You guys are always so good about suggesting fixes that I never would of considered on my own. A couple of you mentioned boiling fuel in the lines….. I didn't know that is a thing… I went to Oreilly's yesterday and picked up some fuel line heat shield and wrapped the fuel line between the pump and carb and the fuel line running along the frame closest to the headers. Today I drove the truck quite a bit and the hesitation issue never showed back up. I hope the problem is solved… Although when I got in tonight from driving the battery was fizzing and making a funky smell. Overcharging? I just replaced the alternator a couple weeks ago… Fix one thing and another problem shows up.. I love old cars and trucks.
Howdy, new guy. Notice that you have resurrected an 8-year-old thread. I'm sure the tree, the vehicle, and the owner are all fine by now. Welcome to the madhouse. LOL
I have a plan to avoid vapor lock on my truck, by running a return line. I wonder if anyone else has done it with success? I have a new fuel tank, 5/16" nicop lines. I want to add a brass T right before the carburetor, from the T run the line back to the tank. Using the nicop & brass T it might look period correct. As long as the fuel is flowing, should not sit long enough to overheat & turn to vapor? I will need to add a restriction in the return line, so not run the carb out of fuel. Anyone else besides me thinks this would be a good idea? @Bigstar Sounds like you found your problem. .... Is common with old cars & modern fuels. Your fix works in January. Wonder if it will work in July? You can add a electric fuel pump to your system. Just use it when you notice the symptoms. Shut it off when you are cruising & no longer need it. Many people use the electric assist pump. My idea above is to simply try to avoid the fuel from getting hot in the first place.
You'll need to put a restriction in the return line sized appropriately as to maintain adequate fuel pressure& volume at the carb .
On OT vehicles, the restriction is the return line (from the two I have) and it’s smaller diameter that the feed line. But no clue how to do it with a carburetor, what do you use?
I think Tony explains the issue pretty well here, & a simple fix .... but his fix includes rubber fuel lines which I can not have near my exhaust. So I think using the nicop line & a brass fitting will be ok & serve the same purpose. And just look cool while serving a purpose. The restriction can be a smaller line .... I'm thinking to add a blank metal spacer inside the fitting & drill a hole through it. .... Tony claims in the video they may be .060" - .0100" This video does explain it pretty well though. Just not a cool fix for a Hot Rod ... maybe ok for a shop truck.
You can do it if your problem isn't chronic. Just be aware that it's a bandaid and not a free lunch. The thinking behind the "return line fix" is that flowing more fuel from the tank reserve past the hot exhaust pipes and hotter manifold will mean that less heat in the mix will be carried to the carb. This is true if you're just looking at a snapshot of the carb and tank return junction. But what's happening is that the same amount of heat is picked up although diluted because some part of that heat is going back into the tank which gradually raises the reserve temp. Then the slightly warmed fuel is cycled through and picks up a little more temp until eventually, you have well-warmed-up fuel cycling past those same hot exhaust pipes and manifold and it's possible to reach that temp tipping point where the fuel percolates into bubbles or pockets of vapor. Of course, every vehicle is unique and different as is every trip, every day's ambient air temp, bumper-to-bumper gridlock situations, and even possibly your engine tune running a bit "hot". I'm not saying that a fuel return is a negative and thus will automatically add to the cause of vapor lock. I'm just saying to be aware of how things work. Think of the fuel delivery as a system and also give some thought to rerouting the fuel lines away from hot exhaust and heat shields or insulation, as has been mentioned. Weigh the impact of all the possible causes in your fuel delivery and address as many as you find issues with. Good luck and Happy Highways, my friend.
I think what you say carries a lot of merit. A constant flow of fuel may eventually bring all the fuel to 1 temperature. I think that would depend on if you have a 1/4 tank of gas or a full tank .... & the size of your tank. I have never driven my truck on the road, only in the yard. I have never had the issue .... others with the flathead 6 has. They have installed a electric fuel pump, then only turn the pump on when the problem happens. This will cure the problem (until next time) <--- imho that sounds like a bandaid. I have never built or restored a car, I'm a retired carpenter playing with a truck. Repairing it to use as a daily driver. Just thinking it would be really easy to create a return line at this stage, or even easier to forget it & just install a electric pump. I'm picking up a vibe that nobody else has bothered to install a return line. Soooo, maybe I should just throw in a electric pump for emergency use. Also suggest @Bigstar add the electric pump as well. Same time their fuel line is routed out & over the exhaust. Then the fuel pump absorbs heat from the motor. I wonder if the motor is running 165F or a nice cool 210F? Sure seems like a electric pump is the cure all to old cars with carburetors.
Some OEMs in the 60s and 70s had fuel filters with a 3rd fitting for a return line. The earliest one I saw was on a turbo-charged Corvair. Plenty of heat with the turbo in the engine compartment. Emissions era V8 with AC etc get plenty hot too. https://www.chicagomusclecarparts.com/products/511191 I do not know if those filters need a restriction in the return line.
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