Yamaha ox66 250 HP WOT 4900 @35 MPH. Is this normal?

[Hammertime]

I have the 232 Gulfstream model with a single 250 HP ox66. What should the WOT be with this motor. I was told there is a normal range between 4600 and 5700 RPM. I installed a new SS prop, 15.8x15 .. I was told by 3 different prop shops this is the proper size prop to go with, so I took their advice.

I noticed though, with 50 gallons of fuel, driving into the wind, with 2 people on board I am only going 30 MPH @4600 RPM. Heading the opposite direction the RPM will rise to 4900 and 35 MPH at WOT. This is with the boat trimmed up..

Is this normal speed at RPM for this boat/motor combo, or does it sound like the motor is only running on 5 cylinders.. How do I check if the motor is running on all 6 cylinders when she is idling. Can I use an old check light and ground each cylinder one at a time to hear if the motor bogs down?? I was told the OX66 will run on 4 cylinders at low RPM and jump to all 6 at higher RPM..

Any advice would be appreciated..

 

[1998sailfish]

5600 rpm is a better target.

When was the last time the High Pressure Fuel pump (VST) was serviced as well as internal (inside the VST fuel pump body ) cleaned or replaced? Also, the same for the O2 sensor.

Low pressure pumps (3) , O2 sensor, as well as a sensor called "altitude" (yes , there is one) will all affect low idle /smoothness of low idle when the motor firing has been retarded to 4 cylinders. The VST filter is very fine and will become partially clogged from fuel degradation in the VST tank. You can check the output pressure of the VST to verify its functioning as new.

Low VST output pressure will affect top end performance. The VST screen is a fairly easy cleaning /replacement procedure which should be done if you have not already.

Hope this helps

 

[Parthery]

The max RPM range for that motor is 5500. If you are at WOT at 4900 something is wrong.

A couple things to look at:

1.) Assuming you are trimming the motor up a bit? If I bury mine all the way down, I too will max out at 4900-5000 rpm
2.) What kind of shape is the bottom of the boat? Growth? Drag? Anything like that will slow you down
3.) I too would clean the VST and replace the filters.

225 and 250 OX66 idle on 5 cylinders. You would know it if the 6th cylinder wasn't kicking in.

 

[Hammertime]

The max RPM range for that motor is 5500. If you are at WOT at 4900 something is wrong.

A couple things to look at:

1.) Assuming you are trimming the motor up a bit? If I bury mine all the way down, I too will max out at 4900-5000 rpm
2.) What kind of shape is the bottom of the boat? Growth? Drag? Anything like that will slow you down
3.) I too would clean the VST and replace the filters.

225 and 250 OX66 idle on 5 cylinders. You would know it if the 6th cylinder wasn't kicking in.

I am still figuring out what the proper trim should be for this boat/motor. So far I find the trim indicator at half way works best, #3 on the Yamaha indicator, but still only revving at 4900.

The bottom of the boat was freshly sanded and painted so that is not the issue.

I have NOT checked out the VST or O2 sensor and filter. I only changed the Micron gas filter on the boat. i didnt change the plugs because they look fine, but looks can be deceiving..

Any chance the Injectors would cause this issue and or the "Throttle position sensor"??

 

[Andrew93]

Defiantly check the VST filter, if you are handy at all it is pretty easy. The gasket on the tank can be difficult to say the least to get to sit back in place, sometimes a little axle grease helps hold it down in the trouble spots. Also if it is stretched out a little place the tank and gasket in the freezer for 5-10min to help shrink it some.

You can check the low pressure fuel pumps by unbolting them and leaving the fuel lines in place. Squeeze the primer and they should not leak, if they leak out the back the diaphragm inside is broke. You can buy the rebuild kits but they are not too expensive to replace the whole thing.

I have had 2 different OX66's have issues with the O2 sensor. One was a bad sensor and the other the sniffing tube was clogged solid when I got the motor. Both times the RPMs fluctuated and would not stay steady. If you are at 4900rpm WOT and staying there and not surging up and down every 45seconds or so I would concentrate on the VST and low pressure pumps first.

Good luck and post back your findings!

 

[Bumpye]

I have a 2003 Gulfstream. It is powered with a Yamaha 225. I've been a bit anal about getting the best trim/best speed combo. At WOT, trimmed up to about 4 on the trim gauge, she maxes out at 5500 RPM and 37.5 MPH. That's with full tanks, calm seas, and just myself onboard. I can't say what prop is on it-I don't know if it's OE, but I feel that the speed and RPM is pretty good for the 225. I only use the trim tabs to trim side to side.

