Newer 18hp carb on an early 60’s Johnson 18?

[johnyrude200]

There is a difference between the air silencers in the evinrude and johnson early to mid 1960’s 18hp motors. Apparently you can’t interchange these two carbs. The issue is that the choke orifice on the lower cowel for the johnson 18hp motors is slightly further to the SB side of the motor, compared to the evinrude fastwins.

I found that the newer 18/20/25hp carbs from 1970-1976 DO fit the older 18hp johnson cowels.

HOWEVER, the 2 studs for the older manifold are about 1/8th inch narrower, along with the manifold orifice is a smaller diameter. The carbs are different between the older style, 6-screw, and newer style 4-screw (referring to the # of screws holding the bowl to upper carb piece).

The newer style also uses the fixed high speed jet, and is just a bigger carb in general.

Has anyone tried installing one of these on an older motor? Results?

The reason why I ask is the older 18hp johnsons aren’t something I have come across many of. I modified a 1970 evinrude 18hp carb by wallowing out the stud orifices on the carb to fit on the older johnson, and ended up having to grind down the edges of the 2 nuts for those studs because the newer style carb has a larger throat. I considered just swapping out the manifold to attain the correct stud width as well, but decided against it for now. I suppose this would be fine (please let me know if NOT), as I can’t tell what difference there is between the early 60’s and early 70’s 18-20 hp motors (the 25 is a different story of course), less the oil recirculating tubes that start getting implemented in 1968.

This is a motor I ended up buiding 100% from scratch, and tomorrow I’ll be able to put some gas to it and see the results. I guess I’ll report back either way but was wondering what people have done. I actually have a 2nd motor just like this one (built from scratch) that I may have to do the same to in terms of the carb, if I can’t find an older 18hp johnson carb.

I’m guessing the larger diameter intake manifold and carb throat were to allow cheaper production costs between the 18-20-25’s, because the intake manifold on the 18-20-25’s is the same in the early 70’s (the leaf plates are different, then of course the internals between the 18-25 are different). Trying to think of how constricting the fuel intake at the manifold could possible cause issues…given the older carbs weren’t delivering any more than the newer, larger throat style ones? If anything, perhaps a ‘run rich’ condition?

Guess we’ll find out tomorrow!

[johnyrude200]

Well I gave the motor a run today and it all worked out, so that is a viable compatibility option for those who wish to do so. You’ll have to do the modification to the carb orifices and nuts to account for the narrower intake on the older motors, but it works.

It took a little bit longer to get the link and sync set up properly, but it idles down fine and runs smooth at all ranges.

I will note that with the early 70’s 18hp carb, this early 60’s motor behaves much like the 70’s version in that when cold, they can stall out suddenly, and blubber initially when throttling up fast from idle. Out goes a big plume of smoke, then the motor clears out and runs as usual.

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