I have a Zenoah G45 I have been tuning and have it running pretty good. I am pretty new to gas engines and was curious, where do you place the tack when taking an RPM reading? I put it about 2" being the prop about mid prop at it's widest point. I have noticed that it does vary based on where you place it. Where is an accurate reading taken from? I am turning about 8100 on the high side and about 2550 on the low side with a Zinger 20X10 prop. Does that sound about right?

Thanks all ! ! !

Sorry all....I meant 2" behind the prop.

That seems like a very high idle speed? Both my G-62 and GT-80 idles at about 15-1700 rpm. Any higher, and the plane won´t land..

I'm also very intersted for an answer to the original question.

I just bought a Tower Hobbies tachometer, and no mater where I place it on my Homelite 25cc, I get readings from 6000 to 9000 rpm at full throttle, but it never stabilizes, it keeps going up or down.

Should I place it in front of the prop instead? and where should it be, at the base, middle or tip of the prop?

Thanking all in advance.

My Glowbee tach is also erratic, I have found that the background behind the propeller will make a difference, try a shady area or have a dark background.

Huey, you might be on to something. I did not think about it, but most of these tachs use photo cells that measure movement based on disruption of the light. I will try a solid colored background.

As far as using it in front of the prop, it should not make a difference. From a safety perspective though, I would always hold it behind the prop. Never know when something could turn loose and the plane lunges forward. Good comments though. I have a friend with a different tach than mine, and it is erratic +- 1500 rpm. Maybe these things are just not that accurate and are intended to give you a ball park reading.

Thanks guys.

Also I am thinking it should probably be placed at the widest point of the prop. What do you guys think?

Chris

I am new to using a tach also I have the tower hobbies mini digital it was also reading very eratic until I put a incandescint light on the opposit side of the prop aimed at the engine.
Now I get very steady readings. Don't use flourescent as they cycle 60 times a second
your tach will read 3600 rpm soo it may affect your readings below that. Give it a try.

Pete

An incandescint light also cycles 60 times a second if its powered from the ac power. Use a flashlight.

I'll try that. It didn't seem to affect the tach maybe it was far enough away?

Pete

Just point the plane to the sun and take the reading from behind the prop.

In low light conditions it can help to have a light coloured background to aim the tach at,with the prop rotating between the light coloured surface and the tach obviously...

In the dark, use a battery powered light to take readings against. Any AC powered light source will mess things up.
If you have a white or light coloured prop, you can also hold the flashlight behind the prop. It will reflect on the prop and activate the photocell.

What you are looking for with these photocell tachs is contrast...

Dirtybird and Rudeboy are right, I think what was helping me get a cosistant reading more than anything was my garage door being in the background.

Pete