Week spark on magneto bike

[Moto Chuck]

I am resurrecting a 1967 250 Mark3 after a number of years and have been having a problem with weak spark. There is a spark if I lay the plug on the head but I can hold the spark plug with my fist against the head and don't get that much of a jolt.

Possible problems as I can see are: weak magnetism in the magneto bucket, weak coil, bad plug wire, or bad capacitor. I have no figured how to test each option and was wondering if anyone has run into this problem on a magneto bike. I have tried starting fluid in the bike and there is not enough spark to fire the starting fluid. The plug is brand new and gapped correctly.

I know that Ducati has made a gizmo to re-magnetize the magneto buckets but I want to be sure that this is the problem before I tear into the engine.

Ideas?

Chuck

[double diamond]

Start with the basics: points cleaned/gapped/insulated/opening/closing properly. Contacts not pitted. Condenser connected properly. Check all connections in the system for corrosion, contamination and ensure good contact, including grounds. Inspect high tension lead for good contact with the coil connection as well as the plug cap. Check the plug cap for signs of tracking (or simply install a known good plug cap). Verify you have a good taillight bulb and that the taillight bypass switch is in good order and wired correctly. The brake light switch also needs to be in good order, the correct type and wired correctly (providing the wiring is still in standard configuration). Are there any non-standard components added to the system such as kill switch or ignition switch? I would suggest removing these. Once you have ensured good system integrity and still encounter poor spark, then you can dig into the components.

Matt

[Moto Chuck]

Start with the basics: points cleaned/gapped/insulated/opening/closing properly. Contacts not pitted. Condenser connected properly. Check all connections in the system for corrosion, contamination and ensure good contact, including grounds. Inspect high tension lead for good contact with the coil connection as well as the plug cap. Check the plug cap for signs of tracking (or simply install a known good plug cap). Verify you have a good taillight bulb and that the taillight bypass switch is in good order and wired correctly. The brake light switch also needs to be in good order, the correct type and wired correctly (providing the wiring is still in standard configuration). Are there any non-standard components added to the system such as kill switch or ignition switch? I would suggest removing these. Once you have ensured good system integrity and still encounter poor spark, then you can dig into the components.

Matt

Have already gapped, cleaned and checked the wiring under the points cover. The bike should be wired correctly as it was running well when parked a number of years ago.

Will check the high tension lead components.

I will check the wiring to the tail light but as I remember those components are not added to the ignition circuit till the rear brake is engaged. It is only when the rear brake is applied that all the juice is routed to the brake light or the switch. OTOH, that brings to mind that I should check the grounding of the one leg of the brake light switch at the brake pedal. The bullet connectors are another possible source of dysfunction.

Good ideas! Thanks.

Probably won't get much chance to work on the bike till this weekend.

[Bevel bob]

I know it is possible to get these systems to work, however I would swap the parts to finish up with a 60 watt minimum 12v alternator ,a battery, an advance /retard as used on the coil ign bikes and a 12v coil. To this system you can add an electronic ignition booster or even pointless electronic ignition. Decent ignition will also help to keep the thing running while you sort out the iffy carburation!!.

[Jordan]

Fresh fuel?

[Moto Chuck]

Fresh fuel?

Less than two weeks old from a test bottle not the gas tank. Clean carburetor.

Have tried pouring gas down the carb throat and spark plug hole. Can't get the thing to even pop. With fuel problems you you can always get a bang or two as it goes from rich to lean or vice versa.

[Moto Chuck]

I know it is possible to get these systems to work, however I would swap the parts to finish up with a 60 watt minimum 12v alternator ,a battery, an advance /retard as used on the coil ign bikes and a 12v coil. To this system you can add an electronic ignition booster or even pointless electronic ignition. Decent ignition will also help to keep the thing running while you sort out the iffy carburation!!.

Would much rather keep it original at this time.

[JimF]

A bad condenser could negatively affect the DC voltage at the points. You can disconnect the condenser and see if your spark increases in intensity as a test. The problem with having no condenser is extremely accelerated points deterioration, but for a short duration test you could disconnect it.

It could be the coil has developed some internal shots which are serving to decrease the amount of effective windings, either in the primary or secondary side of the coil. In either side the result is the same, less spark.

[Moto Chuck]

A bad condenser could negatively affect the DC voltage at the points. You can disconnect the condenser and see if your spark increases in intensity as a test. The problem with having no condenser is extremely accelerated points deterioration, but for a short duration test you could disconnect it.

It could be the coil has developed some internal shots which are serving to decrease the amount of effective windings, either in the primary or secondary side of the coil. In either side the result is the same, less spark.

Yeah that is my understanding of the behavior of a bad capacitor. I do also have some spare coils in the pile from both magneto and alternator bikes, so I can try swapping out parts there.

[Bevel bob]

My 67 Mark 3 with conventional ignition and electrics goes to the Isle of Man every year and gets a thrashing ,its turned out to be the most reliable of the bikes (I take 3) despite a really very poor restoration by the previous owner and my own efforts. A strong ignition is the basis for a happy bike. I would not consider taking it if it had the Mag type system.Good luck with that!.