No compression?

[cordell]

Hello I am back with another question on my merc 1000 1965 in year. I took all the plugs out and put a finger over each hole while I spun the flywheel. Well things were going good until # 5 piston gave NO compression at all.! I then looked in the spark plug hole with a flash light to see the top of the piston looking a little rough if you will. The other looked nice and smooth. I take it I would need a new piston, correct? If so the job of replacement is easy or hard? I have a clymers book. And if I were to replace the one would it be in my best interest to change all, or just change rings on the other?

thanks

cordell

 

[CraigC]

Re: No compression?

From your description it sounds like it probably burned a piston, time for a rebuild! Personally, I would put new pistons and rings in all the way around, but I guess if the rest of them look good AND you measure them with a micrometer they are within specs, you could re-use the pistons. Either way you'll want to put new rings on them all, since you're going to have it apart anyway...doesn't make sense not to. Take a look at the cylinders, paying close attention to the one with the bad piston, to ensure no damage was done to it. I'd get a dial bore gauge and check them as well...basically if you follow instructions in the manual very closely during entire rebuild, and don't take any shortcuts, then you should be all set. (and it'll run for a long time if you take care of it)

 

[cordell]

Re: No compression?

Is this a painful task, the replacing of one piston and rings on the rest? I know nothing is easy when it comes to rebuilding motors. I have rebuilt Chevy 350 engines, pre 1980's of course. And once this is done I would have 100 horse power motor, would this 100 horse motor be a big difference is speed than the 1987 85 horse power Force motor I have now? I am thinking the merc 100 horse is more powerful than the force 85 horse, not just in the 15 horses but it would seem that the mercs horses would be better horses. Say you take a merc 90 hp and put it up against a force 90 hp, the over all performance I would expect the merc to out preform, speed, gas usage, and other areas as well. Am I right about this??

thanks
cordell

 

[gregpro50]

Re: No compression?

The older motors were crank rated for HP while the newer ones were prop rated. Actually the two motors would probably be pretty even for power.

 

[Fuzzytbay]

Re: No compression?

Before I would tear the motor down, I would get a "scope" and check the cylinder walls for scoreing/scratches. They are very common now, and inexpensive to either buy, or rent, or borrow.
On the horsepower note, I am not sure there is a 15% loss between the powerhead, and the prop. I am no expert on that, just seems like a lot of drop in HP just through the driveshaft, water pump and bottom end.

 

[cordell]

Re: No compression?

It is my understanding that sometime in the 80's a standard was set to measure horsepower, at the prop. I heard a few times that Force never followed this procedure, leaving the horsepower at the crank. With both at the crank witch one would have the most power. The force is a newer 85 hp. The merc would be a rebuilt 100 hp...Just wondering if I did put the time and $$ into the merc if my performance would be improved and noticeable?

thanks
cordell

 

[Kev144]

Re: No compression?

It is my understanding that sometime in the 80's a standard was set to measure horsepower, at the prop. I heard a few times that Force never followed this procedure, leaving the horsepower at the crank. With both at the crank witch one would have the most power. The force is a newer 85 hp. The merc would be a rebuilt 100 hp...Just wondering if I did put the time and $$ into the merc if my performance would be improved and noticeable?

thanks
cordell

Well the Force 85 is probably at thje prop about 80 or maybe a bit less HP. The merc has at least a good 20 hp on it. You will see some gains, maybe not massive but there will be some, so long as your boat is rated for it