Yamaha ox66 issues

[afterhours]

Striper56,

I've owned an OX66 250hp for several years now. Sounds like a fuel supply issue. If you haven't replaced your 3 low pressure fuel pumps recently, it is easy to check if they are leaking. You won't know if they are leaking because they are bolted to the manifold and use vacuum and positive pressure from the manifold to operate. The fuel just leaks into the manifold, causing your engine to run rich at idle and starve the engine at higher RPM's. Surging is a classic symptom. The engine will run fine until the vst tank doesn't get enough fuel from the lp pumps. RPM's will rise and fall as the vst tank fills and empties. You may not notice this at higher rpms, except that WOT might be lower than expected.

Simply loosen the two bolts holding each pump to the manifold and squeeze your fuel bulb. Do this with your engine off and cool and check one pump at a time.

If the diaphragm is shot, fuel will dribble from behind the pump. If it leaks replace it, or for that matter, replace all 3 as they are only about $30 each. Don't forget to replace the $3 gasket behind each pumps. You can also rebuild the pumps, but for $30 why bother.

[striper56]

Thanks for the replies Olds cool, erebus, and vancouver.
I will check the fuel pressure and LP pumps first. If that doesn't work I will dig into the injectors. I did send out the injectors three years ago, 2 needed to be replaced and the other 4 needed a little cleaning.
I will report back the results. Thanks again.

[striper56]

Changed the LP fuel pumps and took it for a hide. Same issue except it is now cutting out SOME of the time when pulling back the throttle from WOT down to 2400. I run it up to say 4000 + RPMS then pull the throttle back 10 times and it will only cut out once or twice. Injectors next?
Thanks

[fredbrad]

check your gas tank for a anti-siphon devices,sometimes are inside of the pickup tube similar to a little wire mesh (like on my case) or little steel ball inside the elbow fuel pickup

[Capt Terry]

I just replaced a Sierra filter with a Racor filter and vacuum gage on my '76 Seafari with an ETEC 150. On the first run I was at the BRP recommended limit of 4 in Hg. Bushwacker advised to look for an anti-siphon valve and remove it per ETEC ownersgroup recommendations to avoid running too lean & hot. Removed it and partially blocked screen on fuel tank vent. Have not rerun it yet, but fuel bulb is harder.

[afterhours]

Hey Striper56,

Just in case you don't have this link:

http://www.boatinfo.no/lib/yamaha/ma...003yamaha.html

Section 3 deals with fuel and there are some good write-ups on OX66 EFI engines. On 3-58 and 3-59 there is a discussion on CCS also. The engine only fires on 4 cylinders between 850 RPM and 2000 RPM and 5 cylinders below 850 RPM. There is also a shift cut switch that if actuated (for example by moving the throttle into neutral from WOT) causes the engine to drop two cylinders, no matter what the RPM's are. This is to avoid shock to the drive train.