1983 F150 - 300 six - 1975 Carter YF carburetor - low altitude conditions
I've owned this truck for 5 years and it started having vapor lock-seeming issues within the past year. The 1975 YF carburetor went on 1.5 years ago.

It will start up and drive around in hot weather just fine. But if you park it for 10-15 minutes and come back, it will stall after you get 1/2 mile down the road and refuse to start.
It'll fire with starting fluid, and if you keep feeding it starting fluid it will pull enough fuel to run by itself again.
Same thing will happen if you sit in traffic or a drive-thru for too long - once you pull away it will run out of gas and stall.

Last time this happened I performed some diagnostics.
The float bowl was almost completely empty, which indicates the engine stalled because there was no fuel left in the bowl + the bowl was not being refilled.
By taking the fuel line off the carb and turning the engine over, I found there was virtually no fuel coming out of the fuel pump.
After starting the engine on starting fluid, the volume of gas being pumped into the bottle increased significantly and it was able to run by itself once I reconnected the fuel line to the carb.

The tank vent line is still connected to the carbon canister and is free to vent fumes. The canister isn't vented to the carburetor anymore - I have it venting to atmosphere (which should work just as well).
Most other major issues (bad fuel pump, clogged fuel lines, etc) can be ruled out by the fact the engine runs perfectly fine as long as the truck keeps moving.
Coolant temperature never exceeds 200, even when left to heat soak.

It seems like this is vapor lock, where gas flashes to vapor before the fuel pump.
Then the pump can't deliver liquid fuel until it turns over enough to purge the vapor from the line and pull fresh liquid from the tank.

People talk about retrofitting an electric fuel pump at the tank with a regulator, and others talk about making a return line and soldering a return fitting into the filler neck or sending unit.
But before I make big changes or band-aid fixes, I want to figure out if something changed since the truck didn't use to do this.
As a point of comparison, I had a 1984 van with the exact same setup (same carburetor even) and it never had problems with vapor lock.

Any ideas or other things to check out?
Anyone else in here successfully running the stock system (mechanical fuel pump + deadhead) without encountering vapor lock?

 

Although, vapor lock is most definitely an issue using modern gas in all carburetored vehicles. According to your description I’m willing to bet it’s not your main problem. I say this, because vapor lock doesn’t cause a fuel starvation problem while the engine is running. Vapor lock causes more prevalent issues during shut off, start up. Based on your description I believe you have a fuel pump, carb or fuel filter problem. In reference to vapor lock that will need to be addressed regardless. A wooden spacer between the carb and intake, combined with a vapor lock fuel filter that allows the fuel and fuel vapors be recirculated to the tank. Creates cooler fuel because it’s being circulated. The wooden spacer absorbs heat and acts like an insulator without transferring the heat from the intake to the carb. Allowing the fuel to be much cooler. Todays fuel boils at a much lower temperature compared to fuel from years past. A carburetor on any vehicle is comparable to a carburetor sitting on top of an oven ( your engine). Fuel today is designed for fuel injected vehicles. Fuel injected vehicles don’t have this boiling problem because fuel is circulated and the fuel doesn’t sit on top of the oven. And modern vehicles don’t have a fuel pump attached to the oven creating more heat to easily transfer to modern day fuel. A wooden spacer combined with a vapor lock filter will eliminate vapor lock. You will also pick up a few miles to the gallon and your truck will also run noticeably better. It will also eliminate stalling, hard starts. When ever you shut a carburetored vehicle off on a warm day. You have to hold the pedal to the floor for it to restart. Because a lot of fuel has boiled over into your intake when you shut the engine off. Flooding the intake.

 

I agree it does sound like vapor lock. I just had this same exact problem with my Bronco II, it has a carb. For some unknown reason Ford ran the fuel line up the driver's side frame on the inside. When they got to the engine they ran it around the top of the frame right where the exhaust y-pipe was running down.

