1966 427 425HP Vapor Lock

[Joe T.]

Now that the weather has heated up, I am getting vapor lock. The fuel line from the pump to the carburetor is just about touching the lower radiator hose. I am planning to wrap the hose with heat shield tape. I know this will not meet NCRS specifications but I don't know what else to do. Is this a common problem for a 66 427 425HP? Any suggestions on solving the problem while keeping the original look?

[Bob W.]

Re: 1966 427 425HP Vapor Lock

Joe, read the post below as some of these may help you out.

[Samuel J.]

Re: 1966 427 425HP Vapor Lock

Percolation can be a real problem for the L72, especially on very hot days and/or when the car is idling for long periods of time. The list of possible additional contributing factors is as long as the list of potential solutions. I recall reading an old article in the Corvette Restorer that mentioned that av-gas (due to its much higher autoignition temperature), vaporizes at a higher temperature than pump gas. Therefore, using av-gas actually helps to prevent percolation on a hot day, according to the article.

Compared to the high-octane, ethanol-free "Sunoco Standard" race gas I was using, the Phillips 100LL av-gas at the local airpark has a 50% higher autoignition temperature (824 degrees versus 536 degrees).

if you want to keep your L72 stock or at least looking as close to stock as possible, you may want to experiment with a few gallons of av-gas, assuming it's not too difficult to acquire in your area. I tried it and it worked for me.

Good luck with your L72!

[Timothy B.]

Re: 1966 427 425HP Vapor Lock

Now that the weather has heated up, I am getting vapor lock. The fuel line from the pump to the carburetor is just about touching the lower radiator hose. I am planning to wrap the hose with heat shield tape. I know this will not meet NCRS specifications but I don't know what else to do. Is this a common problem for a 66 427 425HP? Any suggestions on solving the problem while keeping the original look?

Joe,

Is it possible to tweak the fuel line to allow more air space around the pump to carburetor fuel line.

Having the line to close to the lower radiator hose is like putting a heater on it, I hope this helps some because it's little things like that that can make a difference.

[Carl N.]

Re: 1966 427 425HP Vapor Lock

Be sure gas cap is vented and I have found it best to keep tank as full as possible. Below 1/4 tank will result in faster warm up of fuel in tank. If you have to pull intake for some reason, block off heat risers. Old drag racer trick. I blocked mine on L78 many years ago and do not drive it on cold days anyway.

[Joe T.]

Re: 1966 427 425HP Vapor Lock

Would 110 leaded racing fuel help? I can get it locally and have mixed some in the past.

[Samuel J.]

Re: 1966 427 425HP Vapor Lock

Joe, to be clear, it's not the higher octane that will eliminate percolation, but rather it's the higher autoignition temperature, and just because a gas has 100 octane doesn't mean its autoignition temperature is high enough to prevent percolation.

If the 110 leaded gas available to you is known as "Sunoco Standard" (which I believe is the same formulation as the old Cam 2), then the autoignition temperature will be 536 degrees, which, based on my experience with my L72, may percolate on the hottest days and/or in stalled traffic, for example. As I said, my L72 stopped percolating when I switched to the av-gas which has a 50% higher autoignition temperature.

I should add that if your cooling system is not operating correctly and the engine tends to boil over, all bets are off.

[Duke W.]

Re: 1966 427 425HP Vapor Lock

It has nothing to do with the "autoignition temperature". It's about the distillation curve. At any given temperature, avgas and race gas has a lower mass fraction of components that are at or above their boiling temperatures than E10.

See the attached graph of distillation curves. Note that at nearly all temperatures the summer blend straight gasoline has a lower mass fraction at or above their boiling points compared to E10. At about 150F the summer blend straight gasoline has about 20 percent of its components at or above their boiling temperatures. The summer E10 is nearly 40 percent!

Avgas and most race gas blends would have even less their mass fractions at or above their boiling temperatures than the summer blend straight gasoline, which is why avgas or racegas can eliminate or significantly mitigate both vapor lock and percolation.

Duke

[Chris H.]

[Joe T.]

Re: 1966 427 425HP Vapor Lock

I have 10 gallons of AV gas (Phillips 100LL AV Fuel) in the car now. As far as my vacuum, from the distributor to the base of the carburetor. I would consider that full time advance. I also have a new DeWitt radiator along with a 165 thermostat. After looking really close, My fuel line is about 1/8 inch from the lower radiator hose. I plan on changing the fitting and moving the line about 3/4 inch away. I have the timing at 8 degrees and the R45 plugs set to .035. My next step is to replace the plug wires.

[Joe T.]

Re: 1966 427 425HP Vapor Lock

What is 2-inch miracle cure?

[Joe R.]

Re: 1966 427 425HP Vapor Lock

Joe,

Here's is Duke's tuning seminar:

http://www.metroli.org/pdf/2012%20Nationals%20tuningseminar.pdf

It should answer some of your questions about the 2 inch rule and ported vs. full time vacuum advance. You can't tell by looking if it is ported or full time. It has to do with whether the carb is set up to pick up the vacuum signal with the throttle blade closed or not. I hope I have that right. You will need to change your vacuum hookup and vacuum advance to convert to full time. You can also modify the carb, but that probably comes later.

Good luck!
Joe

[Duke W.]

Re: 1966 427 425HP Vapor Lock

The VAC nipple on a carburetor can be set up internally for either ported or full time vacuum advance. The OE L-72 carb is set up for ported. It's easy to test be removing a hose and attaching vacuum gage.

The easiest way to convert is to remove the original choke vacuum break hose and make a new harness with some 1/8" tubing and tee. Run a hose from the base of the tee to the VAC.

The OE 360 12 VAC is okay to use as long as it works and meets spec. If not, replace it with a B26.

After the conversion is made the idle speed/mixture and fast idle speed must be reset to spec. The procedures are in your CSM.

Duke

[Joe T.]

1966 427 425 HP vapor lock

With the hot temperatures (85 - 95) driving the corvette is fine. It does not over heat. It's when I park for a half hour or so, all indications point to a vapor lock and it will not start. I was going to install an electric fuel pump in line with the mechanical pump.

Any suggestions or thoughts?

[Joe T.]

Re: 1966 427 425HP Vapor Lock

Thanks. I have used AV-gas. On really hot days and parked in the sun for a half hour or so it will not start. I keep my idle up (around 900) so it does not stall in traffic.