 

[seasick]

Unless the revs surge up and down, I wouldn't suspect the VST nor the O2 sensor. I also think that motor idles on 4 cyls not 5 but I could be wrong. Regardless you would know if two cylinders weren't firing at speed.
So one way to help figure out what is going on, would be to take a run with less fuel and no passengers and see what the numbers look like. If the numbers look good, then it is a thrust or drag issue; wrong prop, excessive drag etc.
Take a look at the water line when loaded and see if it is lower than it used to be.
looking at the trim indicator is not the way to optimize. Select a cruising speed, pick any down angle. note your speed using gps. Now make very small trim changes either up or down and watch the speed. Normally from down towards up trim, the speed will increase to a max and then start to decrease. At that point you just passed the optimal trim for that cruise speed. During this test, the revs should NOT change.

 

[Hammertime]

Unless the revs surge up and down, I wouldn't suspect the VST nor the O2 sensor. I also think that motor idles on 4 cyls not 5 but I could be wrong. Regardless you would know if two cylinders weren't firing at speed.
So one way to help figure out what is going on, would be to take a run with less fuel and no passengers and see what the numbers look like. If the numbers look good, then it is a thrust or drag issue; wrong prop, excessive drag etc.
Take a look at the water line when loaded and see if it is lower than it used to be.
looking at the trim indicator is not the way to optimize. Select a cruising speed, pick any down angle. note your speed using gps. Now make very small trim changes either up or down and watch the speed. Normally from down towards up trim, the speed will increase to a max and then start to decrease. At that point you just passed the optimal trim for that cruise speed. During this test, the revs should NOT change.

Thanks seasick. I will definately try that. I know there is no water in the bilge to cause excess weight and i personally sanded the bottom down gell coat, primed, and gave the bottom one good coat of ablative paint.
I'll keep you posted with the trim thing when I go back out on the boat this weekend

 

[Hammertime]

I have a 2003 Gulfstream. It is powered with a Yamaha 225. I've been a bit anal about getting the best trim/best speed combo. At WOT, trimmed up to about 4 on the trim gauge, she maxes out at 5500 RPM and 37.5 MPH. That's with full tanks, calm seas, and just myself onboard. I can't say what prop is on it-I don't know if it's OE, but I feel that the speed and RPM is pretty good for the 225. I only use the trim tabs to trim side to side.

I bought a brand new Quicksilver SS prop last week and installed it.

size: 15.8x15.. I got a little more speed out of the prop but not much. The RPM stayed about the same or went up about 100 RPM.

I feel this motor should be doing 5500 RPM at WOT. This is bugging the crap out of me. I feell as if she is firing on all 6 cylinders.. This is the fustrating thing..

One other note. I dont have a gas burn indicator and dont know if one can be installed on the ox66, but this motor is a pig on fuel!!! I feel like she is burning more then it should be but dont have anything else to compare it to

 

[Hammertime]

Defiantly check the VST filter, if you are handy at all it is pretty easy. The gasket on the tank can be difficult to say the least to get to sit back in place, sometimes a little axle grease helps hold it down in the trouble spots. Also if it is stretched out a little place the tank and gasket in the freezer for 5-10min to help shrink it some.

You can check the low pressure fuel pumps by unbolting them and leaving the fuel lines in place. Squeeze the primer and they should not leak, if they leak out the back the diaphragm inside is broke. You can buy the rebuild kits but they are not too expensive to replace the whole thing.

I have had 2 different OX66's have issues with the O2 sensor. One was a bad sensor and the other the sniffing tube was clogged solid when I got the motor. Both times the RPMs fluctuated and would not stay steady. If you are at 4900rpm WOT and staying there and not surging up and down every 45seconds or so I would concentrate on the VST and low pressure pumps first.

Good luck and post back your findings!

I certainly will.

Do you know what the gas burn rate per mile should be with this motor/boat combo? I feel like she is sipping a ton of fuel, But have nothing to compare it to. Can I install a gauge on this ox66 to see how much she burns per hour?????

 

[seasick]

Defiantly check the VST filter, if you are handy at all it is pretty easy. The gasket on the tank can be difficult to say the least to get to sit back in place, sometimes a little axle grease helps hold it down in the trouble spots. Also if it is stretched out a little place the tank and gasket in the freezer for 5-10min to help shrink it some.

You can check the low pressure fuel pumps by unbolting them and leaving the fuel lines in place. Squeeze the primer and they should not leak, if they leak out the back the diaphragm inside is broke. You can buy the rebuild kits but they are not too expensive to replace the whole thing.