First fix was to split and put a old heater hose around the fuel line right in the area where it ran around the frame. That helped it but did not totally cure it on the very hottest days. I then found about half-way back on the frame the factory had spliced the steel line with a piece of hose. I took this loose, pulled that whole front section of fuel line out, straightened it and ran it on the outside of the frame. I then ran a longer piece of hose at the original splice point that now runs up and over the frame back there where there is no exhaust. I then have one short piece of hose running from the outside of the frame straight to the fuel pump on the block. Problem solved so far this summer.

Look at your fuel line and it's relation to any heat source. Don't worry too much about the line going from the pump to the carb, worry more about the suction line of the pump.

P.S. If you are wondering why you did not have this problem much before, it's because of the fuel. Besides being outrageous in price, they keep changing the formulation and it vapor locks very easily.

 

My over-inflated opinion, FWIW:

Be careful troubleshooting "vapor lock", because that is something of a catchall term for generic heat-related issues. A fix that worked for one guy may not necessarily work for you if the root cause is different.

Heat-related problems fall under two broad categories. It's very common to have both issues present at the same time:

1) Legitimate "vapor lock", where the poor mechanical pump has trouble drawing liquid fuel all the way from the tank to the carb.

2) Fuel percolation, where fuel sitting in the carb gets hots and foamy, and can even overflow into the intake manifold.

I'll come back to those areas in a minute. Something else to keep in mind is that modern fuels just aren't optimized for ancient carburetors. I'm not saying modern fuel is bad, but is intended for modern fuel-injected vehicles. Whine all you want, but we have to live with it and make do. Some thoughts on fuel quality and octane:

https://www.ford-trucks.com/forums/1...g-and-you.html


Also, consider experimenting with different brands of fuel. (Per the link above, use the lowest octane) I've noticed a big difference in heat-resistance between brands. My truck is much better now after some modifications, but one of my Jeeps (unmodified '74 Chev V8) is still VERY sensitive to the brand of fuel. I'm very partial to Chevron, and it has worked best for me. On the other hand, I've had the most trouble with 76, even though it also considered a Top Tier fuel. Go figure, must be slight differences in the additive package. I'm not bashing any brand of fuel, just thinking some stuff is perhaps not optimized for ancient carbs. You could try switching brands and may notice an immediate improvement.

Back to "vapor lock". Remember modern fuels are not very heat resistant. The fuel pump, way at the front of the engine, is trying to suck fuel from several miles away at the back of the truck. When everything is warm, unfortunately it's fairly easy for this suction to turn the liquid fuel into vapor inside the long run of line. The pump keeps trying to pump away, unaware it's now moving useless vapor instead of liquid fuel. The carb quickly runs dry and forward motion ceases. You can mitigate this problem this issue by reducing the heat reaching the fuel line, by rerouting the lines away from the exhaust and adding insulation. That will help to some extent, but the best fix is to convert to an electric pump. This keeps the line pressurized and vapor never gets a chance to form.

My conversion to an electric fuel pump:

https://www.ford-trucks.com/forums/1...tallation.html


Back to fuel percolation:

Remember, this is a separate issue from actual "vapor lock" in the lines. The carb itself gets too hot. Warm fuel entering the hot float chamber overheats, and kinda boils over and gets foamy. The carb is designed for liquid fuel, not foam. Fine passages temporarily get clogged with foam, plus this foam overflows and floods the engine. From my experience, the engine quits as if you've turned off the key.

Everything reverts to normal after sitting for a while and cooling off. If the heat was just a little bit above the trouble point, you might get lucky and the engine restarts without much delay.

Another symptom of percolation is the starter cranking speed will be higher than normal when trying to restart. This is because the flooding washes oil from the cylinder walls. This reduces the resistance the starter must overcome, letting it spin faster. The spark plugs can get wet with too much fuel and won't fire, further reducing the load on the starter.

Yet another symptom of possible percolation: If you turned off the engine normally and started it up within a few minutes (such as stopping to knock over a liquor store), the engine will fire up just fine. But if you let it sit a while (maybe 20-30 minutes), the underhood temperature climbs and the fuel in the carb gets foamy. The engine is hard to start and the cranking speed seems higher than normal. You may see a small cloud of black smoke from the exhaust when the engine finally starts.

