Handheld Blower RPM question

[vibblueser]

Was trying to measure idle RPM of my 2 stroke handheld blower. Used the following two methods.

1. Treysit Sirometer https://www.amazon.com/Briggs-Stratton-19200-Tachometer/dp/B0044AV5VK/ref=pd_sbs_86_1/130-4122328-9532053?_encoding=UTF8&pd_rd_i=B0044AV5VK&pd_rd_r=bb595c39-ca1f-48be-86b2-f3ea8653c163&pd_rd_w=p5WGW&pd_rd_wg=sVOgA&pf_rd_p=d2a3ee7d-5d69-4331-b32f-a3ce40b5e0d3&pf_rd_r=982KEKXAMB63PZ7P975P&psc=1&refRID=982KEKXAMB63PZ7P975P
This method worked fine and showed RPM about 3000

2. This is the confusing one to me.....RPM using HZ function on multimeter
I measured using the HZ function on multimeter and found it be between 98 and 101HZ. Avg about 100 so 100 Cycles/second times 1 spark/revolution times 60 seconds/min equals 6000 RPM.

I realize that is double my actual RPM.....is my logic flawed on the calculation. Blower is this hitachi https://www.amazon.com/Hitachi-RB24EAP-Handheld-Lightweight-Commercial/dp/B003VYC31Q

It is a 2 stroke so 1 spark per revolution right but why do I get the right RPM if I make it 2 sparks per revolution? Any help or insight would be greatly appreciated.

 

[wayne55]

On a Husqvarna 125b the std. idle speed is 2800-3200 and max speed is 8600 just to give you a comparison. I have a sirometer and confirmed 2900 and 8500. Can't help with HZ.

 

[pancho400cid]

Yes - A two-stroke fires the plug once per revolution.

As you say, 100 HZ per your meter would be 100 revolutions per second = 6,000 RPM "IF" the meter was doing what you expect.

My guess is the Hz function of of your DMM is only made to understand/deal with sinusoidal AC inputs in a comparatively low voltage range and a limited frequency range. I suspect it isn't accurately reading a high-voltage DC square wave input as a plug presumably has. I'm not an expert on small engine ignition systems.

...

 

[driz]

I bought a $10 no name no touch reader off Ebay last year. Works great to 30,000 rpm so you can check your jet fighters turbine with it. All you do is stick on a tiny reflective sticker that looks like a 1960s cap pistol cap on a rotating shaft or pulley. Then fine it up and point. It even has a memory function. Runs off a 9V radio battery. It isn’t made too terribly and even has a cheap but functional case. Well worth the investment .

 

[Steve_P]

I bought a china digital tach off of ebay also. With mine you wrap the wire around the spark plug wire. It seems correct but I have no way to verify. If it's off, I know it's by less than 10%

When I worked in a mower shop we used something like the BS tach in your link on mowers. After a while we got a no touch tach from Stihl to set WOT on chain saws and blowers.

Why do you care about the idle speed? You just want it to not stall. If it's just for fun, then ok, but you want to set the mixture to control the WOT speed- that's what's important.

 

[American Locomotive]

Basically, the Hz function of a DMM is really only good for pure AC and MAYBE a squarewave. I assume you must be measuring on the kill switch wire, or putting a meter lead against the spark plug lead?

 

[larry_g]

If your meter is reading 2x then it is measuring transitions. Not just the rising or falling transition but both of them.

lg
no neat sig line

 

[ridervfr]

https://www.ebay.com/itm/Tech-Tack-...983654?hash=item2afb5fc3a6:g:pc8AAOSw8JRf8Tok

This is what I have