# Tachometer Question



## mh9162013 (Jul 28, 2018)

So I'm in the market for a tach, but it looks like they don't work like I thought they did.

From watching youtube videos of small engine work (2 stroke engines, like chainsaws, string trimmers, etc.), I thought you place your tach next to your engine when it's running to get the RPMs. But after looking online to buy one, it looks like I'm supposed to connect a wire?

What am I missing here?

Thanks ahead for your help.


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## spyder62 (Jul 28, 2018)

With the one I have, I can do either or. It has to be within a few feet to do wireless. Mine is an opama


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## wcorey (Jul 28, 2018)

Most of the small ones do either or, the wire is typically used on fixed installations like on a m'cycle where its not close enough to the motor for wireless.


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## mh9162013 (Jul 28, 2018)

Ahhh, that makes sense.

I'm considering this one: https://www.rcpw.com/echo-parts/gen...6e3CpQmayFPJAKbl1fjYbcehN2Kbq9UYaAnAuEALw_wcB

This is a really good deal, right?


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## spyder62 (Jul 28, 2018)

I’m thinking opama and pet are made by the same people (Japan) no expert but I think it is a good one. Good price


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## spyder62 (Jul 28, 2018)

I guess mine is just a different model. Like it alot. I paid about $100. It has a sealed battery, which some people don’t like. But it should last longer than me


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## mh9162013 (Jul 30, 2018)

I ended up buying the one I linked to in this thread. I'll let you know if there's anything especially notable about it.

Thanks again for your help.


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## rngrchad (Jul 30, 2018)

I run this and love it. I think the refresh rate is the most important thing on a tachometer. It will run off the pulse or a cord around the ignition or plug. I’ve had mine for about 8 years. Maybe more. Still works like a charm and I don’t baby it. It bounces around in my chainsaw box summer and winter. I keep it in the original plastic ziploc. It’s scratcyed and cracked and still kicking. 
http://www.baileysonline.com/Chains...-01-OEM-Pulse-Engine-Tachometer-502711401.axd


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## rich450es (Sep 18, 2018)

how did that tach work out for you ?


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## mh9162013 (Sep 18, 2018)

I can't say. My workshop was supposed to be set up a few weeks ago, but has since been delayed. Until it's set up, I won't have a chance to work on my 2-stoke engines. When I do, I'll update you all and write a blog post to provide additional details and pictures. I will be taking a detailed look at a Echo handheld blower by taking pictures of the piston, measuring compression and measuring RPMs.


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## mh9162013 (Oct 20, 2018)

rich450es said:


> how did that tach work out for you ?



It worked out well. I used it to examine a number of my power tools, including my Echo PB-251.

I'll post a more detailed review blog post on the tach soon.


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## LegDeLimber (Oct 21, 2018)

Judging from the rpm numbers you mentioned in that blog and mainly because of the oil all over the engine, I'd guess it's in need of rings (and I'd measure the piston for wear too).
That's assuming there's no cylinder scoring.
Is the inside of the recoil/flywheel area oily too?
From my limited amount of time with my echo backpack units (bought used), low revs and oil buildup all over the motor, has been an indicator of ring wear.
The blow by builds crankcase pressure and pushes the oil out around the crank seals.
Of course make sure all of the crankcase and cylinder bolts are tight and the gaskets good, too.

Also, you need the air tubes installed to get the full factory rpm. not having the pipes on will drop the revs a bit, but I don't think it would be more than several hundred rpm though.


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## mh9162013 (Oct 21, 2018)

Thanks for the comments and advice!

I don't know about oily/wet areas on the recoil, but I can check the next time I use it. I noticed fuel/oil coming out of the muffler, but I figured that was because of my bad habit of pulling the trigger when pulling the starter cord. But what you're saying makes sense. I can take my pencil/fiber optic camera and see if I can take a closer look at the piston and cylinder.

Assuming I need new piston rings, how could I be able to tell?

My air tube was on the blower when I tested the RPMs.


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## LegDeLimber (Oct 21, 2018)

Ring wear signs? Compression readings if you trust your gauge.
I'd like to see about 15~20 psi more than what your pic shows.
My pb-413 & pb-500 backpacks are in the 155~160 psi range and don't show oil oozing.
my pb-620 was in the 130's range and it was oozing oil.
Will the blower idle at 2700 rpms or does need to be at 3800 to stay running?
aside from carb/fuel issues, low compression can cause troubles with not being able to get a proper low idle speed.
Low comp will definitely cause loss of power at top rpms, so that's why I wondered about your max rpm reading with the spark screen and tubes.

But fuel issues will too. 
A clogged fuel tank vent or fuel filter can negate your carburetor work.
I've had fuel filters in the blowers that would cause a loss of revs after a few mins of running. Loosening the fuel cap will tell if it's a vent problem (revs will slowly recover) but a fuel filter will still cause rpm loss, even with the cap off.
For curiosity, I shot carb cleaner through one of the fuel filters and blasted air through it... still didn't help. New filter, end of that problem!
The backpacks have a fairly long fuel hose, so it can take a moment to drain it to point of slowing the engine.

To actually physically inspect the rings, of course you'll need to remove the cylinder and remove the rings from the piston.
The rings will need to be inserted back into the cylinder. Use the piston top to push them squarely in the bore.
You need to have the ring in its running area of the cylinder for this.
The ring gap at the ends needs to not be to large. 
Wiseco web site calls for .004"~.005" per inch of cylinder bore.
http://www.wiseco.com/FAQs.aspx

Assuming a cyl bore of 1.34" (per the Echo pb-250 owners manual)
https://www.echo-usa.com/getattachment/f44e3018-5220-459d-92cb-6bf526dc7ee9/PB-250LNes_111226_031413
Then minimum gap would be about .007"
The Echo doc doesn't give a minimum gap spec, So just using the Wiseco info.
Echo shows maximum gap of .020" in this service supplement, for the 1.34" bore.
echo-usa.custhelp.com/app/answers/detail/a_id/1714

Also when you have the rings out, look at the width and thickness/height of the rings. If you can visually see a narrowing of the ring in any area, It's worn out.
The Echo service supplement only specs the ring gap and thickness (image G & H , near the bottom of document)
The ring gap will show wear long before you can spot it from looking at the ring for narrowing, So Echo doesn't bother with width dimensions. 
Notice there is also a spec for clearance between the ring and the sides of the ring grooves of the piston (image E).

I tend to only be online at irregular intervals, So please forgive me if I'm slow at spotting any later questions.


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## mh9162013 (Oct 21, 2018)

Wow, great advice, thanks! I definitely have my work cut out for me.


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## LegDeLimber (Oct 21, 2018)

I forgot to mention that the ring clearance in the ring groove of the piston matters
because too much play leads to slapping/hammering.
This can chip the piston in the ring areas and/or lead to outright broken pistons. This often tends to trash the cylinder when the piston breaks.
If you've ever seen a mushroomed chisel or splitting wedge (from the hammer strikes), then you know what happens to the edges of the ring grooves.
I sometimes suspect a few of the mysterious piston failures we sometimes see here are from ring hammering (from various causes).
Take a look at where the rings cross the exhaust and transfer ports and see if you notice any excessive rounding or other unusual damage to the ring edges.

I've got some photos (somewhere... on the PC) that show some uuuggly port edges in the pb-620.
I'll see if I can find them later.


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