Merc 300 Seized?

[timmeraw]

Well after a lot of work re-awakening a 1960 Merc 300 and getting it running, what I thought pretty well..but no serious sea trials, it appears it is now seized.

I took it on vacation last week to Canada and on it's maiden voyage it ran well for about 15 minutes and then after crossing a wake at near WOT it cut out to about half speed. When I throttled down and put it in neutral it died. I tried several times to get it started and a couple of times had it running for a few seconds and it would die out. After a few more attempts it just clunked. When I tried to pull the cord instead of using the starter it would not budge. It is frozen. The flywheel won't even wiggle so lower unit problems are probably excluded.

One thing I did notice today when finally pulling the plugs are that two were very dry as were the cylanders and one of the plugs actually has the gap collapsed as if the cylander came up too far and hit the plug.

Another thing...when I was running out of the bay the motor sounded like something was knocking a bit which I had heard before on my first run down the canal before I left, but thought that maybe these old Mercs had that sound naturally(having read in here or somewhere about the classic throaty or something sound of the old mercs).

Any thoughts? Do I have a seized engine with a broken rod or bearing or something? What are my options .... tear the head down to investigate? I took off the back plate on the head but it doesn't really let me even see the pistons.

Seriously disappointed after being so proud of getting this old girl going and with new paint looking great.

Thanks,
Tim

 

[thefairlaneman]

Re: Merc 300 Seized?

I too have a 60 merc 300 that I just put back into service. A couple of questions,what oil mix did you use and was it peeing water from the pee hole. Another question is did you de-carbonize it . The timming and the carbs work close together and too lean of carb settings can cause it to overheat from lack of oil. I would soak each cylinder with marvel mystery oil for a few days and try to free it up first. Then run a comp test. I suspect it overheated and most likely ruined the powerhead. By the way did you have to replace any magneto parts?

 

[corm]

Re: Merc 300 Seized?

Hi,

Well Tim it sounds like a rod bearing went out. If you pull the upper cowl off that holds the recoil you should be able to turn the engine flywheel counter clockwise. This is not always true depending on how things stuck in side. I would pull the intake port covers and look into the crankcase. You may be able to see what happened. This is a bummer, it looked great.

 

[thefairlaneman]

Re: Merc 300 Seized?

I must have read the OP too fast,,Yep with the smashed plug I agree with the last post.

 

[timmeraw]

Re: Merc 300 Seized?

The flywheel cannot be moved at all in either direction and yes I had feared a blown rod/bearing. I am sure that the knocking I heard was that it was going.

Ok so if I take off the cover(which is the intake cover btw...the flat plate on the port side?) and see that is the case, is it a wright-off or just a teardown and rebuild(which would include what?)

I really love the look of this cloud white beauty and would like to keep it alive.

 

[coolguy147]

Re: Merc 300 Seized?

i dont know why it would do this. i remember your old posts. did u get the water pump key int he right place? did it overheat? clean carbs?

 

[timmeraw]

Re: Merc 300 Seized?

i dont know why it would do this. i remember your old posts. did u get the water pump key int he right place? did it overheat? clean carbs?

It was pumping water fine and did not over-heat. It has got to be a rod/piston/bearing issue unfortunately. I have confirmed it is not in the lower unit as I just pulled it and the flywheel is still stuck. I can also see the mark in the face of the top piston where it banged the plug.

Not good. I might know someone with a couple of Merc 300's and hopefully one has a good power head. How hard is it to pull/replace the whole head?

 

[Chinewalker]

Re: Merc 300 Seized?

Not hard at all. You'll spend more time getting it freed up than actually swapping around. Most any of the 40-cubic inch powerheads going back to 1956 will work for you, as long as the intakes match up. I am pretty sure the 300 uses AJ-series carbs, so be sure the powerhead you end up with uses the same. Or you could make a sleeper and use the powerhead from a Mark 58 or Merc 400....

Sounds like you might have had a carb plug up, which would lean out those two cylinders. If you do swap in a different powerhead, be sure the recheck the carbs and fuel pumps.