To reduce fuel percolation, you have to reduce heat reaching the carb. A phenolic spacer under the carb will help.

My truck (351W) had previously been giving me lots of grief due to hot fuel. This was more than just the nuisance of long cranking times for a hot restart. I've actually had the engine quit on me shortly after working hard, while just loafing along. My best guess was when using lots of fuel while the engine was working hard, the fuel stayed relatively cool quickly passing through the carb. But when I throttled back, and fuel flow dropped way off, the fuel had enough time to absorb heat and turn foamy.

After being stranded several times, it was time for drastic measures. I made several changes to reduce heat reaching the carb. I'm pretty sure the primary culprit was the exhaust crossover passage inside the intake manifold (not applicable to the straight 6), but I also made some other changes. Long-winded details of my travails here, post #6:

https://www.ford-trucks.com/forums/1...l#post16083555


With fuel percolation, a little change in driving technique may help. After a hot restart, I like to hold a fast idle (1500 RPMish) for about 30 seconds. This helps move some relatively cool fuel through the carb, and gets the hot stuff out of there before it acts up. This has put an end to the problem of stalling half a mile down the road after a hot start.

 

Thanks for the resources! A lot of useful takes and ideas here.

Carburetor spacers have been mentioned a few times. I like the idea as long as the stock air cleaner will still fit under the hood. I plan to measure that very soon and order one - i can't imagine it being the cure but it should delay or reduce the underhood heat soak if that's contributing to my issue.

It should be noted I don't seem to have much of the "percolation" issue. Lots of people describe an engine that is hard to start when hot, but mine only takes at most a second of extra cranking. It's not until I use up the contents of the float bowl that trouble comes.
The 300 six does have a heat riser between the manifolds which uses a bimetallic spring to only heat the intake when cool. I don't feel like this is suspect, mainly because the isn't any exhaust flowing when it is parked.
That said, after a stall the carb and other underhood surfaces are hot to the touch. I used ice once to cool the fuel pump, and then the post-pump fuel lines, and finally the carburetor itself, but none of those allowed it to start.
Further, I have taken the carburetor "air horn" off immediately after stalling. While the bowl barely had any fuel in it, it at least wasn't foamy.

Fortunately, the exhaust only runs on the passenger side of the truck, so there is no exhaust anywhere near the fuel lines until it connects to the carb. I do have the factory line running inside the driver's side frame rail. It shouldn't be influenced by heat other than what radiates from the hot asphalt and the transmission/transfer case. I wouldn't have expected those alone to get the line very hot, since it isn't exposed to sunlight.

There have been a few times where I pull away from a traffic jam or drive-thru and reach 35-40 before the engine stalls. By leaving it in 4th gear the engine keeps spinning as I slow down, and sometimes I have enough momentum for it to draw fuel again and start back up before I stop. Unfortunately, I'm more often waiting to turn out of a parking lot when the truck stalls, and then I'm screwed.
I wonder if it would be better or worse for me to idle the engine higher after a hot restart. On one hand it would empty the bowl faster, but it also would run the fuel pump and engine fan faster.

Haven't yet resorted to ethanol-free gas. I refuse to believe that everyone else driving carbureted cars is doing that (so why should I, right?). I get almost all my gas from Walmart, so changing brands is an option.

The electric fuel pump seems like the most effective solution to me, but I am hesitant on spending money on a pump and a decent ($100) regulator.
The return line would be cheap but I'd have to add a place to return fuel to. Also doable but I'm not sure how I'd approach doing the extra fitting.
I have to move in ~6 weeks and I'll need the truck to be up to the task, so I may end up adopting one of those solutions anyways.

This would be more fun to brainstorm on if I wasn't running out of time!

 

Almost forgot this important clue. The present carb was installed about a year and half ago. That would have been during the fall or winter, right? All seemed good for the first 6 months, but was that during cooler temperature? Sounds like the problem didn't appear until running this particular carb during warm weather.

Some model carbs are just more sensitive to heat. I've got to wonder if this particular carb is one of those critters. If from 1975, that's eons ago, and the poor carb was never really designed for modern fuel. Heck, after the dinosaurs met their demise and were turned into gasoline, they probably hadn't even cooled off yet, that's how long ago that was.

