Logo
FULL SITE MENU

1978 Mercruiser 3.0

J

joystick

New member
Alright... Im semi mechanically inclined and a bit stumped... I purchased a boat a month ago.. Its a 1978 baja with a 140 mercruiser engine. When I first got it I changed oil, gear oil, plugs, wires and cap. Took it in the water and seemed lag a bit but eventually got the rpms up enough to plane. Once on plane it ran quick and smooth.... Towards the end of the day it would not plane at all and would only stay at around 1700 RPMS. I was in the Fox river and was too eager to drive without thinking about how shallow the river was. Immediately figured it was my prop. Sure enough when i pulled it out the prop was eaten up badly. Changed the prop and put it back in the water and its still not planing. Rev stays at around 1700 rpms.

Couple notes that may help...

1. Im running the old gas the guy had in it (it did plane with the old gas)
2. Carb filter looks to have a decent amount of oil soot
3. Missing the overflow line that goes into filter (overflow line on carb seems to be non functional/clogged but note it still plained!!!!)
4. Boat has full rpm when out of water and plugged to the hoes
5. But when in drive and tilted all the way down sounds fine no noises but when up is noticably louder
6. When boats in the water in reverse rpms go up as they should...

Did I..........

Bend the props shaft?
Is the carb in need of cleaning?

PLEEAAASE HELP IM GONNA CRYY!!!
 
Start w/cleaning the flame arrestor. Keep RPM below 1500 out of water. Did you check/change the points? Look under the points plate at the dist. advance mechanism for rust and binding.
 
check compression, sounds just like what mine did in a v-8, run good on trailer, but when it went into water would not plane out 1500 rpms tops,do that and get back with us!!
 
Alright so i checked compression of all four cylinders and here was the reading....

1.120
2.90
3.100
4.100

I also took out all the spark plugs i had replaced and noticed one of them was loose and when i took it out the electrode was burnt out. Bought a new plug and the engine seems to be running alot better!!! I noticed all the new plugs had black soot on them. Is this normal... Anyways.. I'm gonna take it out tomorrow and see what happens...wish me luck..
 
let us know how it runs, and will go from there, good luck. a good number is 120, thought i would throw that in!
 
Last edited:
joystick said:
Alright so i checked compression of all four cylinders and here was the reading....

1.120
2.90
3.100
4.100

I also took out all the spark plugs i had replaced and noticed one of them was loose and when i took it out the electrode was burnt out. Bought a new plug and the engine seems to be running alot better!!! I noticed all the new plugs had black soot on them. Is this normal... Anyways.. I'm gonna take it out tomorrow and see what happens...wish me luck..
Click to expand...

Ayuh,.... Yer motor is worn out, 'n needs Rebuildin'....
 
Before I bought a new engine I would spend $10 on a can of Seafoam and run it thru the carb airhorn to see if the valves are just full of carbon. Black soot on the plugs means it is running rich...verify the choke is working...it may just need the float adjusted.
 
We'll find out if he does...you're not stupid...if there are no burnt valves and only soot/carbon buildup on them Seafoam can help clean the carbon.
 
Back
Top