idle rpms

[Keithwpg]

hey guys i just purchased one of these tachs http://www.northerntool.com/shop/tools/product_524744_524744.

what would the Rpms normally be at idle? just so i have a reference as to whether this tach is working correctly

i was looking through the tiny instructions it came with and you can set it to: a) spark fires twice per revolution b) spark fires once per revolution c) spark fires every other revolution. which would apply for me?

its a 1975 mercury 402 (2 cyl)

 

[BomerFett]

Re: idle rpms

I don't know what type of engine that is, but if it is a two stroke it should fire every revolution because it has a combustion stroke every revolution. If it's a four cycle then it would be set for every other revolution because it alternates between combustion and intake every other stroke. I don't know what type of engine fires twice per stroke. Maybe a rotary? I would imagine that your RPM should be in the 700-900 rpm range.

 

[Keithwpg]

Re: idle rpms

thanks man!

 

[Keithwpg]

Re: idle rpms

finally got around to hooking it up today and at idle using "spark fires every revolution" it gives me a reading of about 1900 rpms at idle, i can tell just by listening it ain't doing 1900 rpm, so i set it as "spark fires twice per revolution" and it gives me a reading of around the 900 rpm mark at idle. im trying to figure it out in my head but does that sound right for a 2 cyl 2 stroke motor? i have the lead wrapped around one of the spark wires