question Magneto short?

[HunterXL565]

Hi all,

 

Has anyone ever had the problem of a spark when the plug is out of the cylinder, but no spark when in the cylinder under compression. I thought it might be a short in the magneto. Does anyone know any good ways of checking for this, because i'm unsure wether this is the engine, or the lack of a spark. Sorry, I'm not that clued up on electrics :nut: This is on a 1979 Can-Am Bombardier with CDI ignition by the way.

 

Thanks in advance,

 

Tom

[fv1609]

Does anyone know any good ways of checking for this,

 

Tom, what about one of those old colour tune kits. It allows you to have a sort of glass sparking plug thing, so you can look at the spark. Then you can see if conditions are favourable for a combustion.

[mike65]

Tom, what about one of those old colour tune kits. It allows you to have a sort of glass sparking plug thing, so you can look at the spark. Then you can see if conditions are favourable for a combustion.

 

Used one of those in the past on various vehicles, very handy when you can get at the plugs. Not very good for VW beetles though.

Apparently you can still buy them but depends on spark plug sizes.

 

http://www.baconsdozen.co.uk/tools/Colortune.htm

Halfords also list them on their website and theres bound to be one on ebay.

 

Had a missfire on a 2.8 capri once, similar symptoms could get spark when plug was out but apparently not when installed. turned out it was sparking but would not ignite fuel. CAn't remeber exactly what cause was but seemed to be cured after a bit of fiddling and new plugs.

 

Has anybody tries a colour tune on a 24volt Land Rove with shielded cables?

 

Mike

[Richard Farrant]

Could be a duff plug. Years ago, most garages had a spark plug tester, where a plug could be subjected to sparking under pressures similar to the cylinder. We used to check new ones before taking out on a job, not unusual to get one that failed under pressure.

[HunterXL565]

Thanks all, I think i just might invest in one of those testers! I have tried many different plugs (correct types of course) with no joy. I've always prefered the points, as you can see the problem easier, whereas i've not a clue whats happening inside this sealed CDI!

 

Thanks,

 

Tom

[philb]

First, try another plug (if you haven;t already) Second, try reducing the plug gap to about half what it should be. The engine should still work OK and it will be much easier for the spark to form. If it works with a small plug gap you then have to find where (if not in the plug, as Richard suggested) the juice is leaking before it gets a chance to spark.