2004 300 hpdi mystery

Ok Guys,
This takes the cake. I have a twin 300 HPDI rig with my port motor doing the lost rpm suffle. That is, I plane out with both motors syc'ed up to lets say 3200 rpms, in 15 to 20 secs after everything stabilizes my port motor will drop, 200 rpms and about 1 to 2 minutes later it slowly comes back to syc. With that said, I have monitored both medium pressure and HP during this event (ie medium press 50 psi, HP 986 psi). Additionally, all variables monitored using YDS (ie TPS, Spark, Fuel PW, temps, etc..) remain constant except for of coarse the fuel PW; it goes up ever so slightly with the rpm drop (that is expected given the load appears to be increasing for no reason). This has been happening for about 2 yrs and now and I have finally decided to fix it but I'm at a lost. One more detail, the motor will not achive max rpm, hence the concern.

Somethings I've done: swapped entire VST assembly with good motor (no change), swapped HP pump with good motor (no change), swapped entire coil and spark plug wire assem with good motor (no change)....any ideas anyone? Oh, and good motor remains running good. Compression is good and even, no codes. Any help appreciated. I even swapped fuel line just incase and still no change.

Re: 2004 300 hpdi mystery

Calling on Rodbolt,
I have read many of your post throughout the years and have come to the conclusion that your experience and knowledge on these outboards is second to none.

I'm not sure what to do next, injectors? Have you ever seen this behavior out of an hpdi?

Re: 2004 300 hpdi mystery

Rod has not posted here for some time, but he has on the other forum that you posted this same thing on.
Hopefully he will post on that thread

Re: 2004 300 hpdi mystery

Thank you. Hopefully someone has found the root cause. When I do discover the cause, I'll be sure to post.

2008 300 hpdi mystery

2008 300 hpdi mystery

First timer question. My 2008 300 hpdi blew up after 100 hours, cylinder #1 smoked. Luckily Yamaha manned up 2 years out of warranty and covered most of the bill...
However, now it appears I have a charging problem. Killing batteries,
Volt meter drops to 10 or so while running. If i disconnect the battery while the engine is on it dies. I havent ventured under the hood yet but have located the stator and rectifier on the parts manual. Anybody have any experience in this department and a good starting place

Re: 2008 300 hpdi mystery

Re: 2008 300 hpdi mystery

It would be a good idea to start a new thread on your motor and problem, not put it on some one elses.

And It is never a good idea to disconnect battery from running motor , it will smoke parts

Re: 2008 300 hpdi mystery

Re: 2008 300 hpdi mystery

Check AC output on stator. If voltage good, bad regulator. If voltage low, bad stator. If you disconnected the battery you blew the diodes in the regulator anyway.