The International Cessna 170 Association

I was looking at the 170's gascolator yesterday and noticed the primer supply tubing runs out of the top of the gascolator. I have also noticed for years when starting a warm engine, when I actuate the primer, sometimes I have to pump the primer several times before it loads with liquid. This also happens sometimes when the engine is cold but not as much as when warm. This same thing also happens with the C-180 and C-195, both of which have similar gascolators.

So why does the primer pump vapor for several strokes before it loads with liquid and where does the vapor come from? I always thought it was just "vapor lock" of some sort but never tried to figure out where it came from. From the above observations, I'm guessing the gascolator on all these planes collects a vapor bubble above the fuel on a regular basis like we have seen in the video. Why it happens is still not clear to me but I'm thinking they all do it. My planes have aluminum gascolators that you can't see thru. So there is no way to tell if a vapor bubble is there except when the primer pumps vapor for the first few strokes. That sure indicates a bubble is there. I have never seen any ill effects from a bubble if in fact there is one.

Take some fuel lube and put it on your orings in your primer or replace the orings. That is where the air is coming from. Stop flying it till you get it fixed. If the seal goes you will have fuel starvation and the engine may quit. There is sufficient leakage to cause the starvation scenario. If the O-rings are in poor shape you may look down the cylinder and see if there is oring debree in the bottom of the cylinder.

Richard - I've wondered the same thing, it always seems like the first pull of the primer sucks mostly air/vapor. Since the fuel pickup (output to carb) is at the very top of the gascolator, I don't see how it can't be a problem to have air/vapor there. But maybe I don't understand physics well enough.

Doug - I've done both of those things... lubed and then replaced the primer o-rings. No change.

I can't reproduce this on the ground.

I just bypassed the fuel flow sensor and its two short fuel lines with a single fuel line. A first test of 45 min. produced no air bubble, but that has happened before so it's not enough data. Will fly it more and see.

I modified my post while you posted. So you looked at the o-rings. did you look down the fuel primer cylinder for debris? Your problem is in the fuel primer. I have seen this before. If you had an air leak downline of the gascolator you would see evidence of fuel leaking on shutdown and you would have a lean operation on shutdown as well. It is only at high vacuum pressure that this air bubble causes it to get air in the gascolator. Mic the o-rings and the fuel primer cylinder.

No debris in the cylinder, the old o-rings were a bit hard but completely intact. I haven't checked the one-way ball valves/spring in the primer lines, however. I wonder if that's it.

What ever you do, please stop flying it till you get it fixed. It is not good.I had to replace the entire primer assembly on a Cherokee because the cylinder bore was galled. The engine quit due to fuel starvation.
I want to emphasis why it is important not to fly this till it is fixed. Where is the air bubble positioned when you have a high angle of attack, like in a go-around or a three point landing? The bubble can be drawn into the fuel line. Once it is there, it is not a question of if the engine will stumble, but when? DO NOT FLY THIS AIRPLANE till it is fixed.

I don't understand how there would be a vacuum there which would pull air into the fuel gascolator from a leaky primer. There should be around 1.4 psi of positive head pressure at the gascolator. If anything fuel should be leaking out from a worn primer. Wouldn't it be easy enough to isolate whether the primer system was the source of the air bubbles by either capping off the AN816-2D nipple (#63) or replacing it with a pipe plug and then taking another video?

Yes if I crack open the primer line connection on the gascolator then fuel spews out. That's the easiest way to get rid of an air bubble, actually.

I have thought about capping off the primer line but the engine just won't start without a bit of primer, even when warm it need a little. The carb does not have an accelerator pump. (Forgot to get the model number of the carb when I was out there.)

And I just can't get this to happen on the ground, although I haven't tried an extended high-power setting. Sitting there running for 45 minutes at 2300 rpm on the ground just doesn't seem like a good idea, but perhaps with the camera I could see something sooner.

I've landed with a large air bubble and no indication of any engine hesitation. So it must take quite a large one, such as I had in Utah, to cause a problem. That was after almost 3 hours of flying.

Gary, capping it will isolate it. Rather easy to do. Since I have had experience with this already, it is not necessary for me to do that. 4bravowhiskey says he has already had a problem in UTAH. Im not sure how much of a problem you need to get you to consider not flying it other then you let all of us know after the statistics come out. Sometimes I do not understand pilots reasoning. At least you are on here telling everybody including the FAA what you are doing. SMH

Doug, I think I agree with Gary on this one. If it is the primer as you suggest (and your probably right) it certainly should not be a problem to isolate the primer system to confirm.

Apparently nobody is reading my posts. HOW DO I START THE PLANE IF IT NEEDS PRIMER TO START??

Pump the throttle WHILE cranking the engine.

As I've said multiple times, that does not work. The carb does not have an accelerator pump.

4bravo Whisky, you can start the plane by setting it at low idle and shutting the engine down with the mag switch leaving it full rich. It should start with pumping the throttle to allow air to get into the chamber allowing a stoechemetric ratio to attain lightoff.

That's a good idea, I'll try that. And will do more ground tests with camera on it. I did install that fuel line bypassing the fuel flow sensor, and so far so good... but may not have enough run time yet to know.