floats correctly adjusted. I suspected a problem with the fuel pump.
I then rebuilt the pump with new valves and seals and just after that, the problems of overflow appeared.
It really looks like the pressure is now too high. I am going to check this very soon . I don't think Rover fitted a pressure regulator on
their cars but I will do it if necessary,however I would prefer to keep the original configuration and find why I have this problem.
I would also like to apology for all the mistakes in my english text.
I will let you know anyway.
The Rover was never fitted with a pressure regulator as it does not require one. Whether the mechanical fuel pump is fitted with the large valves or the small valves, the flow parameters do not exceed the design parameters of the carburettors. Rebuilding the fuel pump does not result in fuel overflowing, there is another problem that is now evident from having rebuild the fuel pump. For fuel to overflow, either there is an impediment to flow within the spill return line, or one of the floats has a leak. Looking forward to reading what you find!

Ron.
 

jp928

Well-Known Member
#9
Pressure from the mechanical pump is set by the size of the diaphragm, and the spring behind it. If the carbs wont accept a pump-full, the diaphragm wont return fully , and the operating lever will not fully contact the cam, and the effective stroke will be reduced. I suggest fitting some insulation on the front pipe along the block under the exhaust - something like Firesleeve - the correct size will slip over the nut at the pump end. 500-600mm should be ample. With more you could run it all the way back to the reserve tap. As above, also check the return line is clear - with minimal fuel in the tank, try blowing down the return - I would not use compressed air on the reserve , as it might blow out the nylon filter in the tank.
Firesleeve - ADL Insulflex
 
#10
I don't think that there is a problem with floats, they are both new, but it may be a problem with the return pipe. I have tried to push
compressed air (nothing happened when I blew in the pipe) and with only little fuel in the tank I could hardly hear some air bubbles
in the tank. When I disconnected the return pipe on the o/s carburettor and put a small hose on the carb to let the fuel go away
without constraint, the flow was important but the carbs don't overflow. As soon as I close the hose with my finger and run
the engine at 3000 rpm, I can feel the pressure in the hose and immediately the 2 carbs overflow. Then, if I let the fuel go in to a
jerrycan again, the carbs don't overflow anymore. I could fill a one gallon jerrycan in about 2 or 3 minutes. The flow is strong.
 
#11
When installing the new floats did you adjust the needle height? I suspect the needles are not closing the seat properly, allowing fuel to push past them with only moderate pressure.
 

jp928

Well-Known Member
#12
New floats should have new needle/seat units (IMHO), along with resetting the float height. Hard to see how pump pressure could change as long as the spring wasnt changed.
 
#13
Yes the needles and seats are new (vitton WZX1101A) and the floats are perfectly adjusted
(about 1mm below the level of carb). I have dismantled the carbs again and rechecked
both carbs.
I am now waiting for a fuel pressure regulator. I also don't understand what happens.
I have another Rover P6 1974 and have checked if the pressure on return pipe was identical
or weaker. It is difficult to be sure as I only closed the pipe with my finger but I would say
that it is nearly the same. I also can't blow air into th fuel tank without using compressed
air, but these carbs don't overflow.....
 
#14
Blowing air down the fuel line to the tank can cause another problem. Local garage did this on my 3500S to get the reserve working. Blew the reserve clear but disturbed all the sediment in the tank and soon blocked the in-line filters I had fitted when installing a Facet pump. Twice stopped on the road with fuel supply blocked. Had to remove all filters and clear them out and checked or the next week or 2. OK ever since, and filters did their job and keep the Facet clear. With any older car, there could be muck at the bottom of the fuel tank, blocking the fuel pick up nd waiting to get where it will do no good.
Ideally, I should have cleaned the fuel tank out, but could not be bothered.
 
#15
I've had the same thing happen on my car. measurement of the height just didn't work. I incrementally adjusted the needle driving tab till the fuel stopped coming out then checked the fuel height in the jet and adjusted a bit more for that. One carb at a time. The actual float heights are different to each other and i cant swap the tops over as a result. But I have no overflow and the fuel in the jet is the correct height. so the mixtures are correct.
 
#16
I previously had a lot of trouble with my fuel tank that was full of "muck". The fuel was so bad that the valves
stiked into their guides. I could not clean the tank correctly so I gave it to some people for repair and they cut
it in 2, sandblasted it and rewelded. So now, the fuel tank is "new". I had no problem at all before I repaired the
fuel pump, the fuel never went out from the overflow pipes, that is why I am nearly sure it comes from the
pump....
 
#17
That leaves two possibilities.
First is that the two parts of pump driving linkage are stuck together, making the linkage one piece which prevents the pressure regulation from happening. If you can hold your finger over the return line connection and the fuel overflows the carbs without first spraying all over you ie. pressure is still low; Id be adjusting floats first as they aren't working against any real pressure.
Second is that the "old" pump was running so badly it couldn't push enough fuel through to feed the carbs properly so the float needles never actually came into play. This would also explain your vapor lock issues prior to fixing pump.
 
#18
I have received and fitted my fuel pressure regulator in line, just after the fuel pump and before the filter and have
tried the car about 10 kms. The carbs don't overflow now. I will try again for a longer distance, but it seems to be ok.
By the way, does anybody know if it makes a big difference if we use WZX1101 instead of WZX1102 (needle and seats) ?
 
#19
Despite the fact I have totally rebuilt my fuel pump, the problem I had before still persists. The car runs perfectly until I stop
it for about 10 to 15 minutes. When I want to restart the engine (still hot) it does not run correctly for 5 to 10 minutes (see my post
21 August).... I have to pull the choke if I want to accelerate (it is not perfect but much better) and push it back again, and so on...
It is particularly annoying in town (automatic gearbox). This car 1973 has AUD669 carbs (big damper CUD2901) Japan. My other
Rover P6 1972 has "normal" HIF6 carbs, and starts better when hot (but not perfectly anyway)
 
#20
my rover v8 has electric pump fitted. tank cleaned and lined fully. all fuel lines blown with compressed air ( clear) float levels checked (OK) and new fuel (no?) low ethanol ( 99 octane premium) used. when cold runs perfect .runs Ok when warm and hot .stop at a garage and if I am quick ..( 2-3-mins?) it restarts ok but loses power and struggles for abut 500 or so yards until (cold fuel?) kicks in and sudden gush of power as we get back to normal. leaving car for say 7-8 mins or more with a hot engine = fails to start unless fresh fuel squired into air intake tracts. again poor running and no power for short time. checking clear glass (new) fuel filter..I note if I stop and immediately pop bonnet and check fuel .it is full. switch off engine and you can slowly see top of fuel line gathering air ( no fuel leaks) I think its ethanol vapour bubbles forming from heat and joining to create lack of fuel and the air I see in filter. currently shielding fuel lines near engine (no mechanical pump) I now stop at shows and leave bonnet fully open and check every hour .can see large air gap in filter (30%) that slowly goes. when no air gap engine starts and runs OK. was suggested to me that I ought fit a larger bypass ( remove restrictor) in return and wire pump up so it has a manual switch with a timer to carry on running for min or so when engine stops. supposedly t carry hot fuel (vapour) back to tank and leave cooler fuel next carbs. Though that does not allow for carb bowls with hot fuel to get cooler supply until it empties ( probably the 500 yards drive I do with lack of power and rough running engine) not sure theres a cure other than to convert to fuel injection ( have to sell wife and kids!)
 
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