Wed Apr 28, 2021 3:01 pm
#1843475
My aeroplane is currently at its CAMO, to have its mags taken off and sent off for inspection, as I've been having trouble starting up for some time.
I'd be very grateful for any insight, especially into any possible causes of my starting problems other than the mags, so please bear with me and I'll describe the symptoms as best I can.
A little background first....
How did it start before?
How does it start now?
So.. thoughts...
If it was plugs or HT leads, I'd notice a mag drop during the power checks.
If it was priming nozzles, cleaning them ought to have improved it. The primer syringe stills feels the same as it ever did... the first shot is mostly air as it fills, so three shots is really about 2.8 shots, but that hasn't changed in almost 30 years. And in any case, the problem is not at all restricted to starting with primer.
It's not battery, as mostly it's fully charged, and AIUI the quality of the spark doesn't depend on the battery, providing the engine will turn over enough for the impulse coupling mag to provide a spark.
So what's left?
Carburettor and Mags. Either there's no fuel, or there's no spark.
If the impulse coupling isn't working consistently right, that would account for everything.
I think (but I'm not sure, as the situation was a little confused with the battery not holding its charge - I bought the CTEK charger around this time) that this issue started around the same time as the replacement of the starter motor.
One question, I'll put this in bold, as it would account for a lot... Is it possible that the impulse coupling worked ok at a slower cranking speed, but no longer works properly with the engine being spun more quickly?
Apparently the mags were new with the engine in 2004, so now about 570 hours old. Last inspected 2019.
It's got to the point where I'm genuinely concerned about getting stranded somewhere, so I've been very reluctant to go anywhere other than locally recently.
So I'd be very grateful for any suggestions, thoughts, corrections to my thinking... anything really.
Thanks in advance.
I'd be very grateful for any insight, especially into any possible causes of my starting problems other than the mags, so please bear with me and I'll describe the symptoms as best I can.
A little background first....
- I've been flying the aeroplane for a long time, nearly 30 years, over 1000 hours, over two engines (O320-E2G, high compression STC). So I know how to start it. The problem is definitely new.
- The engine itself, once started, runs well. The compressions (measured about a month ago at annual) are 76, 78, 78, 78. There was no metal in the oil filter.
- The mag drop during power checks is virtually identical on each mag, less than 40 rpm. It burns a reliable 26 litres per hour the way I fly it, and although I haven't measured it accurately, a quart of oil every 8 hours or so. These consumption figures haven't changed for many years, if ever.
- The priming nozzles were removed and cleaned a few weeks ago at annual, without making any difference to the ease of cold starting.
- The starter motor was replaced with a Skytec model in September 2020. It spins the engine considerably faster than the old one did. See below for a comment on the possible relevance of this.
- The battery is getting a bit tired (last replaced Jan 2017), but it spends most of its time on trickle charge with one of these, and is good for quite a few starting attempts before it starts to lose enthusiasm.
- AFAICT, the alternator is fine. There's no low-voltage indication. The ammeter registers a charge after startup which gets smaller quite quickly.
How did it start before?
- When cold: three shots of prime, 1/4 inch of throttle, 2-3 blades, vroom.
- When hot (or just later in the day), no prime, 1/4 inch of throttle, 2-3 blades, vroom.
- Very occasionally, when very hot, no prime, if it didn't fire immediately I'd pull the mixture to lean, it would catch, and then push back to rich.
How does it start now?
- When cold: three shots of prime, 1/4 inch of throttle, at best, 5-6 blades, then it fires, the prop starts to spin. There's no enthusiastic vroom any more.
- When hot (or just later in the day), without prime, 1/4 inch of throttle, it often spins for quite a few seconds, nothing. No coughing as if it was trying. Sometimes it coughs. Sometimes if I then give it a bit of prime it starts, albeit without enthusiasm. I've over-primed it twice this way and had to wait ages ... but I've stopped doing that and the problem hasn't gone away!
- I've tried the 'lean then rich' trick a few times, no good.
So.. thoughts...
If it was plugs or HT leads, I'd notice a mag drop during the power checks.
If it was priming nozzles, cleaning them ought to have improved it. The primer syringe stills feels the same as it ever did... the first shot is mostly air as it fills, so three shots is really about 2.8 shots, but that hasn't changed in almost 30 years. And in any case, the problem is not at all restricted to starting with primer.
It's not battery, as mostly it's fully charged, and AIUI the quality of the spark doesn't depend on the battery, providing the engine will turn over enough for the impulse coupling mag to provide a spark.
So what's left?
Carburettor and Mags. Either there's no fuel, or there's no spark.
If the impulse coupling isn't working consistently right, that would account for everything.
I think (but I'm not sure, as the situation was a little confused with the battery not holding its charge - I bought the CTEK charger around this time) that this issue started around the same time as the replacement of the starter motor.
One question, I'll put this in bold, as it would account for a lot... Is it possible that the impulse coupling worked ok at a slower cranking speed, but no longer works properly with the engine being spun more quickly?
Apparently the mags were new with the engine in 2004, so now about 570 hours old. Last inspected 2019.
It's got to the point where I'm genuinely concerned about getting stranded somewhere, so I've been very reluctant to go anywhere other than locally recently.
So I'd be very grateful for any suggestions, thoughts, corrections to my thinking... anything really.
Thanks in advance.