Fuel Bypass System...The Simple Way VS The Hard Way

By now most of us have had first hand experience using modern gasoline in our antique vehicles. We know from experience that the gasoline we buy today is not near the quality the gasoline was when out antique vehicles were new. If you have read my book "The Official Guide To Modern Gasoline And Oil for Modern Vehicles"... you know why that a quality gear driven electric fuel pump with the same working pressure as your mechanical fuel pump is almost a necessity.

This is our 92 series electric fuel pump taken apart so you can see how it is made...

A 30 micron fuel filter is included with every 92 series electric fuel pump

One other thing the modern fuel systems do is circulate the gasoline between the fuel tank and the fuel pump via a return line. This helps keep the gasoline cool, keeps the gasoline from turning to vapor and blocking the flow of fresh gasoline to the carburetor (in our case), and any extra volume of gasoline can be returned to the fuel tank.

Ok...fine you say, "but how am I supposed to make the fuel circulate in my antique vehicle fuel system...?" There are a couple of way to do it but at Fifth Avenue we believe "simple is good!" I have prepared cars for the Great Race for close to thirty years now, and the one thing I always find interesting is how a group of 100 antique vehicle owners can come up with so many complicated solutions to a simple problem. Building a recirculating fuel system is a good example.

                            You could do it the hard way like this...

This solution, while good in theory makes the job a lot more difficult that it has to be... and there are a few flaws in this design. In this example the by-pass portion of the fuel system that is closest to the underside of the car ...those 90 degree elbows will prove to be a restriction to the fuel flow as will the check valve installed in the fuel line, and we all know from experience, the more joints there are...the more places you can expect to have a leak. It is just Murphy's Law.

Or you could accomplish the same thing using this...

This is the simple solution. This is a special application fuel filter that works with our 92 series electric fuel pump. This special application fuel filter has 5/16"  inlet and outlets, so it will work in the same fuel line you are using now. It has an extra 1/4" outlet so you can run a 1/4" return fuel line back to the fuel tank to circulate the fuel. You want to install it on the output side of the electric fuel pump. It is a simple installation and do not have to add a bunch of plumbing to complete your mission.

I know what you are thinking...but this filter needs to go AFTER the electric fuel pump so the electric fuel pump can help push the fuel thru the system. If you put this filter before the electric fuel pump it will be difficult for the electric fuel pump to circulate the fuel. With the alcohol in the gasoline today you can not have too many fuel filters... Simple is Good!

This filter is part number 17415DOF and is available in the Parts section of the website.


ONE MORE THING...as Lt. Colombo used to say, this concerns check valves in fuel systems.

Example Of An In line Fuel Check Valve

You might be thinking about adding a fuel system check valve like this one in your fuel line to prevent vapor lock. While that might sound good in theory if you understand how a mechanical fuel pump works and how a carburetor works then you will know that is NOT the answer to your problem.

A mechanical fuel pump works (in simple terms) with two valves, one on the inlet and one on the outlet. The inlet valve opens to draw fuel in using the vacuum created from the diaphragm. Then the inlet valve closes and the outlet valve opens and the fuel is forced out the outlet side of the fuel pump to the carburetor. If one valve is open the other valve is closed.

What this means is...there will always be one valve closed in your mechanical fuel pump, so there is not much chance of fuel draining back to the tank thru the mechanical fuel pump. The same thing happens in the carburetor. The fuel travels into the inlet of the carburetor and the fuel bowl fills with fuel. Once the fuel bowl is full the needle and seat close off the incoming fuel supply thus preventing the fuel from draining back into the fuel line. It works that way (at least in theory) so there is always enough fuel in the carburetor to start your antique vehicle. So there is not much chance for the fuel to leak back towards the fuel tank from the carburetor either.

SO...Where does your fuel go and how come your car takes so long to start after it sits for a week in the garage...?

One of two things is going on. In most cases the fuel in the carburetor is simply evaporating. Modern fuel has a low boiling point to help with emission standards, which is fine for modern cars with high fuel pump pressures. It is not fine for our antique vehicles that have four pounds of fuel pump pressure or less.  So a simple explanation is the fuel simply evaporated out of the carburetor while your car was parked,  most likely out the air horn vent.

The second thing that happens is that the fuel turns to a vapor while in the fuel line. This is common if the fuel line runs next to an exhaust or if there is a lot of heat under the hood from exhaust headers (for example) which will help speed up the evaporation process. That under hood heat will also boil the fuel out of the bowl of the carburetor as well.

When the fuel turns to vapor in the fuel line it will expand and block the flow of fresh gasoline to the mechanical fuel pump. This is common after you have driven you antique vehicle for an hour or so then shut it off.  The heat soak from the exhaust manifolds and it being a 90 plus degree day will help the problem along and make it worse.

The fix of course is an electric fuel pump mounted back close to the tank so it can force the fuel to the front. Most always... the vapor lock will occur between the fuel tank and the mechanical fuel pump. The mechanical fuel pump simply cannot pump the fuel after it has turned into a vapor.

Besides an electric fuel pump and the dual outlet fuel filter shown above you also need to add a pint of diesel fuel to every ten gallons of gasoline. It will do two things. First it will raise the boiling point of the gasoline so it will not vaporize so easily, and the diesel fuel will lubricate the gaskets in the carburetor to keep them from shrinking. (the alcohol in modern gasoline will dry carburetor gaskets out causing them to shrink)

So now that you understand how your fuel system works and what is causing your vapor lock, you understand that adding a check valve into the fuel line going to the carburetor will NOT solve your vapor lock problem...and more important you also know why.