A7 speedex tickover speed,

[Mgibson]

hi I have a tuned engine with an SU carb and 6v electrics ,setting tickover at 650 rpm , if that’s correct speed it seems high and engine seems to top out at 3000pm, would the Rev counter if it’s a 12v unit read half maybe?

[John Cornforth]

Hi

A tickover of 650 RPM sounds about right. 
My road engine happily revs to 4000 (just over 30 mph in 3rd gear) so 3000 sounds too low.
I usually change up at 3500, my wife prefers 4100 as she is used to an MX5 ! 
The fact that the rev counter is 12v won't change the reading.
If you are running with copper HT leads and no plug suppressors the very high electrical noise can easily upset electronic devices, even ones with their own battery supply. Assuming coil ignition i.e. not magneto, try some 5000 or 10000 ohm plug cap supressors. They don't weaken the spark significantly.

Cheers,  John.

[Chris KC]

If the tickover 'sounds fast' it's more likely at 1300 than 650 I suggest.
A modestly tuned special ought to be able to touch 6000 rpm, the most likely barrier would be single valve springs which will kill it at around 4500 rpm or so.
There are several unknowns here but my money's on x2 tacho scaling error.