11 replies to this topic

[Jordie]

Hello all

Just built a 1275cc engine to put in my car. Turned it over for oil pressure etc, fine. Went on to fire it up and its sparking but the plugs arent getting fuel. Tried cranking with the leads off and the plugs stayed dry.

If i pour petrol down the carb, it fires and runs for afew seconds as expected.

I have a SPI tank using the pump, which at first i put straight into a inline regulator set to 3.5psi. This began to blow the pipes off, so I t-pieced the inlet to the regulator to the return on the spi tank. Still wasnt fueling.

Tried afew things, different carbs inc HS4, HIF38 and a HIF44. Luck of having 3 carbs not to fuel the engine at all is too unlikely, so i am looking elsewhere. Any ideas?

You can hear the fuel fill the float then stop, so the float is full, its just not drawing it through.

Jordie

[miniboo]

blocked jet in the carb?

[Jordie]

In all 3. Wouldnt have thought so.

Going to try my mates inlet and HIF44 off his car tonight as we know that works.

[Jordie]

Anyone?

Surely once the carb float is full, it should try firing using that fuel. Not getting a thing with 3 different carbs, HIF38, HIF44 and a HS4.

Cranking over with the plugs out, theres no vapour coming from the bores.
Cranking with the plugs in, they stay dry.

Jordie

[Dan]

I think you'll find you have massively low fuel pressure. You have created a loop in your fuel system whereby you are simply pumping fuel back into the tank constantly. There will be little resistance to flow in this circuit. The other side of this loop has to overcome the resistance of both the pressure regulator and the float valve. It sounds like the float is getting to a point where there is just too much load against the fuel flow compared to the resistance of the return and from then on all the fuel is running along the return line instead. If these carbs have the spring loaded type float valves then it is possible for the valve to close early if there is minimal pressure available. Try pinching off the return line while the engine is cranking and see if that kicks it into life. I think you really need a proper, low pressure fuel pump or a regulator designed to work with a fuel return system.

[Jordie]

Try pinching off the return line while the engine is cranking and see if that kicks it into life. I think you really need a proper, low pressure fuel pump or a regulator designed to work with a fuel return system.

Tried this. Ill get a metty turbo regulator and try that. Just sold one too, buggar.

That doesnt explain why it doesnt use the float fuel when its full. Surely if the float was full, then it would use that fuel to fire, even if the fuel pressure was too low for it to run after the float emptied. We are getting zilch fuel past the carb.

I will see if i can lend a metro turbo regulator off a local lad to try. If not ill have to order something if that is the problem.

Edited by Jordie, 22 August 2008 - 11:42 AM.

[Dan]

You don't quite understand how carbs work. A carb can't really empty it's float bowl, especially not at cranking speed because there simply isn't enough venturi generated. If the fuel in the bowl is not at the right level, a weak low speed venturi will never draw any fuel off. Fuel level / float bowl angle causes many starting problems in home built cars. The fuel must be at the right level in the jet and that is set by the level of the float, if the pressure is too low then the spring loaded float valves will close early. They have a spring loaded section after the main valve is shut, allowing fuel to weep slowly past when the bowl is full and fuel being drawn off at a steady rate. The valve only fully opens when the level in the bowl is very low. The problem with this is that when there is very low fuel pressure the spring is too much for it and the weeping will never occur. Your float is allowing a surge of fuel to part fill the bowl but it is never reaching the full level. I would imagine that you have your regualtor on the bulkhead in the normal position which is to say higher than the carb. The flow to your carb is probably nothing to do with the pump at all and is simply gravity feed from the regulator, gravity feed of a couple of inches will never fill the bowl past the spring loaded valve.

[dklawson]

You tried three different carbs and none worked. I know it seems unlikely that they would all have problems. However, if they have all been sitting on the shelf for a while, you really ought to take just a moment to remove the float bowl from at least one of them and make sure the float valve is opening and set correctly. As Dan said, with your "T" diverter the pressure reaching the carb may not be anything near what you anticipate... perhaps not enough to unseat a stuck float valve.

I also agree that you need a different pump. The cost of a quality regulator is equal to the cost of a Facet pump. I would consider looking at how the SPI pump is connected and what low pressure pump you could fit in its place.

[Jordie]

I'll put a manual fuel pump on it for now. see if that works. Thanks

[dklawson]

The manual fuel pump should get the pressure right, but if the float valve is gummed up and stuck closed the problem will not be resolved.

[Jordie]

Changed the fuel pump and tank and same problem. Tried my m8s HIF44 and inlet on it and it worked. So i changed inlet manifolds and it fired up using the carb i had been trying.

Ditched it all in the end for a set of twin carbs mmmmm