'01 250HP OX66 Surging at 3700RPM

[rodbolt17]

dunno.
did a OX66 two stroke today that everyone has had their mule skinners on.
had to clean the VST, the VST was the 66K style not the 65L style yet they had a 65L filter in it.
had to pop out the injector screens and clean them.
then I had to correctly set the tps and the shutter links.

[DennisG01]

I hope I don't have to go through all that! I was not aware there were different "styles" of VST's and filters. But I do know that the style (as far as "looks" are concerned) of VST and filter is the same on this 2000 model as my "spare" 1997 model.

Over this last winter I had the injectors cleaned and rebuilt by a local (reputable) injection shop. The report said (and I'm paraphrasing here) that they were pretty good to begin with in terms of spray pattern. The baskets were slightly dirty - new baskets and o-rings were used.

[rodbolt17]

no I don't think they are.
an easy way to tell??
the 65L uses four larger screws on the fwd end of the VST cover.
the 66K all the vst cover screws are the same size.

[DennisG01]

no I don't think they are.
an easy way to tell??
the 65L uses four larger screws on the fwd end of the VST cover.
the 66K all the vst cover screws are the same size.

I could be mis-remembering. The '97 is at home in PA right now, and the '00 is with me in Maine right now.

Hopefully everything is at least correct for the year - the guy who owned the boat before me had it serviced at a Grady dealer... hopefully!

[DennisG01]

I'm beginning to think this issue is throttle cable related. It acted up today, but I wasn't in a position (family was with me and it was a bit choppy out) to start experimenting. Anyways, I bumped/tapped the throttle down and then back up again and it held RPM's fine. It did it one more time and the same "fix" worked.

[zenoahphobic]

I'm beginning to think this issue is throttle cable related. It acted up today, but I wasn't in a position (family was with me and it was a bit choppy out) to start experimenting. Anyways, I bumped/tapped the throttle down and then back up again and it held RPM's fine. It did it one more time and the same "fix" worked.

The friction adjuster needing tightening can lead to dropping of revs as the springs will gradually make the lever creep a little towards low.
Quite common I would think.
Seems like too much emphasis on "fuel" when perhaps this might have been a mechanical issue
Look for the simplest first.

[DennisG01]

The friction adjuster needing tightening can lead to dropping of revs as the springs will gradually make the lever creep a little towards low.
Quite common I would think.
Seems like too much emphasis on "fuel" when perhaps this might have been a mechanical issue
Look for the simplest first.

The shift/throttle lever, itself, does not feel 'sloppy'. Meaning, I don't think the throttle lever is falling back. Could it still be something in the control/binnacle, or something back at the engine? Or maybe it's just old cables? It's just weird that it never happens at other RPM's.

[zenoahphobic]

The shift/throttle lever, itself, does not feel 'sloppy'. Meaning, I don't think the throttle lever is falling back. Could it still be something in the control/binnacle, or something back at the engine? Or maybe it's just old cables? It's just weird that it never happens at other RPM's.

Depending upon the angle of your control, weight and vibration can gradually act on your lever as you drive. We don't know what control you have.
You can only check this by disconnecting the cable(s). I'd start at engine because this might be the easiest. With cable loose tugging on the ends do they readily move the lever? Move the lever through it's arc, does it move smoothly and easily? what about near the RPM setting? how about moving the throttle at the engine where you have disconnected it? often there are points where resistance increases (various cams) and check at maximum ignition advance (seems your issue at the revs max advance occurs). What I 'm getting at is that resistance at max ign may be pushing back on your cable.
If you found your control arm extremely easy to move with the cable disconnected then I would be looking at it for friction dampener. Once of course if you are at the back of this control you may need to get behind and into it and disconnect the cable there and see how free things move.
Eliminate this system as having anything to do with your issue I would suggest at this point.

[DennisG01]

Thank you, guys. It'll be some time before I get back to the boat, but everything that is being said makes sense and I'll work on that next time I'm at the boat.

I'm not sure of the exact model number of the control box, but from the looks of it (and pictures I've seen/compared online), it "appears" to be a #704. It's a top-mount, single lever with the gear-disengage at the "pivot" point of the lever. Has a single button trim/tilt switch on the handle (thumb activated).

[zenoahphobic]

That control has a slotted screw on the front side to adjust friction on the lever.
Remember if you don't get back to your boat for awhile then it might be due for maintenance and you might have to start again.

[DennisG01]

Slow, but still working on this. Got a question...

... If I monitor the fuel pressure at the Schrader valve above the high pressure pump:

If the surging happens and the pressure doesn't drop, does that eliminate the fuel supply from this equation (other than the line from the HP pump to the fuel rail)? I can imagine it being a problem with the injectors - the same issue was going on last year and I had the injectors professionally cleaned over the winter.

All I'm trying to do here is narrow down the options of where the problem may be - in other words, I'm trying to eliminate variables. In the end, I may very well be dropping it off at a shop, but since the problem is not consistent, I thought it might be best to be able to provide as much information as I can.

The only fuel pressure tester I have only reads to 10psi (was/is used for my lift pump on my '98 6.5 Turbo Diesel Suburban). I was thinking of this: https://actron.com/content/professio...ure-tester-kit Thoughts?

[DennisG01]

Oh, I should add that I haven't been back to the boat since the end of July so I haven't done any further checking with the cables. My brother will be at the boat in about a week and I explained how to check the HP pump for pressure, along with showing him the information in the service manual (I have one at home in PA, along with one on the boat)

[DennisG01]

My brother will be at the boat for the next week or so. He's going to hook up that fuel pressure gauge and also observe the throttle cable (at the engine). "Hopefully" it will act up for him....

[DennisG01]

He didn't get a chance to hook up the gauge, but I was there last week to pull the boat and winterize. Went for about a 30 minute ride with the fuel pressure gauge hooked up, but the engine ran flawlessly (which isn't a bad thing, but it makes it tough to figure out an occasional problem!). I took a video of the pressure gauge and one of the things I was wondering about was the 'fluttering' of the needle. NOTE: My phone's camera didn't catch how quickly the needle was actually fluttering and in the video it 'appears' to be fluttering very slowly. In real life, it was fluttering/bouncing very quickly. In the video you'll see that the needle flutters for a bit, then becomes rock solid, then flutters again, then rock solid, etc. This back and forth thing was pretty consistent. Could this be happening when the VST tank is filling up (float valve)?

What do you guys usually see the fuel PSI running at? About 35 like mine? I thought that about 35psi is correct?

The side-mounted (port side of engine) fuel/water separator filter (little red ring) stayed about 2/3 filled the whole time.

I later drained some fuel from the VST tank - very, very clean. I then checked the VST tank and filter - spotless. I also checked the fuel pressure regulator filter - also spotless.

https://youtu.be/HIE0RyLEPl8

[zenoahphobic]

Fluttering gauge

I would not be worried about the gauge flutter. The pump and engine produce vibration, I suspect your gauge tunes in and out , or resonates with the frequency. Hysteresis is another explanation.