1976 Merc 650-Pros and Cons to Rebuild

[tallcanadian]

So, I've been doing some research and getting prices on rebuilding my 3 cylinder 650. I do hear a lot of negatives in wasting money on these motors. So I decided to come here where I've got a ton of help from you Merc pros before. Any input is appreciated. One thing I did hear is that these motors have high dome pistons and don't and won;'t last long with low grade gas. I was told that I could get low dome pistons and they would work better. Does anyone have any input on longevity for a rebuilt 650. And when I say rebuild I mean a total overhaul. New pistons, rings, rods, etc. Thanks.

 

[GA_Boater]

This is more a bump than anything else. I don't know about the domed pistons. How is the motor running now, TC? Do you have any numbers on the cost of a total rebuild? If you put a 1000 bucks into a $500 motor, you have an expensive $500 motor.

 

[Faztbullet]

All aftermarket pistons are low dome pistons except for a few that offer high dome if in stock. Like GA stated dont put$1000 in a $500 motor as you cangetr a new year for a little extra $$$

 

[gm280]

First, has you verified that you will actually need new pistons and such? That would be the better idea to verify what is really wrong with the engine before making any decisions. If the actual compressions numbers are good but it isn't running properly, it could just be a carb issue and not a total rebuild. Figure out what IS wrong and then go from there. JMHO!

 

[tallcanadian]

The motor's compression is from top to bottom 105, 95, 120. It fired right up after 2 years of sitting however, the idle kept getting rougher and rougher. It started smoking real bad out the exhaust. The cost of the rebuild with all new parts is coming in around 750 dollars. The reason why I'm looking to replace everything is the fact that I don't want to tear this down and only do a fix. Maybe I'm thinking wrong but if I tear this down, I want it all replaced, hoping to avoid further issues. I bought my 850 for 800 dollars, hoping that it would save me time and trouble of rebuilding the 650. However, I've had more than my share of bad luck with it. I can go and find another motor for around the same money, but who knows what I can end up with. I hope you all see where I'm going with this. I actually lose sleep over this.. lol. Thanks again for all the input.

 

[GA_Boater]

I see what you're saying, TC. Better to work with the devil you know than finding another one with a longer tail. :smile:

Since the motor has been sitting for a while, some routine things might work out. The roughening idle and smoky exhaust kinda say the fuel system needs work. I would rebuild the carbs and check or replace all rubber parts in the fuel system, including the fuel pump and hoses. I would also do the Seafoam mosquito fog mega clean out. That won't cost much and the carbs/fuel pump would be part of a rebuild anyway - so that's not wasted money. It's possible the compression will improve after just the Seafoam treatment and some longer running times. But my point is you would have a better starting place to base a decision on. And get a good night's sleep.

 

[merc850]

Remove the transfer port cover (green arrow) and check the pistons for damage, you can push on the rings gently with a pencil to see if they are still springy and you will need a new gasket.

 

[tallcanadian]

Wow, thanks guys. A ton of info, and a glimmering of hope. This is why I come here.

 

[racerone]

The lower unit on that motor is not considered to be " robust " for a 65 hp model.----Look into the option of finding a good used powerhead with lots of life left.--Some might be available in Ontario.

 

[tallcanadian]

The lower unit on that motor is not considered to be " robust " for a 65 hp model.----Look into the option of finding a good used powerhead with lots of life left.--Some might be available in Ontario.

I've had my radar out for other options for quite some time. However, I'm hoping and trying to keep everything original.

 

[tallcanadian]

I have a chance to get a fuel pump from a 1977 Merc700. However the part number for the pump assembly and check valve kit are different from my 1976 650. What could possibly be so different, since they are essentially the same motor. Will the pump from the 700 work on my 650. Looks the same,..