Fuel line insulation to remedy vapor lock?

[The motor guy]

Originally Posted by DoubleWasp
If your intake manifold has an exhaust crossover, that is the first thing that needs to go. Easy way is to use one of the FelPro intake manifold gasket kits that includes stainless restrictors and block-offs for the crossover. I've watched water boil off of the intake manifold where the crossover comes through.

Phenolic carb spacer or the "thick" carb gasket (if Quadrajet) is the next thing to do.

Your carb engine almost certainly does not have a fuel cooler, so I wouldn't worry about that.

Unless you have some serious restriction in your fuel lines, I wouldn't worry about that or insulation. Your engine doesn't have bare exhaust manifolds glowing with exhaust heat. You also don't have an engine bay baking with 200° heat after shutdown like a car.





I think it does have the crossover. There are two spots on each side if the manifold where the paint is flaked off from the heat. It's really rusted in that spot too. It does have a thick black gasket at the bottom

 

[Traction]

A common problem now when if you use ethanol fuel on a carb engine. I NEVER do.

 

[DoubleWasp]

True enough, but we have so many guys using ethanol and carb without this issue that I can't really say that's a given in this situation.

The "cool fuel" vapor lock problem mostly came about from Mercruiser using a single stage high pressure fuel pump for fuel injection duties (nobody does this).

The pump has to suck fuel all the way from the fuel tank into the engine mounted fuel water separator, and then across the engine to the fuel pump module mounted on the other side. In the middle of all of this, there is no vapor separator tank (nobody does this either) and fuel is being constantly recirculated onto itself. The module itself is also enclosed with no water or fuel jacketing. The pump itself has a thermal bond to the fuel cooler body, but runs seriously hot, being a single stage high pressure pump.

Once the brass/copper cooler takes on a good bit of corrosion layer in the raw water flow, the battle between the heater (pump) and the cooler (fuel cooler) becomes a losing battle for the cooler.

That's when the engine is running. On shutdown, the raw water flow path becomes convection heated by the engine oil cooler (if applicable) power steering cooler, engine, or heat exchanger, and also the fuel pump.

Vapor lock is pretty much unheard of on the carb systems. Hence my greater suspicion that he in fact has fuel boiling or vaporizing out of his carb.

Vapor lock can still happen, but that would take a ridiculous amount of restriction in his lines.

 

[DoubleWasp]

Originally Posted by motor_oil_madman
Originally Posted by DoubleWasp
If your intake manifold has an exhaust crossover, that is the first thing that needs to go. Easy way is to use one of the FelPro intake manifold gasket kits that includes stainless restrictors and block-offs for the crossover. I've watched water boil off of the intake manifold where the crossover comes through.

Phenolic carb spacer or the "thick" carb gasket (if Quadrajet) is the next thing to do.

Your carb engine almost certainly does not have a fuel cooler, so I wouldn't worry about that.

Unless you have some serious restriction in your fuel lines, I wouldn't worry about that or insulation. Your engine doesn't have bare exhaust manifolds glowing with exhaust heat. You also don't have an engine bay baking with 200° heat after shutdown like a car.





I think it does have the crossover. There are two spots on each side if the manifold where the paint is flaked off from the heat. It's really rusted in that spot too. It does have a thick black gasket at the bottom


Easy test: Give a push on your accelerator pump on the carb once the engine is in a state where it would usually fail to start. See if you get good, strong shots of fuel from your pump Jets, or if it looks piddly or you get nothing at all.

If you don't know what I mean, post a pic of your carb with the flame arrestor removed and I'll circle the accelerator pump arm and the pump Jets.

If they're not spraying correctly under the typical failure situation, then we know your carb is evacuating fuel. If they shoot good and strong and the engine still fails a hot restart, then we know it's something else.

You don't have to buy one part or spend more than 2 minutes on this test.

 

[The motor guy]

It starts up great when cold even after sitting a couple weeks. Just as long as I pump the throttle a couple times and set the throttle so it runs around 1500rpm when warming up. That's what the instructions Say. I can cut it off and start.it right back up and you would think it would crank back up instantly with no additional throttle, but nope. I have the pull the throttle lever out so I can give it more fuel without it being in gear to get it to restart. This is a pain cause it's kind of hard to pull the lever out.

 

[WyrTwister]

Originally Posted by motor_oil_madman
It is on a boat with a 350 and Holley carb. A buddy of mine showed me how the fuel lines go along the block on each side of the front of the carb and are very close to the engine. I'll try the split fuel line trick the other poster mentioned to put over the steel lines.


