Re: Help identify older Johnson
Those were a very popular motor in their day (which was a long time ago). I have a 1950 TN-25 as well that dad bought around 1952. I don?t ever recall seeing it in use because when I was 3 dad replaced it with a new 1965 Johnson 9? (I logged a lot of hours with that one). I am due to get my old TN back after a 1 year restoration.
Just yesterday I received a ?General Instructions? (operator) manual for the TN(L) 25-26 through Art Dekalb. I already had an original for an older model TD (made 1941-49; same motor as the TN without the neutral clutch that was added for 1950) but I wanted the official word on the new neutral clutch operation. Remember that prior to 1949 no motors had a reverse gear or neutral. The 1949 Johnson QD-10 & Scott-Atwater pioneered this feature.
In the manual they say to set the magneto lever (throttle & speed lever) to somewhere within the START to STOP speed range in order to shift. DO NOT FORCE because there is a cam within the magneto advance that will not allow shifting beyond the START position to prevent over-revving. The shift lever mechanism on mine was broken, go figure. Now it does not say that the motor will return to neutral when stopped. Nor does it actually state explicitly that it must be running; however the action of the various components (drum, bushing, spring) implies that there must be rotation (i.e. running) in order for the spring to tighten on the drum and bushing.
The following excerpt can explain what it actually says about neutral operation. I quote: ?The neutral clutch consists of steel bushing keyed to the lower driveshaft, an accurately ground steel sleeve driven by the upper driveshaft and a spring which is coiled both around the steel sleeve and the bushing.?
Let?s just say that flipping the neutral level controls a latch in the lower gearcase to lower the steel sleeve, thus allowing the spring to unwind and relax it?s grip between the bushing and sleeve. With shaft rotation and the lever in the engaged position, the spring will wind up in a Boa Constrictor fashion, thus firmly locking together the driving and driven elements in this scheme.
I would be careful to limit shifting to the lowest rpm, as broken springs can happen (good luck on finding a good replacement). I believe that from 1950-53 Evinrude also used the very same system on the early Fleetwin 7? .
Another caveat about these early motors (in general anything before the switch to 50:1 mixture and/or anything smaller than the model QD 10-horse) is the use of bronze bushings as main bearings to secure the main journals in the crank. You cannot simply restore correct clearance in that area by use of an overersized bearing insert; the bronze bushings are cast into the engine block. When excessive clearance happens there you get excessive compression leakage in the crankcase chamber. You have 2 cylinders & chambers operating at opposite phases; one pressurized, one in vacuum. Leakage between chambers will result in a lot of strange running characteristics like not being able to slow down & idle properly after higher rpm running. This is especially critical on rotary valve motors, the last of which at OMC was the TN 5-horse (the Evinrude 7? was a reed valve). Most people do a compression test on the cylinders and declare the motor fit; however there really isn?t an easy way to check chamber compression. The only true way to do it is to split the block and use a micrometer to measure clearance between the bearings and journals. Service manuals tell you how to deal with excessive clearance: ?replace the block and crank.? That is why mine has a low hours 1952 TN-27 block & crank, which leads me to another caveat about these sweet old motors...
Points. Up to about 1951 the old Johnsons and Evinrudes used guillotine-style points. They are obsolete and hard to find, expensive when you do find them, and a bit of a pain to set as well. My 1952 block uses newer conventional point which are readily available, will fit 6-horse motors through most of the 1970s.
The water pump was much improved on the TN. Since it was possible to engage neutral, the water pump on the lower unit was out. It was replaced by a better variable volume centrifugal pump just above the clutch.
I just wanted to prepare you for the idiosyncrasies of these long obsolete but great running motors. They were a Johnson best seller in their day. The 8.84 cu. in. displacement allows you to use piston rings for the much newer 6-horse. The 5 ? was the modernized replacement that used a full F-N-R lower unit, better and quieter mounts and isolation, plus a twist grip throttle. They also switched from a 3-port rotary valve design to a 2-port reed valve design. The 6- horse was an updated 5 ? that was basically the same through 1979.
