The International Cessna 170 Association

Hello all..

Recently I have noticed a fuel stain on the bottom of the cowling. After further investigating, we can see fuel dripping out of the air box after I prime the engine. After starting the engine all is smooth and no dripping. This is an 0-300..Any ideas? Thanks Pete

How many squirts of primer?
It sounds like it's leaking out of the intakes back into the carb and out the box.

Try priming less or even see if it will start without priming at all.
Mine will start without priming unless it's really cold outside.

That was 2 squirts, it always needs a prime..I also feel my primer is a little harder to push in than in the past. I did replace the "o" rings last year and it has been fine..

This doesn't sound right , just 2 pumps and it's running out of the intake tubes. I do 3 pumps on the throttle and NO primer and it starts first blade over . I can't remember when I last used my primer. Maybe it's not cold enough in this part of the world.

Hi Pete,

I seem to recall something about a dirty or partially plugged primer nozzle causing that. Instead of atomizing, the fuel just sprays in a heavy stream. You may want to pull the primer nozzle out an clean it.

Do you pump the throttle as well as prime? The accelerator pump will have the same effect. FWIW, many moons ago my Dad would do this on his old biplane before hand propping it because the primer was broken. One day the throttle cable broke (unbeknownst to him) in the wide open position. Needless to say the engine caught and the plane headed across the field, dragging the guy he had guarding the throttle from the outside. He said it had just started to lift off when it quit, since as a safety precaution they always started it with the fuel off. Subsequently they fixed the throttle cable....and the primer.

DEM

It is not normal unless as stated, you over prime by pumping the throttle or using the primer. What ever your doing I'd do slightly less and see if the engine starts. If it starts then check to see if any fuel dripped on the cowl.

Personally, a little engine compartment cleaning fluid so long as it is not excessive, corrects some of product application from the automatic anti corrosion spray system I spoke about in another thread.

We did pull the primer line and nozzle and all was clear..I don't pump the throttle because I was told that method could start a fire, not sure about that.
My friend told me he pushes the primer while pulling the starter, I guess another way of doing it. Bottom line is I have always used the primer and never noticed fuel coming out of the air box...does anyone think there could be an issue with the carb? Thanks

Perhaps you may not have the proper primer fitting in the botton if the intake manifold. If someone has inadvertantly installed a standard flare fitting. the primer pump will pump way to much fuel in on each stroke.

What Bl is talking about is the AN fitting in the intake does not have a straight hole. It look like what I call sintered. Multiple small passages through. This helps to atomize the fuel.

Chances are though if yours didn't drip fuel before and now it does and no one touched the AN fitting, it's not the fitting. Unless of course your nozzle was plugged and now it's not.

Nope nothing has changed in the 3 years I have had the plane. Just recently noticed a harder to push primer.

pmel22 wrote:Nope nothing has changed in the 3 years I have had the plane. Just recently noticed a harder to push primer.

Harder to push primer sounds like an obstruction.
Something might be clogging your tip enough to prevent it from atomizing and thus running down the intake.

I'd pull the tips and verify they are clean and clear.

Hi we pulled the tip and it appeared to be atomizing the fuel correctly, its kind of like the primer is all of a sudden putting much more fuel than it had been...It is really weird..thanks

If you do not have a membership just this book is worth the price of membership. (Owners Manuals on the members only page)
Personally when the O-300 was still installed I never used the primer especially with the engine not turning over. I always pull the starting cable and give the engine a pump on the throttle. Using the primer or the throttle without the engine turning over just dumps fuel into the carburetor heat box. A real good possibility for an engine compartment fire.
Regards,
Jim
Excerpt from the 170A operators manual.
Page 3 of the 170A operator’s manual;
C. STARTING THE ENGINE
(1) Set carburetor heat to “cold.”
(2) Set mixture control to “full rich.”
(3) Set fuel tank selector to “both tanks.” (Take-off on less
than ¼ tank is not recommended.)
(4) If engine is cold (50◦ F or below), prime the engine as follows:
Turn master switch “on.”
Clear propeller
Make certain magneto switch is “off” and throttle “closed”
Then give the engine three or four strokes with the primer as
The engine is turned over. (Ordinarily not required except
at winter temperatures.)
(5) Turn magneto switches “on.”
(6) Open throttle 1/8 (to idle position) and start engine by
pulling starter control. Note: In extremely cold weather
a few strokes of the primer as the engine fires will enable
the engine to keep running. (Avoid over-priming) After
priming push primer all the way in and turn to locked
position to avoid possibility of engine drawing fuel through
the primer. Do not pull out the starter for a second starting
attempt until the engine has come to a complete stop from the
first attempt. Failure to do this may result in damage to
the starting gear

Thanks metal master...I should of mentioned this is a 56 172 . Just doing what the owners manual say to do (4) in normal weather temperatures use 2-4 strokes of primer just before the engine starts
(5) clear propeller
(6) master on
(7) mags on
(8) open throttle 1/8"
(9) pull starter

its the way Ive done it for 3 years, this started a couple weeks ago, when I noticed a harder to push primer

spoke to Cessna support today for over an hour, and came to the conclusion it must be primer...we will see...thanks guys