1971 Merc 7.5 power head swap.

[cmeilleur1337]

Hey all. Just to revisit the facts from my last thread, I have a 1971 Merc 7.5 Hp ( red label) with bad electrical. From what I can tell at least 1 coil and the Stator are bad. Other then that the rest of the outboard is clean. I Picked this up as Parts for my 1974 7.5Hp Merc ( that is running fine) and decided I may as well put the work in to fix this one and have a spare.

So I left off the old thread stating that I could pickup a functioning power head from the same vintage for around 125 CAD. Well, I picked it up. It looks to be a blue label unit, possibly '76 or so. This is what the serial tells me, but wether or not the power head is original to the mount is any ones guess. It is missing the lower unit. Upon inspection, this unit has CDI ignition not points. A few Questions.

1. Will the entire power head fit onto the rest of the '71?
2. When installing the power head, are there specific requirements for the mating Gasket, or can I cut my own / use RTV sealant?
3. Failing number 1, Can the CDI ignition, coils and stator be put on the '71 power head?

I have started removing the power head from the '71, but am hung up on the mount for the throttle. On the '71 it is bolted to the power head, whereas the 76 it was bolted to the housing.
The 2 also use different style carbs, and choke mechanism. Which i do not foresee being a problem

Thoughts?

 

[cmeilleur1337]

Bump?

 

[jimmbo]

I was going to suggest seeing if the 71 lower unit would fit the 76. But you need to check if the shift lever operation is the same. The 76 has the shift lever vertical while in neutral and moving it forward shifts the unit in to forward. But in some older motors the lever is almost horizontal when in neutral, and lifting it up puts the unit into forward. If that lever was somehow attached vertical, the shifting selecting would appear backwards. I think the 71 was the same as the 76, but I have been mistaken before.

 

[cmeilleur1337]

Well, I was aiming at swapping the power head with all the electrical, carb etc from the 76 onto the '71 lower unit adapter plate, keeping the look of the '71, with the hood, etc, however there are minor inconsistencies, mostly with the way the throttle control mounts. on the '71, it is bolted to the block. on the '76 it is bolted to the adapter plate. So without drilling a couple holes and tapping them to fasten the throttle control to the block, it wasn't going to happen.

Also, as a side note, the carbs are not cross compatible, The carb from the '71 has an external fuel pump and filter, with an external vacuum line going to the crank case, whereas the '76 carb has the built in pump, and a direct vacuum port built into the intake.

All that being said, after some part number cross referencing, I noted both the power head base gasket and the adapter plate to mid gasket, had the same part # on both the '71 and the '76. So I tore the '76 completely apart and transplanted the entire engin compartment onto the mid/lower unit of the '71 and it fits perfectly. There is a small difference with the reverse lock, but being as that portion os built into the mount, it was not an issue. Physically, it fits, and it runs great

As to the shift lever etc, it was not an issue. It all fit together, and shifts beautifully.

 

[Chinewalker]

You would also have to swap the belly pan and cowling as the choke bits are different. 71 has a Tillotson while the '76 has a MerCarb. Recoils are different, too. Lots of small differences that will drive you nuts, but can be done.