1971 Mercury 650 Thunderbolt long shaft convert to short?

[jbcurt00]

Been chasing a 65-70hp motor while I'm away & unable to work on the boat. Found a great deal on a 650 Thunderbolt long shaft. What other years parts swap to this 1971 & can I convert it to a short shaft?
I don't have ser# in front of me, but it checks out correct 1971 650 Mercury, I've been trying to run down a manual too, but no luck there yet, except Seloc.

Anything I really need to march for w this 650? Weaknesses or problems?


Thanks

[sutor623]

Hmmm, I have that same motor, but it is a 67. Is yours the 3 cylinder model? I believe it is. This motor is very compatible with other motors of the same era, although I believe that the 650 is a little tougher to find parts for than it's inline 6 cylinder brothers. I know a fair deal about the lower units of these guys as I have been extensively researching them, although I have not heard nor seen anything about a shortshaft for it.....

[sutor623]

Now, I have heard lots of bad things about the 65hp 3 cylinder motors. One bad thing that I have heard is that the top cylinder is typically the first cylinder to lose compression. I have the Seloc for that motor and it does a great job, at least for me it did. And by the way, they do have this motor in a short shaft, but they are VERY rare.

[jbcurt00]

I think its the last year for 4cyl. If I remember correctly, Merc changed 650 thunderbolts to 3cyl in 1972, then back to 4cyl some time later. 68 or 69 thru 71 650's are supposedshare some parts with either (or both) 800's & 850's.

I think the 650 came as either long or short LU (maybe exhaust housing tube not LU) as Merc data lists both 650E & 650EL, and these are both electric start & w/out the "L" is supposed to be a short shaft version. It seems as though short 14-16" transoms were still around in 70-71, so Merc made both long & short. May be unable to convert mine, so raising transom is only way to go. I'm ok w that, and have been working through a plan to lay that out when I get back.

I really would like not to raise it if possible, parts hard to find may be why its a good deal, complete w everything. We are a ways off of needing to raise the transom or having to change to a short LU, just thinking & planning ahead & down the road.

My other question was also what years & models parts crossover & fit my 1971 650.. I have a few more details now thanks!

I had details reverses sorry, I now know there is few parts (but still a few) that also fit mine from other model thunderbolts. May keep this for parts if I can find a short shaft before raising the transom (a couple months off).

[Faztbullet]

The 4 cylinder 650 was from 1963-1971 and the 3 cylinder 650 did not come out until 1972. No 4 cylinder 650 engine parts will interchange with a 800/850hp, midsections,lower unit and some electric is about all.

[79Merc80]

As far as conveting from a long to a short shaft, I don't believe it can be done. It is possible to go from a short to a long with an extention kit. Only way I can think of is to raise the engine up 5 inches on the transom. You might have to build the transom up to achieve it though.

HTH, Craig

[ThumbPkr]

The short shaft engines with the 5" extension on them are not that uncommon,in fact I have one myself but I am not sure how easy it would be to remove the extension without doing damage to the short shaft leg.Mine is on there pretty solid likle it was glued in place so prying on it to remove it could cause more troubles.
I think that the extenion might have to be destroyed to take it apart.You can still buy them as far as I can tell if you ever needed one.Ron G

[mr 88]

Probably the easiest and cheapest way to go is to buy a basic jack plate.Yes it can be converted but you will end up spending more money/time converting vs jacking it.

[jbcurt00]

Thanks, I was out of town & had the 4cly 3 cly order backwards above. I hadn't gotten my hands on the 650 or an online copy of the manual. I had hoped it would be similiar (easy, may be not) to the 19597 Johnson 35hp SeaHorse, that had an optional extension to make the Long LU. But that doesn't seem to be the case, I'll probably keep hunting for a short 650, I really like the look of the motor on the FireFlite. But it needs to go WAY up, so it may not be a transom raising either.

Thanks for the info about which motors & years share a few parts too. Going to search for a manual. Will probably buy one this weekend. Don't want to wait looking for a free 1.

Thanks again