Well Norm, that depends.

On whether you want a permanent installation, or just to be able to check revs on one occasion.

For the former option, small digital tacho/hour meters with inductive sensing and internal battery power can be had very cheaply, these days. The sensor wire is just wrapped around the plug lead 3-4 turns.
See some of them in the list that this search brings up: http://www.ebay.com.au/bhp/digital-tachometer.

For the latter, one of the above units could be used, or one of the optical/laser tacho units that also come up in that list. Using these would require putting a reflective mark on the flywheel screen, but that's easy to do. Even a silver paint pen might do it.

Or there's the old vibrating wire tacho, like the one I've been using for years;
[Linked Image]

These are still available through Briggs dealers as P/No 19200. Particularly useful for tuning chain saws, where an optical tacho can't really be used. I'm not sure how well they work on engines with balancer shafts, like the GXV340, though.

Cheers,
Gadge

"ODK Mods can explain it to you, but they can't understand it for you..."

"Crazy can be medicated, ignorance can be educated - but there is no cure for stupid..."