The BSA A7-A10 Forum

Technical => A7 & A10 Engine => Topic started by: a101960 on 17.08. 2008 14:25

Title: Smiths Chronometric Rev Counter
Post by: a101960 on 17.08. 2008 14:25
I have often wondered why the Smiths chronometric rev counter never returns to zero. It always registers the RPM at which the engine was cut. Does anybody know why this happens?
Title: Re: Smiths Chronometric Rev Counter
Post by: a10gf on 17.08. 2008 18:59
Mine returns sometimes 'close to' zero, sometimes not so close, I'd say this has some mechanical cause, either needs adjustment or lubrication somewhere inside the counter, not looked in there yet.
Title: Re: Smiths Chronometric Rev Counter
Post by: dpaddock on 19.08. 2008 15:22
The working principle of the Smiths chronometric instruments is not well understood by most people. Suffice it to say that they are a clock-like mechanisms - which is why the tacho is called a rev counter.
I have a set on my Goldie and the rev counter needle often fails to return to its zero position - which is actually 500 rpm - unless I give it time to idle before shutting down. I suspect this because the mechanism has a limit below which it simply can't count in real time. I've looked on the Internet to find an explanation of the principle but there's nothing out there.
In any case, it seems to me that as long as the instrument functions satisfactorily above 500 rpm, it's of no concern whether it returns to "zero" each time.
Someday I'm going to look inside the instrument and figure out how it works, but today I'm just going to ride!