Fuel out of the Primer Knob

Dayron Nunez

Dayron Nunez

Filing Flight Plan
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Dayron N.
Hi guys

I don't know if my questions is too dumb or not, but hopefully someone will know about this!
A couple days ago I went to fly on and old 1957 C172, the aircraft had been seating for about 10 or 15 days before I used it, it was parked with the fuel valve in OFF position, I started doing the walk around and did not change it to ON yet, I took drained fuel from the wing checking for water or dirt as always, then I went to check the fuel drain valve under the cowling and just a couple drops only (Of course as I have the fuel selector closed), so I went into the cockpit and changed it to the Both position and re try to take a sample from under the cowling, all good this tame.

My issue then was when I tried to start the engine, I went to prime it, and the first time I pulled the primer knob back, it let some fuel out through the space between the knob tube and the knob base, it was fast and just a little bit of fuel, but I'm not sure if that is normal or not, or maybe because the way I handle the fuel valve, I've never seen this before.

Anyone has ideas of this?

Thanks in advance.
 
Ideas? Yeah, don't smoke.

Did it leak a second or more times?
 
Not normal. There are o-rings in there that need replacing. Cheap and easy fix.
 
Chip Sylverne said:
Not normal. There are o-rings in there that need replacing. Cheap and easy fix.
Click to expand...

Thanks, I will definitely check with a local shop about that, just kind of wanted to have an idea before calling, to avoid being dragged into doing extra stuff
 
Dayron Nunez said:
It did not, just that time, after that I primed the engine 3 times, with no issue and started just fine.
Click to expand...
As Chip noted it should be a simple o-ring replacement and the primer has "told" you about a potential problem. If it were just me I'd put it in the keep-an-eye-on-it category. Other folks will probably say replace the o-rings and move on.
 
those o-rings are probably nearing end of life. The first "dry pull" caused a bit a leak, and then subsequent "strokes" were lubed by the fuel from the prior strokes so they held better.

I wouldn't ground the plane for it, but I wouldn't put it off to annual or 100 hr either.

Too cheap and easy to fix.

OTOH, if it was leaking on every stroke I'd do immediate maintenance.
 
Ravioli said:
those o-rings are probably nearing end of life. The first "dry pull" caused a bit a leak, and then subsequent "strokes" were lubed by the fuel from the prior strokes so they held better.

I wouldn't ground the plane for it, but I wouldn't put it off to annual or 100 hr either.

Too cheap and easy to fix.

OTOH, if it was leaking on every stroke I'd do immediate maintenance.
Click to expand...
Thanks, I will start looking for the correct parts on the aircraft manual, and check for that repair, seems like it might be a pilot owner fix possibly, if its only the O-ring.
 
Chip Sylverne said:
Not normal. There are o-rings in there that need replacing. Cheap and easy fix.
Click to expand...

THIS!

This is a serious problem, not just because you get avgas in the cockpit (which is bad enough) but it also means you're leaking air into the fuel system most likely which can be fatal.
 
Dayron Nunez said:
Hi guys

I don't know if my questions is too dumb or not, but hopefully someone will know about this!
A couple days ago I went to fly on and old 1957 C172, the aircraft had been seating for about 10 or 15 days before I used it, it was parked with the fuel valve in OFF position, I started doing the walk around and did not change it to ON yet, I took drained fuel from the wing checking for water or dirt as always, then I went to check the fuel drain valve under the cowling and just a couple drops only (Of course as I have the fuel selector closed), so I went into the cockpit and changed it to the Both position and re try to take a sample from under the cowling, all good this tame.

My issue then was when I tried to start the engine, I went to prime it, and the first time I pulled the primer knob back, it let some fuel out through the space between the knob tube and the knob base, it was fast and just a little bit of fuel, but I'm not sure if that is normal or not, or maybe because the way I handle the fuel valve, I've never seen this before.

Anyone has ideas of this?

Thanks in advance.
Click to expand...
http://www.aircraftspruce.com/catalog/appages/doorsealsAccX.php

I replace them every annual, less than seven bucks and under 10 minutes.
 
Dayron Nunez said:
Thanks, I will start looking for the correct parts on the aircraft manual, and check for that repair, seems like it might be a pilot owner fix possibly, if its only the O-ring.
Click to expand...

Two O-rings. Get Viton instead of Nitrile rubber, will last forever. Use fuel lube (EZ-Turn), just enough to make the rings shiny, plunger will glide like buttah even in the cold. Takes about 30 minutes, use a rag to catch and few drips of fuel that may come out when disassembled. If the hangar fairies do it, make sure to tell them use care not to bend or crack the little copper fuel line.
 
Unless you own this plane, you have no business making repairs to it. Get it to a mechanic even though it is a trivial repair.
 
flyingron said:
Unless you own this plane, you have no business making repairs to it. Get it to a mechanic even though it is a trivial repair.
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Yes I understand that, I own 1/3 of it and will be switching to full ownership soon.
 
Actually, in retrospect. It's not legal to do it even if you are an owner.
 
Probably a dried out seal,an easy fix for your mechanic.
 
It starts ok, but sometimes there is a 3' long blue flame coming out of the primer. Better get the O rings replaced...
 
Last edited:
Yes, you can spend $5 or more for a primer O-ring, OR you can purchase the correct O-ring recommended by Cessna for 25 cents each.

A 1978 Cessna Service Letter SE 78-73 recommends substituting VITON O-ring NAS 1593-012 in the primer. Two are required. This O-ring has since been revised to M83248/1-012, which is also Viton. Skygeek sells them for 25 cents each. The Service Letter contains an affectivity list, but most primers use the same O-ring.

Best to lubricate both O-rings before installation. I use a very light application of Dow 4, which allows them to 'squirm' into place and seat as they should. Keep things scrupulously clean, for a single grain of sand can score up and destroy your expensive primer.

O-rings can easily be cut or damaged during installation on the relatively sharp edges of the plunger grooves. I roll a couple of turns of tinfoil around the plunger end up next to the O-ring groove so the O-ring won't have to pass over a sharp edge. Rolling the O-rings into place works better that trying to stretch them on. After you are done, look closely at the O-rings with a magnifier for any evidence of twist after they are seated before installing the plunger. Actually, look at the O-rings with a magnifier before lube & installation to check for any cuts or defects.

Jerry King
 
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