Yamaha F90 / 65 low RPMs

[Kanadakid]

Guys,<br /><br />Changed the impeller (Jet lower unit) to a lower pitch and still can't quite hit 5000 RPM. My selling dealer is not a Yamaha dealer. The boat came as a package from Triton. <br /><br />The closet Yamaha dealer to me can't water test due to the fact their is no water near him. <br /><br />I can get up to 4900 RPMs if I use 89 octane. 87 octane is more like 4650 RPMS.<br /><br />My speedometer says I'm running at 33 MPH. Who knows how accurate that is as it is just a pressure stick.<br /><br />I do smell a little gas smell in the oil but doesn't seem to be making oil. If it is making oil its just a little.<br /><br />I'm using Mobile one so there should be no problem there.<br /><br />Pulled the plugs and they seem Ok. Tan tips and dark at the base. So it doesn't appear that any of the cylinders have an issue.<br /><br />I found a dealer who is a yamaha jet dealer but is nearly 3 hours away. He says the engine should do 5500 rpms but the speed seems OK.<br /><br />Checked the pulse rate on the tach and it is set right 6P.<br /><br />Rodbolt / JB others ..........do you know of a very good Yamaha tech in the Trenton New Jersey / Easton -Allentown Pa area ? <br /><br />For some reason Yamaha won't specify a dealer.<br /><br /><br />Thanks,<br /><br /><br />Kid

 

[rodbolt]

Re: Yamaha F90 / 65 low RPMs

hello<br /> what year is the engine?<br /> what tach do you have? is the motor set to low? what pitch impeller are you running now? has it ever turned up correctly? and has the tach been verified as accurate? the F90 should turn about 6000 when set up correctly.

 

[Kanadakid]

Re: Yamaha F90 / 65 low RPMs

Rodbolt,<br /><br />You have helped me before, thanks. It is a 2003 purchased new as leftover feb 2004. The tach is the one that came with the boat, Triton 176 sport. As long as I recall it was always at 4900<br />RPMs.<br /><br />As far as the tach is concerned, I took the engine back this past Jan. and the dealer said they hooked it up to another tach and ran the engine. It said 4900 RPMs on their tach. <br /><br />Whether he really did use another tach I don't know.<br /><br />Th ecurrent impeller is a 7 3/16 th. Orgininal was a 7 3/8.<br /><br />**** at specialty marine said that the Yamhas run about 5000 rpms with the reccomended impeller.<br /><br />In order for me to get 4900 I have to run 89 octane.<br /><br />The dealer I spoke with in Pa. said that I should get into 5500 rpms.<br /><br />Rod, this is really eating at me. I'm concerned that I got a bad engine here.<br /><br />Yamaha will pay tp have this looked at, but want to get the straight scoop from a good tech who will find the issue. You know ?<br /><br />Thanks again for your help. Wish i was in NC so you could get this right.<br /><br />Don

 

[bigbrownbuku]

Re: Yamaha F90 / 65 low RPMs

the f90 has a 12pole alternator. recheck your tach setting.

 

[Kanadakid]

Re: Yamaha F90 / 65 low RPMs

Ben,<br /><br />It is a 2003 F90 / 65 Jet. I called Yamaha and they gave me the 6P spec. <br /><br />6P is where the pulse setting was from the MFG<br />Triton. <br /><br />How can we verify what the pulse setting should be ...........otherwise I need to focus on the engines performance.<br /><br />Rod........do you know the pulse setting on the 2003 F90 /65<br /><br />Thanks for your help / Imput.<br /><br /><br />Kid

 

[bigbrownbuku]

Re: Yamaha F90 / 65 low RPMs

i checked the spec today kk. its a 12 pole alternator.

 

[rodbolt]

Re: Yamaha F90 / 65 low RPMs

12 poles =6 pulse<br /> sometimes there is confusion over pulses and poles. a 2 pole altenator will give 1 positive and one negative pulse. since we have a rectifier one pulse is shunted so the tach would only see 1 pulse. a 4 pole will have 2 pos and 2 neg but if shunted(rectified) you will "see" 2 pulses. and so on and so on. about the only way to test is to use an inductive pick up tach and compare it to the altenator driven tach. be aware that the f90 is a waste spark system. means the coil fires twice per complete cycle. so if your inductive tach has a 2 and 4 stroke setting either use the 2 stroke setting or divide the 4 stroke reading by 2. about the only way to confirm the power output of the engine is to install a standard lower unit and run it on a test wheel or a dyno. no other way. that motor should still be able to run 5500to 6000. but there is no way to test the power output of a jet. so far I have seen no standard test impellers. so other than checking all fuel system, ign system and set up there is no relable way to say if you have a engine problem or a impeller or hull set up problem. a couple hours to switch from jet to prop shaft and back for the test is easy. finding a test gearcase may not be. but I would test the engine with a known good inductive shop tach. its rare to have a system problem that affects the factory tach but its possible. is this a yamaha tach or aftermarket?<br />if the engine is running correctly your still 1000RPM low.

 

[Ray Neudecker]

Re: Yamaha F90 / 65 low RPMs

The three cylinders used to require a special adapter in line to make the tach read correctly on the factory tach.

 

[Kanadakid]

Re: Yamaha F90 / 65 low RPMs

Rod / Ray<br /><br />Rod, thanks you really know your stuff. Wish you were here in NJ. One of the dealers said since me speed seemed OK that he thought the RPM was related in some way to the Tach. <br /><br />The tach was already in the boat along with speedometer, gas guage and Amps guage. So the answer is no it is not a yamaha tach. It is factory installed Teleflex.<br /><br />How would I go about hooking up and inductive tach.<br /><br />Ray, I'm not following you. The engine is a 4 cyl engine. If you would be kind enough please explain a little further.<br /><br /><br />Thanks,<br /><br /><br />Kid