1978 Mercury Black Max V1500, Surging at high throttle.

[NateDJ]

How about vid when it acting up as motor can sound fine and rev up fine on 3 cylinders..

 

[NateDJ]

Does the engine rev up, or just run at just above idle? 6MPH is probably somewhere around 2000RPM, on most boats. Do your RPMs exceed 2000?

If so, the prop hub may be bad. if not, your motor is not making proper power for some unknown reason.

Engine off, boat in gear can turn engine over with the prop going CC. In neutral prop spins freely.
On hose engine seems to reach full speed on prop, in water it doesn't get to 2k RPM.
Previously when it was surging it would push the boat at about 12-20MPH so I have no question about it being capable of spinning that prop at speed, If everything else was working properly.
The only remaining question is the Unknown Reason it isn't making power.

 

[444]

Did you do the link and sync as per the instructions in the service manual? See link below. I only ask because I have a V1750 and out of the water it would rev up fine but in the water it wouldn't do over 2000rpm. I chased all sorts of minor issues wasting my time until I finally did the step by step link and sync and problems went away. It was weird because the previous owner claimed to be driving the boat before I bought it but that must have been a lie.

manuals

outboard motor manuals

www.tinboats.net

 

[NateDJ]

Did you do the link and sync as per the instructions in the service manual? See link below. I only ask because I have a V1750 and out of the water it would rev up fine but in the water it wouldn't do over 2000rpm. I chased all sorts of minor issues wasting my time until I finally did the step by step link and sync and problems went away. It was weird because the previous owner claimed to be driving the boat before I bought it but that must have been a lie.

manuals

outboard motor manuals

www.tinboats.net

Well I used a different manual than that one but will try it that way and see if that helps. Also replacing the champion spark plugs with NGK after work today to see if that makes any difference.
Going with NGK Standard Spark Plug (BU8H) Part # 6431
which seems to be the recommended ones. Worth a try.

 

[NateDJ]

I love finding gems like this one when reading tech manuals, This just makes you feel like you are standing there talking to an old guy who absolutely knows what he's talking about! LOL

 

[NateDJ]

On this one, I did try the timing light on all 6 cylinders to make sure they were all 6 firing and they were. Forgot to state that above.

 

[Faztbullet]

1)Get rid of the Selco crap book and get a factory manual. 2) Why is max timing screw run out so much?