New to the Johnson SPL 28hp ~1986 - Questions on operation & prop

[Bam_Bam_of_Atlanta]

I have a Johnson SPL 28hp longshaft ~1986

1. There is mixture screw just above the air intake.
What setting does it adjust and is there a starting point? (Turns out?)

You can see a picture at: https://picasaweb.google.com/1166542...80218351490050 I can not seem to get the link to work.

2. Just to the left of the air intake is a black box with a short red lever and small fuel lines coming out. What is this and where is the lever supposed to point? I don't see it in the parts diagrams.

3. The small fuel lines to the left of the air intake are rotted. What size lines?

4. The engine runs fairly good. It is mounted to a 36' catamaran on the Starboard Hull. The factory rates the speed of my boat as 14mph (They are known to exaggerate) with a 25hp motor. The previous owner got 11mph and I am getting 9mph, but that is with the engine canted in about 15 degrees so it steers better under asymmetrical thrust and the motor is pointed down 1 to 2 holes to much on trim (Added nylon sliders to the S.S. lowering system that changed the trim & the pin is rusted solid). I am planning to get the trim set this week and try the motor lined up with the hull once I get away from the dock and shore since the rudders can not counter the asymmetrical thrust at low speeds. This will only get me back to 11mph.
It has a Michigan #012015 9” Dia. X 10” pitch prop. I am thinking that I need the 9 ¼” x 7” OMC 389067 (Work Boats) to push this boat ~3400 to 4500lbs at these low speeds. On line calculators show it should be running a 10.3" x 7.7" pitch for 5000rpms and 9.7" x 7" pitch for 5500rpm. But there is no clearance for much more than 9.5" diameter and then pray the rubber does not let it move much.
I don't know if 14mph is to slow for the Michigan 10" prop, or to fast for "Work Boats" or maybe perfect for one of these. I also don't know how much the factory exaggerates. On line calculators show the 28hp motor to be 0.8mph faster than the factory specs for 25hp. Typical for non planning hulls to get little results with HP.
I wish I could say what the rpm is with the installed prop, but a tack is somewhere on the long to do list.
Last, I am thinking of cruising at about 75% power since displacement hulls don't respond to HP very well, but the mpg does!
My experience with outboards for 20 years is a 1.75hp Game Fisher so I am open to any tips on getting this one up to par.
Any opinions on props? Other props? Opinion on the OEM 9 1/4" x 7" work prop?

5. Ventilation - The engine can be lowered almost to the air intake. But a 3' wave will drown it. By the time you get it high enough for the engine to not drown, ventilation starts if the rudder is turned much. I don't get it. The anti ventilation plate is well under the water. Does anyone think that any air leaking around the rudder post can cause this? It is a fairly tight fit so I can not imagine much being able to leak if there is suction for some reason around it. But then this cavitation thing acts like an on/off switch so I don't know how much it takes to set it off. So far all I can do is back off the power ... a lot. Maybe the motor running about 15 degrees sideways screws up the anti ventilation engineering.

You can see the set up at: https://picasaweb.google.com/1166542...99744023922370

Thanks as I know this is a lot to ask about. Just say newbie.
Dana

[boats.net]

Usually the initial starting point for mixture screws are 1 to 1.5 turns out. Then you adjust from there. The arm next to the air box is the manual primer operation. The little primer fuel lines are 1/16" & 5/32". The hand for electric primer operation is pointing to the solenoid. Your RPM is a strong factor with choosing the proper propeller, if you are bordering red lining, then do not change. If you have some RPM to play with, you can try the smaller prop.

[Bam_Bam_of_Atlanta]

Thanks for the reply.

Once I used the search word “primer”, I found that lever. Google also said to turn it full clock wise to close for normal operation.
The picture shows it in the middle. That may be the speed problem right there.
The previous owner said that pushing in the key turned on the “electric choke” and I hear it click.
So does the amount of time you push in the key control the amount of primer?

Sounds like I need to move installation of a tack up the priority list.

Thanks for the fuel line sizes. Picking up today on the way home.

[boats.net]

Yes, the length of time you push the key in, determines how long the primer is enguaged.