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#3320651 - 03/23/14 08:48 AM 1954 Flatheads and gas in oil.
Plumber Offline


Registered: 10/23/13
Posts: 331
Loc: USA
Both my 54 Plymouths have the 230ci flathead. One car is manual, the other Powerflite 2 speed automatic. Both carbs are different.

I always get gas that is leftover in the manifolds when shut off leeching into the valves then into the oil pan.

it is such a common thing the factory manual mentions it, and explains that the oil cap expels some of the gas in vapor form....But, you may want to change the oil more frequently.

Which oil would be better at handling oil/gas mixing?

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#3320656 - 03/23/14 08:51 AM Re: 1954 Flatheads and gas in oil. [Re: Plumber]
Red91 Offline


Registered: 12/09/13
Posts: 1222
Loc: Alabama, United States
I would say a 30HD would be adequate. Probably any multigrade HDEO would do as well.
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#3320659 - 03/23/14 08:59 AM Re: 1954 Flatheads and gas in oil. [Re: Plumber]
spasm3 Offline


Registered: 05/30/10
Posts: 6720
Loc: North Carolina
Amsoil makes a 5w 30 hdeo.
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#3320666 - 03/23/14 09:09 AM Re: 1954 Flatheads and gas in oil. [Re: Plumber]
eljefino Offline


Registered: 06/15/03
Posts: 29983
Loc: ME
In the 80s cars had some sort of solenoid in the carb that either closed it beyond idle or shut off fuel, to keep the engine from sucking fuel down after spark was shut off. Not sure if it's part of the ISC or what.

Seems it wouldn't work well in the fuel line though, as the carb bowl would have enough to supply fuel to the oil during spin-down.

Do you want to stay 100% stock? A PCV system would be easy to rig up. Do you have a road draft tube?

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#3320675 - 03/23/14 09:21 AM Re: 1954 Flatheads and gas in oil. [Re: Plumber]
Astro14 Offline


Registered: 10/10/10
Posts: 6966
Loc: Virginia Beach
I'm guessing it's just vented to the atmosphere. I also suspect that the engine called for a -40 in summer...it was built in the days of single viscosity oil...

My personal recommendation for this would be a 15W40 HDEO. Something that you buy on sale for about $12/gallon and change often. Good zinc levels, covers all seasons in which you'll run this engine and it has good viscosity for fuel dilution. Cheap enough to change when the dipstick starts smelling like gasoline...


Edited by Astro14 (03/23/14 09:22 AM)
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#3320713 - 03/23/14 10:10 AM Re: 1954 Flatheads and gas in oil. [Re: Plumber]
toneydoc Offline


Registered: 12/16/11
Posts: 1477
Loc: War Eagle
No oil itself will really HANDLE this for a long time which is why they suggest changing it frequently. Gasoline readily breaks down oil. I use gas to remove oil from surfaces from time to time and it breaks down the oil easily. Use a decent quality oil and change the oil frequently. Especially if you crank and drive them often.
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#3320842 - 03/23/14 12:59 PM Re: 1954 Flatheads and gas in oil. [Re: toneydoc]
Plumber Offline


Registered: 10/23/13
Posts: 331
Loc: USA
The vapors are vented. There is a draft tube. I figure any gas leftover in the manifold is the culprit. The engine mounts at a slant, so any remaining gas will drain back.

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#3320935 - 03/23/14 02:22 PM Re: 1954 Flatheads and gas in oil. [Re: Plumber]
flanso Offline


Registered: 12/26/06
Posts: 344
Loc: Manhattan
The solenoids that became common in the 1970s were used to decrease the throttle opening, when the car was shut off, to prevent run on. They did not shut off the fuel supply to the carb. Carburetors from that era used steel inlet needles against brass seats and that design is prone to leak. If you can find modern carb kits for those carbs, the kits will likely have viton, or other synthetic, needle tips that will dramatically reduce fuel leakage into the carb throat after shut down. You can observe the leaking by getting the car up to operating temperature, then shutting it off, remove the air cleaner and then observe the nozzle in the center of the venturi with a flashlight. You'll probably see a very slow, small drip, drip...

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#3322335 - 03/24/14 06:49 PM Re: 1954 Flatheads and gas in oil. [Re: Plumber]
artificialist Offline


Registered: 09/23/07
Posts: 8356
Loc: Florida
In the "European car oil section" I read about Renewable Lubricants Biosyn, and there were reports of it handling fuel dilution better than the other oils in use.

That in mind, it seems that numerous cars with carb engines ran for ages with conventional oil changed every 3000 miles, so long as the engine was a good design.
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