1996 150 has intermittant low power after new reed valves

I have a 1996 Merk 150 offshore. It has always idled a bit rough, but I was told thats how they are.
Recently I replaced the reed valves, the newer ones are much stiffer than the old ones. I fired the motor up and it seemed to be running rougher than usual and I can't get it to idle below 1000 rpm. I called a local mechanic to have a look at it. Compression is fine and everything else seems to check out. He said take it out and give it a good run, and see how it settles in. I did just that. The first thing I noticed, is it seems to be way quieter. It really used to whine and now it hums if that makes ant sense. Initially the boat ran fine, acceptable hole shot, 30 mph at 5000 rpm. pretty much like she used to. I did several stop and starts, all good, so I gave her a 45 minute run. It ran like a top. I stopped to fish for about an hour, trolling on my kicker. Started the engine up for the run home, so far so good. But when I went to open her up she just quit. for almost 15 minutes the best I could get under load was 2000 rpms before she would stall out. In neutral she seemed to rev up fine except now she is puking blue smoke, a huge cloud. I ran it at 3000 rpm until it cleared up and put the boat back into gear. Still the same problem 1500 -2000 rpm then buhhh. Finally I started backing the throttle up and down, then all of a sudden she just opened up and ran like a champ all the way home. No smoke, no funny noises. I didn't try to shut it down and start it back up because it was almost dark and I had a bit of a run to get home. I'm going to pull the plugs and have a look at them later. Does anyone have any ideas why this might be happening?

A guess? It fouled a plug.

Jeff

Nope I checked them today, all looked good. Also if it was a fowled plug, why would it all of a sudden start to run well, and continue to run well for more than half an hour on the run back?

What are the compression numbers ?------How / why did you decide to replace the reed valves.-----Not much goes wrong with them.

Compression is good. 100 -110 across all cylinders. .............I replaced them because one of the old ones had a big piece missing from it. I couldn't get exactly the same type that was already in the engine, as mercury has changed them, so put all new ones in of the same type and flexibility.

Water in gas?

nope good gas. Plugs all clean looking. can it get over oiled somehow? or over fueled? I have rebuilt the carbs, and in my mind if it was a sticky needle valve it would be consistent and would over rev, no?

A sticky needle valve could flood cylinders with too much gas and motor would slow down.-----But run with a timing light to see if to have ignition issues.

Im not sure what you are suggesting I do here. Do you want me to take it out on the water with the timing light? I have checked it, I forget what the reading was but it was slightly advanced and seemed steady. I am at the extent of my knowledge here, so if you have a suggestion, could you please also provide some basic instructions?

Hook the timing light to the # 1 cylinder.-----Lay the timing light on a seat.------Take boat for a spin.-----Observe the flashing of the light.------Repeat with other 5 cylinders.------Bad spark will be obvious as the light will be intermittent or no light at all if cylinder is dead !!

Ahhh Ok that is easy enough. It is going to be a while before I can put the boat back in the water. I just started a project that is going to suck up most of my time for a couple of weeks, but if I get a spare couple of hours I will try to get it done. Do you think I can do it in the driveway on muffs? Or should it be under load?
Oh and thanks for your help, it is appreciated.

Gotta be under load! Takes no power to turn an empty propshaft!

I have a 1996 200hp EFI merc. Was bogging at acceleration wouldn't plane off after going from fear to literal several timed it would take off. Smoked like a freight train, would stall out and die. Replaced everything under sun ECU, coil, plugs, wires, fuel pump, rectifier a ton of others. It turned out the older models once they get so old the knock sensor is no longer needed and the idle stabilizer is no longer needed. Have motor timed after those two things are removed should fix your problem. Easy way to test it is when on the lake and it starts acting up unplug the knock sensor it should run better.

Brianxt said:

I have a 1996 200hp EFI merc. Was bogging at acceleration wouldn't plane off after going from fear to literal several timed it would take off. Smoked like a freight train, would stall out and die. Replaced everything under sun ECU, coil, plugs, wires, fuel pump, rectifier a ton of others. It turned out the older models once they get so old the knock sensor is no longer needed and the idle stabilizer is no longer needed. Have motor timed after those two things are removed should fix your problem. Easy way to test it is when on the lake and it starts acting up unplug the knock sensor it should run better.

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Sorry I have been busy with a project that is taking all my time, I have not logged in for some time, as I have no time to look at the boat. I will definitely try this, might take another month before I can get to it as I am busy seven days a week right now. Thanks for the suggestion