tachometer reading and loss of top end speed

[brokenboat]

My Faria Tach. for my 1998 Yamaha 150 has a specified pole setting of 5, but the Tach will go to 7000 rpm. The engine sound is not in sync. with the tach. A mechanic told me to set it at 6 and the readings are more in line with engine sound, but the needle will sometimes do it's own thing. The tach. needle will also jump up or down when trimming the motor.<br /><br />Second, my boats top speed has been between 42 and 47 mph. I have lost about 10 mph and this occurred after I decarbed the engine. Going by sound I would have to say the engine isn't reaching WOT even with the throttle tool blocked. I can't trust the tach. Any thoughts on either issues would be appreciated.

 

[Fouled Plug]

Re: tachometer reading and loss of top end speed

Check those spark plugs. Decarbing can let loose deposits that can foul 'em out. Not sure on the tach problem...might beg/borrow a known good one to check your RPM's.