Motor question for the Guru's...
 

I have 525sc motors in my boat. Just last month, the port motor began to "miss" and the rpm's would drop significantly. They would then go back up. Almost like the motor was either back firing but without the bang. It was more of a hiss.. At the same time, I had lost about 500 rpms on that motor from where it should have been running. Initially, the motor just did this at wide open, then at a high rpm, then, simply if the motor had a load on it as in heavy current or getting on plane. It got to be a big enough problem that the motor began to stall out following the episodes. It would then start back up like normal. The time frame from problem start to stalling was under 20 minutes.

I took the boat to my local guy who is very competent and had them look it over. Initially the thought was a misfire as the plugs, wires, etc.. had not been changed in a while. So, new plugs, wires, rotor and cap. The problem persisted. So, the changed the fuel pump along with filter. Did the same work on both motors. At this point the motors ran really good. Got the rpms's back up to 5000 at wot. The boat is running 26 pitch 4 blade bravo props. No more issues while running here in Pittsburgh on the rivers. Even sustained runs.

So, we go to Buffalo and partake in the poker run. The problem came back. But not as frequent. During the run, the port motor would do the same "hiccup" when we would have a hard landing on the water and send a jolt through the boat. If the water was calm or smooth, no problems. Just when we would hit hard. The voltage, oil pressure, temperature and fuel pressure gauges would not move when the rpm's would drop then pick back up. No stalling either. Kinda baffled. Any thoughts or ideas would be greatly appreciated.

What ignition do you have, TB IV or TB V?

I had an ignition issue with an HP500 with TB IV that started out intermittently then got progressively worse. It would run great for some time, then hiccup, and eventually stall and not re-fire until it sat for awhile. It turned out it was the rev limiter, as it heated up it caused the ignition to fail. Probably not the same issue, since yours seems like a loose wire or ground loose and the jolt is moving things enough to cause a momentary issue, but at least something to consider.

Not sure which one I have but I can find out. Havent had a chance to get together with my mechanic here since we've been back as I have been busy with work. Was thinking the possibility that the jarring could be causing an interruption in the fuel but a loose connection could be it too.

I would guess ignition also. Check all your grounds and battery cables. Sometimes the coil can get hot and "go open"

Pretty sure your 525's have the thunderbolt 4, and the extremely problematic rev limiter mounted right behind it on the bracket. Get rid of it.

Don't be afraid to check your actual ignition switches and the connections to them also . Have seen them do this quite a few times in the past on boats. They eventually wear out from either the keys bouncing around in them or them getting wet repeatedly.

Do you have kill switches? On two different boats I have had the kill switch (lanyard side) become loose enough to allow the button on the switch to come out enough to make the engine momentarily cut out when hitting a large wake or wave. Drove me crazy till I figured it out.

Padraig

Post your serial # - early early apps had firepower ignition and later years went to t-bolt set ups.

Thought they all were thunderbolt

I'll go a different direction assuming the first running issue was solved which it sounds like it was.

If your boat was really being jolted hard it could simply be fuel sloshing out of the bowls, thru the vent tubes, back into the carb's throttle bores. This is not uncommon if you are driving like in an Apache Boat video. LOL.

I have rebuilt 525SC carbs (Holley Dominators) and don't remember seeing any slosh baffles installed in the top of the metering blocks.

If your boat continues to run great in water that doesn't make you 'jolt/land hard' then this may very well be what's up. In the meantime, you can make sure that yuor float levels are correct. Too high a float level can cause this too - especially when jolted.

Will add to this, what I have remember as we had to fix some of these that would run good and then at any giving moment would not or just shut down, let sit and would run good all over again. Back in late 1998 if I remember correctly was that mercruiser had a bunch of engine serial runs affected by excessive internal ignition module heat and if they got to hot or stayed hot for long periods of time the module could completely fail.

mercruiser came out with a heat sink plate kit which goes on the module to help draw heat away from it and this solved a lot of those sporadic running problem - shutdowns and failures. Just bringing this info about so maybe something to check out or possible module failure to whomever.

All 525 SC with engine serial # runs 0D456000 thru OD763831 used firepower ignition set ups, After those serial runs from Mercruiser came T-bolts ign. Sometimes I know about this marine stuff you just have to believe. LOL

+1
I had this happen on a poker run.. We were in 2' tight chop and when we approached our first stop.. one engine loss rpms for a 1/2 sec and came back, wife asked, "what was that?" I was like, wtf, I don't know?? Then it just killed. Cranked right back up and did it again. This time when I turn the key to prime, it didn't sound off, I went to move the lanyard and it beeped!!

We shoved some paper from my bro n laws cig pack and lasted the day. I changed both kill switches and the initial beep sounded a lot better.... That was my problem! It had missed a lil before but nothing that bad. Easy check!

Check the main power plug to the engine and all grounds associated, coil would be another option

cannon plug and the pins having good solid contacts with none broken nor any corrosion.

I will definitely start with replacing the safety/kill switches. I know I had to adjust the port motor switch a couple of times to make contact so the motor would start up. Then I will start checking everything else. Hopefully I will get to duplicate the conditions here on the rivers and see if it works. Otherwise, I will have to wait till I run in rough water again to test it out. Thanks for all of the inputs. Spoke with my mechanic today and he tends to think it might be a contact or adjustment issue at this point as well.

At least with twins you can roll components between the engines, and eventually you will come across the issue, either by process of elimination or you will find the issue as you are removing/reinstalling parts.

C-Deezy, I saw the boats line under your avatar and thought for a moment you might have been the buyer on my old 240 Sport. But it supposedly went to the Cincinnati area of Ohio.

I could be a kill switch problem which is not adjustable, but I highly doubt it has anything to with the neutral safety switch adjustment.

The problem sounds electrical but it could also be float bowl sticking.
I know n my 525SC Pantera, I launched it nose high a couple times and pulled back on the sticks. The angle would change s much that fuel bowls would flood and stall the engine.
I have also heard of the Merc TB IV add on rev limiters having issues