340 - 440 stroker (512) vapor locking

[A3404bbl]

I have a 74 challenger that I just swapped a 340 for a stroker 440. The fuel system is original with the exception of a small electric kicker fuel pump run in line by the tank. I am having issues with vapor lock. The car will idle and drive for about 30 minutes on a summer day before cutting off. My question is could I use the line that was run to the charcoal canister in the engine compartment labeled fuel tank, to run from a fuel filter with a vapor separator back to the gas tank?

 

[Chryco Psycho]

I assume you are using a mechanical fuel pump , have you checked or replaced the pushrod for the pump they can wear down & not fully cycle the pump .
I doubt the vent line will work properly to return as it goes into a mainfole with 4 vent lines & would likely fill the manifold before allowing fuel back into the tank , I would recommend replacing the fuel lines & sender with 3/8" & proper return line just based on the amount of fuel required by a 512" , 5/16 is too small already .

 

[Cuda Hunter]

Stock radiator?

 

[Cuda Hunter]

Electric pump by the tank, do you have a mechanical pump hooked up in the line?

 

[cuda joe]

I put in the 440 fuel lines bigger lines with a return you also need a 440 fuel pick up; fuel pump ;vapor seperator .. did away with the electric pump by the tank so much better

 

[MoparCarGuy]

Lots of good info so far in this thread. I also run a stroker 440 (500CID). I think the telling item you mentioned is that it runs find for 30 minutes then you have a problem. I am suspecting you could have debris in your fuel tank that is collecting on your fuel sending unit "sock" and restricting flow. The debris collects on the sock and then falls back off when you turn off the engine.

You will need to check a few things to figure out if this is your issue. Pulling the fuel tank sending unit is no fun so start by visually inspecting the entire fuel line including any rubber hoses from the tank sending unit connection up to the carburetor. You want to rule out collapsed rubber lines, possible fuel line damage (crimping), an unknown fuel filter that is clogged, etc.
If all looks okay, consider removing the fuel line at the carb and putting it into a good-sized bucket. Turn on your electric fuel pump and see what happens for fuel flow into the bucket.

If you do not get good flow, you have a restriction somewhere from the tank to the carb OR a problem with the electric pump and/or mechanical pump. You may want to bypass the mechanical pump altogether and see if your problem goes away. The electric pump should be pressurizing your entire fuel line and that makes fuel percolation (vapor-lock) less of a problem. The mechanical pump, as noted by @Chryco Psycho, is not really doing much for you and may be contributing to a possible vapor-lock situation. Plus, it is a safety issue using both pumps. I do not recommend running an electric pump and a mechanical pump together as a failed mechanical pump diaphragm can lead to the electric pump filling your crankcase with gasoline. Very BAD. If you do abandon the mechanical pump, remove it and the fuel pump pushrod and install a block-off plate.
Good flow, then it tapers off? You have a restriction somewhere and it could be that fuel tank sending unit sock.
Good flow and it appears to stay good? Install a fuel pressure gauge to see what you get near the carb. 5 to 7 psi is all a carb needs but pressure does not prove flow capacity. The potential for vapor lock is still there even with an electric fuel pump.
And, last but not least, you may have to pull that fuel tank sending unit out to check the sock and look into the tank for debris. An upgrade of your entire fuel system to 3/8" is highly recommended. The factory 5/16" fuel tank sending unit and fuel line is marginal for the engine's needs. Hard tubing is measured as Outside Diameter (O.D.) so you have <5/16" internal diameter (I.D.) now. Fuel line hose is measured in I.D. so 5/16" hose is really 5/16". For your stroker motor, 3/8" fuel line tubing is minimum. A good option is to repurpose your existing 5/16" fuel line as a return line and purchase a new 3/8" fuel line for your supply line. This will keep fuel flowing which is easier on your electric pump and can prevent vapor lock since the fuel does not remain in the line to "boil".

Let us know what you find and good luck troubleshooting.

