Electric primer stuck open

Allan Moore · #1 07-22-2020, 01:41 PM

My RV6A is a O-360, has an electric primer solenoid to the #1 cylinder. its wired off the electric boost pump through an independent momentary push switch. So normal ops, boost pump on - push primer button 5 seconds- boost off and hit the starter.
one time in flight, boost pump was selected on to switch fuel tanks and the #1 cylinder EGT went to zero, the CHT plummeted and the engine ran very rough to indicate only 3 cylinders were working. As soon as the boost pump was turned off the #1 cylinder came back to life. I have since installed a kill switch going to the primer solenoid.
Has any one had any issues with this type system. Next time I remove the hood I will access the data plate for more information.

DeeCee 57 · #2 07-22-2020, 02:30 PM

well, ain’t the reply u hoping for, but haven’t had any problem with my primer system... logical, since I rarely, if at all, use it. Mine’s on #1 and 2.

Seeing your location as Fl, and guessing you have a similar set-up to mine, e.g. carbed O-360, I suggest starting the engine without that primer: using the accel pump of the carb by pumping that throttle lever once or twice, depending how long you haven’t run that engine, should be enough. The only times I use that primer is in freezing or sub temps.

Even at my place, where temps are usually way lower than yours, I’m thinking of removing that system altogether, saving weight and gaining simplicity

kaa · #3 07-22-2020, 11:03 PM

It sounds like either a push button failed short and energized the valve, or the valve itself failed. If it's the push button, I'd replace it with something more robust (I use a Honeywell 1TL momentary switch). If it's not the button, I'd check that the solenoid valve is plumbed correctly (the ones that ACS sells are directional, and I think there is a possibility that the pressure will open it if it is incorrectly connected.

Also, what would a kill switch do that that the push button (or a switch) doesn't do?

avrojockey · #4 07-28-2020, 08:32 PM

I'm with DC on this...completely remove the system. Save weight and eliminate points of failure. I have no primer and I start off the accelerator pump. The number of pumps is temp dependent but usually no more than 3 will light it off in cold WI winters. After you pump the throttle, return throttle to idle to act as a choke. Engine should fire up within a couple blades, then adjust throttle to normal idle. This starting technique is great because you don't accidentally get a hugh increase in RPM after light-off causing all sorts wear.

If you are still hesitant about getting rid of primer make sure and solenoid you have installed makes the system fail-safe. Solenoids are powered open and/or closed. A primer solenoid should only open when power is applied and spring-loaded closed when power is removed.

Allan Moore · #5 08-06-2020, 01:22 PM

Thanks to all who replied. I found the primer momentary on switch had an intermittent short. On the bench with a VOM I tried it several times and it opened and closed fine (on and off). Then after a few more attempts It would fail ON even with the button all the way out. It was fickle and I obviously replaced it with a new switch. I am still glad to have an Emergency cut off switch in line so the electric boost pump could still be used with another failed primer switch. I know a duel failure not likely BUT! NOT LIKEY HAS CRASHED MANY AIRPLANES.

Snowflake · #6 08-07-2020, 09:10 AM

For those pumping the accelerator pump to "prime" the engine... Are you doing that with the starter engaged, or just at standstill? I find if I do anything more than one pump on the accelerator with the prop not turning, my airbox starts draining fuel onto the ground.

Ralph Inkster · #7 08-07-2020, 10:56 AM

Good thought, Snowflake
"For those pumping the accelerator pump to "prime" the engine... Are you doing that with the starter engaged, or just at standstill?"

While turning over - that is my preferred procedure on carb'd engines. Instead of using the primer, I just pump the carb while turning over the engine. The engine usually starts much smoother this way, avoiding the 2 cylinder chugging event you get from un-uniformly primed cylinders.

As for over priming - aside to the mess on the ramp which could be caused by either over priming or over pumping, check the recent thread where Walt found a filter that wore thru the bottom of the airbox, accumulated fuel/dirt goop probably acting as wet sandpaper there.
Another potential is the increased risk of a start-up engine fire if it decides to pop back.