Do you still have the previous carb? Might be worth swapping it back to see if the fault remains.

 

From 75 to 83 is only 8 years. I'm thinking that there would have been very few differences in a single barrel carb that might affect vapor lock issues.

I understand that nation wide we'll be going to 15 percent ethanol. So more people will probably have more issues with carbed engines. It won't take long for the higher ethanol fuel to show up at your local filling station. I understand that the ethanol is added from a separate tank and system right at the point of delivery. Apparently blending is not done at the refinery as they find the ethanol is too corrosive and hard on the transport systems, pumps, valves, etc.
​​​​​​
If you suspect the fuel blend as your issue at least try a tank full of ethanol free. If that works for you then you know that it's a fuel issue. Then start blending in some E-15 and see where the break point is. Maybe only do the blend in the summer months? Ethanol free fuel when available is usually more expensive, which makes no sense to me.

 

It's mostly the same design as the original carb, but the original has extra emissions features added to it.
You are right about the variability but there's not much fundamentally different. Bowl is still vented just like a later carb, and I rebuilt and adjusted everything on it within the past month in case it was the issue.

I put it on January of 2021. It stalled and restarted while rolling a few times that summer, but I never got stranded until July in a long drive thru. I had a lot more problems this year, which got worse after I went through it and set the float, metering rod, etc to factory specs. Originally the float was too low, everything was just set the way it was when remanufactured by the supplier. Making these adjustments coincided with outside temps going up and the passage of time - I wouldn't expect the carb adjustments to affect this problem unless I had LOWERED the float.

I have 2 original spec carbs but they were swapped off because of throttle shaft problems and too much air bleed for my setup (ran lean, detonation, etc). I still might try to swap good parts into 1 carb and run it for at least a day or two.

I almost forgot! the 1975 carb is entirely reflective silver. The old ones are brown or green. I'm sure it absorbs much more heat that way.
In my head, other symptoms don't point to a problem at the carb, but I'll try swapping it anyways.

 

Yeah, who knows? It's free to try, other than your labor, which doesn't affect me.

Most likely the jetting is different between the two carbs, and perhaps the 1975 configuration is just more sensitive to heat.

You may have more than one active fault, too. Let's say there's been some undiagnosed issue all this time reducing the flow of fuel to the carb, although not killing it completely. Just thinking out loud, but maybe the 1983 carb was jetted lean enough that it never ran dry on this reduced supply. Swap in a 1975 carb, probably jetted a little richer, and now the engine can consume fuel faster than the substandard fuel delivery system (pump, lines, tank pickup, etc.) can supply it. Whatever's going on, heat makes it worse, either by increasing consumption or decreasing delivery.

That's my story, and I'm sticking to it. Squirrel!

 

If you want to try a electric pump, I would try one of these. I have had very good longevity and performance out of this type of pump. I just bought one for my diesel and it's still going strong. And it's cheap. It probably would cure your problems you are having. But it would be a little bit of a hassle to install. I would try to install it with rubber lines back at the top of the tank area. And then you will have to run a 12v line back there to power it. If your truck had that evap solenoid coming off the top of the old carb with that large rubber line and a red wire, you can use that wire to power the pump with a small fuse.

Take note in the instructions on that angle that you should mount this pump when bolting it the frame back there.

https://www.ebay.com/itm/15343704594...kAAOSwpwBcKsU5

 

Well, experiment completed. I put one of the original carbs back on and it vapor locked in the exact same way. At least that rules out any problem with the 1975 carb.

Franklin, did you install this electric fuel pump on a carb engine without a regulator or a return line? If all I need is the pump itself, then I'll do it.
Not too daunted by the wiring side of things. Would probably run it on a relay and fuse.

 

Watch out, that particular example looks to be a Chinese clone of a Facet brand pump. Buried deep in the description, it says this is "Carbole" brand. I guess it's better than Fling Dung or Wok Hoam, but I still wouldn't trust that one.

Genuine Facet pumps have an excellent reputation. They make several styles. The cube is what Dave suggested:

https://www.facet-purolator.com/

Determine which model you want and they are available from many vendors.

