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Fuel related, I had the same issue with mine and I have 2 gas tanks, the first one had no issues or anything, switched over to the second tank and started getting the same issue you’re having. Definitely check your vents/fuel line to make sure nothings clogging it or restricting the flow of fuel to your motor
100% fuel related, i had the same problem on my boat and turned out my fuel line was to long and fuel filter was getting clogged. I would check for clogged vents or anything preventing fuel from flowing properly.
Check your tank vent. Like a smaller engine you open the vent on a portable tank. Should be a vent on the tank or fuel cap. You can try to loosen your gas cap (Hope it has a chain so you do not lose it if it falls off.) Put a strip of duct tape over it if you are worried about it falling off. That's the easiest to check venting. If the motor comes around then find the vent hose and replace it or see if it is pinched. Good Luck sounds like a solid engine. Hope it is just a vent issue.
Had the same issue with a Suzuki 115. Would sound fine at lower RPM but started cutting out once I got about 2500 rpm. My fuel lines on the engine were coming apart on the inside, squeezing the fuel lines felt mushy, like gel inside. That allowed small pieces of rubber to go to the filters. I was using Startron with "advertised" non-ethanol gas and the fuel lines still fell apart. Seemed like when I asked for more throttle, the soft fuel lines were collapsing and sucking closed. I replaced all fuel lines in engine with auto ethanol rated fuel injection line, had to change the low pressure and high pressure fuel pumps. But ran that boat for 6-7 more years and never had any other problems.
I had the same problem on my outboard several years ago, I found that issue was ：the fuel line from the tank to the engine was almost clogged. To make sure that was the issue, you need to connect the fuel line to a portable gas tank and test it on the lake, if the same issue happened, then you check the pump and the other things. That will save you time and money. Good luck 🙏
Definite fuel delivery issue either from pump (fluctuating pressure), fuel line breakdown from inside due to rubber degradation or carburetion / injectors (fowled).
It could be ignition coils failing. A weak ignition coil could still work and produce a spark but as the motor warms up it can’t produce enough spark to get the motor the run the way it is supposed to. A way you can tell is by pulling the spark plugs and seeing if the spark plugs are dirty or not. If they are dirty then that could be the issue.
Coil pack or power pack. I had the same exact issue, and then it finally went out on me. Replaced the power pack and ran like a sewing machine. You can check the spark by sticking a screwdriver in the spark plug wires and turn the motor over. No spark means no power.
I had the same issue with my 250 pro xs and it drove me crazy till I finally figured it out. Were the fuel pumps live in the vvt tank I think it's called "big aluminum housing" I removed that and tore it down. I found there was a piece of small plastic that was floating around blocking fuel from entering into the housing. Basically starving the fuel system. I used compressed air to blow out the fuel passage way and a piece of hard rubber shot out! Put it all back together and never had another problem.
Most likely you have a leak in your fuel line causing it to suck air instead of pulling fuel. And hard to check idle on muffs due to no back pressure. The idle will be higher in the yard. Will not surge either because it isn't under a load
I don't know Mercs like the Ox66 Yamahas, but when I hit weird problems on mine, it was a ground issue on the motor. Check your ground blocks for any corrosion, especially the one for the HP fuel pump. As the motors age, those grounds can get some corrosion that impacts the grounds and creates weird problems.
What others have said about Fuel delivery. Might be getting some air in the fuel line via the oil injection. Make sure your oil reservoir is air tight.
I had a Johnson 150 on my old skeeter. It would idle great and run like hell until the motor got up to temp. Then it would start to surge like your motor, but only if the cowl was on. It ended up being a bad coil on cylinder no.3
VST is similar to Yamahas VST. Check and replace all fuel filters and screens in VST. Lower pressure from fuel system can contribute to this. Saw it many times in our shop
I replaced the controllers on my 150hp Mercury EFI. Worked for me; I now get my full 5600 ROM range. Just a thought for ya, but you've probably fixed it by now. Good luck!
