carby set up with fuel return line?

[unfamilia 1,938]

Thinking about my bike carb set up I picked up a regulator from a mate. He was telling me his old rotor had a fuel return from the carby fuel bowl to the tank.

 

have been reading up a bit and some old school racers used to just flood carbs with fuel and hook up the overflows back to the tanks with a return line.

 

wondering if this is a solution for bike carbs dribbling out the overflows at random???

[gerg 10,871]

Your float level is there not just to stop the carby from flooding, but is actually a precise adjustment that works in conjunction with your emulsion tube, main jet and idle circuit. Get this wrong and the fuel will sit too far up the bowl and thus emulsion tube, meaning less air will have a chance to mix with the fuel in the tube and you'll be way rich. Same goes for idle circuit. Less emulsion air results also in poorer atomisation once the fuel hits the Venturi. So in short, run the float level that's designed for the carby and get around this flooding issue some other way.

[unfamilia 1,938]

Cheers. Just asking after my friend said was on his rx7 stock

[gerg 10,871]

Cheers. Just asking after my friend said was on his rx7 stock

If the Japs did something like that it would be engineered to death to ensure it wasn't dodgy in any way... They are totally anal about everything working right. I suspect that setup would be there to run a higher pressure fuel supply to prevent starving fuel from the thirsty rotary but to reg it back down at the carby so it runs as it should.

[unfamilia 1,938]

So floats set to 1mm travel let idle 20mins no leaks at 1psi fuel pressure. Took for drive. Leans out at just over 5k...got detonation :-( . but getting to 5k much better even with retarded timing.

15min drive idles at 1700rpm still and no leaks. Yay. But now worried floats too low lol.

[unfamilia 1,938]

Ok found my datsun has the option of return line in efi and twin carb models.

Ive got another line for a return now.

 

I also found in the spares from previous owner I have a return metal fuel pump to carb line. Wont work with bike carbs as line now runs against fire wall not front of car like stock.

 

so reading up about bypass setups, using a t peice fuel will run to carbs and return line (like the check valve set up on here but constant) its adviseable to put a one way check valve so fuel doesnt flow back into carbs.

 

does this sound like a viable option. Seems popular In jap cars to stop vapor lock

[gerg 10,871]

Dunno what pressure you'd be generating but it would be determined purely by the cracking pressure of your check valve and the back pressure of your return line. Not real accurate in my experience. Your bike carbies run on gravity feed so fuck all pressure really. I tried this setup on a red motor ski boat with an electric fuel pump running a tee off to the twin strommies and a return to the tank. Turned out just the flow capacity of the pump and restriction of the return line was enough to generate excessive pressure and overwhelm the float needles and flood the crap out of the carbies. Total failure! We just fitted a mechanical unit and never looked back.

[unfamilia 1,938]

Got a stock mechanical pump. It reads 4 psi at idle. Too much. Regulator set at 1psi currently.

Figure set at 2psi with return line same size as feed line, any fuel carb not using goes down return line. One way valve stops flowing back to t peice feeding off regulator into carbs.

 

I watched fuel bleed into carbs via gauge bleeding from 1psi to 0. Carb floats went from 55ml to 85ml. So needle and seat not holding back pressure of 1psi.

 

Gravity feed with a tank would need a petcock valve to prevent similar issues once stopped from my understsnding

[gerg 10,871]

Have you mounted your carbs the correct way yet? If not, I still reckon this is the main drama

[XFChris 126]

Gotta be careful with running return lines controlled by check valve. Check valve has to be a quality one, test it too with air if you can. I had issues when I tried that sort of set up, which I think came down to faulty check valve.

 

It works well for guys who run pumps like holley blue and red.

[unfamilia 1,938]

Im thinking I can almost run no check valve, its only for 1psi pressure. Just means can up pressure slightly for more flow now too

[gerg 10,871]

That's the same as the boat setup I mentioned... Electric pump, tee-offs to the carbies, return loop and no check valve. The sheer flow was enough to create excessive pressure and flood the strommies.

[unfamilia 1,938]

Upping from one to 2 psi should be ok ;-)