Yamaha 250 mid range rpm's change

[AQUADJ]

I have twin 1998 250 ox66 fuel injected motors. They idle perfect at 700 and will stay steady to under 2000. Above 3700 to 5200 they run perfectly. It's the between range, 2000 to 3700 where the rpm's will bounce as much as 500, sometimes only one or both motors, primarily in the 3000 to 3700 range. There is no consistancy to when it happens and it seems more infrequent than before but still happening. I've had the fuel filters, plugs, and the resistors that didn't meet spec changed, and they cleaned and tuned as much as they could. The mechanic seems to think it's either the design of the Yamahas where they only run on as many cylinders as is necessary to meet the demand at a given speed or possibly the O2 sensor. I'm not familiar with the operation of the Yamaha regarding the use of cylinders and if at these rpm's it would happen.

 

[rodbolt]

Re: Yamaha 250 mid range rpm's change

its rather confusing but usually gets the blame for other problems, most the time with heavy hulls you cannot maintain RPM below about 3800. it will move around with wave action. below 850 rpm all 6 fire, 850 to 2000 #2 does not fire, below 2000 #5 does not fire. check the shift cutout switch, but above 3000 rpm it has no affect on cylinder firing.

 

[AQUADJ]

Re: Yamaha 250 mid range rpm's change

Thanks rodbolt. This is a heavy hull - a 27 Blackfin but it's not a typical change in rpm associated with wave action. One engine, usually the starboard, will maintain the rpms but the other will fluxuate as described.

 

[rodbolt]

Re: Yamaha 250 mid range rpm's change

has the O2 sensor readings been checked at the suspect RPM and the high pressure fuel system been tested?<br /> also a faulty low pressure pump diaphram can create issues, did they change the filter in the VST? and occasionally the filter in the pressure regulator will stop up. I have seen some regulator issues drive techs nuts but usually it will cause rapid flucuations in fuel pressure. if the needle swings more than about 3 psi it will indicate a problem with the high pressure fuel components.<br /> the ECU does not monitor fuel pressure on that engine and cannot alter injector on time to compensate for pressure changes, to much and it goes rich to low and it goes lean. these motors are actually simple to work with and will usually trouble shoot themselves if the tech understands how each part works with the next and starts eliminating subsystems as a culprit.<br /> I would also monitor the TPS voltage at the problem RPM.<br /> if you suspect a lean cyl you can always squirt premix directly down the throttle bore at the problem RPM and note the engines reaction.

 

[AQUADJ]

Re: Yamaha 250 mid range rpm's change

I think they changed any suspect filters and cleaned the O2 sensor, I can only assume it was tested. I don't believe any of the other testing was done. The last recommendation was to log the fuel consumption at normal operation and compare it with the consumption when the idle is off. Although I'm dealing with a Yamaha mechanic I'm not confident with his diagnostic skills. I'll try your suggestions, Thanks.