25 XD to 25 XS Merc

**HGipson** · 01-02-2014

I've heard the 25 XD Merc from the 80's is the motor you can turn into the 25 XS. Trying to figure out what all can be done to these motors to slightly improve the horsepower. I know 400 CC is the limiting factor on these engines. Once again I'm not going to be racing and this motor is going to be on a 1436 flat bottom Monark boat. I built this engine last year from about 5 - 25 mercurys we had laying around. It's like a Johnny Cash Cadillac! 25 XD powerhead and the rest is 98-2004 parts. Took a little grinding and machining but I got a new/old combo that's running good. But wouldn't mind for a little better as always! Thanks

**zul8tr** · 01-02-2014

What is a 1436 Monark? What does it weigh with boat motor, fuel, driver, other gear?

I assume you have the non race lower unit on the XD? The 25XS uses the fishing power head and came with a short racing lower unit with a quicksilver gear case with forward only. If you did that switch you could run with special props but not on a heavy boat?

Do you have OEM steel reeds? You gould go to fiber reeds from Chris Carson marine.

http://www.chriscarsonmarine.com/reed-valves.htm

What sealer did you use for the split crank case on assembly?

What is timing set at? What fuel. oil and mix are you running?

Is the carb fixed jet or adjustable?

What spark plugs are in the engine?

What compression readings do you get?

What max rpm, accurate speed at that rpm , prop pitch and gear ratio is in the lower unit?

These XS, XD powerheads are loop charged design and are very efficient and deliver good power unless you get into porting the exhaaust and mod exhaust the things noted above are about it .

Note on a hydroplane set up to run this engine with a 400 lb total weight right prop will do 70 + mph, kilo record is 76.2 mph.

**HGipson** · 01-02-2014

14'36" flat bottom aluminum boat was made in Monticello, AR. Boat weighs about 180-200 lbs. Battery 25, motor 114, gas 6.6 gallons, me 250. Not trying for a record just heard you could hop these 25XD powerheads up a little. It's got the factory lower unit don't know the gear ratio. Short shaft 15" motor. Was rebuilt completely last spring and still has perfect compression but I can't remeber the exact number. Don't know how many RPM but it'll run around 34 mph 15 pitch stainless prop. This will be for hunting/fishing just like to get all I can out of my motors. Thanks man