25xs rpm's

[hemij51]

Does anyone know what rpm you should be running for an early model 25xs!! I have a good Dewald prop (6 3/4 x 12) on a 10'3" pavlick hydro from the early 80's. It is running at best 5860 rpm .. 145 psi of compression in both cylinders.. Any help would be great??? Thanks !!

[ZUL8TR]

RPMs

Does anyone know what rpm you should be running for an early model 25xs!! I have a good Dewald prop (6 3/4 x 12) on a 10'3" pavlick hydro from the early 80's. It is running at best 5860 rpm .. 145 psi of compression in both cylinders.. Any help would be great??? Thanks !!

I know you asked about the 25XS but since I never raced that model here is the data for my 1973 Merc 25SS.

When I was racing 25SS hydro from 1973 to 1980 with a 1973 25SS Merc with 16:21 gears on a 10 ft round nose Wetback hydro at 355 lbs total min weight for the Class I was pulling 7100 rpm with a Pinner 2 blade. Competition set up prop shaft center about 3/4" below bottom with top speed about 63mph in good water.

Your compression for the 25XS is about right but rpm to low at 5860. I would guess that a good Merc 25XS on a hydro at correct weight for that class in the early 80's should be in the high 60's.

What speed at 5860 rpm?

What is the total operation weight of your rig (boat, motor, gas, driver, etc)?

What gears are in the lower unit: 16:21, 1:1?

What is the timing set at?

What jet size is in the carb or is it adjustable jet?

Is this the long shaft 25XS which is same as the early 25SS or is it the later short shaft 25XS?

What plugs are in the engine?

What centerline prop shaft depth below bottom are you set at?

Is the prop shaft level with bottom or tucked in or out? How much?

Are your sponsons clear of the water at speed?

Do you have full length air traps? If yes do they taper to zero at the bottom rear or is there some trap left at the rear of the bottom?

[Jeff Brewster 59s]

Hemij51,
I have sent you my phone number in an email in an effort to help you.
Jeff