I have had 2 different OX66's have issues with the O2 sensor. One was a bad sensor and the other the sniffing tube was clogged solid when I got the motor. Both times the RPMs fluctuated and would not stay steady. If you are at 4900rpm WOT and staying there and not surging up and down every 45seconds or so I would concentrate on the VST and low pressure pumps first.

Good luck and post back your findings!

I certainly will.

Do you know what the gas burn rate per mile should be with this motor/boat combo? I feel like she is sipping a ton of fuel, But have nothing to compare it to. Can I install a gauge on this ox66 to see how much she burns per hour?????

The OX motors are not known for fuel efficiency. The burn at WOT is often close to twice what it is at its sweet spot. I would think yours burns 20 gal per hour wide open, maybe more.
I forgot to ask, what prop was on it before you changed it and what were the performance numbers? (Basically, why did you change props?)

 

[Hammertime]

Defiantly check the VST filter, if you are handy at all it is pretty easy. The gasket on the tank can be difficult to say the least to get to sit back in place, sometimes a little axle grease helps hold it down in the trouble spots. Also if it is stretched out a little place the tank and gasket in the freezer for 5-10min to help shrink it some.

You can check the low pressure fuel pumps by unbolting them and leaving the fuel lines in place. Squeeze the primer and they should not leak, if they leak out the back the diaphragm inside is broke. You can buy the rebuild kits but they are not too expensive to replace the whole thing.

I have had 2 different OX66's have issues with the O2 sensor. One was a bad sensor and the other the sniffing tube was clogged solid when I got the motor. Both times the RPMs fluctuated and would not stay steady. If you are at 4900rpm WOT and staying there and not surging up and down every 45seconds or so I would concentrate on the VST and low pressure pumps first.

Good luck and post back your findings!

I certainly will.



Do you know what the gas burn rate per mile should be with this motor/boat combo? I feel like she is sipping a ton of fuel, But have nothing to compare it to. Can I install a gauge on this ox66 to see how much she burns per hour?????

The OX motors are not known for fuel efficiency. The burn at WOT is often close to twice what it is at its sweet spot. I would think yours burns 20 gal per hour wide open, maybe more.
I forgot to ask, what prop was on it before you changed it and what were the performance numbers? (Basically, why did you change props?)

The prop that was on the boat when i got it was a Yamaha, 17-M..15x17?? It was beat up and had a one inch split on one blade.. The new size prop is a Quicksilver, SS,15.8x15.. I noticed a light improvement after i installed it. Not so much with an increase in RPM, like I was hoping for but faster speed at WOT..

 

[Parthery]

29 Gallons per hour if you run it full throttle.

You CAN install a fuel flow motor....two different ways to do it but it depends on what you have for electronics.

If you have a GPS/FF that is NMEA 2000 (N2K) you can add a fuel flow sensor which goes in the fuel line and connects to your GPS/FF. It tracks GPH and with a GPS feed to determine speed will give you MPG. Garmin and Simrad/Lowrance both make the sensors. I have the Simrad/Lowrance sensor hooked into my Simrad GO7 and it gives me MPG. Total cost was about $200 for the sensor and the N2K Backbone to hook everything up.

The other - more expensive option - is to purchase the Yamaha Fuel Management guage and sending unit. Expect to pay upwards of $500 if you go this route.

 

[Hammertime]

29 Gallons per hour if you run it full throttle.

You CAN install a fuel flow motor....two different ways to do it but it depends on what you have for electronics.

If you have a GPS/FF that is NMEA 2000 (N2K) you can add a fuel flow sensor which goes in the fuel line and connects to your GPS/FF. It tracks GPH and with a GPS feed to determine speed will give you MPG. Garmin and Simrad/Lowrance both make the sensors. I have the Simrad/Lowrance sensor hooked into my Simrad GO7 and it gives me MPG. Total cost was about $200 for the sensor and the N2K Backbone to hook everything up.

The other - more expensive option - is to purchase the Yamaha Fuel Management guage and sending unit. Expect to pay upwards of $500 if you go this route.

Thanks Parthery. I just recently installed a new garmin 7 series side view chirp Plotter. it it does have the feature for fuel burnt ratio built in. I'll have to look into this..

 

[against the wind]

I have a ox66 200hp. i called yamaha service they told me at idle the motor runs on 4 cylinders, after 2000 she runs on 6.As far as prop and rpm's i have a yamaha prop 13 3/4 x 17M. Im pretty satisfied with results WOT 5100 at 47 mph but there again, were am i going im just enjoying the day and in no rush to go anyway. lol my fuel burn is about 17 gallons per hour WOT For a good prop size call ken at the propgods he is very helpful and can give u great advice on prop size good luck