The International Cessna^® 170 Association

Hello all.
Was about to take a flight yesterday eventing and as I ran through the checklist the Fuel Primer didn't work.

There was no resistance whatsoever. I rolled the airplane back in the hangar and checked the lines and the fittings but I didn't see anything obvious.

I've moved the fuel selector valve and verified that there's fuel in the gascolator.
I didn't find any loose fittings or obvious cracks or kinks in the copper line either under the dash or from the firewall to the intake manifold.

My question is this: How do I trouble shoot the primer?

Thanks
Jim

This is usually caused by a output check valve of the pump stuck open. try this, unlock the primer plunger then pull it fully out quickly, then see if the pump works as it should. the check valves in these pumps are nothing but a chrome ball that sets on a seat built into the pump casing, and held on its seat by a tiny spring. if this does not change anything, remove the check valve and see if the spring is in place. It only requires a straight slot screw driver to remove the cap that covers the valve in the base of the pump.

Tom's suggestion is one thing that can go wrong.

Another is the o-ring has worn/shrunk. Spinning the plunger out by turning the knurled knob off, removing the plunger and just wiping or wetting it with fuel and reinserting it might be enough to draw fuel and making it work again. If you have a dab of fuel lube on the o-ring instead of fuel even better. Of course wetting it will probably only get it to work for a little while till it dries out again. Of course a new correct o-ring with a dab of fuel lube would fix it long term.

Bruce Fenstermacher wrote:Tom's suggestion is one thing that can go wrong.

Another is the o-ring has worn/shrunk. Spinning the plunger out by turning the knurled knob off, removing the plunger and just wiping or wetting it with fuel and reinserting it might be enough to draw fuel and making it work again. If you have a dab of fuel lube on the o-ring instead of fuel even better. Of course wetting it will probably only get it to work for a little while till it dries out again. Of course a new correct o-ring with a dab of fuel lube would fix it long term.

It's been my experience that when the plunger "O" rings are that worn, they leak. he didn't say they are leaking.

Thanks for the suggestion Gents.

I pulled the plunger to the rear stop rather vigorously a couple of times and it started working again. Thanks!!!

I'll make a note to replace the O rings at annual this winter..


Thank you again for sharing your wisdom.

Jim

Your C-145/0-300 has a very poor primer system, it only primes the left 3, and the fuel hits the intake system at the flange of the "Y" pipe, so it will drain down and out very quickly. It is better to never prime unless the engine is cranking, and better yet stroking the throttle while cranking will prime all 6.

I've only felt I needed my primer once in 14 years. It was 17 degrees F and I gave it three shots, then pre-flighted. The primer was exceedingly stiff to operate.
Two more shots and cranked it while delivering another shot... it fired on the third blade, but didn't like it and I had to give it another shot to keep it from dying. It idled poorly so I pulled carb heat and it quickly warmed up over the next minute or so. Oil pressure was slow to build, of course. I let it idle slow for about several minutes before I closed the carb heat and brought it up to 1200 RPM.

A major concern in these conditions is an intake-fire. Like Tom posted, the primer delivers the fuel to the left intake-spyder and can drain right down to the carb. (Pumping the throttle like he posted will also create a hazard because the accelerator-pump will spray fuel into the throat of the carb.....but unless the engine is simultaneously-cranking when you prime/pump ...the fuel will drain downhill into the carb air box. A backfire can light it off and you've got a fire.
KEEP CRANKING to draw the fire up into the intakes.

If ever you do this.... have your fire extinguisher in-hand should in case things get out-of-hand. (It's always best to have an observer outside and away from the prop with the extinguisher, of course, and be prepared to stop cranking should he approach the prop.)

If you do ever experience an intake fire, remove your air filter and inspect for fire damage to it....and to your carb air box/carb-heat and hoses.
Obviously, priming has it's hazards and is to be avoided if at all possible. PRE-HEAT is best....warm air is preferred, IMO. (Electric pads and cylinder heat may not help the intake however, and that is where the fuel must atomize.)

I never prime unless it's below freezing.

OK George, I agree with everything you mentioned, except priming only below freezing, what procedure do you use to start normally?

gahorn wrote:
If you do ever experience an intake fire, remove your air filter and inspect for fire damage to it....and to your carb air box/carb-heat and hoses.
Obviously, priming has it's hazards and is to be avoided if at all possible. PRE-HEAT is best....warm air is preferred, IMO. (Electric pads and cylinder heat may not help the intake however, and that is where the fuel must atomize.)

I never prime unless it's below freezing.

Also check the carb Venturi after a carb box fire. I've seen plenty of damaged venturi up here in Canada after a fire as described by George. As per the manual, I prime even on warm days but only for the first start of the day otherwise a couple strokes of the throttle is sufficient on a warm engine. A trick on a very cold day after priming is to leave the primer fully out and charged ready to give it a small shot after initial cold start to keep it going, leave carb heat too. Don't forget to lock the primer after start, just feed it in slow enough not to choke out the engine. I always pre-heat when below freezing before attempting a start.

hilltop170 wrote:OK George, I agree with everything you mentioned, except priming only below freezing, what procedure do you use to start normally?

If it's not freezing but cold, I pump the throttle once or twice (max) and then immediately start cranking. It always fires on the third or fourth blade. (But unless it's below freezing I never use the primer.)

gahorn wrote:If it's not freezing but cold, I pump the throttle once or twice (max) and then immediately start cranking. It always fires on the third or fourth blade. (But unless it's below freezing I never use the primer.)

The other way around works better.

I prefer to use the primer. The primer nozzle shoots a fine spray into the manifold and would take a while to run out the carb. Pumping the throttle however shoots pretty much a stream of fuel into the carb throat and runs out much quicker. If you are going to pump the throttle I would have the engine cranking first and then pump.

lowNslow wrote:I prefer to use the primer. The primer nozzle shoots a fine spray into the manifold and would take a while to run out the carb. Pumping the throttle however shoots pretty much a stream of fuel into the carb throat and runs out much quicker. If you are going to pump the throttle I would have the engine cranking first and then pump.

Actually the primer places the fuel about 3 " from where the accelerator pump does. When the engine is not cranking any fuel in the intake system will run out very quickly.

T. C. Downey wrote: Actually the primer places the fuel about 3 " from where the accelerator pump does. When the engine is not cranking any fuel in the intake system will run out very quickly.

3 inches on a horizontal surface (atomized) vs. the vertical opening opening of the carb throat(streamed). I'll stick with the primer, I get a much smoother start without the over revving without oil pressure common with throttle pumping technique

lowNslow wrote:

T. C. Downey wrote: Actually the primer places the fuel about 3 " from where the accelerator pump does. When the engine is not cranking any fuel in the intake system will run out very quickly.

3 inches on a horizontal surface (atomized) vs. the vertical opening opening of the carb throat(streamed). I'll stick with the primer, I get a much smoother start without the over revving without oil pressure common with throttle pumping technique

The primer nozzle does not as atomize the fuel as much as you think. and it is only in one "Y" pipe.

The accelerator pump will deliver fuel up the intake pipe, and when you close the throttle the fuel will fall onto the throttle plate which is open a tiny bit, The air coming thru the idle setting opening will atomize the fuel for all 6 cylinders.