Measuring engine rpm??

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bluedragon

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Does anyone have any bright ideas on measuring engine rpm on my single cylinder Sabb engine? I'd like to set the variable prop maximum pitch to give an optimum engine speed of 1500-1800 rpm at full throttle but don't have a tacho. 400 "donks" per minute is a bit fast to count!! I need some method of measuring flywheel rpm visually or by some simple mechanical counter?
 
On my 6 cylinder Gardner I use a Bicycle speedo. The head unit is on my dash. The sender unit is mounted on the engine on a piece of stainless tube, same size as handle bars. A small magnet is mounted on the big pulley on the front of the engine. The sender unit is mounted so the magnet passes with in 3 mm. The unit is set up to read kilometers per hour with a 24 inch wheel. This gives a figure with the decimal place in the wrong place, 900 rpm shows as 90.0 for example. My engine operates between 350 and 1000 rpm and we have compared the tacho with an electronic test unit through the range with an error of about 4%.
 
What you need is a cheap little German-made gizmo with a vibrating wire that you reel in and out until it resonates. Mine is invaluable; cost about £8 mail order from US tool shop. Unfortunately it's on the boat and I can't remember the name.

Regards, Mudhook
 
Bluedragon; I have a small digital tacho on my yacht. Its made by an American company "Tiny Tach". So simple to install, 3 wires, 2 for pos and neg, 1 to a tiny transducer that clamps round injector pipe. Thats it! Have a look at www.tinytach.com/tinytach/diesel.php The UK contact is Richard Dunn E/ mail itco@btopenwold.com. this guy is extremely helpfull, quick delivery and doesnt want payment until you are up and running. How bad is that in this day and age!
 
I knew you guys would have the answers! I'm trying the bicycle speedo idea right now. Cost £5 in Halfords so nothing to lose. I've calibrated it to show 100 kph when the engine is doing 1000 rpm...so little mental arithmetic involved and it could be fitted permamently if I wanted to. The diesel Tiny-Tach looks very interesting as it'll give engine hours as well as rpm and will certainly do the job I'm sure. I've contacted the UK distributor for a price. Many thanks to all.

PS - who noticed my not so deliberate mistake? 1600 rpm on a 4 stroke single is 800 donks per minute, not 400 isn't it...
 
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"Tiny Tach". So simple to install, 3 wires, 2 for pos and neg, 1 to a tiny transducer that clamps round injector !

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Thats another good idea. Never thought to use an injector pipe. Could probably adapt a heart monitor or one of those things walkers use to measure distance they cover. I am amazed there have not been cheap tachos developed from the many electronic counters about, thats all a bike speedo is. The "tinyTach" looks good, just expensive for what it is.
 
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