Vapor lock?

[MaxHarvard]

Any good tips on how to get rid of it?

It only happens after a long (over 1 hour) drive and i slow down and idle. the car is a 1968 camaro with a small block. stock fuel tank, stock fuel pump.. 750 demon carb, 1" phenolic spacer.

Any help?

[XcYZ]

Eric, make SURE your gas cap is venting. I had the same problem years ago and it was a bad cap that wasn't that old.

[BC69]

That makes me ask this ? as far as its "venting"...when I drive for a decent amount of time...20min or so and then stop for gas. When I take off the cap there is a POP and a Whoosh of fumes...the cap is supposedly a venting cap,but my thought is it isnt venting..

Agreed?

Thanks
Tim

[MaxHarvard]

That makes me ask this ? as far as its "venting"...when I drive for a decent amount of time...20min or so and then stop for gas. When I take off the cap there is a POP and a Whoosh of fumes...the cap is supposedly a venting cap,but my thought is it isnt venting..

Agreed?

Thanks
Tim

Mine is doing the exact same thing!! How do i find a vented cap for a 68?!

[XcYZ]

The restoration places carry them; Ricks, Year One, Classic Industries, etc.

[MaxHarvard]

The restoration places carry them; Ricks, Year One, Classic Industries, etc.


Ahhh ok, i hope this works... its only after long drives... really weird.

[BC69]

What is the best cap? Are there aftermark ones that are better? I just went to yearone.com and found what is supposedly the 69' vented cap...it is exactly what I have. My cap has VENTED stamped on it, so I just dont think buying the same cap again will make a difference.

Didnt I read a long time ago about someone drilling a small hole in their cap. Wasnt that you Scott? Or am I just nuts.

Tim

[XcYZ]

Tim, you have a good memory. That was me that drilled a 1/16th hole in the cap as a troubleshooting measure. At the time, it was extremely hit or miss. The car would die once a week or maybe once a month. It was acting like an electrical problem, but as it turned out, the cap was acting goofy, it just didn't fail, it was intemittent or vented extremely slow. The hole I put in the cap fixed the problem, but I ordered a new one for obvious reasons.

[MrAngry]

The same exact thing happens to me with my 440 68' Charger. Only happens after about 1 hour of constant driving. I have a vent hose from my gas cap that I blew out with my air compressor - thought that may have done it, but then it actually happened last night after an hour drive, totally annoying. I also have a spacer under the carb. - I've made sure all fuel lines are clear from any substancial heat sources i.e. headers, exhaust. I have also replaced the fuel tank and sending unit. I may try heat wrapping the fuel lines as I don't what else to do.

[Y-TRY]

What is the symptom of vapor lock? I know causes but don't know what it feels like. Does the engine just stop abruptly for a second?

[MaxHarvard]

Y-TRY - It feels like the motor is starving for fuel and dies and then wont start back up. Whats happening is the fuel is so hot that its actually boiling inside the carb.

Scott, where would i drill this hole in the gas cap to vent it?

[XcYZ]

Anywhere so it can equalize pressure. All you're doing is allowing air to enter the tank.

[Rick Dorion]

I would either drill a small hole or order another cap from Rick's or whoever you prefer doing business with. With a phenolic spacer I don't see how the fuel is getting so hot as to perculate in the carb. I have the Holley thick gasket, and some thermo-wrap in the engine compartment , nothing exeotic, where my fuel line is within 1" of my uncoated headers and never see this. The cap sounds suspect obviously since you're seeing/hearing pressure build-up.

[MaxHarvard]

I would either drill a small hole or order another cap from Rick's or whoever you prefer doing business with. With a phenolic spacer I don't see how the fuel is getting so hot as to perculate in the carb. I have the Holley thick gasket, and some thermo-wrap in the engine compartment , nothing exeotic, where my fuel line is within 1" of my uncoated headers and never see this. The cap sounds suspect obviously since you're seeing/hearing pressure build-up.


I have a vented cap, but i think its faulty because i hear the unmistakable sound of vacuum sucking when i take the cap off.

[BC69]

I have a vented cap, but i think its faulty because i hear the unmistakable sound of vacuum sucking when i take the cap off.

Me and you def have the same problem...I am just gonna get a new cap and see. My cap is pretty new now and is just not working. I will spend the 15 bucks and see if the new cap works...if it doesnt I will send it back and drill a hole in the old one.
Just concerned for some long drives I have planned soon.

Tim

[jannes_z-28]

The reason for your problem is probably after the long drive the fuelpump have sucked fuel out of the tank but no air as entered to fill the space the gas had. After a certain volume the underpressure in the tank is so low that the fule pump can't pump any more. Remember that a pump can only suck maximum about 30 feet of a liquidpillar.

When you drive a short distance you will not produce the same amount of vacuum to reach the level where the pump can't pump anymore.

So the rule of thumb is "the volume that is pumped away must be replaced with air." That's the reason for the venthole in the cap.

So if I can correct you, the problem is not Vapor Lock.

Vapor Lock is the opposite where hot gas vaporize inside the carburators fuelbowls and create an overpressure that prevents fuel from entering the carb or in other cases overflow the bowl.

Heat in all forms is bad for gasoline (Except inside the cylinder). I had vapor lock problems and solved it by blocking the heatrunners under the carb. You know the channels that lets exhaustgas run under the carb.


Another thing is the bad quality of gasoline nowadays. When our musclecars were constructed the gasoline was of a much better quality and heatproblems was not that common.

We still have 98 octane gasoline over here but twenty years ago we had 100 that was more up to it and held for high compression.

Those were the days...


Jan

[MaxHarvard]

Jannes, i wrote during one of my early posts on this thread that the fuel was boiling inside my carb... i about plotzed!!