Measuring engine RPM

[lightning]

I need to measure the RPM of my Tohatsu 9.8 so I can determine which propeller to fit when using it on my 2.85m SIB.
Tests on the water have proved inconclusive and I need to determine the RPM at WOT although Tohatsu have recommended the higher pitch prop on this small boat (9.5 as opposed to the 8.5 fitted as standard)

I can see the "Tinytach" at £65 or so, but as I will only really need it once, I wondered if anybody has tried the one from Hong Kong widely sold on ebay for under £10.

It only needs to work once....

Alternatively can somebody sell me a used Tinytach for a reasonable price, then I can sell it on when I've finished with it?

[lockieboi]

I've got the cheap £10 shanghai special, does the job although I can't vouch for the accuracy.

I do think about putting it in the car to see how accurate it is.

[TomKat]

tried one and seemed to work OK.

[TRevor Lawson]

Got cheapo version , wired mine into ford ka to check, not to far out from cars tacho prob about 200rpm, but calibration may not be too accurate on all ?

[lightning]

Thanks, I'll try the Hong Kong one then.

[lightning]

I actually ended up buying the Tinytach as it's available from some online buyers for £40 including postage.
I'll probably install it permanently on the engine, and attach it to the transom with velcro.

[lightning]

This arrived already from the USA.

The instructions say to wrap the pickup wire round one of the spark plug leads. The unit asks how many sparks per revolution the engine produces.
The engine is a Tohatsu 9.8 2-stroke. Would that be one spark per revolution? Bearing in mind the unit is connected to only one of the spark plugs.

[Downhilldai]

Yes..

[lightning]

Thanks

[jamie.rib]

I thought the tohatsu had a wasted spark, so dont that mean it will fire twice per revolution?

[lightning]

Wasted spark? Sounds like my wife after a night out with her friends

The unit does have a 2 sparks per revolution setting. I guess it will have to be trial and error.

[jamie.rib]

Yes on the tohatsu you have a wasted spark system, both plugs fire at the same Time, even if you mix the plug wires round it will run the same, both on top stoke and bottom stroke (which is the wasted spark). So you need the 2 spark setting so it can half the calculation of your rpm.

Regards

Jamie

[lightning]

Well I fitted the Tinytach and it works fine, but the RPM on the Tohatsu changes all the time at idle according to the Tinytach, from 900 to 1100

With the engine in gear it varies all over the place from 700 to 800rpm
although it actually sounds alright. It would be better if the Tinytach had some sort of electronic damper on the display to stop it being so unstable
I am guessing that a 4 stroke engine would not suffer this issue!

Still it should be OK for deciding what prop to go with