The Cessna 120-140 Association Forum

Hello,
I have a Left, Both, Right and OFF fuel selector valve. The left tank burns 9 gallons and the right tank burns 4 gallons out of a total of 13 gallons burned on average over a few hours of flight. It does this with passengers or solo.

Does anyone have an idea how to get them to burn more evenly and the cause(s) of this? Or do I just have to run Left, Right and alternate as if I did not have the Both option?

Thank you for any information,
Brady

How is your fuel system vented? Uneven flow between the two tanks is almost always due to uneven pressure between the two tanks.

Here are links to four related technical articles you might want to look at.

viewtopic.php?f=10&t=105
viewtopic.php?f=10&t=103
viewtopic.php?f=10&t=138
viewtopic.php?f=10&t=106

Or, switch to one tank when the imbalance gets too much.

Do you notice the unbalance in flight or just when you fuel?

viewtopic.php?f=10&t=106

Scroll down to page 3 on this link, if you have the forward facing tube/caps here's your issue.

Thanks for the input. I can see the imbalance on the fuel gauges in flight as well as when I fill. I fave Scott fuel gauges which seem pretty accurate.

I do not have forward facing caps.

I am considering swapping caps from left to right to determine if it is an issue with the caps venting. IE, after switching, the right tank should burn more now.

I know I can run tanks separately but the aircraft burned evenly until a few months ago and I would like to find the reason this is occurring.

Can you tell us a little more about your bird? 140A model? Does it have the inter-connect tube between tanks? There are a bunch of Neal Wright articles on gas caps and fuel systems.

It is a 1946 140. It had a major overhaul and update in 2001 and I believe that is when they put the new fuel system in. It has the red vented caps with the small holes inside them when you luck at them from bottom up. I am sorry I don't have more background but it is not a 140A model.

Three years ago, it burned 2 gallons more over 3 1/ 2 hours on average from the right tank (right tank burned more). Then it seemed to burn evenly after a few months of flying and was not an issue. Now it burns more from the left tank. As mentioned, it took 9 gallons to fill the other day left and 4 on the right.

As mentioned, I am happy to operate in Left/Right switching mode but would like to know what is going on.

Thank you.

OK, so the 1946 140's did not come with a Left-Both-Right selector option. They only came with a L-R-Off. So, your bird if it has the L-B-R-Off selector it ought to have the vent inter-connect between the tanks and hopefully a 337 or some such logbook notation for the modifcation. If not, then something is very incorrect.

I strongly suggest you read the Neal Wright articles to get you educated. The best one is this I think...

viewtopic.php?f=10&t=105

The gas caps should look something along the lines of this...

The caps do look like that except they are solid (the two holes are not present).

I will continue to read the articles and yes, I agree with everything you have state.

Thank you again.

The caps do look like that except they are solid (the two holes are not present).

This is getting more interesting....

If the caps are not vented, and it's not a 140A system, there has to be some sort of venting somewhere to allow the fuel to drain out of the tanks. There ought to be an interconnect between the tanks across the top of the headliner somewhere. Please do keep us informed as we don't want you to have any fuel starvation issues. Pictures are worth a thousand words.

I am working with a new owner here in WA that has an 'interesting' fuel system also.