Are you looking to go full electric and remove the engine-driven pump? Or would you prefer to have a boost pump arrangement, plumbed in series to feed the stock pump? I've gone full electric on my truck, going back about 7 years with no issues. Before that, I ran an electric boost pump back by the tank for about 12 years. I didn't run the boost pump constantly, only when vapor lock in the lines might be an issue.

Some considerations for both methods:

1) If you go full electric, you must choose a pump with an output pressure similar to the stock engine-driven pump. On my 351W, that was 6-8 psi. I think all engines were similar, but don't quote me on that. If you can't find an electric pump in the desired range, you'd have to go a little higher and add a fuel pressure regulator.

2) If you install a boost pump, and don't plan to run it constantly, you must make sure fuel can still be drawn through even when not powered. For example, I think gear or vane style pumps act as a shut-off when not powered. A diaphragm style (similar to an engine-driven pump) uses a pair of check valves and should let fuel be drawn through at all times. I'm pretty sure the Facet cube is like that, but couldn't find any verification at their website. I had used a Carter P740222 and it worked fine in the boost pump configuration:

https://carterengineered.com/electric-fuel-pump-p74022

The pressure output for a boost pump is not critical, as long as it is below the output of the engine-driven pump. The electric pump just needs to maintain some positive pressure in the lines. The stock pump is what controls the fuel pressure. If the input to the mechanical pump is less than specs, it will increase pressure to the desired range. It can't reduce the pressure if too high, though, so the output of the boost pump must not be higher.

 

I have not had a running problem with my 81 F100 that has a 300. I am running stock carb & log intake but EFI EXH manifolds and stock pump on motor.

I have had this year when hot, have had a few days high 90's, that if I shut down and run into a store I would come out and smell gas and found it was flooded so hold throttle open 100% a little more cranking and it would start.

It would run ruff till it cleared out then be ok, no stalling when driving.
I also found this happened more so with the front of the truck lower than the rear but not really steep but could tell.
Other wise no issues starting or driving.

Before doing the electric pump try a different brand of gas for 2 tanks and see what happens.
I buy the same brand from 2 maybe 3 different stations 2 of them are 40 miles apart the 3rd is maybe 5 to 10 miles from the main one I get gas at.
Besides the pick up I also use it in my 02 Durango and my JD garden tractor..
Dave ----

 

Before you two replied I went on a whim and ordered that Chinese Ebay pump since It was only $19... Lol the quality might not be as good as a "genuine" pump but I figured it was worth a shot.
I would love to keep the stock mechanical pump if I could make it work better. Considering replacing mine before doing an electric swap since they are also $15-20. My current pump is 4 years old from a parts store.

I think I'll go ahead and replace the mechanical pump, then add the electric one if still needed. Also going to order a carb spacer, looks like they are also around $20...

Trying different gas is still a good idea. Unfortunately I just filled up before making this thread so it will be a while. The price of a tank of gas is more than the cost of converting to an electric fuel pump. Lol.

 

I would not add the electric pump to the mechanical pump. There is too much risk of the mechanical pump diaphragm tearing and then the electric pump will fill the engine crankcase with fuel.

This type of pump is self regulating, no external regulator needed and no return line needed.

I do not know if this is true, but it would not surprise me if all these pumps are made in China, and some get a Facet sticker and some don't get a sticker at all.

Little story; I just got a deal on a john deere zero turn with a diesel engine. It uses a little pump as a lift pump that is similar but it's round instead of square. The original fuel pump went bad, I did a bunch of searching and my first thought was to use one of the little square pumps like the one I used on my truck. The original pump from john deere that would plug right in the wiring was about $100. But on Amazon they had a knock-off version that had the plug in also for $25. I bought it, installed it mowed my yard twice and it quit on me dead. The Amazon pump and blown the fuse and crapped out all at the same time. I decided to call out Amazon on it, and they sent me a new pump in just a few days. So I am on my second one and it's working so far. If it goes out, I will see if I can get my money back and put one of those little square pumps on it, I will just have to splice the wires.