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engine stumble

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pfstelwagon(at)earthlink.
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Posted: Tue Feb 20, 2018 8:18 am Post subject: engine stumble

I think Jan is correct, the coil probably fires at the wrong time rather than shutting off.  In the past the standard in auto racing was the Vertex Magneto.  The Vertex had an internal coil, Mallory came out with a magneto which used an external coil, eventually Vertex went to an external coil. In Top Fuel racing dual mags are used (MSD Mags) with external coils.  It seems that some people learned that packing everything in one place generates a lot of heat, heat and electrical components are not the best of friends. The Russian and Chinese engines were developed quite a while ago and were state of the art at the time but time moves on.

 

Frank

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pfstelwagon(at)earthlink.
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Posted: Mon Feb 26, 2018 10:58 pm Post subject: Engine stumble

One would suspect that if the fuel in the intake tract is burned it would backfire thru the carb. That won’t happen as the super charger impeller is a still forcing the fuel/air towards the cylinders preventing the backfire thru the carb.



Frank  N23021

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Looigi

Joined: 20 Apr 2015
Posts: 80
Location: Christchurch, New Zealand

Posted: Mon Feb 26, 2018 11:28 pm Post subject: Re: engine stumble

Hi Frank,



I can tell you from personal experience on P&W engines it is entirely possible to backfire them out of the carburetor.  In fact the old guys who flew DC3's  around southern New Zealand back in the day used to intentionally lean out the mixture so the resulting backfire would clear the ice from the intake screen.



The supercharger produces relatively low pressure compared to a bunch of fast burning fuel!



But good call dude, I had not considered the condenser.  If that were to fail with a short circuit the mag would cease to fire the plugs on that mag, but I am not sure how to explain the other mag going out in sympathy??

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wlannon(at)shaw.ca
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Posted: Tue Feb 27, 2018 9:43 am Post subject: engine stumble

Hi Looigi;



The other mag is not going out in sympathy.  It continues to run normally 

and fire in sync.  The subject mag. just fired a plug in the intake stroke 

igniting the incoming charge essentially stopping the engine for a second.



Walt



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Looigi

Joined: 20 Apr 2015
Posts: 80
Location: Christchurch, New Zealand

Posted: Sun Apr 22, 2018 1:55 am Post subject: Re: engine stumble

Hello again All,



I have pulled the dud coil apart and found an intermittent connection between the secondary coil and the plate that connects to the cigarette.  Out comes the soldering iron!



I have also been looking into the cause of these failures.  



The internal resistance of my plugs varies hugely.  I didn't do a good survey when I had them out last time, it was just a passing observation.  I have an annual check due in a few weeks so I will have a good measure up of them then.  I am thinking that the high resistance of the plugs is causing the 'spark' to take an easier route to earth through the coil somehow.... As you can see, my theory is a work in progress.



Can I please take a bit of a survey of you guys?



Of the guys who have had a magneto coil failure, how many were running the standard Russian spark plugs?



Cheers

Chris

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