Petrol starvation elderly Fiat Talbot engine

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swifty999 · #1 Post by **swifty999** » Mon Sep 21, 2009 10:07 pm

As I'm new I thought I would take advantage, van started missing badly running on petrol, ok on gas.

Wouldnt start at all on petrol after it had been stood for a week, on investigation the carb was dry, definately set to petrol, checked fuel was pumping from pump (old fashioned diaphragm job) Did pump Was a bit feeble but I didnt know what to expect, removed pipe from switch/cutoff solenoid, no flow, removed in line filter, no flow, by passed return path tee, carb filled and van started

Look like the return provides the path of least resistance it is close to the pump and lower than the cut off switch which feeds the carb. petrol is not reaching carb
Have I got an inefficient fuel pump or has something wierd happening with return path.
GAS install is 6 years old and has been working fine

Thanks
Swifty

Gilbertd · #2 Post by **Gilbertd** » Mon Sep 21, 2009 10:36 pm

One of my varied fleet had a mechanical diaphram type fuel pump with a restricted return on the inlet to the carb to allow excess fuel delivery back to the tank. The pump started to leak (directly above the exhaust manifold!) so I decided to do a mod.

If you think about it, the cam driven diaphram pump doesn't have much going for it. If the carb is dry, you've got to crank the engine for ages to get fuel through and the pump supplies fuel in direct relation to the engine revs. So at high revs on the overrun, the pump is pumping fuel at a fair old rate but the carb doesn't want any so it has to be returned to the tank. If the return has too much restriction excess fuel forces it's way past the needle valve flooding the carb.

My answer to this was to blank off the hole where the mechanical pump used to be, blank off the return and fit a demand type electric pump. I intially used one of the old SU type clunk, clunk pumps fitted to old XJ6s but later found that I could buy a modern solid state type that did the same job (but without a set of contacts that needed careful cleaning and adjustment every so often). I wired it via a relay that took it's signal from the starter (so it pumped while cranking) and the oil pressure sender. This way it fires up as soon as you start cranking the engine and continues to be energised while there is oil pressure giving a safety cut off in event of an accident.

This would also solve your problem with the petrol shut off solenoid. You wouldn't need one, just kill the power to the pump.

swifty999 · #3 Post by **swifty999** » Tue Sep 22, 2009 8:42 pm

Interesting and appealing solution, not sure if I have the know how but will look at possibilty carefully.

If I blank the hole when pump is removed will it need a gasket, is that area subject to oil pressure?

Do you reckon I could use existing petrol cut off supply for new pump which is ignition fed and currently energises the cut off solenoid when switched to petrol ie will power the new pump when ignition swiched on and lpg/petrol switch set to petrol (or will the fact it wil pump just with ignition on be a problem?) do i need to use the oil pressure sender?

thank you very much

Gilbertd · #4 Post by **Gilbertd** » Tue Sep 22, 2009 11:04 pm

swifty999 wrote:If I blank the hole when pump is removed will it need a gasket, is that area subject to oil pressure?

It's not subject to oil pressure as such but there is oil sloshing around behind it. I used a lump of 10mm thick aluminium that I had laying around. Cut it to roughly the same shape as the base of the original fuel pump. Drilled two holes to match the pump mounting holes and then used the original fuel pump gasket between it and the block.

swifty999 wrote:Do you reckon I could use existing petrol cut off supply for new pump which is ignition fed and currently energises the cut off solenoid when switched to petrol ie will power the new pump when ignition swiched on and lpg/petrol switch set to petrol (or will the fact it wil pump just with ignition on be a problem?) do i need to use the oil pressure sender?

If the pump is on all the time the ignition is on you sort of defeat the object. Use the supply that currently goes to the shut off solenoid. It is powered when you are set to run on petrol and the ignition is on, exactly when you want it powered. As soon as you switch to gas, the power to the pump is disconnected so no more petrol flow. You don't need to use the oil pressure sender trick. I did it as a belt and braces safety feature. If you don't use it, and a huge number of older cars didn't, in the event of an accident that fractures a fuel pipe but doesn't damage the battery or wiring, the pump would continue to run and squirt fuel over a hot engine. A pretty rare set of circumstances when you think about it.

