Hey guys, I apologize about this being extremely long but I want to give you the most information to see if anyone can make sense of this.. I’ve got a 1996 Hustler, originally came with a Gen V 502, but was replaced with a Gen VI long block. It’s got Stainless Marine through hulls and about 200 hrs on the new motor (replaced in 16). Runs on MEFI 1 w/ VST. When I bought it, it ran the same way as I describe below, just much more severe symptoms. I’ve been trying to get this thing running right and it’s absolutely killing me at this point. I’m reaching out in hopes that someone will have some insight on what could be going on.. so here’s a description with some videos, a list of things I’ve already done and videos of the actual scanner recording. I’m posting these because a lot of the parameters don’t seem to be a steady value and I’m not sure if they’re supposed to be.

I’m using the Rinda Scanner and Merc manual 16.

-Symptoms:

•IDLE: good/smooth at 600rpm. with the Rinda scanner, all parameters appear to be in check and it appears to be running “good/as it should.” Let’s call this good running condition, “condition A.”


*Only concern here at this point is Map voltage which is 2.3V at idle. According to the book I have, it says 1-3v at warm idle, then in another part of the book, says 1-1.5v so I’m not sure if that could be part of the issue.*

•NO load/in neutral: on the trailer or in the water, I can slowly raise rpm, and at about 1k-1100rpm, the boat starts surging a little. While still raising the rpm slowly, it smooths out at like 1500rpm. Slowly raise it more and at about 2k rpm, the rpm shoot’s up at an insanely fast rate. This is 100% repeatable, every time. Prior to putting in a new coil, it misfired like crazy this identical rpm range instead of surging. See video 1.

•ON THE WATER: it seems to idle fine, in neutral and in gear as described above. I don’t have data while just idling at 600, but column (A) of the chart and video 2 at the bottom of the post is 950rpm in forward gear.

•ATTEMPTING TO GET ON PLANE: When trying to plane out and getting up on the water, it sounds like something is holding it back. it’s down on power big time, takes longer than it should to plane out, seems very rich, the bow is extremely light/bouncing, some misfiring and max rpm is about 4300. I can even put the tabs all the way down and it still bounces like it’s crazy back heavy. Let’s call this poor running condition, “condition B.” The data associated with this is in column B and video 3. “Condition B” is HEAVILY affected by how much weight is in the boat (fuel/people) and gets much worse with more weight.

*Side note: When I first got the boat, max rpm/speed was 30mph at 3300rpm with these same symptoms/conditions but as I was slowly going through everything, it was getting better and better to where I’m at now but something is obviously still very wrong.

This probably sounds ridiculous/unsafe, but the only way to correct poor running “condition B,” whether it’s 30 seconds after plane, or five minutes, is to cavitate the prop even just for a quick second. I do this by trimming up a little and yanking the wheel quick just to have it catch some air to be able to spin past that rpm point…..Once that happens, it seems like it’s running perfect..immediately, you can feel the transom pick up, nose of the boat goes flat/zero bouncing, immediately picks up a ton of speed, sounds like it’s running perfect, no longer rich, no misses. So right away I’ll trim back in, throttle back to a cruise and it’s good. I can go WOT and it will spin to 5100/5200rpm and go 69mph, no issue. I can throttle back enough where the boat is about to fall back off plane, (say like 2800rpm at 30mph) wack the throttle again and it still runs perfect. I recorded some data of it running great. Data associated is column C and video 4

****BUT, if I come back down to an idle, or just above, even for one second, then try to plane out again, I get the SAME poor running “condition B” over again and I have to cavitate it again… This happens every single time, at the same exact rpm.


This last time I took it out, it felt like something was literally holding the boat back bad like the anchor was out. It seemed like it was almost drive related because the difference in how the boat handled, was positioned in the water at similar speed, and even threw much different wakes/where the water was being thrown from the back of the boat. But it makes zero noise. I’m at a loss. If anyone has any ideas, I’m all ears.

I need to grab a go pro and stick it under the swim platform and see what’s actually going on back there.

*I will attach the videos in a moment just below*

here’s a list of things I’ve done in the last two months since I bought it:

IGNITION/ELECTRICAL: -Plugs/Gapped to spec

-MSD super spark Wires

-Merc Cap/Rotor. Original was corroded.

-Merc Ignition module.

-Merc Ignition Coil. Previous one caught fire in driveway

-Fixed grounds x3

-ignition timing 8* off. Fixed.

-Merc Knock sensor. Bench tested bad.

-Separated, rerouted, and added loom to all ignition/charging/knock sensor related wiring to mitigate interference.

-Ignition circuit breaker replaced. Was frozen.

-was still misfiring. Re-removed dist, retaining clip for rotor was broken. Fixed, but no fix.

-replaced “fixed” distributor with Summit Racing Billet Aluminum distributor but no change.



FUEL:

-Fuel filter

-Crimped/Braided stainless feed and return fuel lines (VST to rail, rail to VST). Originals were kinking.

-Ultrasonic cleaned and flow tested injectors (two were faulty)

-New injector bushings/orings/gaskets

-Cleaned/tested Fuel pressure regulator (clogged).

