3.0 mercruiser running rough

[Fireman115]

I just did a motor swap on my 94 Bayliner rendezvous with a 3.0 mercruiser. It's been running great the past week or two since doing the swap until today. At full throttle it started running a little rough the rpms would dip a little now and again. Then I had to let my dog to go to the bathroom shut it off then when taking off again it was doing it again but worse. It normally runs 5100 rmps at full. And it was dipping to 4900 now it's dipping to 2800. And it was doing it last year so I put it up and winter came too quick and I didn't get to winterize it so the block cracked hence why I replaced the motor. If I put it to about 4100 it will run fine. I'm lost at what it could be. I was thinking the only thing that's the same is the fuel/water separator and the carburetor. I took the new/used motor down to the long block as it was set up for through hull. I've also read it might be timing or the carburetor needs rebuilding. I am lost and at the point of just giving up and selling. Please help thanks

 

[alldodge]

No serial number of replacement motor

Assuming the original motor was a 3.0 as well, it looks maybe like your carb is running the motor lean. If it is running lean then the valves can get to hot and warp (tulip).

I'm hoping your tachometer is not accurate, Max rpm for 3.0 is 4600, and for the LX is 4800

Pull plugs and see what the color is, and also do a compression test

 

[Fireman115]

I will do that. How do I adjust the carb to run better. I've never done it. I will post pictures of all 4 spark plugs. The motor is a LX, and not sure if the tach is accurate or not. And for the serial number I can't read it.

 

[alldodge]

If the motor is running lean then the jet size in the carb is to small, carb needs cleaning, have a vacuum leak, incorrect timing or float setting is to low.

 

[Fireman115]

Even for it to do it all of a sudden on both motors. Last year with the old motor it was running fine then it did the same thing. And then this year with a different engine it was running great and then all of a sudden running like crap.

 

[Scott Danforth]

start looking at your fuel tank and its contents. you may have changed the motor, however you have not cleaned the tank

and running a 3.0 at 5100 RPM is a good way to destroy that motor

 

[Rick Stephens]

All it takes is crap in your fuel. A little water, becomes a dirty carburetor, goes a long ways toward running like the crap.

 

[Fireman115]

Ok so how do I get it to not run the rpms so high other than throttling it back

 

[alldodge]

Ok so how do I get it to not run the rpms so high other than throttling it back

If (the big IF) your tach is accurate then just change the prop to a higher pitch. That said, I'm not sure your tach is accurate

 

[Fireman115]

It runs at about 25 mph