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Upon troubleshooting an issue today it occurred to me to check and see if my gas cap was venting so as to not build up a vacuum in the gas tank. When i took it off to examine it, it seems that it is not of a venting type. After looking at some other examples online, it would seem they're not either.

Is a 50 bathtub supposed to have a venting fuel cap, and if not, how is the pressure in the system equalized?

As far as I know, all Packards had vented caps.

Yes, you should have a vented cap. Some very early cars that used a hand pump to supply compressed air to pressurize the fuel tank for fuel delivery didn't have vented caps, but that was all over with by the 20s.

Some gas cap vents can be very subtle. Look it over close. I'm running a modern gas tank on my 56 Exec with plastic screw on type cap. UNVENTED. I drilled a 1/64 hole streight thru it. HOWEVER my cap is protected by a the fender and a service door. I donlt know what your 50 has. IF cap is directly exposed to weather with no type of protection then u'll have to get a cap that is vented for such an application.

Test current cap with a radiator hose or something and suck thru the hose from the INNER face of the cap to determine if it really vented or not.

Attached are pics of the cap...unless it's really well engineered and it doesn't seal against that seemingly cardboardish gasket that tight, i'm guessing it's not.

After ironing out some other issues, there's one that i've noticed i've had since the motor swap and i think it's causing a different issue.

After the car has been running a bit in the driveway (maybe 10-15 min?) or after taking off about 1-2 miles down the road from a cold startup, or from a warm startup, the car has 0 psi fuel pressure at the carb.

Today i took the glass bowl filter off, all the lines and put a new rubber line from the tank, to a round plain new cannister pass through style filter, to the fuel gauge and into the carb.

Tested and the same symptoms. At first i thought maybe i was running into the dreaded vaporlock, but then i did the cold test today and the car wasn't even all the way warmed up, or the choke all the way open and it did it again. You can let it sit and go start it and get 3 1/2 - 4 psi instantly.

When it loses pressure, i believe the car is WAY leaning out. It knocks and pings, especially at part throttle cruising when it would seem the vacuum advance is in. I have it retarded almost all the way and the vacuum advance disconnected and it can still knock some, and has just no real power.

I have a new fuel pump, new carb, all new lines, all heat shields in place and then some, and moved the new rubber line WAY out of the way of any heat source. I started to consider maybe the tank was building a vacuum and after running a bit idling or even sooner when driving, it was keeping the pump from keeping the carb full.

Strangely, the car still runs, just without power and kind of out of breath around 50mph. I'm sure the timing is contributing.

I'm banking on trying it without the gas cap tomorrow, and hoping that's it. Also thinking maybe what Kevin said about debris clogging the pickup and when the car shuts off, it floating away. Again, strangely, the car doesn't stall or anything. When i had the glass bowl on, i did noticed once, right after shutoff, it was only half full, and it's usually full all the time.

At least i got the rough idle solved with the manifold and carb swap....is anyone running a bathtub and a 4 bbl? Solving this and setting the right timing should solve the running out of breath at higher rpms.

Edit: also strangely, I've run this cap since i got it a few years back, it was on it when it came to me. I used to run this car with the 288 like crazy down the highway. Maybe it did vent some somehow or i never had it tight? Hard to imagine it never worked when i got 2 solid summers of mostly highway driving out of it.

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Just as comparison, here is what a vented 47 cap looks like--although different part number than yours. It has the locking flanges on the inside of tube while later appears to have them on the outside. Yours should be the same cap as used thru 56. Maybe someone could post a pict of theirs for comparison to see where it vents.

The half full bowl, if not caused by a vacuum, could also be caused by a partially clogged element. I had that problem and the gas couldn't pass into carb fast enough. You could blow thru the element but gas couldn't flow well. You'd think the bowl would stay full in that case, but it didn't and bowl eventually dropped in level to about half.

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I can see in yours the breather hole at least, this looks like a solid gasket riveted to a solid piece of metal. I saw one on ebay that was a round dome and it had breather hole also.

For the glass bowl, it has the paper style element, and it was new also. Now i have the new pass through metal can type, and i could blow through it easily. If the gas cap does turn out to be an issue, why wait til now to show up? Can't wait until tomorrow to go and try it out.

Another way to make sure the tank filler is vented is to file a groove on the surface where the cap gasket meets the filler tube. This way, no matter what cap is on the tank, the tank will always be vented.

(o{I}o)

Here is the cap on my '51. It has c-2 stamped on it as well. Don't know if it is original to the car.

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That "c-2" cap is the one I have on my '54 and what I have seen on other 51-54 Clippers.

When my tank was full of junk, I could idle it forever and even rev it in the garage and it would work flawlessly. But once out of the road with the gas sloshing around in the tank it would stir up the debris in the tank, and clog the filter within a couple of blocks.

Also don't forget there is a screen in the bottom portion of the fuel pump that can get partially clogged.

I'm really think a fuel filter back by the tank is good front -line insurance to keep the any tank crud from migrating forward into the lines, pump, or carb.

As others have said, those ceramic filter elements up near the carb can look clean, but the pores in the element can be clogged, or partially blocked.