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1975 Mercury Thunderbolt 850 Need help.

WalleyeFishing

New member
Ok first of all let me start by saying I am very very limited experience with boat motors. This is my first venture into saving some money. I do however consider myself a very mechanical person and figure thing out quickly.

So lets start from the beginning. I got a great deal on this boat but it had been sitting for a number of years when I bought it. I didn't do my homework on maintenance before dropping it in the water though. The first few times I got it in the lake it ran like a top. A bit hard starting but she flew. I was extremely happy with its performance. The last time I brought it to the lake to fish was our Minnesota Fishing opener last weekend. Right away on the lake I gave it throttle and it made a bit of noise and then I noticed right away it took alot longer to plane out then usual and its top speed was alot lower then the few previous times I had it on the water. I did some research and found the most likely culprit to be a bad impeller. I dropped the lower unit and upon taking the impeller housing apart....it was literally in pieces. I thought "Awesome. I figured it out and saved myself some shop fees". Upon putting the lower unit back together I immediately ran into problems. I couldnt get it to close the gap between the last inch or so. So i set the motor aside for a few days and did some research.

My research showed that upon taking the motor apart I should have had it in forward gear when I dropped the lower unit. I had it in Neutral. I then read that turning the prop would turn the drive shaft and that is how you get the splines to line up and it to go up that last inch. So i threw it into forward and turned the prop while lifting up and she popped right in. Screwed it in and decided to head to the lake to test it. I tied the boat to the dock at the lake and upon starting the motor she lurched forward and tried taking off on me....while in neutral! So i killed the engine fast and got her back on the trailer and headed home.

I did a bit of research and figured I must have messed something up with the gear selector when I dropped the motor in neutral and then attempted to put it into forward with the lower unit disconnected. So today I attempted to fix this but can't for the life of me figure out how to tell which gear is what! I made a couple attempt to fix it with different positions..but every position just does the same thing and the prop spins like crazy when the motor starts..where as before the prop didnt spin until put into gear.

Secondly I will attach a link to a youtube video. This is showing the side of the motor and how it reacts when the motor is shifted from Neutral to Forward and back to Reverse and so on. And then it also shows the bottom "shift linkage?" where the lower unit gear connects up. This piece seems to only move a very very small amount in each direction when the boat motor is shifted. Leads me to believe perhaps something is bound up with the linkage from me not putting it back together in the proper gear?



With the supplied information can ANYBODY tell me what I am doing wrong or if something is wrong with the motor and if It is me making the mistake what I can do to fix it.
 

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Ok, so I am relatively certain i have figured this out! While waiting for a comment I decided to do a bit more playing around. I took a pair of pliers and spun the shift shaft on the lower unit all the way counterclockwise. Suddenly the prop was spinning freely and I knew I had found neutral. At this point I was happy but I couldnt get it to spin any further counterclockwise and I knew that neutral was in the middle so then i would only have neutral and one gear. I clicked it back clockwise and turned the prop...turning it clockwise which I guessed to be the way it would spin in forward produced an audible clicking noise. So i assumed that meant i was in reverse actually. SO i went to put it back into neutral while still spinning the prop and it went PAST Neutral this time and into FORWARD! The clicking stopped and I became super happy. So I figured it out guys. Please save this in case anyone has problems like this!
 
That lower unit when in forward will------Click if prop is turned clockwise and lock if prop is turned counterclockwise.--------Spin freely in neutral------Lock both ways in reverse.
 
You MUST take all the slack out of the lower shifter shaft before stuffing her up there. Also, since the upper shifter shaft is 'floating about', it's best to have the motor vertical, not on an angle. I use a stout bungee cord to handle the weigh of the LU as I wrestle the thing together. Finally, that last inch or two is the splines engaging the crankshaft, then the shifter splines engaging. Hand turning the flywheel helps the first, and wiggling the shifter helps with the second.

Jeff
 
Are you 100% sure about this? I brought it to the lake today and sure enough when I put the shifter in reverse to come off the trailer it shot forward. So this is my only other option to try!
 
They often jump into forward ( from neutral ) when you are not looking.--------So remove the lower unit and find neutral on the lower unit.-----Turn the shift shaft Counter clockwise.-----May take some effort as there is a strong spring on the clutch dog.----Reassemble with control in neutral and lower unit in neutral.-----Or forward for the control and forward for the lower unit.------Work carefully amd make sure both are where you think they are.
 
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