OX66 RPMS dropping???

[GreatWhite23]

Hey got a new boat fri. 91 Marlin with 2001 Yamaha 250 1200 hrs. She has a few issues.
1. Backfires hard enough to stall until warm

2. Alarm seems to go off after sitting for fifteen minutes fine. Noticed it is not spitting water out the
exhaust. Peeing fine. Oil res is full.

3. Electrical-hit a wave all electronics off- hit another back on. Motors have juice all the time. When on shore power everything came back on and stayed on.

4. The RPMs drop about 300 than go back up I can hear the motor surging??? Fuel pumps??? She takes off just fine. Compression tested 108-110 both motors. Onely when over 3000 rpms. There was some lag when I got it up on plan for they first time but only the first time.

Any impute would be appreciated> The marina is to busy to work on it ,3 weeks out. Thanks

 

[Mr Honey Do]

GW23;
I wish I was an expert and could help. :sorry
I do have '02 Twin 250 Yamaha's on my Marlin 28 and can tell you that they're a great engine!
It took my mechanic awhile to get them purring, but once we got there, hell they're quieter
than 4 strokes!!! Our problem mainly was old gas in the tanks. Of course we put new filters on.
Make the appointment with the Marina and get them fully serviced!
Then you have a starting point/base reference in which to proceed and develop a history with them.
Don't count on what the guy who sold them to you said.

Good luck stay in touch

 

[family affair]

My experience over the years has been most ox66 issues are fuel related. If your gas is not old, check low pressure fuel pumps. A ruptured diaphragm will cause a rich condition resulting low, smoky idle and surging at higher rpm's. If this condition has existed for a while, your O2 sensor may be fouled contributing to the issue.
As for the electrical issues, I have no experience and could only guess you have a loose or corroded connection somewhere.

Good luck!

 

[jbrinch88]

Hey got a new boat fri. 91 Marlin with 2001 Yamaha 250 1200 hrs. She has a few issues.
1. Backfires hard enough to stall until warm
This can simply be due to old dirty plugs. Can't hurt to throw a new set of plugs in the motor.

2. Alarm seems to go off after sitting for fifteen minutes fine. Noticed it is not spitting water out the
exhaust. Peeing fine. Oil res is full.After sitting for 15 minutes idling? Have you narrowed it down to which alarm it is? Water can be coming out the indicator hole just fine, but the thermostats can still be seized/corroded causing an overheat. Only way to tell his hit each bank with a temp. gun while idling. You shouldn't really see anything above 160-165.

3. Electrical-hit a wave all electronics off- hit another back on. Motors have juice all the time. When on shore power everything came back on and stayed on.Can you explain this more? Where are you losing power? Gauges? Dash? DC Panel?

4. The RPMs drop about 300 than go back up I can hear the motor surging??? Fuel pumps??? She takes off just fine. Compression tested 108-110 both motors. Onely when over 3000 rpms. There was some lag when I got it up on plan for they first time but only the first time.If it is a slow surge, most likely a fuel issue either pumps/filter etc. If it is a sudden drop in rpm it can be due to an intermittent coil on one or more cylinders going out.

Any impute would be appreciated> The marina is to busy to work on it ,3 weeks out. Thanks

 

[GreatWhite23]

Thanks,
Fri I will be changing thermostats, water pumps, plugs, low pressure pumps,fuel filters. Hopefully this works it cannot hurt. The electrical might be a switch connection. The batteries are all new but does not mean they are good. When it shuts off everything goes dead except for the power to motors. It came back on when I got back to shore and plugged in to shore power. On the way to the docks it would flicker if the wave was big enough

 

[The_Chain]

The RPMs drop about 300 than go back up I can hear the motor surging??? Fuel pumps??? She takes off just fine. Compression tested 108-110 both motors. Onely when over 3000 rpms. There was some lag when I got it up on plan for they first time but only the first time.

I had some similar symptoms...this may help.

viewtopic.php?f=2&t=19209

 

[Tucker]

You got a few hours on that girl. Suggest going over to THT and search OX66. If service history is unknown I would replace EVERYTHING that is recommended on THT maintenance threads for this motor along with the fuel system maintenance.

 

[GreatWhite23]

Thank you for the help. Heading down tomorrow to get her, will post some pics on Mon. Planning on running maintenance on motors fri afternoon and sat. Do have a new powerhead on one motor. The marina says all maintenance had been done every year??? I can say all hoses had been replaced the rest is hard to tell . She seems solid no stress cracks or soft spots ect. Worried about getting the lower units off hopefully the come off without a fight.

 

[Graybeard]

My old Parker had a 200 OX-66 and several circuits in my onboard computer went up causing the engine to do all kinds of unusual stuff. YAMAHA has a troubleshooting tree you can use and it may help

 

[Lil Lucky]

If your motor is dropping RPMs and then coming up... change your VST filters!!

 

[GreatWhite23]

Thanks for your help. Replaced fuel filters, low pressure fuel pump, Cleaned O2 sensor, water pumps, Plugs and oil res. sensor. She ran great :dance . Just need to put new headliner.

 

[The_Chain]

Great! now its time to get out and enjoy her...

 

[seasick]

Ditto on the VST filter. Also change the inline fuel filter. The shutdown when hitting rough seas is a loose connection somewhere. Check the ground connection at the motor first.

Backfiring at idle until warm can be due to bad LP pumps as well as a low idle setting. That can be an adjustment or a bad temp sensor that is causing the ECU not to know that temps are cold.

Get a manual and follow the diagnostics.
Good luck

By the way, remember that at low revs, two cylinders shut down. Lots of newbies buy coils that they don't need

 

[Tucker]

Man, back to back spam posts, dang,
Did give me a chance to re-read the thread. Alarm at low speed, overheat alarm? This is the classic case of the PRV valve. The old style was prone to failure and they upgraded it to a mushroom shaped valve. Inexpensive repair and easily a DIY job just take your time.

 

[SkunkBoat]

Electrical--- check the "House" push button breaker near the batteries. By check I mean take it off, throw it away and replace it with a brand new breaker.

Also check the connection of that wire (which is usually on the back side of a battery switch ) and the negative connection of House to the battery.

 

[Tucker]

Replace breaker?! That's an expensive part, why would it wear?

 

[SkunkBoat]

Replace breaker?! That's an expensive part, why would it wear?

I was 90 miles out at night and lost all house. Luckily the PO had told me about that breaker or I never would have known to look there first.

I pressed it an nothing happened, was tracing the wires from it, pressed it again, tapped on it , lights came back.
Turned out the screw connections (hidden by cover) were crap. I have heard others here had same issue.

A 40 amp breaker cost $25