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By TopCat
FLYER Club Member  FLYER Club Member
#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....

  • 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.
User avatar
By Paul_Sengupta
FLYER Club Member  FLYER Club Member
#1843487
TopCat wrote: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?


Technically, yes, and not unheard of, but unusual if it's working how it's supposed to. Usually even with a high speed starter it's not fast enough to kick out the impulse coupling.

Maybe they'll find something when the look at the mag.

I seemed to have the opposite problem with mine the other day, there was a clacking sound with the engine running, which I assumed to be the impulse coupling still engaging. I shut down, but gave it a go again, and this time it wasn't happening. Just lack of use over the past year I guess.
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By rikur_
FLYER Club Member  FLYER Club Member
#1843490
Sounds a bit like our experience in 2019. In our case it turned out to be a mag fault, albeit no other noticeable impact except poor starting.
User avatar
By Rob L
#1843626
TopCat wrote: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?.....


I have heard anecdotal evidence of this being possible. Impulse springs only work less than about 600rpm; above that rpm they just work like non-impulse mags (as you would expect). The very anecdotal stories I've heard is that using new higher-speed starter motors and Slick Mags is that the mags need adjusting in some way to do with the E-gap (which is to do with when the magneto rotor provides the maximum magnetic effect).

I'm not sure of the details...I've seen B&C and/or Skytec starters (can't remember which) fitted to C152 (Lycoming 0-235 engines) with no apparent discrepancies, so I might be talking out of my elbow.
User avatar
By Rob L
#1843628
Further thinking: what make of mags? And how many hours use between new in 2004 and 2019?

I see you're having them "inspected", so may I assume they're Slicks?
User avatar
By Charliesixtysix
#1843638
You are doing the right thing by having the mags checked. I would await the results of that and try it when they are refitted before altering anything else.

It is sound policy to change only one thing at a time, or you stand the risk of not knowing what effect each action has on the problem
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By johnm
FLYER Club Member  FLYER Club Member
#1843653
My money would be on the Mags having some sort of fault so agree with advice above....
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By PaulSS
#1843655
My limited understanding of batteries shows that a battery can be fully charged up (12 volts on the voltmeter) and yet still not hold the cranking amps. Modern batteries (according to a battery shop I once spoke to) are designed to stop working when they’re knackered, unlike the older ones which tend to fade away. Maybe this is what yours is doing.

Are you able to borrow a known, good, battery to test your starting? That would be a cheap and easy way of ruling out that possibility before getting complicated with induction leaks, mags etc.
By riverrock
FLYER Club Member  FLYER Club Member
#1843676
If it had been before disassembly you could have tried starting on the non-impulse mag and see if you got the same starting result.

We had issues sharing our Bulldog which required much fettling but that's an injected engine so I won't confuse the issue!
By Peter Kelly
#1843702
Perhaps not specifically helpful to the OP's case, but there often appears to be a disproportionate focus on the ignition system at the expense of the fuel system both during maintenence and operation.

We often see carburettors in very poor condition, some of which have no recorded maintenance for 50 years or more. Many small Continentals are even fitted with the wrong carb. They all look the same so must be interchanagebale - no?

A fully refurbished or exchanged carburettor often makes a world of difference to starting and running. An exchange carb looks expensive until you work out is save a couple of litres an hour in fuel-burn... Reliable starting is a bonus.

Specifically addressed to the OP, these engines are so simple that any change in performance should be a cause for concern. I always think the engine is trying to talk to you and if you listen very carefully, you will be able to ear and understand what it is saying. Keep listening and don't ignore the problem hoping it will go away.

P
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By GrahamB
FLYER Club Member  FLYER Club Member
#1843707
I agree @Peter Kelly .

It's easy to thing 'sparks' straight away and ignore other things. The lack of 'enthusiastic vroom' speaks fuel to me.

Some other possibilities:
- inlet leak - we had one of those once on a Grumman, albeit a Tiger not a Cheetah
- wear or something working loose in the carb so the correct mixture is not being delivered
- blocked idle jet

equally - crappy plugs. It's not unknown to have more than one go sour within a short period of time of one another.
By TopCat
FLYER Club Member  FLYER Club Member
#1843745
Peter Kelly wrote:Specifically addressed to the OP, these engines are so simple that any change in performance should be a cause for concern. I always think the engine is trying to talk to you and if you listen very carefully, you will be able to hear and understand what it is saying. Keep listening and don't ignore the problem hoping it will go away.

I absolutely get this. I've been flying it so long that I don't so much fly it as wear it. It does talk to me, as you say.

I wouldn't ignore it, that would be rude :)
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By Peter Kelly
#1843783
Also check the resistor in the plugs. Often missed during service work.

Should be about 1K ohm when new. They slowly increase in resistance with time (in a very variable and unpredictable way). Anything above about 4-5K ohm needs put in the bin.
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