coil question

[the_last_hour]

Since we figured out that the coil was causing my problem for the "i need the experts on this one" post, I now have a question about how the coil affects things. `
Can a analog tach have an internal problem that somehow messes up the coil? Through trial and error we noticed the following...
When tach wire is removed from coil, the boat runs fine.
When the tach wire is re-connected the boat surges at 2400 rpms and up. The analog tach drops to zero at 2450 rpms,and sometimes bounces everywhere, but the floscan digital tach keeps measuring signal upwards of 3400, when I slow the boat down, the analog comes back in at 2450 again. Why doesn't the digital drop out when they share the same wire, why does the analog drop out at the exact rpm that the surging problem presents itself? Do I run a new tach wire, or replace the tach?
Thanks again for all of the great help.
STeve

 

[makomark]

I think your issue is the tach effecting the coil's function.

a classic failure mode for the 'analog' tach is an internal failure of its circuitry which provides a short from its input lead to ground. The digital tachs usually are built with an electronic "buffer" at their inputs; if they failure, the classic mode is to open (vs short to ground).

I've guess that your anaolg tach has an intermittent failure. I'd pull the tach wire from it's input, tape up the connector, and ty-wrap it so it can't become grounded. If that resolves the issue, time to get another tach.

FWIW, I've never seen or heard of one that failed and induced a "lean pop" thru the carb. I can envision a failure mode where you could get the "pop" thru the exhaust due to the ignition cycling while the engine is spinning.