 

[73Dodge]

I had issues with vapor lock on my 440 stroker as well. Solution for me (and I mean totally solved): Upgraded fuel lines to 3/8 pickup, 3/8 line, 5/16 vent line and 5/16 return (from Vans and Inline tube). New Carter M6903 Fuel pump and pushrod (the electric pusher isn't needed and can actually create restriction, so ditch it), 426/440 style fuel filter/vapor separator and DEI fuel line insulation near the headers up front and exhaust in the rear. The return style system eliminates 99% of vapor lock issues, even in the extreme heat we've been having this summer.

 

[Drivun]

I run a 3/8 line from the sender thru mechanical pump to carb then return style regulator and back to tank sending unit via original emissions return fitting. Two 3000 mile longhauls on Power Tour with +90 days and long lines without issue.

 

[A3404bbl]

Thanks for all the great info! I put in a new 3/8 sending unit with 1/4 return, wrapped the fuel lines with DEI insulation, and am still running both the mechanical fuel pump and the electric kicker pump by the tank that was installed by the shop that put the motor in for me. Drove around this past weekend for about an hour with no vapor lock issues. The only new issue to come up after hooking everything up is the fuel pressure gauge at the carb now constantly jumps around, where with the old system it had a constant 7lbs.

 

[Drivun]

In my case with a magnum small block stroker and a mechanical Carter pump, the gauge bounced between 5 and 7 very quickly but consistently…up down up down. Was told it was a function of the lever action of the pump but the values were healthy so no worries.

 

[MoparCarGuy]

The 5 to 7 psi jumping around is a result of the electric pump force-feeding the mechanical. EDIT: Many people use the factory mechanical pump to regulate an electric pump and the mechanical pulses the pressure gauge needle.
T̶h̶e̶ ̶m̶e̶c̶h̶a̶n̶i̶c̶a̶l̶ ̶p̶u̶m̶p̶ ̶i̶s̶ ̶a̶c̶t̶i̶n̶g̶ ̶a̶s̶ ̶a̶ ̶r̶e̶s̶t̶r̶i̶c̶t̶i̶o̶n̶ ̶t̶o̶ ̶t̶h̶e̶ ̶e̶l̶e̶c̶t̶r̶i̶c̶ ̶p̶u̶m̶p̶ ̶j̶u̶s̶t̶ ̶a̶s̶ ̶@̶D̶r̶i̶v̶u̶n̶ ̶n̶o̶t̶e̶d̶.̶ The issue with this setup is a failed mechanical pump diaphragm will allow fuel to enter the crankcase.

 

[Drivun]

For clarity, please note my set up is a mechanical pump only so the fluctuations dont have anything to do with an electrical pump being used with a mechanical pump.

 

[MoparCarGuy]

For clarity, please note my set up is a mechanical pump only so the fluctuations dont have anything to do with an electrical pump being used with a mechanical pump.

The info is for A3404bbl's fuel system. He is using an electric fuel pump feeding a mechanical fuel pump and the needle on his gauge is bouncing. You said your setup (without an electric) has the fuel pressure gauge needle bouncing as well. Needle bouncing can be helped with a liquid-filled gauge but it is not really necessary. The bouncing is usually seen at idle (lower RPMs). The bouncing needle should stabilize a bit more at higher RPMs as the cam driven eccentric spins faster.

 

[Drivun]

The mechanical pump is acting as a restriction to the electric pump “just as @Drivun noted.”

For clarity I never said the mechanical fuel pump was restricting the electric pump as you attributed to me in the quote above. I never mentioned an electric pump.

 

[MoparCarGuy]

For clarity I never said the mechanical fuel pump was restricting the electric pump as you attributed to me in the quote above. I never mentioned an electric pump.

I will edit out the sentence. That sentence was a bad attempt at giving you credit for pointing out mechanical pumps "pulse" and can cause a bouncing pressure gauge needle. Instead it implied you knew the mechanical pump was a restriction for the electric pump.