Virtually all cars from the 60's and 70's that used electric fuel pumps had them powered directly from the ignition supply. IIRC there was an accident involving a fairly famous actress who had a head-on in a Rover P6, the fuel pipe fractured, fuel was pumped over the hot engine, it ignited and she burned to death. This highlighted it and car manufacturers went about solving the problem in a variety of ways. Jaguar went for an inertia switch that disconnected the fuel pump in event of an impact (it also killed the fuel pump if you hit a big pothole too so wasn't the most popular of methods). BMW and Porsche (on the 924 and 944) went for a timer relay. The fuel pump ran while cranking and would shut off 4 seconds after you stopped cranking unless it saw a pulse from the ignition system that kept resetting the 4 second delay. They weren't particularly reliable either and cost an arm and leg to replace (which when you consider that they only consisted of a 30p 555 timer chip and a relay was a complete rip off!). Ford went for the oil pressure sender method which was fairly reliable unless you ran out of petrol. As the only way to keep the pump running was to crank the engine on the starter, it was always a toss up whether you'd get enough fuel into the carb for it to start or you'd flatten the battery! Modern injected cars have things like crank position sensors to detect that the engine is turning and an ECU controlling the fuel pump so it's no longer a problem.

It's up to you really, I did it because I could although as all my fuel lines are flexible hoses, the chances of them being broken off in event of an accident are very remote.

gavbriggs · #5 Post by **gavbriggs** » Wed Sep 23, 2009 6:36 am

didn't some really old cars have a switch for the fuel pump?

swifty999 · #6 Post by **swifty999** » Wed Sep 23, 2009 7:49 pm

Thanks Gilbertd

excellent info

have found a pump on E Bay complete with mounting rubbers and am going to proceed with the mod, the new pump needs to be close to the petrol tank I understand and also lower than the tank?? Hopefully I can find a safe spot to mount it

Swifty

SimonHobson · #7 Post by **SimonHobson** » Thu Sep 24, 2009 8:56 pm

Gilbertd wrote:If you think about it, the cam driven diaphram pump doesn't have much going for it. If the carb is dry, you've got to crank the engine for ages to get fuel through and the pump supplies fuel in direct relation to the engine revs. So at high revs on the overrun, the pump is pumping fuel at a fair old rate but the carb doesn't want any so it has to be returned to the tank. If the return has too much restriction excess fuel forces it's way past the needle valve flooding the carb.

Actually, the lever type driven off the camshaft don't have most of the problems you've described (apart from the dry carb issue, and that's why they have a hand lever) !

The lever simply compresses a spring and pulls the diaphragm back, it does not force any fuel out, just draws it in. When the lever is released, the spring provides a relatively controlled fuel pressure. If no fuel is being used, the diaphragm stays put and on the next cycle, nothing happens. As fuel is used, the diaphragm comes back up under spring pressure to supply the fuel - and on the next cycle, it gets pulled back again by the lever.

I am fairly certain that most installations with a return line have it to avoid vapour locks in the fuel lines - by allowing fuel to return to the tank, the lines are kept full of fresh fuel that hasn't got hot enough to start forming vapour bubbles and interfere with proper operation.

swifty999 · #8 Post by **swifty999** » Fri Sep 25, 2009 7:15 pm

Thanks

am going to fit an electric pump in the under bonnet area, supplier assures me the one I have ordered has long pull short push spec. this will be turned off when running on gas saving the need for a seperate petrol cut off and eliminating the suspect mechanical pump, is that logic sound??

SimonHobson · #9 Post by **SimonHobson** » Fri Sep 25, 2009 9:33 pm

swifty999 wrote:am going to fit an electric pump in the under bonnet area, supplier assures me the one I have ordered has long pull short push spec. this will be turned off when running on gas saving the need for a seperate petrol cut off and eliminating the suspect mechanical pump, is that logic sound??

As long as the carb is well above the tank, and allowing for petrol surging about under braking etc. If there isn't a lot of height difference, then you can get petrol coming up the line just under it's own momentum.

Also, make sure the petrol tank breather is clear - and pressure in the tank will push petrol up the pipe.

swifty999 · #10 Post by **swifty999** » Sat Oct 03, 2009 9:50 pm

It works!!

Thanks Gilbert and Simon