-Added in-dash fuel pressure gauge

-removed fuel tank pick ups to check for clogs/debris. New braided lines

-new fuel pick up, vacuum tested.

-new anti siphon valve and line. Tested with clear hose and saw fuel backing back into tank/creating air.

-fuel lines purged and anti siphon added. Now lines hold fuel completely.



SENSORS:

-Merc TPS. Changed but no improvement.

-Merc MAP. Original tested bad.

-IAT. Original read very low and changed.

-Merc Knock sensor. Original Bench tested bad.

-Merc Idle air control valve, original was frozen.

-Oil pressure sensor wire fixed and repinned. Loose causing intermittent alarm.

-Replaced stainless speedo pick up tube



DRIVE:

-fluids

-New overflow bottle. Wires broke out of level sensor



MISC:

-Merc Alternator. Was only making 12V, now making 13-14.

-Replaced all 3 belts

-Water pump pulley. Was bent, replaced.

-Fixed two exposed wires in harness

-Two nonfunctional breakers in dash replaced

-Removed PCV system and added catch can.

-Exhaust manifold bolts: 1 missing, 3-4 others loose. Fixed.

-Broken mercathode wire

-Replaced clear/yellow low side vacuum hoses to fuel pumps.

-Replaced all high vac hoses.

-Intake plenum, throttle body gaskets

-calibrated IAC to idle value of 40

-smoke tested motor multiple times looked for vac leaks

-Rechecked timing

Data:
You can see when it’s running like crap (column b), it shows way more TPS, at a much slower speed, way more GPH, increased pulse width, etc compared to column C at almost the same speed.
*I will attempt to attach the chart and videos, not allowed due to post count*

You need 10+ posts before you can put up pics.
There is no video downloading at all, you’ll have to put on youtube or etc and link or embed here.

You need to get some fuel pressure readings. Fuel psi for VST engines is (I think) 32psi. You need to check fuel psi while running the boat and under load.
This all sounds pretty typical for VST issues.

Exhaust under water when off plane ?
What prop ?
what fuel psi at idle, bad rpm/load spot ? Wfo at top rpm ?
Any fuel in vent for vst ?
Vacuum line hooked to regulator ?

I didnt know that and uploaded them all to YouTube already. I do have the links and a chart with each one of those rpm ranges but not sure how to get it up here

I did a ton with fuel pressure. Sorry I wasn’t more specific but it seemed to be all spot on. Key on, pump running. 37psi. Key on pump not running, 34 psi. No leak down. Then while idling, mild rpm and even WOT, it doesn’t go below 31 psi. believe it or not, when it’s running like hell, it actually has more fuel pressure, but I’m assuming that’s because there’s less manifold vacuum going to the regulator on the rail.
I need to get these charts up.

no water in or near the exhaust. No fuel in vent hose from the VST.




I appreciate all the tips guys! One thing that confuses me, is this setup supposed to have a mechanical pump on the side of the block? Some diagrams show yes, and some do not. Mine has one along with the VST. Also, I had a recent random thought. When I checked the pickup tube, I changed the fuel line from the pick up, to the filter housing just so I could see what was going on.

Immediately noticed it was draining back and not holding a prime even minutes after shutting it off. even after running it for a day. (Not sure if that’s normal for a properly working system without an anti siphon valve but in my mind, having that happen in a “closed” fuel system, immediately raised red flags because there’s no where for the air to escape.. Checked the rest of the lines and found no leaks. This could be total bs but my thought was, In a closed system and without a constant prime/one way/siphon valve, the pick up in the tank, all the way to the mechanical pump is constantly getting full of air. Which..the mechanical pump is.. belt driven. Which is directly related to rpm. It’s gotta be that the mechanical pump doesn’t make “X” amount of vacuum to overcome the air in that line until “X” rpm. Which is why, every time you go back down to idle through a no wake zone or whatever, rpm decreases, along with vacuum, and the fuel falls back down the pick ups. Then the process repeats.

I did add the anti siphon, but I still see a small amount of air.


PS: my YouTube username is kyle2614
there is only a few videos there if those help

video 1:
video 2/column 1:

video 3/column 2:

video 4/column 3:

You have a block mounted mechanical fuel pump along with the mech fuel pump on the seawater pump ?

Has it always done this or did this problem emerge suddenly? I recently attempted to help a guy with similar issues on a mag engine, nothing helped it similar to yours. Someone else had supposedly done a compression leakdown test on it, customer pulled the heads off and found a tuliped, bent valve hanging open, not saying that what you got going on but id recommend a compression test then leakdown the worst cylinders at least. I was convinced it was fuel or ignition related, it was not

Oops sorry, I have a seawater pump mounted mechanical fuel pump that goes to the VST.

I only got this boat about three months ago and it’s done it since I got it. Each thing I fixed definitely Seemed to improved certain areas of drive ability, but It’s certainly still there. Now it’s almost more obvious when it switches from running like hell, to running good, because when it’s good, it’s GOOD.

God I hope this isn’t the case, although I wouldn’t be surprised, I don’t know what’s left. I was going to put a carb on it in hopes that would take care of it but just have been looking for an intake manifold. I want to at least know wtf the problem is before I cover it for the winter.