With the rev limitter issue you had, Id check the fuel hose between the tank and the fuel water seperater in the boat and if there is a liner on the inside of it, it possible collapsed and thats the reason it keeps bogging down, my 225 optimax did the same thing and that was the fix. replace that hose with a pure rubber hose and it wont ever happen again
I’m going to say it’s fuel related. The first thing you need to do is isolate the problem to either the boat or the motor. The best way to do that is to run it on an external fuel tank. If it keeps having the problem with the external tank then it’s obviously motor related and I would look at your fuel pumps. If it runs right on the external tank then it’s on the boat side and I would start by checking your vent. I had a similar issue and replaced damn near every component before I found that it was just a venting problem.
I would take it out and when it starts doing that, use a spark tester to see if you are getting spark, or just pull the boots off and if you get zapped then you have spark. If they all seem to have strong spark, then maybe move to troubleshooting your gas systems. If you have no spark or intermittent, then you could change the boots around to make sure that’s not the issue. Then change the coils around, and if that’s not the issue, then it should be between power pack, trigger, or stator. That would just be how I would check for spark. Doesn’t take too long if you have the tools on the water. If this isn’t your issue, then atleast it’s one less thing to worry about. Good luck
I am in your exact position I just bought a 2001 nitro and rebuilt the motor a 200hp mercury and I’m having fuel issues and because there’s no one out there that really works on these older motors I’ve done lots of research. There’s ways you can check if your fuel pump (sending unit) on the side is bad and losing pressure. U can try when on water when that’s happing by squeezing your fuel bulb aka losing prime is the term. And clean out water fuel divider thing.
A $0 diagnostic you can do is pull the sparkplug and read them. See if you have any outliers. I don't recall if you replaced them yet but they are cheap enough you could do a new set.
Definitely sounds fuel or spark related. Replace fuel pump, see if it’s fixed. Another comment mentioning overheating coils could cause this as well. You could also hook up the engine to a NEMA 2000 network and monitor fuel pressure during the event to see what it’s doing, having the boat under no load will not give you any meaningful data as the engine is not really needing to work to sit in gear or at idle out of the water.
You can look up how to adjust the throttle position indicator using a volt meter/ohm meter. Always check fuel lines 1st…especially the one from the tank to the motor with the bulb.
had similar issue years ago with grey fule line and ethanol gas. ate everything apart and gunk would cause it to bog down. I would start with someone elses suggestion try an external tank with noew hose and clean fuel and see if it still happens. gotta eliminate somehing and thats easy and low cost to do good luck
That's a tough one. Could be coils, plugs, or the vst filter. I would see if you can acquire a fuel pressure gauge and then to check the coils when it gets hot dont turn it off and see if it will do it at a lower rpm.
First of all I love those seats!!! Maybe a different color but it matches. Seems like fuel isn't moving fast enough to the pump. But I would think if that was the case it wouldn't start up so great if it sat for a lilbit. I don't kno if you did already but I would check the fuel line
Pump or injector pulse or air / vacuum leak. You can change that engine over to a carburetor. First ck compression then fire. Disconnect the battery for 30 minutes to rest the ECM.
Well change the fuel line from tank to engine, the line looks good from outside but it breaks down on inside causing air to get into system Also do not let these motors sit for long periods of time before starting them and running them run once a month on muffs to push clean fuel through entire system run for at least one hour not sure if your ethanol free but that also helps keep fuel system healthy as well
Get an external gas tank and run it on that first. Then depending on what happens diag from there. If issue still occurs then check spark on all cylinders and power pack. Also check voltage. Could be an alternator going at high speeds. Sounds rough at idle.
Theres 2 fuel/ air regulators. One in each fuel rail. When they start going bad it will surge. When it goes bad it will surge in nuetral and u will have fuel/oil mix running down the lower unit coming out of the mid section weep holes. I have a 300xs and have had this happen twice. U can change the one on the starboard rail without the tool. They are spring loaded and the one on the port side has a stronger spring in it.
