Engine performance question

[MechanicMike]

I have a 2000 Yamaha 150 SWSII OX66 SX150txry.
If a cylinder was cutting out at speed what kind of RPM drop would i see? I've been trying to chase down a lack of WOT throttle RPM all season. Boats been running well under normal use. I checked the plugs the other day and one plug was real clean, like its not firing all the time. Plugs were originally replaced back in May. Changed the plugs again and the boat ran great. Picked up top speed and RPM. After running the boat for a half hour, the boat started to surge at cruise speed. Would drop a 100 rpm or so. Seemed to be less noticable at lower RPMs under 3600, but did notice it on the holeshot. Fuel system had been gone through at the beginning of the season, all filters were replaced or inspected including VST,regulator screen, injector screens. Low pressure pumps replaced, 02 replaced, spark plug caps replaced. Compression check 115-120 all cylinders (previous owners definition on routine maintenace differed from the shop manuals). Pumping the primer ball did not change performance. It seems like a cylinder is cutting out. Im thinking its something ignition related because the new plug seemed to improve things temprorarily. Are coils common failures on these engines? I'm gonna start checking it out this week but would like the problem resolved before I pull the boat for the winter. Any info is much appreciated.
Thanks
Mike

[99yam40]

anything can happen over a few months, so all I can say is start testing things all again.
Compression, fuel pressures, and ignition system components like the service manual says. maybe test voltages
Hooking it up to a computer would help a lot.

Plug boot was my 1st thought, but you just replaced them all.

Too rich or too lean of a cylinder will not let it fire properly, and if it is too lean you can damage a piston. So be careful

Would not hurt to check the fuel pumps for a pinhole

[jf123]

Clean plug may indcate water. Try a leak down test. The heads sometimes develop smalll pinhole leaks that a compression test will not pick up. If your brave put your tongue on the plug and see if you taste salt.

[MechanicMike]

Thanks for the replies.

I was thinking water myself but thought being this engine is run in saltwater, there would be some kind of residue on the plug. The only residue I saw was oily. The boat was been sitting a week when I replaced the plugs. Also the low pressure pumps were replaced in the beginning of the season.

Are headgaskets a maintenance item? I'm not sure of the total engine hours, but believe it to be somewhere in the neighborhood of 600-800 hours.

Thanks again.
Mike

[jf123]

The oil helps cover any salt crystals until it gets worse. Taste seems to be the best indicator unless it gets worse and noticable more on the plug.

[MechanicMike]

As I cant bring myself to lick the tip of the spark plug, i did pull the plugs again last night. Theres only about a half hours run time since install, I couldn't see anything wrong with the plugs. I did look in the spark plug holes with a flash light and all the tops of the pistons were carboned up. Didn't seem too bad but if water was entering a cylinder and creating steam I'd suspect the piston top would be clean or noticably cleaner than the rest. They all seemed about the same. Hovever I did notice corrosion around one head bolt. I've read this can be normal with older engines. People refer to it as salt creep?? Still kinda new to boat stuff
The boats getting pulled this weekend. I'll do a final de-carb on the engine and stabilize it before pulling, and retest everything once its home.

Thanks for the replies
Mike

[jb123]

which hole is giving you the issue ??
have you run motor up to 1500 + rpms and pulled a plug at a time ?
checked lp pumps ?

[MechanicMike]

Low pressure pumps are new this season. Pulling plug wires is worth a shot. I avoided this because i wasnt sure if certain cylinders are suppose to cut out at lower rpms. Didnt wanna mis-diagnose. The cylinder in questiin is cylinder 4 i believe. Port side middle.

Thanks
Mike

[99yam40]

unbolt pumps and pump up primer to see if fuel leaks from the pulse ports.Just to make sure they are OK
Never know when something will go bad after replacing

[jf123]

Leave the plug out that is in question. Over a few days once the fuel evaporates it will show corosion if there is salt present.
I would recheck the fuel pumps as mentioned. You can also send the injectors out to get flow tested.

[MechanicMike]

Quick update:
Ran several tests yesterday before I pulled the boat, hooked up auxillary tank ran engine, ran with diagnostic tester, no codes. measured tps voltage & o2 sensor voltage- o.k.. Did a decarb of engine. Engine ran better but still same problem.
Pulled boat. When I got it home, started winterizing it. Unbolted low pressure pumps & pumped primer ball. Wouldn't ya know it the top pump was leaking badly. Pumps were replaced in May with Yamaha pumps. Only reason I could figure was I was chasing a lean sneeze condition back in may. Had one bad pump then, so replaced both. Ran the boat and it still sneezed. Traced it to a bad o2 sensor, which fixed the lean sneeze. Im thinking the sneezing might have ruptured the fuel pump diaphram? Only ran the boat briefly like that. Live and learn I guess. Only bad thing is I gotta wait til spring to run the boat. I hate north east winters.
Thanks for all the replies
Mike

[99yam40]

Quick update:
Im thinking the sneezing might have ruptured the fuel pump diaphram?

Mike

I believe you have hit the nail on the head.

Never assume just because you replaced some part a few months ago, it is not the problem.
It is too easy to test all things over again to be sure

You should be good to go next spring.