testing compression on an old tower of power

[johnny howe]

can you compression test an engine that is stuck in forward? if i pull all the plugs, does it matter what gear its in? thanks and what compression should a 1978 mercury 1400 have in each cylinder?

 

[Chris1956]

First, remove the prop and spark plugs. Then jumper the solenoid to make her spin. Actual compression numbers are not important. If the compression is within 15% in all cylinders, the motor is good.

Why is she stuck in gear?

 

[johnny howe]

remove the prop? why do you have to do that?

 

[johnny howe]

im looking at the motor to buy, i want to do a compression check on the motor. the seller said he just changed the impeller and it is stuck in forward. i know i can pull the lower unit and move the plastic shifter to what gear the selector is in. i think he thinks there is more wrong with it then what it is. i could help him with the selector if i dont buy it. but he is discounting it more thinking their is a lower gear problem, i know , i know. also i have a 95 hp on my boat now, can i put a 140 hp motor on it if it weighs only 26 pounds more then what i have. the boat is rated up to a 120 hp motor, if i go bigger how will this affect it?

 

[Chris1956]

When you crank the motor in gear, the propshaft will spin. If the prop is installed, it could hit your leg or foot.

It is quite possible the seller screwed up the gearcase install, and the gears are fine. That happens all the time. However, there is no way for me to tell.

Weight will probably not be an issue in replacing your 950. Legality of that much power on your boat is the issue.

 

[johnny howe]

I have a 1966 Larson all American 16ft with a 1966 mercury 950ss on it now it did about 30mph but the motor dropped the 4th cyl with 0 compression and 120 in each of the others, now it does about 10, but it starts up great. I actually don't know the actual top rating for my max motor size for this boat , I found a cheap 140hp that runs good. I think it would fit. what legality issue are you speaking of?

 

[wired247]

Just put the 90 HP cowling on it. No one will care. No one does anyway. I have yet to meet anyone that has received a ticket for overpowering a boat and I am one of the poster children for boat overpowering. Its only really an issue for your insurance IF you have boat insurance. They could disavow any knowledge of your existence if they could prove your motor is overpowered on your boat and you cause an accident. Thats why I don't bother with insurance and stay away from other boats.