The place for technical discussions about GA and flying.
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By Flying_john
#1843813
Agree with most of what's been said about the mags, don't forget to get the mags properly timed when put back in. If the plugs are out do the Ohms check from top, to electrode <1k is ok. Does the impulse coupling "clack" on manual turning of prop ( on both if two fitted). Ignition switch fault where the mag(s) get intermittently grounded on the cranking position - test with continuity tester (battery disconnected) and then see if P lead(s) get grounded on cranking position of the starter.
Get carb overhauled if not been attended to for many years or hours.
Have you had the fuel lines pressure tested (if rubber) to check if the walls are deteriorated and limiting fuel flow.

I think thats enough straw grasping !
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By Paul_Sengupta
#1843902
I would guess if there was a fuel/carb issue, you'd get it firing on the primer fuel but then stopping after a few blades. If it doesn't fire at all, I'd say it would be more likely to be ignition.
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By TopCat
#1843904
Paul_Sengupta wrote:I would guess if there was a fuel/carb issue, you'd get it firing on the primer fuel but then stopping after a few blades. If it doesn't fire at all, I'd say it would be more likely to be ignition.

Once it starts, it idles fine at about 750-800 rpm if I close the throttle (I don't leave it there for long so I'd need to check again to be more precise).

No asymmetric mag drop during power checks.
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By Sooty25
#1843920
TopCat wrote:
Paul_Sengupta wrote:I would guess if there was a fuel/carb issue, you'd get it firing on the primer fuel but then stopping after a few blades. If it doesn't fire at all, I'd say it would be more likely to be ignition.

Once it starts, it idles fine at about 750-800 rpm if I close the throttle (I don't leave it there for long so I'd need to check again to be more precise).

No asymmetric mag drop during power checks.


Paul is right, if it was carb it would fire on the primer. Been there, in our case it was a sheared fuel pump drive. ( different engine).

Sounds like a weak spark to me. That would be less noticeable when running as well. Are both mags impuse mags?
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By Awful Charlie
#1844266
I've got a Skytec started, and yes it spins the engine quickly but not enough to cause a problem for the impulse coupling (which I have on only one mag)

Hopefully your mag overhaul will find the problem (make sure they set the correct E gap), and they were due a check at 500 hours anyway ;-), but you'll want to exclude the potential before you move further forward.

In my case the major reason for hard starting (particularly when hot) turned out to be the engine driven fuel pump - finally isolated after basking in bright sun and 40dC for a couple of hours, and with the fuel level as low as I've ever let it go - having got it started it needed to electric pump to pick up from the idle. (mine is an injected unit, so maybe a little different) No other problem showed itself (ie it was fine at full fuel flows in nose high climbs), it was only at very low engine speeds that it wasn't doing a good enough job
By TopCat
#1845903
Thanks for all your replies and suggestions, folks, the hive wisdom of this parish greatly appreciated as always.

Friday I brought the aircraft back to base after a 500-hour overhaul on the mags. Apparently there was quite a bit of rust inside and the points were corroded. Who knows whether the two lockdowns were responsible.

Anyway, both cold and hot starting are without doubt hugely improved. I refueled when I got back and whereas before the restart would take two or three goes, sometimes 8 blades or more, it was the good old two blades and a vroom.

Likewise today - cold start with prime was 3 blades and a vroom, then after 45 mins I came back and let it sit for half an hour, after which it was again two blades and a vroom.

I hesitate to count chickens, but it's definitely a lot better. And my wallet considerably lighter, but that's a hell of a lot better than getting stranded somewhere, or the engine stopping in flight.
By TopCat
#1846184
Flying_john wrote:Could you tell us who overhauled and at what cost please and what was replaced during the overhaul.

Gama Aviation at Bournemouth (mags shipped there by my CAMO at Shoreham).

Labour: 180, parts: 315 (both mags)

Parts list per mag:

Image

Plus a couple of seals.

Including VAT and shipping back: £618.80 :pale:

Plus the CAMO's labour (and a few bits and bobs in parts) for removal and refitting. £1200 all up inc VAT.
By TopCat
#1846202
Charliesixtysix wrote:Interesting to see you needed a pack of 25 cotter pins in each mag.

That hadn't escaped me. Still, hopefully it won't come apart...
By TopCat
#1846203
Any thoughts on the cost of that folks?

My aircraft has always been maintained by the same people, and generally it's been very reliable, so they must be doing something right. I have no idea whether I'm paying over the odds though.
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By Flyin'Dutch'
#1846209
Very reasonable!

They have charged you 580 inkl VAT.

Or 480 before VAT.

How much do you think they end up taking home from that?

This is not a pop at you and clearly as a Dutchman it is a compulsion to negotiate prices but it is not all gold that glimmers.

From a Class 2 Medical I do I do well if the bottom line gives 30 quid for my family to spend. Not a moan, just a fact.
By TopCat
#1846242
Flyin'Dutch' wrote:This is not a pop at you and clearly as a Dutchman it is a compulsion to negotiate prices but it is not all gold that glimmers.

For the record, I'm not trying to negotiate prices. The aircraft has been at the same CAMO for 30 years this summer, the last 23 of which in my ownership. Generally, it's been very reliable, so I figure they've been doing things mostly right. They send me the invoice, I pay it.

By saying "I don't know if I'm paying over the odds", I mean nothing more, nothing less than exactly that; I am certainly not implying anything else at all, and I'm certainly not looking to do anything on the cheap. I know very well from other walks of life that "cheap no good, and good no cheap".

I have zero experience of paying anyone else for aircraft maintenance, so I just don't know.

It would take some extraordinary information for me to consider moving.

Hope that's clear.
By riverrock
#1846411
Well people normally budget £1000 for a mag overhaul, so it's not hugely out of the ball park. Don't know how much time your CAMO spent checking other things before looking at the Mags either.