'78 85 hp johnson, on a 17' dixie. max hp is 115.  early this morning,55 - 60 degrees i was turning 5100 - 5200 rpm at 39.2 mph gps at WOT.

Later in the day (noonish), 75 - 80 degrees i was turning 4800 rpm at 35.5 -36.5 mph. hole shot was still good. There was 2 ft waves, and it warmed up a good bit. would the waves/current cause my loss in rpm? Still have power on all 4 cylinders, butterflies fully opening. good compression on all 4.

I'm not sure if i usually lose rpm in the current, because i havent really fished in that much wind before.

I was talking to my mechanic this winter and pretty much ask the same thing. He told me that the boat will actually run faster on cold water than warmer and that humidity plays a roll in how it runs also. He ask me if I had ever noticed that when it is real hot out that the boat goes slower? Yes it does. I also have a 115 Merc and I can get 53-55 00 rpm in cold but only 48-5000 while it is real hot out!

Strange aint it. Kinda like fishing, sometimes it works good and sometimes it dont!

A couple of things came into play. One is the chop, smooth water boats run slow, mild chop will give you your fastest speed, heavy chop and boat will start to slow again.  Moderate chop reduces the hull wetting/drag, heavy chop lets the hull break loose, but the larger waves make the hull bite in too much so you loose more than you gain.

Cool, heavy air, makes more horse power.  the colder the water, the more dense and you get less prop slip.  All this adds up to a faster boat in the mornings than in the afternoon.

Take note of your afternoon rpms and chop, come this summer when the water and air are 20 degrees warmer, you rpm will be noticably less with the same amount of chop.

I haven't run small motors in many years so can't comment on those but your average high performance, big V-6 bass boat running at or near max rpm in the early spring will easily loose 300 rpm in the summer.

I think it was the conditions also. I went ahead and pulled and cleaned the carbs, and checked for any fuel leaks. Compression is still the same it was, so I think I'm good to go. All except my control box froze for some reason - Have to take it in tomorrow.

Is it adviseable to have a summer and a winter prop? Or just run it a few hundred rpm under max recommended in the summer?

Your control box probably overran the cable travels and let one gear jump travel too far and lock up or the shift/throttle cables are not properly adjusted so it can't find neutral. This is something you need to learn to fix yourself and make the adjustments to prevent this, old control heads are subjuct to do this. Too many times, I see shops telling the customer they need a new box at several hundred bucks and the only problem was it was out of adjustment.

I prop it so it turns max rpm in the cold water/weather and don't worry about the summer rpm drop.  Everybody's boats are going to be slower in the summer, so you don't have to worry about somebody going faster than you when you went faster than them in the cold water.

Your control box probably overran the cable travels and let one gear jump travel too far and lock up or the shift/throttle cables are not properly adjusted so it can't find neutral.  This is something you need to learn to fix yourself and make the adjustments to prevent this, old control heads are subjuct to do this.  Too many times, I see shops telling the customer they need a new box at several hundred bucks and the only problem was it was out of adjustment.  

sounds like the gear jumped. Its still locked up even when the cables are removed. Is there any diagrams on what to pull and check?

go hear http://epc.brp.com/default.aspx?lang=E&brands=skidoo,seadoo,atv,sportboat,ej Use the pull down and select Johnson, select your year model and hp, model number doesn't realy matter, and select the control head. The shift assembly (ref 2) is usually what gets out of place.

wow, thats a great reference. Shows a good many part diagrams for all of the motor. I'll tear it apart in the morning and see what I find. Thanks a lot!