Tach question

[madolive3]

1978 Johnson 115. I bought a used Tach for the old boat I'm putting together. From the help I've received here in the Iboats forums, I know that my motor is a 6 pulse system. The Tach I have only lets you select between the number of cylinders the motor has. Here's a pic or two. Will this Tach accurately reflect my rpm???

 

[emdsapmgr]

Re: Tach question

Looks like a Tempo tach. This type tach may be setup for stern drive only.

 

[Silvertip]

Re: Tach question

The answer is yes it is an ignition coil driven tach for use with 4, 6, or 8 cylinder auto-style (I/O or inboard) engines. Depending on the design of the electrics inside the tach it may respond to the pulses from the "alternator" on your outboard but it's accuracy will be off as follows:

A 4 cylinder engine produces 2 pulses for each revolution, a 6 cylinder produces 3 and a V8 produces 4. Your outboard produces 6 pulses per engine revolution. Therefore if you selected the 6 cylinder terminal the tach would read 1/2 the RPM the engine is actually turning. So at an actual engine speed of 6000 rpm the tach would read only 3000. Do the math for the other two settings. My suggestion is to use the correct tach. Try Great Lakes Skipper (greatlakesskipper.com) for new/old stock gauges.

 

[madolive3]

Re: Tach question

Yes, I'll get a proper tach. When I was running the motor with the tach connected, I was having some trouble getting the linkages and timing synced. I was idling at 3000 on the tach so I know it wasn't at 6G Rs. I had the tach set at 6, (I thought that the 6 on the back of the tach meant pulse not the number of cylinders.) So now I have the idle as low as I can get it and this tach is reading 1100 to 1200. Is there a way to measure the Rpm with a multi meter?

 

[boobie]

Re: Tach question

Just get the proper tach and your probs will be over.