1978 Mercury 700 70 hp linkage broke after installing hydrofoil

[adriancooke]

I have a 1978 Mercury thunderbolt 700, it is in great condition in my opinion and it has had no problems since I purchased it. The other day I installed a new hydrofoil, a doelfin, we took the boat out and were cruising around 20 mph when all of a sudden the engine just stopped and would not restart and the prop was making a clicking sound, upon getting the boat on shore I have realized the top part of the linkage on the power head moves freely and seems to be disconnected, I am not sure if it is broken or if it came off, I have also seen some oil type fluid coming from the lower unit at the top of the lower unit which I feel indicates I need to replace the shift rod gaskets in the lower unit and more than likely replace the water pump and impeller as well. I am trying to figure out first of all why I can not get the lower unit to drop all the way down after unscrewing all of the bolts and also whether the shift rod broke at the top or if it can be reconnected, and did installing the hydrofoil cause this to happen? possibly from too much strain/pressure? I appreciate anyones input as I am in the dark until I get the lower unit off, I am also concerned about reverse lock cams, the engine seems to be stuck in gear but it clicks. Thanks for any help anyone can offer.

 

[Texasmark]

Re: 1978 Mercury 700 70 hp linkage broke after installing hydrofoil

On that year model engine, first of all did you remove the screw at the rear top of the midsection.....the part that protrudes farthest to the rear on the engine. There is a plastic plug there and under it is the hex socket screw that holds the trim tab. Unscrew that and remove the trim tab. Then from under the anti vent plate, look up in the hole where the trim tab was sitting and there is another screw there. That is one of your bolts holding the midsection and lower unit together.

Otherwise it's corrosion at the mating surface of the midsection and the LU and probably between the crankshaft and the powerhead.

The Shift shaft connection is splines as is the drive shaft. It too may be corroded but there is some slack in it so if it were your immediate problem the two would separate somewhat.

There are two alignment pins between the two so don't expect the LU to rotate. It comes straight down. You can get to the interface alum castings to soak with penetrating oil, but getting to the driveshaft is a real problem. Some people have even cut a hole in the midsection to get at the top of the drive shaft. Others have suspended the engine upside down by the LU close to the floor with a pad beneath to keep from damaging the cowling.

As far as what fell off or broke, a visual examination should answer that question. On the prop clicking, if it was clicking when the engine stopped and the boat was moving forward, that is the overrun clutch on the prop shaft and is normal.

You didn't say which linkage fell off. I assume the throttle which has nothing to do with the lower unit nor your whale tale addition which shouldn't have upset anything anyway if you just drilled 4 holes in your AV plate and bolted it on.

Pop the cowl and have a look then get back.

Mark

 

[adriancooke]

Re: 1978 Mercury 700 70 hp linkage broke after installing hydrofoil

the part of the linkage in the power head, where the shift linkage meets the lever that turns the shift rod is completely loose and I can see about an inch or so before that that the shift rod is not connected to the swing arm, not sure if they broke our just came apart

 

[adriancooke]

Re: 1978 Mercury 700 70 hp linkage broke after installing hydrofoil

the part of the linkage in the power head, where the shift linkage meets the lever that turns the shift rod is completely loose and I can see about an inch or so before that that the shift rod is not connected to the swing arm, not sure if they broke our just came apart

I have taken all of the bolts out but they're is a gold colored rod about two inches down from the power head in the front of the engine,i think it is the reverse lock to lock the engine into place in reverse, do i need to disconnect this also?