40HP Johnson

I have 2 1976 40hp Johnsons, one on my boat and one for just parts. I have a few questions on something I'm not clearly understanding. The one on the boat is a "Long Shaft", the parts motor is a "Short Shaft". I've been told and also read somewhere that the Cavitation Plate (?) should be even with the bottom of the boat. Now, here's where I'm lost, the short shaft is 4" to short, the long shaft is 3" to long...What gives? I've been looking around for another long shaft motor of a lesser hp to put on my boat and everyone I find that says "Long Shaft" is still to short...3" to 5" too short. I've researched the boat and came up empty...I know everything there is to know about both of the motors, the Long Shaft is exactly 5" longer than the short one. My transom is 21" (Give or take a centimeter). According to measurements, something is off and I'm sure it's not me because I have measured numerous times to make sure. The shorty is 17" from the transom bracket to the Cavitation Plate, the long shaft is 22"...but the Cavitation Plate is 3" below the bottom of the boat? Am I just too stupid to understand it or what?

Something about your math is not adding up. So can you take the long shaft and bolt it higher on the transom or do you need a jack plate?

Agreed; something's off. OMC shafts lengths are 15", 20", 25" and 30", not 17" or 22".

The cavitation plate even with the bottom of the hull is an average rule; my 35hp Johnson runs best with the cavitation
plate 1" above the bottom of the hull.

If your transom is 21" you need the long shaft motor. With the SS it would be hard to control and not get on plane.

You can always put a piece of wood (rip a 2x4 in half) on top of the transom and move that long shaft up just a tad.

I agree with the others that your math or your measurements are in error and this is evident by your own words. I removed some of the wording that was not necessary to show this.

JGibson said:

(the short shaft is 4" to short, the long shaft is 3" to long)
(the Long Shaft is exactly 5" longer than the short one.)
According to measurements, something is off and I'm sure it's not me because I have measured numerous times to make sure.

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LOL, I agree, I totally agree. Maybe my eyes are going bad or I have forgot how to read a tape...I'm sticking with the long shaft tho and will get that piece of wood in place.

I've found where someone has replaced two of the intake bolts on the long motor, I tried to tighten one of them and found that it was stripped...my solution is to change over everything from it to the short motor and convert it to a long. Now, when I say everything, I mean everything from the flywheel down. Can someone maybe give me some direction on how hard of a job this will be? I would rather just switch everything over instead of drilling and tapping a new hole, seems to me that drilling will cause pieces of the motor to be inside of the casing causing me to have to take it down and clean it out, or am I wrong about that?

[url=https://www.tinboats.net/forum/viewtopic.php?p=316189#p316189 said:

JGibson » Yesterday, 20:56[/url]"]I've found where someone has replaced two of the intake bolts on the long motor, I tried to tighten one of them and found that it was stripped...my solution is to change over everything from it to the short motor and convert it to a long. Now, when I say everything, I mean everything from the flywheel down. Can someone maybe give me some direction on how hard of a job this will be? I would rather just switch everything over instead of drilling and tapping a new hole, seems to me that drilling will cause pieces of the motor to be inside of the casing causing me to have to take it down and clean it out, or am I wrong about that?

Click to expand...

what are you changing over? you wanna make your short shaft a long shaft?? if so you just need to swap the drive shafts, lower unit's and replace the upper and lower drive shaft bearings/seals...do both motors run?? a lil more info would help me understand exactly what you are attempting.. and by intake bolts what exactly do you mean????

Intake Manifold is what I should have said. This pic shows it, the number 27 & 29 bolts are the ones I'm talking about, 27 is the stripped bolt.



I will need to change over everything. The magneto is all that was wrong with the short motor (that's what I was told) when I got it so that will need to be changed. All hoses, flywheel, carb, coils, fuel pump, starter, lower unit. At this time, I cannot get the long one to run...it's getting fire/gas but will not even try to hit.

Ok, I got everything changed over but the foot from the motors. Got a problem though, the motor is getting fire, getting gas, turning over good but will not even try to start...what could be wrong here?

Fire may not be strong enough. Do you get a strong blue spark and hear it snap when the plugs fire? Have you checked it out with a spark tester? Whats the compression like?

No compression.

Very strong spark, bright, thick, 7/16" gap on both. Will check compression tomorrow.

Got the motor running yesterday Turned out to be bad plugs. Bought new ones and she started up great. Had to replace a coil because when I pulled the wire off of the plug, the wire came out of it...idles great, starts up fast, gets my 14' on plane quick. Gonna take it back today and do a speed test with my phone to see how fast it is. Only thing I'm probably gonna regret is the gas mileage...but now I can at least get to where I want to go a little faster 8) Thanks for the help guys, greatly appreciated.

you also need to throw that plug gapper away- 7/16s- thats im possible - 7/16s is almost half an inch--

LOL, plugs are gapped at 30, the 7/16 is the gap on the spark checker :wink: