Uneven fuel flow

N2865C · Post by **N2865C** » Mon Nov 14, 2005 3:58 am

I just finished my annual. My "extra" project this year was to remove the RH fuel tank to replace a rivet and give the fuel bay a good cleaning and inspection. Everything looked good and we got it all back together. After a test flight I filled the tanks and took off on a cross country. When I landed I had used 8 gallons from the LH tank and 3 on the right. I filled up again and same story on the way home. I'm trying to figure out what could be wrong. I can't really see anyway we introduced an obstruction in the line. I'm thinking that maybe we somehow kinked the rubber vent tube connector, but thats just a guess. Any ideas any of you have before I head out to the hanger to investigate would be appreciated.

FredM · Post by **FredM** » Mon Nov 14, 2005 8:39 am

Uneven flow is not abnormal. I'm not sure what model 170 you have. Do you have one or both vented caps? with only one cap vented I believe this would probably be more noticeable. perhaps you swapped your caps by mistake or one of the vented caps is plugged. you may also want to check the main vent to make sure it isn't plugged either. Even if everything is fine some uneven flow is to be expected.

N2865C · Post by **N2865C** » Mon Nov 14, 2005 3:12 pm

I have a '54 B model with one vented cap. The vented cap is on the right side (the side we are having problems with). Before I pulled the tank the fuel flow was the same from both sides so something has changed. An almost 3 to 1 difference between tanks seems excessive.

doug8082a · Post by **doug8082a** » Mon Nov 14, 2005 3:40 pm

It does sound like something changed there. I did notice with mine, though, that having two vented caps evened things out some, but not completely.

kloz · Post by **kloz** » Mon Nov 14, 2005 5:10 pm

Might check the little red flapper in the cap. Sometimes they will stick and not allow fuel to flow because of vacuum.

Post by **GAHorn** » Mon Nov 14, 2005 6:51 pm

Uneven fuel flow is common, but in most cases it's the other way around...the right tank usually appears to drain faster.
In a re-cap (pardon the pun) of the scenario: The tank which appears to be draining faster... is in reality NOT the tank supplying most of the fuel to the engine. Instead, it's the tank which appears not to be draining at all. How does this happen?
In this case, N2865C, your right tank would be feeding most of the fuel requirements demanded by the engine...and simultaneously your left tank is supplying fuel to refill the right tank via siphon action across/thru the fuel vent line. This makes it appear that the left tank is doing all the work. (Sounds like you did a good job and got nice, clean, well-working connections/hoses on the right side. This can be verified at altitude by switching and running on only the right tank for awhile.)
Make sure your caps didn't get switched, and that your vented cap is venting properly (if indeed it is on the right tank.)