failing spark plugs

[steve williams 3Participant @stevewilliams3]

[steve williams 3Participant @stevewilliams3]

can anyone help, i have recently finished makeing a 23cc 4 stroke engine of which i a realy chuffed BUT i have a problem. the engine starts fine & runs for afew minutes ishut it down make a few adjustments then start it again and it will run again realy good. after doing this a few times i found it wouldn’t start invastigations found the spark plug was no longer working new one purchased (MINIMAG FROM REEVES) started fine again for a few times then the same happened, no spark. just purchased a more expensive plug (rimfire from hemingways & blow me after 5 mins of runing same again plug has failed. i am runing 12 v gel battery, mechanical contacts with condensor & a small motorcycle coil. does anyone have any ideas as its getting expensive & frustrating

[Pat BraveryParticipant @patbravery]

I would check the coil out as the internal insulation could be breaking down when it warms up,

[AndyBParticipant @andyb47186]

I would check the condenser, coils tend to misfire when they start to break down before totally failing. Are the points burning from too much arcing? They could be blackened and maybe starting to pit.

Please give a bit more detail about what actually happens when it dies as this can give a good clue.

Edited By Andy Belcher on 19/11/2009 20:23:08

[Les Jones 1Participant @lesjones1]

Hi Steve,

                 If the problem is caused by the plug and not the previous suggestions then I think oil is getting past the rings into the cylinder or the mixture is very rich. Is the plug black with a carbon build up? If it has a layer of carbon on the insulator this will short out the spark. If the mixture is correct the plug should be brown in colour. Black indicates a rich mixture, grey indicates a weak mixture. Try sandblasting one of the failing plugs to remove any carbon build up and see if it then works.

Les.

[Martin CottrellParticipant @martincottrell21329]

Hi Steve,

 

It sounds a bit odd that you should be knackering plugs after a few minutes running and it does point towards a coil break-down as Patrick said when the coil warms up. Have you actually tested the continuity through the plugs to check that they are indeed dead?

Have you re-tried any of your “dead” plugs after a cooling down period?

 

Another thing you could try when the fault occurs next, is to remove the spark plug wire from the end of the plug and (using a pair of insulated pliers!) hold the end of the plug wire just off any bare metal surface on your engine and flick over the flywheel and see if you get a spark from the end of the wire. If you get a spark then probably the plug is faulty. If you get no spark then you need to look elsewhere, coil, points, condenser & also check all the associated connections are clean and tight. One thing quite often overlooked is the plug lead itself. Check the continuity through the plug lead. Quite often a broken plug lead will allow a spark to jump the gap in open air when carrying out the test above but will fail to give an adequate spark for ignition in the pressurised confines of the combustion chamber.

 

Hope these few pointers may help but ,as Andy requested above, a few more details of what happens and what you’ve actually tried would be good!

 

Regards, Martin.

[steve williams 3Participant @stevewilliams3]

thanks everyone that replied, problem now solved. it seams to have been a combination of faulty coil & bad conection on plug both rectified now running well

many thanks

steve

[Ian AbbottParticipant @ianabbott31222]

To check for power at the plug wire, get a bystander, wife, kids, cat’s tail, they will tell you if the coil output is satisfactory.

 

Don’t ask me if I’ve done this.

 

Ian 

[Author]

Posts