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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!

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

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

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

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

blethermaskite wrote: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.

ducwiz wrote: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.

DBDBrian wrote: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.