Packard Forum Index » Pre-War (1899-1942)

There was also this service bulletin regarding the tail pipe. The fix was to eliminate the cross over pipe going in front of the fuel tank. The new pipe put the tail pipe on the passenger side of the car, and would also pinch the end of the pipe as it ran along the side of the fuel tank to provide more air flow around the pipe. I understand that on some cars the tail pipe actually would boil the gas right in the tank. With this service letter your car can have either left or right side exhaust and still be factory correct.

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29 Guy -- I saw your video on Youtube and noticed you have engine splashers installed. Would removing them help dissipate the heat? I realize heat rises, but perhaps opening that up will help with keeping things a little cooler?

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Those videos were done by Corky before he brought the car down here for me. Yes, both of the shields under the engine are in place and I'd thought of that, but, given where the carb is and how filthy and wet it is down here, and seriously, the astonishing amount of bugs we have flying around (no kidding), I thought it wiser to leave them in place. Without an air cleaner and not much way to fit one on there, heaven only knows what all the carb would suck into its mouth!

Thanks for the suggestion, I think taking them off would definitely help with vapor lock, but, I'd likely end up sucking endless amounts of trash and horse flys into the carb!

One area to avoid is sharp bends in the fuel lines. Those fancy 90 degree bends may look great, but they create a Bernoulli effect where a low pressure differential is created the same way as an airplane wing. If you look at the Model A the fuel line looks sort of ghetto, but that is the way Ford reduced those sharp bends.

The vacuum tank next to the exhaust manifold is required for the gravity feed. In 1931 they put the pump up front thinking that would be the coolest place and by 1933 moved it to the opposite side on the Eight cylinder cars, but kept a couple of very sharp bends with fancy fittings going through the frame.

These problems continue today. Tanks now are sealed to maintain pressure with relief valves to prevent excess, and pumps are submerged in the fuel, thus preventing any low pressure spots. But misfire conditions sometimes still exist that are remedied by changing the shape of the fuel lines in some cases. Even in systems running 450 psi.

Today we have great ways to use thermo-couples to look at temperatures along the fuel lines. I think Packard was groping for a solution. With a diaphragm fuel pump the intake valve creates low pressure to draw fuel and if bubbles are being created and merging everything can come to a halt. Testing with clear fuel line is one way to analyze the problem.

Higher octane gas has less of the volatile components in it such as butane which are the main culprits in vapor lock. I would suggest using it higher octane gas instead low octane. I use to use 5 to 10% diesel in my ’70 Corvette, which wanted 100+ octane, as an anti-knock work around (along with de-tuning).

I had a friend who had an issue with vapor lock. His fix was to run a metal fuel line along the outside of the frame opposite the exhaust system side. In the engine compartment he avoided all the engine hot spots as best he could. I don’t recall if he installed an electric pump or not in order to remove using the pump on the engine.

In discussions with a fuel company (BP) I was told there is no relationship between octane rating and fuel volatility. What you may find is that the higher octane fuel does not have ethanol

I had two 1929 Packards, on one I re-ran the fuel line down the opposite side for the reasons described, it made no difference. There is very little heating of the fuel in the line from the fuel tank to the vacuum tank. The problem starts with the fuel stored in the vacuum tank located in a very hot engine bay and above the exhaust manifold.

Those vacuum tanks can develop leaks that cause fuel starvation. They should hold vacuum when in the filling position, and the vacuum side should close off when draining. If you put a test gauge into the fill port you see the action on the road. If the vacuum tank never gets near motor vacuum the thing is leaking.

As well, I read an article about winters in the Northeast where the Hudson river regularly iced over and halted commuters for days. The railroads as well were commonly snowed in. That is why they built the H & M tube trains under the river. By the 1940's the problem was already subsiding. So when these cars were built things were different to begin with although the Harvey cars seemed to do okay in the desert.

I breezed over a lot of replies, and not surprised to see the old tried and true "fixes" and the everlasting search for the reasons. I have some questions. When it's vapor locked, have you ever taken the fuel line off to confirm it's actually blocked by excess vapor pressure? As to it happening "...inside the carb..." that's partially true or helpful. How's the needle and seat? If the answer is that it looks new or we just got a kit from so and so, where did it come from?

Let's talk about today's fuels. I'm near Motown and I think Michigan hot summers are probably on a par with Louisiana. High humidity from being surrounded by lakes, temps in the 90s, none of it uncommon or pleasant in a vintage car. However I can't recall a single real vapor lock situtation since the mid 90s. Our fuel today, despite the rapid evaporative nature, is superior to what was around in the good ol days. Way more energy in it, more "light ends" to the blends, and frankly fuel injection CAN NOT have fuel that vapor locks so put 2 and 2 together and come out with Whiskey Tango Foxtrot. Since even our regular is better adding 2 degrees to your ignition timing is a monster drivability improvement and will tend to drop coolant temp just a wee bit, maybe 5 degrees on average. Fuel isn't "late burning" in the exhaust manifold adding a bunch of unwanted heat in the worst possible place when you do that. New formulations are also less apt to lubricate sensitive little carb parts like power valves, check balls, and the ultimate culprit, the needle and seat. Once the needle valve gets stuck in the bore of the seat (due to heat) no gas gets into the bowl and we run home to "VAPOR LOCK" because it acts like it. It ain't.

Look up Daytona Carburetor (in FL obviously) and scope out their solution to "vapor lock" via their exclusive needle/seat. Not the old viton tip or all brass. We fought the same scenario on a fresh 34 in FL on a tour and that's exactly what it turned out to be. The Daytona needle and seat was the answer, and that was back in 2010. No issues since in over 1000 miles of summer driving that I was aware of. I only use Daytona Carb parts in my rebuilds.

Now if you have that already then just stamp me a blowhard and have the mods delete my reply, but that's been the final solution twice in the last several years for me. And for the record this is for everybody reviewing this if they suffer it now and then too. Have fun, here's a link:

daytonaparts.com/daytona-carburetor-float-valve.html

The car in question (1929 Packard) has a vented float bowl so boiling fuel in the carburetor may not pressurize the bowl. The vent is a small hole in a screwed cap, I removed the cap on my cars providing a bigger vent opening, that made no difference
In my opinion the problem is simply that the fuel drawn into the engine is a mixture of liquid and vapor. The vapor has negligible energy so the car starts "bucking", as the vapor part starves the engine of fuel. Pulling out the choke or full throttle can provide slight improvement but once it starts, stopping and cooling the vacuum tank and carburetor with a wet rag will provide temporary relief.
The problem is as simple as that, the cure however is far from simple, how to keep the fuel cool.
It's worth adding that on later model cars vapor lock might be due to the fuel pump not delivering vaporized fuel, pumps deliver liquid not vapor. On cars with a vacuum tank, its not a pump, its simply a tank under negative pressure (when the float drops) and as such will suck liquid or vapor if required.
A final comment despite this being a real PITA when it happens, sometimes a year or two will pass without a vapor lock incident occurring leading to the belief that the latest "fix" has somehow been the cure, then when conditions are "right" it happens again