PA28 technical question
 

Hi all,

Can anyone tell me if theres a reason that the primer on the Cherokee only primes 3 cylinders and not all 4???

Short and sweet, any help appreciated

Thanks

I'm not an engineer, but I believe its to help the engine start when our weekend pilot overprimes. With one cylinder not primed it at least has a chance of starting!

Thanks
 

Thanks very much for your help, fullyfine.

If there is anyone who knows for definite what the reason is, or can confirm fullyfine is correct, I would be grateful to hear from you.

Thanks all

It is number three cylinder that is not primed and this is just a quirk of the engine design.

It's happened to me again!! I posted an answer to this some time ago - it hasn't made it to the screen.

However - my answer was the same as fully fine - it helps start on the "dry" cylinder when Gromit jumps in and overprimes it.

The Fourth cylinder conection is used for connecting the manifold pressure gauge on engines with constant speed props.
Always #4 cyclinder (stbd aft cylinder)

Regards

The primer only primes 3 cylinders as there are only 3 primer lines - OF COURSE!

But make sure it is actually locked on the pre take off checks. A pilot in my club once commented that the engine rough ran at low RPM and wouldn't shut down on ICO on one of the club PA28s, so he shut down on the mags. On inspection, it turned out to be the primer which was unlocked and slightly out, by only about half an inch.

It was obviously allowing fuel to be sucked past it into the cylinders making the mixture rich and causing the rough running at low RPM. It was also allowing enough fuel to run the engine at ICO.

Never knew that ..... been instructing on PA38's for years ... whooops !

BTW : I always demonstrate to my students what happens to the engine if the primer is unlocked - and not to 'ram' it back in if they find it out ......

Primers
 

The primer on the Cessna 152 routes to the front two cylinders only, presumably this is so that when the even worse pre-solo students flood the thing it will start too! One other thing, the copper primer pipes can become work hardened and shear at the connection to the cylinder - check it on the DI, as pilot may end up pumping fuel on to an engine and as it starts and heats up = fire! I speaketh from experience.

Sorry fellas, it's because there is an optional line for a cylinder head temp gauge that can be installed and connected to that #cylinder. (According to FlightSafety Academy Vero Beach) I think it's right, they're located next door to the Piper Plant!!