Weird tached RPM values

Joined
Apr 26, 2017
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15
Me again. I'm trying to figure out some issues on my 77 cobalt bow rider after its first day on the lake in 12 years. It runs great and is able to get up on a plane at WOT, even at maximum weight capacity. The strange part is that the RPM the engine sits at while at WOT is only reading about 1800 on the tach. That doesn't quite sound right... I'm starting to think that for some reason my tach is reading half of the RPM it should be. More evidence to this theory is a sharp thunk and a jolt when shifting into gear as well as a fairly fast speed while in gear at idle. I know you're not supposed to shift above 1k rpm. If my tach is reading half, then my real rpm would be 1200 at idle.

Is there any validity to this or should I take it out again without a full crowd onboard?
 

southkogs

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Jul 7, 2010
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14,783
Can't tell what your tach is doing: You should be idling around 600RPM and WOT should be about 4600. You should be able to hear the difference between 1000RPM and 600RPM, but you need to measure the RPMs.
 
Joined
Apr 26, 2017
Messages
15
I think I just found something! Looking through my select manual, I found a footnote mentioning tacos with different cylinder number settings. Since these gauges were used on 4, 6, and 8 cylinder engines I wonder if it somehow got switched to 4 cyl mode. I won't be able to check until after the memorial day weekend, but I'm confident I found it.
 

southkogs

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Jul 7, 2010
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Yes, check that - but the numbers still sounded weird to me and that shift sound is concerning. If it's idling too fast when you shift, you can mess up the lower gear case.
 
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