Tiny Tach- different RPM for each spark plug

A

apace2

Cadet
Joined
Apr 3, 2011
Messages
12
I bought a Tiny Tach knock-off to re-prop my 1985 Johnson 30 hp. I get two different RPMs depending on which spark plug wire I wrap the Tiny Tach wire around. At idle the top spark plug wire gives me just around 700, which seems appropriate and then without adjusting the throttle and switching to the lower spark plug wire it gives a reading of 350. No matter what the RPM is the lower spark plug always seems to be almost exactly half of what the upper one is. Is it only firing half the time? Any thoughts? It seems like the engine is running smoothly.
 
M

MACH1SCJ

Petty Officer 3rd Class
Joined
Jun 8, 2010
Messages
82
Re: Tiny Tach- different RPM for each spark plug

What about using a couple of inline spark testers?
 

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B

boobie

Supreme Mariner
Joined
Nov 5, 2009
Messages
20,826
Re: Tiny Tach- different RPM for each spark plug

Pull one spark plug wire at a time and see if you have equal drop in rpm when you do.
 
H

Haffiman

Commander
Joined
Dec 17, 2009
Messages
2,454
Re: Tiny Tach- different RPM for each spark plug

You most likely have a defective power pack that sparks upper cylinder same time as lower. Test either by using a double spark tester and hook on both leads, or use a timing light on the flywheel. Add a TDC makr for the lower cylinder on the flywheel, hook the light on uppr spark lead and start the engine. If you are able to see TDC for both upper and lower cyl, your PP is defective.
 
S

schematic

Lieutenant Junior Grade
Joined
Jan 12, 2008
Messages
1,102
Re: Tiny Tach- different RPM for each spark plug

Also be very suspicious of that tach. I have played with a few and found the same issue.
 
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