power loss ? Yamaha v250 ox66

frijolitos · Feb-07-2008, 09:02 PM

I've got twin 2000 yamaha saltwater series V250 hp OX66 on a 30 ft Grady. Situation: cruising at 3600rpm in a tail sea with a moderate chop, the starboard motor will sporadically loose power (1000rpm) and then quickly accelerate back to the normal speed,.....but only when the boat is crushing into the backside of the next wave. In a calm sea or head sea, it is running as strong as new. I was thinking spark issue, then a friend mentioned maybe the O2 sensor (said his friends friends friend had the same problem). Fuel filters are clean, clean fuel. gonna give it the once over first thing tomorrow, but was wondering if anyone has had this symptom on the same motors??

**Ali** · Feb-08-2008, 08:42 AM

Kurt - Call Robby Gordon at Boating Dynamics in San Diego.

He can diagnose over the phone...

WIDEOPENBITE · Feb-08-2008, 08:47 AM

I would bet my $ on a fuel supply problem. Sounds just like a sticky float in a carb, but you have F/I

sixty · Feb-08-2008, 11:13 AM

If it was a bad O2 sensor it would run bad all the time. definately sounds like fuel supply. Try pumping the bulb when it happens. If the low pressure fuel pumps have not been replaced in a while, replace them. They are best replaced every season on a boat with a lot of use & every other season with limited use. Also check the low and high pressure fuel filters.

Oh yeah, Robbie really knows his stuff.

You have a PM!

ShadBurke · Feb-08-2008, 11:27 AM

Most likely VST tank. Do a search on web, grab some carb cleaner and get that screen clean.

VST is the death of the HPDIs.

sixty · Feb-08-2008, 11:35 AM

OX66 is not HPDI, but still does have the VST tank.

Definately check it out. if the Diaphrams on the low pressure pumps are old they will have a hard time filling the VST. My hunch is the LPFP's need replacing and there is something in the tank or fuel lines causing increased vacuum when the boat is pointed down hill. If the Bulb is pumped when this is occurring it should bring the RPM's back up to normal or close. I could be WAY off though

frijolitos · Feb-08-2008, 03:12 PM

Quote:

Originally Posted by sixty

OX66 is not HPDI, but still does have the VST tank.

Definately check it out. if the Diaphrams on the low pressure pumps are old they will have a hard time filling the VST. My hunch is the LPFP's need replacing and there is something in the tank or fuel lines causing increased vacuum when the boat is pointed down hill. If the Bulb is pumped when this is occurring it should bring the RPM's back up to normal or close. I could be WAY off though

thanks guys.....the thermostats (one of them) on that motor wasn't open so I pulled em out, cleaned them in hot vinager and they are clean as new. That same side(the one that didn't open) seemed a little hotter than the other side. I pulled out the fuel pumps and put in a higher quality diaphram than the factory ones about a month ago and checked all 3 again today and no leaks, so they seem fine. Swaped the o2 sensors and same problem so that eliminates that. Gonna check compression tomorrow for shits and giggles, but they run like new, just the very sporadic hiccup. I'll let you guys know whats up, as this is a new symptom to me, I'm not a mechanic, but I know a bit about the motors.

sixty · Feb-08-2008, 05:16 PM

Don't forget to replace the poppet valve when you do the thermostats, or atleast pull it & inspect/clean.

I'm glad I lost the OX66 & went with Hondas!

Quote:

Originally Posted by frijolitos

thanks guys.....the thermostats (one of them) on that motor wasn't open so I pulled em out, cleaned them in hot vinager and they are clean as new. That same side(the one that didn't open) seemed a little hotter than the other side. I pulled out the fuel pumps and put in a higher quality diaphram than the factory ones about a month ago and checked all 3 again today and no leaks, so they seem fine. Swaped the o2 sensors and same problem so that eliminates that. Gonna check compression tomorrow for shits and giggles, but they run like new, just the very sporadic hiccup. I'll let you guys know whats up, as this is a new symptom to me, I'm not a mechanic, but I know a bit about the motors.

frijolitos · Feb-08-2008, 06:16 PM

Quote:

Originally Posted by sixty

Don't forget to replace the poppet valve when you do the thermostats, or atleast pull it & inspect/clean.

I'm glad I lost the OX66 & went with Hondas!

thanks Cory, did that. thanks for the offer on the manual also. I am pretty sure it will be the vst tank/filter. gonna pull it tomorrow and check/clean. If $ wasn't an issue, I would repower, as these do burn more oil than I was used to on my E-tecs (technology has come a long way since 2000). I'd love to throw some Zuki 4strokers on there! thanks again for the input

towerfab · Feb-08-2008, 07:08 PM

Kurt, what shad said is right on, I've had several ox-66's and my hpdi 200 have done the exact same thing, in fact I think shad was on a trip with me when my hpdi was doing that and I leaned it out/burned a cylinder. I can pretty much guarantee that when you drop the vapor seprerating tank you'll find a fine brown colored material clogging the screen. twist the screen cartridge off the bottom of the pump and blast it with carb cleaner,do this several times,if you have compressed air blast the screen while wet with the carb cleaner from both ends. That'll get you back on the water, atleast untill it happens again. With ethanol in fuel these days many think the fine material is the inside of the hoses breaking down, therefore you can clean/polish your tank, run 2 micron filters like I did, and still have the problem... tight lines, Tom R.

towerfab · Feb-08-2008, 07:10 PM

Another thing Kurt, when replacing the rubber o-ring on the tank you need to either buy a new one or use some really sticky gasket material, you'll find it impossible to get the old o-ring back in place without replacing it or using a glue, T.R.
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JUST-IN-TIME · Feb-09-2008, 01:35 PM

if u did the fuel pumps and the bulbs stay hard, it is not those
it is the strainer in the VST, seen it all the time
if it was the o2 sensor your idle would be off and you would be burning more fuel
run seafoam or ring free
o2 carbon up mostly because of the type oil you run

check compression and while your at it clean and re-gapp the plugs

also fix those rigging tubes unless you wanted rusted powerheads

to the guy that switch over to hondas
have fun getting to the poppet valves on anything over 200HP
pain in the but!!
also the last spark plugs are a pain also