PA-28 Primer “Leaking” into Cylinders??

HGeorgeC*

Hi All!
A friend has a very nice older PA-28 with a new engine…
Apparently it’s been running very rich and his mechanic thinks the Primer may be leaking into the cylinders when it’s running.
Quick question….
Is that even possible??
Every Cherokee I’ve ever flown the plunger type primer was, I thought, a sealed pressure system that required you to pump the fuel into the cylinders via the plunger on the panel. You made sure the primer was locked closed after use, but once you used it, no more fuel would be introduced into the cylinders via the primer system. Can the actual Primer Pump leak through and draw fuel from the fuel strainer??
When leaning, the engine does not start to run rough until the mixture is almost full aft.
Any thoughts? Anyone ever seen this?
Thanks in advance!
Happy New Year!

George
Seneca N8434M
KLPR

empannin

It might be possible at idle. The only "pump" is the differential pressure in the intake vs fuel tank. There is no way it is possible much above idle as the primer line is way too small.

If it is too rich at idle then the first choice should be the idle mixture screw, the mixture control, and maybe the float valve.
I am not sure why it is not a practice but a wide band O2 monitor works fine with 100LL (70 hrs on an RV10 with one) and could be used to adjust mixtures with more precision. I would put stochimetric at about 2/3 of full mixture at idle so you can go from rich to cutoff. Could use to check at other settings too.

Eric

HGeorgeC*

Hi Eric!
Thank You for the information!
I’ll pass it along!
George

griff390

George;

Agree with Eric. It's extremely unlikely that a primer line will cause a rich condition. If your mechanic wishes to verify, have him/her pull the plugs from one or more of the jugs that have a primer (Piper systems only prime 3 cylinders), and the plugs from the non-primer cylinder.

Reading spark plugs may be a lost art, but it can tell you a ton about what's going on inside the engine.

Here's what to look for: if the primer is causing a rich condition, the plugs from the cylinders with the primer lines will be blackened with carbon, and the plugs from the non-primer cylinder will show normal burn coloration. If the plugs from the cylinder without a primer are also black with carbon, it would confirm that the problem is elsewhere.

I'd bet the problem is one of the items Eric listed.

HGeorgeC*

Hi Griff!
Outstanding!
Thanks for the info and the Troubleshooting Tip!
You guys Rock!
George

kentshaw

If the primer is left unlocked, you will have rough running, hard to shut down engine, and maybe backfiring. I would rebuild the primer pump with one of the rebuilding kits and eliminate the pump altogether. Takes about 5 minutes to do from the front of the panel. The rebuild kits are inexpensive.

HGeorgeC*

Thanks Kent!
We are looking into that exact thing too!
Thanks!
George