Stray HT sparking

[Geordie]

I'm baffled with this one. Noticed it when trying to adjust tickover. I get a reading of 1ohm between battery neg > frame > engine cases > spark plug body > carb body or any combination of 2.

I don't think the video will load here so I've put it into dropbox. The share link is:-

https://www.dropbox.com/s/hivx3xxo82krz ... S.mp4?dl=0

Any thoughts gratefully received!

[ducwiz]

Are you really sure that a low-resistance value reads between battery minus and the engine case?
Imho the engine is somehow electrically isolated from the frame and battery minus, so the spark's current doesn't see a low-resistance path to battery/engine case, hence a second spark is produced at the point of the closest sparkover distance. What happens when you pull the carb backwards a few milimeters?
You can check the faulty connection by putting a temporary cable connection between your engine case and the negative battery pole. If you do so, please report the result.

Hans

[blethermaskite]

Engine may be trying to earth itself through the throttle/choke cables? put a temporary jumper earth cable firmly connected to battery neg and an engine bolt/screw (not an engine to frame bolt) I think you have a dodgy earth path.
Cheers,
George

[Jordan]

External sparks so close to fuel is a seriously bad situation.

[DBDBrian]

I tend to think a dedicated earth is a must, too ensure a good connection.

P1080017.JPG

[Geordie]

Engine may be trying to earth itself through the throttle/choke cables? put a temporary jumper earth cable firmly connected to battery neg and an engine bolt/screw (not an engine to frame bolt) I think you have a dodgy earth path.
Cheers,
George

Agree, the carb is on a plastic sleeve so only conductor is via cable.

[Geordie]

Are you really sure that a low-resistance value reads between battery minus and the engine case?
Imho the engine is somehow electrically isolated from the frame and battery minus, so the spark's current doesn't see a low-resistance path to battery/engine case, hence a second spark is produced at the point of the closest sparkover distance. What happens when you pull the carb backwards a few milimeters?
You can check the faulty connection by putting a temporary cable connection between your engine case and the negative battery pole. If you do so, please report the result.

Hans

I borrowed a new Fluke with 1/10th Ohm division. Battery neg to spark plug reads 0.0 / 0.1 Ohm. With jumper cable from battery neg to head fin is the same.

To help matters along, the sparking has now stopped.

[Geordie]

I tend to think a dedicated earth is a must, too ensure a good connection.P1080017.JPG

Yes, I'll think on a retro-fit.