engine rebuild. Merc120hp sport jet

[Willor]

Sport jet 120hp

Hi everyone I’m currently pulling my engine out my 1999 sport jet boat with a merc 120hp. I’m looking for info on what I can port and wat I can’t port. I’ve seen cpl forums say I can’t port the exhaust ports seeing it’s a120hp 4cyl. I was planning on boring .040 over an new everything in motor. So while I’m in there I’d like to do some wrk to get some gains. Can I port all ports an raise exhaust port also I’ve also read that intake and transfers should be rough not smooth due to surface tension an I know it’s pretty smooth in there from what I’ve seen online. What do you guys recommend for engine wrk when I pull it apart? Thank you

 

[Willor]

hi guys im looking to rebuild my engine on my 1999 sport jet with. A 120hp merc jet motor. My plan is to do me everything inside an bore .040 over an I’d like to do some porting for some gains. I’ve read I can’t port exhaust due to it’s a 4cyl. I’d like to port all ports an also I’ve read that intake and transfers should be rough not smooth an from on line it all looks smooth in ther. Should I make it not smooth. While I have engine apart what should I do for engine wrk to get some gains. Thank you guys.

 

[Willor]

Engine rebuild. Sry kinda new to this stuff an idk how to fix the spelling now.

 

[Faztbullet]

Why are you going .040 over? Why do you want to port it?

 

[racerone]

It is a FORCE powerhead.-----I believe he wants to force it to pump more water.

 

[Willor]

The motor is tired an has lost compression in one cyl. The complete kit I see easily available is for 40 over. I sent my pump out to mavicks to get over hauled. Just while I’m in the motor I’d like to any wrk that will give me a little more power/torc. That’s all. I have a 1999 sugar sands Heat xr2. Just would like to be able to have a little more pep in its step so to speak. Right now it looks more fun an fast then it really is. I’d like to match the performance with the look. An have a little more fun with it this summer

 

[Willor]

Just hard for me to tear it all down an not do any mods. But boats an boat motors are new to me. If this was any typical motor I’ve wrk on in the past I’d feel comfortable with my thoughts. This one I need help on guys. Thank you

 

[racerone]

composite reed valves.-----Look into that item.

 

[Faztbullet]

Just a few notes for your information.....1) boring does not add noticeable HP maybe 1HP at best. 2) composite reeds only react quicker and add maybe 2-4 HP. 3) Porting the exhaust increases top end but kills low end, where you add to one place you take away from the other. 4)The pump impeller keeps you from turning extra RPM to use any porting mod's.where as a outboard you can use different style/pitchprops and engine heights 5) to make HP you have to increase airflow into engine and that mean you have to get it out also. You exhaust manifold is way restrictive and its a crossflow so cant shave the head either. Just bore it, clean up the casting lines and flashings in intake/exhaust areas and use it till you find one with bigger motor and sell it.

 

[Willor]

Thanks. Also any information on roughing the surface of the intake. The reed compartment an pretty much all the intake others then where crank bearings go on that piece. So is it better rough or smooth like it is stock.

 

[Faztbullet]

Just knock off the flashing /casting lines and go boating as you will not see any improvements in performance. Composite reeds will make response quicker

 

[Texas Prowler]

Faztbullet is the man! I took his advise with my little rebuild and also picked up some Chris Carlson reeds and am one happy boater. The reeds make the difference your looking for. Clean up the casting flaws anywhere even in the midsection under the head . it just makes the airflow much smoother.