Fuel bypass on carburettor engines

[Julian West]

Just out of interest, does anyone run a fuel bypass on a carburettor engine?

Google reveals benefits to running a regulator/bypass plumbed in after the carb. Fuel cooling and preventing vapour lock seem to be the main benefits. I'm wondering if it may be a little easier on the fuel pumps.

Does anyone have any first hand experience in this matter?

Any opinions gratefully received!

Kind regards
Julian

 

[DaveM]

I went to a bypass exactly as your diagram after losing 3 fuel pumps in 10 years. Since then (4 years) have been fine. I would agree with your vapor lock prevention benefit too.

 

[Paul Hendrickx]

Hello,
have a look on this YouTube channel, simple well explained
Paul

 

[Paul Hendrickx]

 

[Cliff Beer]

Yes, a return to the tank works very well. As indicated, helps a lot with vapor lock as the incoming fuel tends to be cooled by drawing from the tank rather than sitting in hot fuel lines in the engine bay.

If you're using a Holley or Weber or other carburetor configuration make sure you have a good low-pressure regulator as each likes a different psi and with some 1psi can make a difference.

 

[Julian West]

Thank you all for the replies, looks like a positive modification to the stock GTD fuel system.

May I ask, those of you that have fitted the bypass, do you also run a twin tank/twin pump setup? If the bypass regulator is added after the carb in a twin tank setup, presumably a motorised valve would be needed to direct the bypassed fuel back to whichever tank is in use at the time.

I like to keep things as simple as possible & as such I assume the system could be simplified by running the bypass regulator before the carb as per the enclosed diagram X2 ?

IE two pumps/filters, two regulators, etc. Each regulator would feed into a non return valve & then into a tee, feeding the carb?

Does anyone have any opinions or thoughts on either system?

 

[Ian Anderson]

A pollack valve works https://www.oilybits.com/pollak-6-port-fuel-selector-valve-with-switch.html
so you can select one tank and the return goes to the same tank.
Put it before the 100 micron filter and you only need one pump and filter set.

 

[JP Verweij]

I run twin tank, twin Holley red, twin one way check valve into twin Malpassie filterking regulators. From there T into one fuel line to the Holley 4 barrel
From the Holley a return line with a 0.5mm restrictor into a T back to both fuel tanks.

With this I can swap easely to run Quad Webers in the future also.

 

[Julian West]

A pollack valve works https://www.oilybits.com/pollak-6-port-fuel-selector-valve-with-switch.html
so you can select one tank and the return goes to the same tank.
Put it before the 100 micron filter and you only need one pump and filter set.

Thanks Ian,

Very interesting, I've found the attached diagram which incorporates the 6 port valve & one pump/regulator & switched return. Looks like it would work well but I'm concerned that valve seems to have a plastic/nylon body & pipe connections.

 

[Ian Anderson]

Correct re the plastic body but they are on a lot of cars and seem to work ok.

I have seen them plumbed in on a Landrover running bio oil (deep fat fryer used oil) and standard diesel.he ran 2 pumps between the tank and the pollack so the whole thing was at extremely high pressure and had no ill effects on the unit.

I have on on my car similar to the diagram you just posted. Works well. That said I did replace the first one after about 6 years as one of the seals on the return side went and it always returned fuel to one side, even at that it was still drivable as I used that tank first!

The thing I like is, unlike a solenoid, where it has to be powered to be open this is only powered for perhaps a second as the valve moves across and is then inert until you want to turn it back the other way.

They only do one size so if you are running a monster, thirsty motor, make sure that this is not going to be a restriction on your fuel flow to the engine.

Ian

 

[Terry Oxandale]

"...Fuel cooling and vapor lock". That is an arguable result, especially with mid/rear engined cars. I don't have carbs (EFI), but have very much dealt with the recirculation issue of fuel temperature and vapor lock. If you plan on running the car for no more than 20-30 minutes, than the short-term gain in fuel cooling is an acceptable argument. Beyond that amount of time ( in my particular case) I've seen (measured) the temperature, in the main tank, increase to about 170F after an hour of driving. The hot engine bay simply heats these fuel lines going and coming such that at some point in time you're going to end up with no gain as the cooler fuel in the tank is warmed by the recirculated fuel. The solution to my Summer vapor lock was using one of the side scoops of my car to direct cool outside air through a fuel cooler (similar to a small oil cooler), just prior to the fuel rails. Even with vast amounts of thermal shielding and insulated fuel lines, nothing worked well until I did that.

 

[Terry Oxandale]

Lastly (5 minute edit limit caught my second part), with the surge tank now somewhat isolated from the convective and radiative heat from the headers, I may dead-head the surge tank rather than constantly be circulating warmer fuel back into the main tanks on long runs.

 

[Julian West]

Lastly (5 minute edit limit caught my second part), with the surge tank now somewhat isolated from the convective and radiative heat from the headers, I may dead-head the surge tank rather than constantly be circulating warmer fuel back into the main tanks on long runs.

Thanks for making me aware Terry. I'm not going to fit a surge tank so I don't think it will be an issue.

 

[Frank Catt]

Terry, I always return hot fuel back to the front end of main tanks, this way it is delivered again at main tank temperature. Fuel pressure dump valve also same way, Frank

 

[Terry Oxandale]

Good point Frank. I have two 10-gallon tanks, and the passenger side one is the primary tank I draw off off for operations. at some point in a long ride on a hot day, that fuel is eventually going to be recirculated completely through the tank, with warmer fuel going in, replacing the cooler fuel coming out. But yeah, what you're describing would delay the inevitable. I also put a NACA duct under the tank to draw cooler air from under the car into the tank housing. There is very limited room in these cars for all the ancillaries, and also trying to balance the car. And off course, I have the water pump in the adjacent compartment next to the tank, so that contributes some amount of heat as well. Great car to drive around in 50-60º F weather, but not when warmer.

 

[Howard Jones]

Here's another track-type thing. I have found that keeping the fuel cans out of the sun and in the breeze make quite a bit of difference (vapor lock). Inside the trailer is best because you don't get hot pavement heat soak either. My SLC uses about 6 gals each 30 min session so I add a can of fuel just before I get into the car. This way the fuel in the tank at least starts out at "in the shade" ambient temp. This can be up to 15 degrees lower than in the full sun.

Leaving the rear clam open between sessions also helps a lot to cool the engine room.

 

[Neil]

Covering your fuel jugs with a wet towel can help keep it cool by evaporation.

 

[Sean S.]

A pollack valve works https://www.oilybits.com/pollak-6-port-fuel-selector-valve-with-switch.html
so you can select one tank and the return goes to the same tank.
Put it before the 100 micron filter and you only need one pump and filter set.

Just curious if anyone has found a similar valve set up that uses npt or other threaded fitting styles vs the slip on barb type fittings. I plan to use a relief style pressure regulator and I would prefer to keep all the fuel lines threaded AN fittings vs slip on.

Thanks,

Sean