vapor lock?

[Pat K]

Ok, we finally isolated the starter problem and fixed it. As the mech, said there is a relay between switch and starter that sometime worked, sometimes not. He wired around it, problem solved.

Now the latest issue is on 100 degree days (black car) going up the hill to my house, engine temp just a little over 200, the car starts hesitating, sputtering like a vapor lock. Only happens when outside temp is close to 100. As my son will probably inherit the car in a few years and he lives in Dallas, so am trying to sory out all of the temp issues.

Any suggestions?

1985 - 305.

RQB 4143

[Devildog]

Everyone says it is the ethanol in todays fuel, not true ethanol has relatively low vapor point. However, due to ethanol being low RVP (reid vapor pressure), the refiners blend in more butane than 30 years ago and even more butane because the modern fleet has electric fuel pumps, injected systems and a closed pressure fuel system. Easy fixes:

1.make sure all the fuel lines do not touch the hot block or exhaust. If any doubt split a length of rubber fuel line and slip around the metal fuel line to thermally insulated it.

2.Put a 1/4 inch thick phenolic or some polymer spacer/gasket under the carb to isolate the heat transfer from the manifold to the carb (thicker spacers are more common if you have adequate hood clearance).

3. If #1 and #2 do not relieve the vapor problem, consider installing an inexpensive electric fuel pump (low pressure < 10 psi) near the tank. This can be activated by a toggle switch mounted under the dash, console, etc. Use to pressure the fuel to the std engine fuel pump which will either (1) purge any fuel vapor do to heat or (2) add enough pressure to re-liquidfy the vapors in the lines. Turn off electric pump when not needed.

#1 + #2 will solve 80% of the vapor problems of modern gasoline in older carb automobiles, + #3 cures 95%. Inexpensive, relatively easy adaptations of the old fuel systems.

Joe

Edited June 10, 2015 by Devildog

[IndyJimW]

Not a big fan of the "vapor lock" bugaboo, in my 40 years as a mechanic I have never seen it.

Start with the basics. This engine was designed to operate at those temperatures so you probably have a problem with one of it's sub-systems.

Is the choke fully opening?

The air intake should have a vacuum flap that draws hot air off of the exhaust manifold when cold. Is it closing when hot or are you still drawing hot air with the engine at operating temperature?

On the passenger side of the engine there is a vacuum operated EFE valve connected to the exhaust manifold. If it is stuck shut it directs exhaust gases under the carburetor to the other side exhaust to heat up the carburetor when cold.

Test the fuel pump. Fuel pressure at idle should be 5.5-6.5 psi. Volume should be 1 pint in 30 seconds. Replace the pump if it can't meet these specs.

Is the EGR valve operating properly? This can cause a vacuum leak and poor running.

The cooling system is designed to operate at 195 degrees so that sound OK. I would check it anyway.

Check the ignition system for heat break down problems.

If you have the big cap distributor I found that the vacuum advance may not work or the advance weights stick. Both of these will cause drivability problems.

Safe motoring,

Jim Wood

[Devildog]

Pat,

Jim is on target (as usual), the vapor problem is usually a 'hot start' situation not a sputter on a running engine as you described. Tell your son in Dallas to get on our Avanti group list www.thirdcoastavanti.com

Joe

[Pat K]

Joe:

He doesn't get to inherit my Avanti until I die.

lol

PK