Ive read alot of the comments and i dont have a motor that big got a 150 but id do a check on fuel tank to the carb or injectors. Check coil pack with a meter but it could be anything on newer motors down to a sensor. Hope you find the problem and be an easy fix
You have a coil issue. I had an older motor that would idle and run fine to 3k rpm then it would bog down. Not sure if you can check the timing on each spark plug to look for a bad coil.
They had issues with primer bulb that seemed good but would cause issues. If you have the issue try pumping the bulb. Cheap to replace if that is issue.
Funny I had an 82 90 hp Johnson for 14 years ran the Hell out of it and never turned a wrench except lower unit oil and spark plugs every 6 months kept cables lubed.Not one issue.And th se Texas Lakes are big.It was a Thunderbolt Bass boat.I SOLD IT year's ago but it must have been built Tues Wednesday Thursday.Ha Actually the tilt and tilt seal had to be replaced.Period
Sounds like water in your fuel tank , or like you said the throttle it sounds like when you have a hot foot and your revving it. So I would check the throttle and empty your gas tank completely and fill it back up you may have water in your tank
I don't think its fuel related. Fist question, is the outboard charging? Check that first. Could be a rectifier. It also could be a coil pack. It wouldn't idle and purr like a kitten at idle if it was fuel related.
Make sure you don't have the gray fuel line because it falls apart and it will get inside your injectors. Also rebuild your high pressure and low pressure fuel pump
replace your fuel hoses if theyre chinese you may have an issue with the collapsing internally also wonder if when the boat is on plane/plain if the pick up inside the tank is some how not staying submerged in the fuel is the fuel tank venting properly
I had my Opti do that maybe not to that extreme but mine turned out to be a bad plug. It would die and come back on plane. I had just replaced the plugs recently before the issue and one of the NGKs went bad. Changed the plugs with the NGK IZ and a fuel filter and it was good to go.
What were your RPMs at WOT before the issue? I have a 175 Optimax that would only pull 4000-4200 RPM at WOT, no codes. I was at WOT one day and it was like a turbo kicked in and it bumped up 1000r's and went back down after a few seconds. Turned out to be a bad Coil on Cylinder#6 (this is common on this motor) and I was only running on 5 cylinders for a long time before i diagnosed it. Replaced the coil, spark plug and spark plug wire and she's running like a top. Its easy to test also, just fire it up and start pulling spark plug wires, it will arc on the block if they are good. I should have started on cylinder 6, when i pulled the wire there was no change in the idle and no spark. I got shocked a couple time checking, haha. Keep us posted!
My friends boat with VST would surge like that and then stall completely. Pressure would build up in the VST and cause vapor lock. The low pressure fuel pump is only like 3-5 psi and even though the float in the VST was down no fuel could enter due to the pressure built up in the VST. I remember opening a little screw or something on top of the VST and hearing the pressure escape. With the pressure relieved fuel came into the VST, was picked up by the high pressure pump and all was good again. There is a valve/solenoid that is supposed to vent the VST pressure into the intake manifold and if it fails it will allow pressure to build up in the VST.
How much fuel in the tank? Vent clean and clear? It maybe pulling vacuum on tank if it’s not venting fully. Half full or lower tank would let sides pulse. Pull vacuum causing tank to cave and over comes fuel pump and tank takes a breath and then allows it catch back. Spiders or dirt dauber could have it partially clogged. Is fuel pump mechanical, electric, or vacuum?
If it was me I would replace the primer ball and fuel line. I think one of those is collapsing. If it was fuel pump it wouldn't run. I almost guarantee it's fuel line or primer ball
You bought an old boat. Haven't seen all the rebuild vids but did you replace the fuel tank or at least really clean it out. Sounds like it's getting starved for fuel. Sucked some goop somewhere.
Have you hit anything with the prop? Mine did this but I nailed a stump and busted up the bushing in the prop. When I hit higher speeds it would do that same thing in your video.
Had this problem and replaced both fuel pumps, still had the same issue. Turned out to be the fuel driver in the ECU was bad. Picked up an ECU off a parts motor as the new replacements are very expensive such as the bracuto
