Update on Vapor Lock and Changes made with Gas Cap/Sig difference in the 2 caps

[Aerocraft]

HOWEVER,, by not allowing fuel to run out of the tank, it can cause a ?Stall?..maybe not the vapor lock but problems?[/QUOTE]

The slight vacuum in the tank when it is not vented will prevent the gas from draining to the carb and the fuel mixture will become very lean, acting quite similar to what happens when the fuel boils in the carb. Gar Williams

[Mike V. Florida]

Originally Posted by James Rogers

An unvented cap/tank won't cause "vaporlock" (wives tale). What happens with an unvented tank/cap is, when you remove fuel/liquid from a container it must be replaced by an equil amount of something else such as another liquid or solid or gas (air).

I never said it was an unvented tank/cap!!

I said "I want to take a little closer look at why a bad vent could cause vaporlock. ... If you have a bad vent in the cap (missing or clogged) and if the gasket is old and not sealing properly".

[Special Coupe Frank]

The end result is the same - fuel stops reaching the carb.

A blocked / restricted tank vent could also contribute to vapor lock (fuel vaporizing in the lines) : as pressure on a system is reduced, the boiling-point of any liquid in that system is also reduced. If you reduce the pressure on a system below regular pressure of 14 psi / sea-level, it will boil (assume a vapor state) at a lower temperature. ( The temperature of boiling water is less in Denver CO, than it is at Atlantic City.)

So, if you have a partially blocked tank vent, you may get by with slow driving on level ground, on a cool day... but when the ambient air-temp gets up around 90 * F or more, and you're running at higher speeds, that partial vacuum forming on the gas-tank could combine with the high heat to create vapor-lock...

I've had various experiences with blocked tank vents on fuel pump vehicles and gravity-fed farm equipment...

I do believe that true vapor lock should be pretty unlikely in a gravity-fed vehicle like a Model A ...

I have not experienced heat-related vapor-lock in terms of the vehicle dying from fuel starvation while running at speed.

I have indeed experienced a no-start scenario when returning to a vehicle parked "hot", due to fuel in the carb boiling and flooding through the jets; this is known as "percolation", and happens (usually) only with down-draft carbs.

For what it's worth, since getting my A back on the road last Monday, I've logged about 500 miles, all during hot weather (high 80's / low 90's), about 50/50 stop'n'go vs fast running. I'm running 87 octane (cheap gas - Valero ), which "may contain up to 10% ethanol".

Temp at the top of the radiator stays between 170 and 180 * F.

No issues with fuel starvation.

[pat in Santa Cruz]

I think your problem was the cap. Watch out for repro gas caps too. There is a hole in the center that has to align with a hole in the inner cap. The center can rotate and shut off the vent hole. The one you have pictured is the style that can do that. Snyders makes a good one (so they say) that is a copy of the Eaton. I prefer an original. A show quality one is pricey, but a driver quality one can be found at a reasonable price.

Some of the gaskets today can also block the vents in an original cap. There should be an airspace between the outer edge of the gasket and the inner edge of the cap. The most recent gasket I got from a quality vendor was too large and the car ran well at 35mph, but failed within a mile at 55. When I unscrewed the cap, there was the tell tale sucking sound from the vacuum in the tank. I had to trim the edge off by about 1/8th inch around the circumference.

I drove my sedan up the Columbia River in a heat wave with a stock carburetor when it was so hot the fuel was boiling in the bowl when I stopped. It made it hard to start, but once started and cool fuel ran down the line, there was no trouble. The bubbles in a gravity system with a straight line to a vented tank do not get trapped and cause a blockage at least with an indented firewall type fuel shut off. It could be the vapor lock is more of a problem with the firewall mounted fuel sediment bowl.

[James Rogers]

Originally Posted by Mike V. Florida

I never said it was an unvented tank/cap!!

I said "I want to take a little closer look at why a bad vent could cause vaporlock. ... If you have a bad vent in the cap (missing or clogged) and if the gasket is old and not sealing properly".

The only place the tank gets vented is in the cap. I still stand by my post that a bad vent no matter where it is won't cause vaporlock. 2 different things vaporlock and a bad vent.

[msmaron]

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Originally Posted by James Rogers

The only place the tank gets vented is in the cap. I still stand by my post that a bad vent no matter where it is won't cause vaporlock. 2 different things vaporlock and a bad vent.

OK lets remove the word Vapor Lock.., dont you agree that with a bad or NON vented cap you can have the car shut down because the gas wont flow in gravity system?
mark

[James Rogers]

Originally Posted by msmaron

OK lets remove the word Vapor Lock.., dont you agree that with a bad or NON vented cap you can have the car shut down because the gas wont flow in gravity system?
mark

Exactly the point I was trying to make. I do believe, your problem is because of dirty fuel system.

[Tom Wesenberg]

Vapor lock vs. vaccum lock!

I too suspect junk plugging the fuel line or jets. The first thing I would do is clean the carb and sediment bowl, remove the fuel line and blow blow through it, remove the shutoff valve and install a fine screen filter. Make sure the fuel line going into the carb doesn't extend more than 1/8" beyond the brass ring.

[Bob A.]

Originally Posted by Tom Wesenberg

Vapor lock vs. vaccum lock!

I too suspect junk plugging the fuel line or jets. The first thing I would do is clean the carb and sediment bowl, remove the fuel line and blow blow through it, remove the shutoff valve and install a fine screen filter. Make sure the fuel line going into the carb doesn't extend more than 1/8" beyond the brass ring.

Tom , Maybe the addition of the "dog leg trap" on the fuel tank line that some one posted awhile back to catch the trash in the tank. I think that would help.
Plumbers call it a "Bull" on a natural gas line before it is hooked up to a appliance to catch the debris before entering the control valve.

[Tom Wesenberg]

Originally Posted by Bob A.

Tom , Maybe the addition of the "dog leg trap" on the fuel tank line that some one posted awhile back to catch the trash in the tank. I think that would help.
Plumbers call it a "Bull" on a natural gas line before it is hooked up to a appliance to catch the debris before entering the control valve.

I think Mark has an indented firewall so that may not fit this application.

[Special Coupe Frank]

Steam fitters call it a "mud-leg".

[Harley03joe]

Has Anyone Tried Clothes Pins On The Gas Line ?
We Did That When I Was A Kid, The 45 To 48 Also The 50 Ford & Merc Used To Vapor Lock A Lot
Joe