Gas - How old is too old and what do you do about it

[KM7]

My 5.8 L Ford is tuned per the manual, 10 Deg Before TDC. The carb was rebuild last summer, The spark plugs are in great shape. It is hard to start sometimes and does not want to idle below 700 rpm. I'm suspecting bad gas, it is 6 months old.

So what do you do about bad gas?

The tank is only about 1/4 full so I could add fresh gas + a fuel additive and then run the motor for a while to burn up the old gas.

Any suggestions?

I understand that a mostly empty tank can lead to moisture condensing on the walls of the fuel tank.

Is it better to keep the tanks full and prevent that condensation or only keep a small amount in the tank and avoid letting it get old? But then what can you do to prevent the condensation?

How old is too old for gas?

Thank you

 

[dingbat]

The gas I have in my tank is 7 months old.

I don't use additives. E10 is the best fuel cleaner made.

I store whatever is left in my tank at the end of the year. Anywhere from 1/4 to 1/2 tank.
I just top of the tank and in the Spring.

I don't believe in the condensation myth unless you live in a jungle on the equator and let your boat sit for years at a time. You can't condensate enough moisture out of the air to worry about it.

 

[kenny nunez]

If everything on the carburetor is right the idle mixture screws should be 1.5 turns out from lightly seated and the idle stop screw should be around 1 turn in against the throttle lever. If those settings do not work there may be something in either an air bleed, or the metering block gasket may have shrunk. If it ever popped back through the intake then the power valve could be blown out.
6 month old fuel should not be a problem unless you have been using any of that ethanol gas.

 

[Lou C]

I'm embarrassed to admit...that I had gas in my boat from 8/2016 when the head gaskets blew....till the spring of 2019 when I finally got my act together to finish repairing the engine and a few other things...I had a newly rebuilt Quadrajet, new fuel pump etc. And added some fresh gas but there was still prob a half tank of 2.5 year old E10 but stabilized gas and...it ran just fine...as well as it ever did...so 6-9 months I wouldn't even worry about it. In fact I started it up just this weekend after the long winter storage period, cranked over 2 x to fill the bowl...and roared into life...same gas....

When I took the Q-Jet apart it was clean as a whistle inside, no crud corrosion or anything else...

 

[KM7]

Thanks Dingbat, Kenny and Lou. So the condensation is a myth. Kenny The idle mixture screws are at 1 1/4 out. That is a little more than the 7/8 the manual says for my engine and carb. (91 Ford 5.8 L w/ a Holley). 1 1/4 out gave me the highest idle as far a setting the mixture screws. What do you think about the book saying 7/8?

Is there a better way to set the mixture?

I was having a hard time getting it to idle. I may have it too low because I was setting it in neutral. Should I set the idle while in forward gear and tied to the dock?

@ Kenny: You said "something in either an air bleed, or the metering block gasket may have shrunk. If it ever popped back through the intake then the power valve could be blown out."
How do I test these items? Is there something I can see? It never "popped" until just yesterday when I was fighting with it. It did pop 1x. What can I look for?

How to I check for vacuum leaks. We used to spray starter fluid around the base of the carb to check. Is that still a good method? My started has a bit of a wide spray. It may be hard to find a leak. Is there something with a finer spray that can be directed to a smaller spot?

Thank you.

 

[kenny nunez]

To check the power valve the carburetor has to be removed then remove the float bowl and metering block. To check the valve push the piston in then with your thumb seal off the diaphragm side and let the spring try to push the piston back out.
if it stays in then the valve is good.
Probably a good time to get another rebuild kit, just be sure it is for a marine carburetor. Of course the “sweet spot” on the idle mixture screws is usually 1.5 out or 1/8 either way. You probably do not have a vacuum leak. If 7/8 makes it run correctly then then use that setting, but every Holley marine or auto usually likes 1.5 .

 

[KM7]

Ok I will try that.

 

[Scott Danforth]

smell the fuel...... if it smells like varnish, its bad. if it smells like gasoline, its still good.