orifice idle jet 40/50hp motors

[johnyrude200]

So I have a 1989-92 M.Y. short shaft 40/50hp Johnson motor with no tag (sticker fell off…WHY did they do away with the aluminum plates and welch plug ID….).

I’m trying to identify if the motor is a 40 or 50hp, and the carbs look to be for a 50hp. The problem is when I pulled the idle jets, they are #29 & 31 upper and lower respectively.

According to parts books, these belong on a long shaft 50hp motor. Now the short shaft motors use jets #32 & 34. Why would a short shaft motor run larger jets?

Can I get by leaving the smaller jets on this motor or will it run oddly? I referenced the 1989 parts guide on Marine Engine.

[billw]

One would guess because of back pressure; but I sure am no engineer. (Too bad I am not going to Yamaha school this year. One of the instructors there is a former OMC carburetor engineer.) Anyway, I can’t wait for Fleetwin to jump in on this one because it seems like there were some bulletins related to idling on those. Maybe not. I don’t have access to OMC literature anymore.

Also, in Marine Engine, did you see the commonality list when you click "more info" on those jets? (You then also have to click "show more.") There is a HUGE list of where those jets can be found on other motors. If you do run into trouble, I bet you have some of those other motors kicking around, that you could use the jets out of, as an experiment.

Long live American manufacturing!

[fleetwin]

It is like Bill says, these engines are so "over engineered", really can be a pain to deal with sometimes. These engines are so reliant on exhaust pressures/back pressure to run correctly, so I’m thinking the shorter exhaust hsg might be the reason different metering air bleeds/jets are used. So, there is no ID tag on the transom bracket, and no serial number welch plug on the powerhead? Oftentimes these carbs have a part number stamped in ink on them or there is an ID tag attached to the carb bowl with the part number for the complete assembly, either of these numbers there?
What is it about the carbs that make you think they are for a 50hp? Those jets you a referring to are actually intermediate circuit air bleeds, they meter air, not fuel. So, a smaller air bleed will create a richer mixture. Changing an air bleed by .003" is roughly the equivalent of changing a fuel metering jet by .001"…So, the different sized air bleeds you are seeing don’t make much of a difference at all. The air bleeds in your engine will create a slightly richer mixture off idle. I wouldn’t worry about it, just be careful setting up the carb sync, follow the book exactly. The engine must be run on a boat in the water for the running adjustments also, don’t attempt this on a flushette or in a test tank. Those little needle valves control the idle mixture, can’t remember what their initial setting is though. I’m guessing the short shaft engines use larger air bleeds to create a slightly leaner off idle mixture due to lower engine height and higher exhaust back pressure.
Sounds like we really don’t know what year/hp you have there. If it was mine, I wouldn’t worry about it, just set it up carefully according to the manual, I’m sure it will run fine.

[johnyrude200]

Good information to have thanks Don.

The carbs do not have any ID tag on them, I will look closer for the stamped ID. They have the larger throat so that is how I arrived at them being 50hp carbs.

I recently discovered that the 40 & 50 hp motors also have different leaf plates, so swapping carburetors doesnt fully convert the motors in either direction.

This seems consistent from at least 1980 through the end production in 2006. So a couple of years ago I had asked how to convert my personal 1993 40hp to a 50 and swapped carbs. I didnt see much of a difference (if any). I will be switching the leaf plates/manifold and retesting this summer.

[fleetwin]

OK, didn’t realize the reed blocks were different…Perhaps the 50hp uses the shims. The marineengine breakdowns are a little lacking, there is no explanation for all those little numbered notes. The difference between the 40-50hp crankcases might be nothing more than a silly little water spray into the exhaust cavity. Again, these engines are so "over engineered", and not in a good way. "Exhaust tuning" is relied on for some of the WOT power and mid range running characteristics. The sync procedures are so overdone as well, the 1988 models are easy to sync up, 89 and newer ones involve a long/drawn out procedure that can be confusing and easy to mix up.
So, in this case, I would just go ahead and do the sync procedure, I don’t think the different sized intermediate air bleeds are going to make much of any difference at all…

[johnyrude200]

On the note of those sync procedures Ive found that just lining up the cam follower with the line on the cam itself and ignoring those sync procedures works OK. I followed those procedures carefully all last season and found none of it mattered in a test tank with a test prop as soon as run on the boat. In other words Id recondition the motor and reset the syncronization following the service manaual, only to have to change everything around once in the tank and on the boat.

What I did find is both with the 40-50 and 70hp motors, all that dwell linkage stuff is just a big headache, but the motors seem to start and run fine up and down the power curve by following the old procedure of lining the cam follower to touch the hash mark on the cam, of course making sure the carb butterflies are closed before that. I mean, even with the 1989-thru motors, the hash mark is still on the cam yet the sync procedures basically ignore this. Why leave it there? Reuse of old parts?

When I set them up following that long procedure they wont start right, have big back pressure issues, and run erratically both in the tank and on the boat.

I question whether that procedure loses it relevance with an older motor? The reason I mention this is having gotten my hands on a few older motors from different years and horsepowers, and seeing the difference between a low hour motor, a middle hour motor, and a well used motor definitely makes a big impact on overall running, starting, and to a certain extent power characteristics. The high hour motors I refer to arent knocking and clanging away, and still show reasonable compression numbers. But a low hour motor with high compression certainly runs and performs much better.

Reconditioning a variety of motors it seems like a better condition one is much more forgiving in general and tolerant to settings perhaps being ‘off’ without really showing any significant difference in performance. In other words, they seem less sensitive in general.

[fleetwin]

I would agree in terms of that idle timing spec…All sorts of things affect idle RPM in including transom height and prop pitch. I too have altered the base settings to achieve proper idle RPM. The only spec you want to confirm is the WOT timing, don’t alter that one….Keep in mind that many of those crazy adjustments can unknowingly affect WOT timing.

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