Old Mercruiser 120 3000rpms WOT rated for 3900-4300

[jesseglu]

I recently bought a 85 cobia 17ft open bow with a 153cid 120 in it. Starts, idles, runs and drives great. Max rpms at wot is 3000 tho which is a bit low. Previous owner had recently replaced rotor, wires and plugs so i don't suspect it's that. Was struggling to get even to 3k and I put in fresh gas with some seafoam and it runs 3k rpm now but not much over. With no one in the boat but me I've seen about 3200 and playing with the trim I maxed it out at maybe 3400. Any riders in the boat it's basically 3k. I don't know a ton about boats this is my first one. I did push the throttle lever all the way with engine off and it seems the throttle still has a decent bit more travel. Idk if that is normal though or if maybe that needs adjustment. Prop is a 19 I can't read the first number on mine, if that helps.

 

[Scott Danforth]

Check compression. All cylinders should be about 140

Go back to the distributor. Set the dwell to 32 degrees with a dwell meter

Set the timing to 4-6 degrees BTDC

 

[jesseglu]

Check compression. All cylinders should be about 140

Go back to the distributor. Set the dwell to 32 degrees with a dwell meter

Set the timing to 4-6 degrees BTDC

Ok. I know how to check compression. The other 2 things I will have to learn how to do.

 

[jesseglu]

Ok so I finally got around to it. Compression looks good. Points have been converted to electronic tho so now I'm stuck there. I watched a bunch of videos and looked at my neighbors old hot rod to learn all about points and mine doesn't even have them. Neighbor also let me borrow his timing light but I havnt gotten to checking that yet. Should I think about changing back to points?

 

[Scott06]

Ok so I finally got around to it. Compression looks good. Points have been converted to electronic tho so now I'm stuck there. I watched a bunch of videos and looked at my neighbors old hot rod to learn all about points and mine doesn't even have them. Neighbor also let me borrow his timing light but I havnt gotten to checking that yet. Should I think about changing back to points?

if you are getting spark I would leave it alone for now. People switch out point for electronic to avoid filing the points, setting dwell, and setting timing annually.

At this point I would use your neighbors timing light to make sure timing is set correctly and you are getting the correct total advance. if you dont have the correct timing for the engine ballpark 406 degrees BTDC as mentioned above and 24-26 total advance at 2800 rpm or so. if you have that plus a good strong spark (check on spark gap tester) then the ignition is doing its job and look at fuel delivery/carb

 

[Bondo]

Ayuh,.... Have you tried a smaller prop,..??
Is the drive original,..?? If not, is it the right gear ratio,..??

 

[kenny nunez]

With the correct drive ratio of 1.98 a 14X 17 propeller should be close to what should be on the drive.

 

[jesseglu]

As far as drive and prop, it's the original drive as far as I know. Prop is currently a 14x19. It's pretty dinged up so I planned to get a new one. Perhaps I'll try a 17

 

[Bondo]

As far as drive and prop, it's the original drive as far as I know. Prop is currently a 14x19. It's pretty dinged up so I planned to get a new one. Perhaps I'll try a 17

Ayuh,...... As per Kenny, that would be a good place to start,..... maybe a 15" if you plan on towing water toys,....

 

[jesseglu]

Ayuh,...... As per Kenny, that would be a good place to start,..... maybe a 15" if you plan on towing water toys,....

I actually just found a nice 15x17 on fb. I do pull tubes and skis so maybe I'll try it out.

 

[jesseglu]

I took the boat out the other day and it made me think of something else. Btw I'm supposed to get a new prop tomm I was going to change that out and then check the timing when I go test that so I will report back. But also sometimes when I throttle up it will not quite want to go and when I back off and rethrottle it's usually fine after that. So maybe it is a fuel issue?

 

[Bondo]

I took the boat out the other day and it made me think of something else. Btw I'm supposed to get a new prop tomm I was going to change that out and then check the timing when I go test that so I will report back. But also sometimes when I throttle up it will not quite want to go and when I back off and rethrottle it's usually fine after that. So maybe it is a fuel issue?

Ayuh,..... No doubt, possibly,....