Spli FCR plumbing question

You can do it either way, a return to the tank does help to stop the fuel boiling in the pipes when the motor is hot but it seems happy both ways. my return will be used but with a restriction in the return line, just to take some pressure off the pump and give it a chance. p.m. me for details.

 

Surely to reduce pressure you want LESS restriction in the return?

 

why do you want LESS pressure? you need max volume with the fcr's.

 

I must have misunderstood you - you didn't literally mean take some pressure off the pump then? :smile:

 

A more clear answer, the pump on the ss will give around 7-8psi, or 1/2 bar of pressure static (without flow). thats a little too much for the FCR carbs, they are designed for 4 psi. (any more could cause fuelling problems through flooding) so if you do not use a return there is too much pressure and the pump is working too hard where it dont need to. if you use a straight return the pressure does not get to 4 psi as its an open pipe back to the tank, so best option i found restricting the return with the correct size jet will lower the pump pressure to give the correct feed to the carbs. easy.

 

Seem to remember somewhere in my dim and distant past being taught that pressure is resistance to flow. Based on that you would want to use the largest bore pipe possible if you wanted it to flow the maximum volume of fuel.

WW

 

Thats what I was getting at in post #3.

 

Yup, I was agreeing with you. I should have made it clearer in my original post.


WW

 

I'm thinking of the FCR route what is considered to be the right sized jet ? for the return fuel pipe
Thanks

 

Aimed at ww, yes the whole idea is to have the largest pipe but if the pipe is open at the end you will have no pressure, you need the pressure of 4psi to fill the float bowls and keep them filled at full chat.
a good idea is to use a mitsubishi pump that is regulated to 4 lbs, that you have to fit outside the tank. for the original internal its got too much pressure so it either has to be bled off with a return or by a pressure regulator.
as for jet size, cram a 250 - 300 in the pipe and give it a try, you need a pretty accurate fuel pressure gauge that reads full scale to around 10 lbs. for more pressure a smaller jet, for less, a bigger jet, but only in the return pipe.
without the pressure at high power levels the carbs will run weak at the top end as you will be empting the bowls too quick.

 

I ran a SS IE fuel pump when i had FCR's fitted to a 944cc engine with CJS Flowed heads and never had probs with this setup.
The Carby fuel pumps can get lazy with age.

 

Quite right, the pumps do get lazy, and full of crap, but surely someone spending around a grand on carbs aint gona use them with a shitty pump?