After you apply insulation , you might apply the silvery aluminum tape to the insulation . The type HVAC guys use to seal duct work . To try to reflect heat .

 

[funflyer]

Originally Posted by BrocLuno
Boiling fuel out of the carb is not vapor lock. That is heat soaking and boiling fuel ...

Vapor lock is vaporizing the fuel in the supply line from the tank. 1940's and 50's Chevy's were famous for this. Pulling a hill on a hot day the exhaust heat would warm the supply line until the partial vacuum between the tank (rear) and pump (front, engine) would allow the fuel to change state to vapor. Then no fuel to the engine. Pull over and let it cool down and hope you had enough battery to reprime the pump when it was cool.

Best fix, install electric fuel pump to push the fuel, instead of sucking it. Shade tree fix, insulate the line. Hot rod fix, dual exhaust moved away from fuel line

Heat soak, allowing the fuel to boil, or simply evaporate, is technically "vapor lock". No question about it, this is what's causing the OP's hard start issue. Here's a good article on the subject. Read the paragraph "Vapor Lock Volatility". Last line says it all, "The answer, particularly in a carbureted engine, is to minimize the exposure of fuel system components to heat in every way possible."


https://www.enginebuildermag.com/2013/11/impact-of-todays-fuels-n-carbureted-engines/

 

[user52165]

Second on the recommendation of an electric fuel pump............. always worked for me.

 

[The motor guy]

Originally Posted by user52165
Second on the recommendation of an electric fuel pump............. always worked for me.


How does that cure the vapor lock issue?

 

[DoubleWasp]

An electric fuel pump mounted close to the tank helps in a few ways. First, it changes the fuel path after itself from a vacuum to pressure. Pressurized fuel is harder to vaporize than fuel under a vacuum. Secondly, an electric fuel pump can prime the fuel system and completely fill the fuel lines and carb bowls with a solid column of fuel. Engine will start as normal.

Carburetors are naturally vapor separator tanks. Vapor separator tanks have a float and needle just like a carb. Keep the fuel flying into a carb, and it can separate a reasonable amount of vapor. Third, once an electric fuel pump is on, it's running full open. If vapor forms, it's going to be pushed out and replaced with fuel really quickly. The vapor will simply leave out of the carb vents, and fuel will keep filling the bowl at full pump rate until the needle closes. Since only liquid can cause the float to close it, the carb stays fed.

Fun fact: The use of vapor separator tanks for some time allowed for ethanol fuel to cause many fuel injected engines to have a no-start due to a stuck float or needle.

Buuuuuuut, it's important to know if that is your issue.

 

[The motor guy]

This might explain why the slight uneven idle after 20 minutes of running.

 

[emmett442]

You mention that:

It's hard to start when hot
It idles unevenly when hot
You have to open the throttle to get it to start when hot.

You could test your theory by giving it a shot of fuel or starting fluid down the carb when you think it's boiled out.

Without knowing your entire setup, I'd say it's possible you have a problem opposite what you think. You aren't fuel starved when you start hot: you're too rich. That's why opening the throttle blades works.

Is the choke opening fully or staying partially closed after warmed up? Any fuel dripping out of the boosters or accel pump nozzle after shutdown?

 

[The motor guy]

I guess I can run it on the hose and then take off the flame arrestor to see what's going on.

 

[mk378]

When it is hot, if you open the throttle before trying to start, does it start right away? If the gas had indeed left the carb, you would have to crank a long time to bring more up no matter what you did with the throttle.

Also when the gas boils out of the carb on an old car, the vapors go out the bottom of the engine compartment and drift away to quietly become smog. On a boat, the results can be more dramatic.

"Sometimes difficult to start" and "idles a little rough" are de rigeur for carb systems.

 

[The motor guy]

Yeah it usually cranks like 5 seconds or so before starting with throttle. If you dont give it throttle it will start but and run at a really low idle barely running for maybe 3 seconds then die.

 

[DoubleWasp]

That's why it's always important to not only use the blower, but to make sure the blower actually has a hose going down into the bottom of the bilge.

One would be amazed at how many have no hose attached at all, or the hose is feet above the bilge.

 

[The motor guy]

I open the engine box before starting to make sure their aren't any gas fumes or anything. Then again once I'm out in the bay at anchor I'll open it to check the bilge for water. Yes my boat does have a blower going down to the bilge.