Hooking up a tachometer to a 9.9 Johnson

[mustard73]

Hi,
I'm currently tuning up my 1985 Johnson 9.9 because it doesn't seem to be revving up as high as it has in previous years. I would like to temporarily hook up a tach for testing purposes and to see what rpm it runs at when held wide open on the lake. Has anyone done this?

Thanks.

 

[boobie]

Re: Hooking up a tachometer to a 9.9 Johnson

I've got a shop tach built by Merc-o-Tronic that clamps on to the plug wire but it's a little to expensive for a one shot deal. I know they make mini tachs and vibration tachs so you may want to google outboard tachs and maybe you'll find one. They have also been mentioned on other posts here.

 

[Rick.]

Re: Hooking up a tachometer to a 9.9 Johnson

A good quality inductive timing light will tell you your rpm. at any throttle position. Best of luck. Rick.

 

[mustard73]

Re: Hooking up a tachometer to a 9.9 Johnson

So I picked up a multimeter from Canadian Tire that reads frequency and was able to use that to measure the rpm roughly. It jumps around a bit but if you watch it for a few seconds, a guy can average it out which is close enough for right now.

The basic setup is to ground the multimeter to the engine ground and then touch the positive lead to the insulation on one of the plug wires. You get a reading in Hz so you need to multiply this to by 60 to get rpm. If your motor "wastes a spark" ie both cylinders fire at the same time, twice per revolution, your reading will be double the actual rpm.

I did notice that I could read the rpm from the top plug wire no problem but the bottom wire would give me zero or sometimes a small reading but for a split second. I tried swapping wires top to bottom but got the same results. I might try swapping the locating of the coils and see if the problem follows. Any other ideas?

Edit: I should note that with the engine running, if I pull off either of the plug wires the motor will continue to run (struggling though) and then eventually die. I believe I'm getting at least intermittent spark on the bottom cylinder.

 

[AlTn]

Re: Hooking up a tachometer to a 9.9 Johnson

coil swap is good...unplug all connections and clean with rubbing alcohol and make sure all male connections are "down" in the plugs and haven't been pushed "up"...after doing this coil test and plug cleaning...clean the powerpack ground, coil grounds, and reattach...still got a weak, intermittent reading on lower cylinder, it's a good chance the powerpack is going....you can test further with a dva adaptor for your multimeter if you want conformatioin