The motor starts and idles great. The issue is when giving it any throttle it will speed up but does not stay constant. We are thinking about draining the fuel tanks but wanted to get thoughts first. It revs up and down swinging really bad upon increasing the throttle. 2000 to 500 to 2000 and so on. But no issues idling for an extended period. Fuel pump has been replaced. Anything else it could be besides bad fuel causing the issue?

Not know details of the motor, I'm going to say it's your VST filter is clogged

I would say same thing VST filter and I would get Injectors clean also

Don't run it that way, take it a shop. but you know that.

We can check your motor out for you

We are going to look into the O2 sensor and VST filter to see if it resolves the issue. Appreciate the help.

if 02 wets most time cant clean

The engine is a 05 yamaha 225 vmax. I do not believe it has an o2 sensor. But the VST filter was extremely dirty a d there was about 1/8" of trash built up in the bottom of the separator tank.

We have ordered new gaskets and filter to replace.

Does anyone know if the fuel o should fall out of the top part? I need a diagram or someone to explain how the fuel pump sits back in the top of the vst. There is a small plastic piece that goes on the discharge of the l u.p that slides up and down. But not sure which way it goes or if there should be an l ring in that piece where the fuel pump discharge tube sits?

Yes it does have an O2 sensor, and they cause lots of problems. I would be looking at the O2 for sure.

If the VST had a lot of trash in it, I would clean it and replace the filter there, but I would also replace all the other filters. Your power head could be at stake if you don't. If just one of the injectors goes lean it could destroy the power head.

There are 8 mystery filters, 2 in the High Pressure pumps and one in each of the 6 injectors. They cost less than $1 each and are a PITA to change.
There is a filter inside the clear plastic housing that is the initial filter that feeds the VST tank.
There is the filter on the bottom of the electric fuel pump inside the VST tank
There is a replaceable inline filter that is just above the VST tank that has a tricky little connector on one end and a hose clamp on the other.
This also might be a good time to replace the low pressure pumps since there was lots of trash in the VST. They are about $40 each and there are two of them.
And if you do not have a water separator in the bilge, add one. If you have one make sure it is replaced at least every year, preferably more often that that.

Most of the HPDI motors do OK if the fuel system is maintained frequently, but can explode in a heartbeat if you don't.

Wayne

you wouldn't no how to disconnect that ( tricky little connector ) would you

Gotta push the sleave in as you pull the hose out. I used a box wrench to slide down and compress the sleeve.

^This

You HAVE to keep the push sleeve pushed all the way in when you pull the hose out. Kinda a PITA because you are pushing one way and pulling the other at the same time.

that is tricky thanks

Search or google HPDI and VST etc and you'll find lots of how-to's with pictures on other forums (the hull truth) is the one I used the most. On the overpriced filter learned this trick and it worked pretty well. Harbor Freight Panel Clip Pliers HF Pliers