RPM's go up; Tach goes down?? Help

B

Blue 17

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Joined
May 12, 2004
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I installed a new Teleflex Vector tach on my 1974 Johnson 85hp. At idle it registers 950 revs and at cruise speed 4000 which I believe is accurate, but after 4400 the tach begins to fall eventhough the engine rpm's go up.<br />I tested the voltage at the battery and get 13.5 vdc at idle so the rectifier seems ok.<br /><br />What should I look look for next??<br />Thanks<br />Tim
 
rickdb1boat

rickdb1boat

Supreme Mariner
Joined
Jan 23, 2002
Messages
11,195
Re: RPM's go up; Tach goes down?? Help

Check for a spun prop hub. MAke a mark with a permanant marker on the inner prop hub and the prop housing. Take it out for a run and see if the marks still line up when you get back. If not, the hub needs reworking. Does speed still gain, even though the Tach. registers low? If so, you may still have a Tach problem. Check all connections to make sure they are clean and tight. Make sure the new Tach. is set on "6" pole on the back of the Tach..
 
B

Blue 17

Recruit
Joined
May 12, 2004
Messages
5
Re: RPM's go up; Tach goes down?? Help

Thanks Rick<br />I don't think its a hub problem. At 4400 you can go WOT and it will set you back in the seat but the tach drops to 3000 at what i believe is about 6000/6200. I do have a low pitch prop, not sure what pitch (it sounds like 6500+ at WOT, it winds way way too tight), that is why I am trying to get the tach to work so I can prop for 5500 - 6000 at WOT (I have kids that ski). All connections are new and the pole is set for "6".<br />Is it possible that the rectifier starts to fail at a higher RPM level? The stator system was replaced new last season.<br /><br />Thanks<br />Tim
 
O

ob

Admiral
Joined
Aug 16, 2002
Messages
6,992
Re: RPM's go up; Tach goes down?? Help

TW ,Does your boat have a volt guage.If so what is it reading at WOT or around 6000?
 
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