Reed petal stops

[Alby1]

Can anyone tell why a shim was added to the reed stops on the 1979 and up engines but NOT on the 1974 thru 1978 Johnson 9.9 and 15hp? Common sense tells me that the engine will breath better at the top end and possibly get more revs. But it's not recommended to shim the stops on the 1974 thru 1978 motors. Comparing a 1975 motor,(has no shims), to a 1979 motor, (that has shims), I looked up the part numbers for these two motors for reeds, leaf plate, manifold, block, crank, and head, there all the same.

What engineering change did they make to the later engines, to require shimming the reed petal stops?

Thanks, Alby

 

[rolmops]

Re: Reed patal stops

Re: Reed patal stops

When this engine was first designed it was supposed to be purely a 15 horse engine.Then a ruling came through that boats under 16 foot length and with less than 10 hp propulsion outboards did not have to be registered.This was the reason why we now have the 9.9 and the 15 horse engine that is basically the same.
Later on, the registration rules changed.
The only difference between the early 9.9 and 15 hp engines was the size of the carburetor throat. This allowed the engine to increase its rpm thereby increasing its horse power.
Inserting the shims was done for the explicit purpose of allowing the engine to suck more air and in 1981 the exhaust system was tuned differently.
Part of the reason why this was done was to give the engine a bit more power so as to better compete with the Japanese engines that started coming onto the market.The Japanese engines had their horsepower measured at the propeller whereas the American engines were still measured at the crankshaft.so the Japanese engines were a bit stronger.
In 1981 the American engines started having their horse power measured at the propeller.
for more info on the 9.9s go here.
http://www.sschapterpsa.com/ramblings/Johnson 9.9_15.htm

 

[ezeke]

Re: Reed petal stops

According to LeeRoy Wisner, the shims were only added to the 15HP motors from 1979 on, in an attempt to allow more fuel to be pulled into the engine. Later, the larger carburetor throats were introduced.

See: http://www.sschapterpsa.com/ramblings/Johnson 9.9_15.htm
Search for "shim".

The shim, part #325038, is listed in the parts catalogs as "15 Models" in the footnotes.

I've attached copy of diagram 3-2 from page 3-3 of my 1980 9.9/15 HP Models Service Manual.

 

[Alby1]

Re: Reed petal stops

Thank you for the prompt reply rolmops and ezeke. Thank you for the link, I have read all of the articles LeeRoy Wisner listed on that site.

My assumption was kinda on track, I just didn't have the details. In other words, it's ok to shim the older 1974 to 78 motors when converting them to 15hp. The only thing that concerned me was the possible damage the reeds would incur opening that far, and also the reaction time for opening and closing. I didn't know if there had been a difference in the engineering that restricted shimming in the 74's thru 78's. I did note what ezeke said about LeeRoy Wisner " in an attempt to allow more fuel to be pulled into the engine ". Is this true? Is this just an attempt? Logically, not knowing the engineered design, increasing fuel and air thru the engine should give it more power and revs.

Thank you again,
Alby

 

[ezeke]

Re: Reed petal stops

re: "attempt". I would say that it was an "attempt" because in 1981 they were still trying to raise the horsepower by adding the larger exhaust and in 1987 a larger carburetor throat.

The 1980 models were rated at 15HP BIA at 6,000 RPM.
The 1997 J15ELEC model is rated at 15HP ICOMIA, ISO and NMMA at 5,500 RPM.