slow hole shot behind 120 merc I/O

[sabastianunf]

I have a lot of good questions about my 4 cyl 120 hp stern drive 18ft cuddy cabin. I say they are good questions because there seem to be alot of people with simmilar problems. So lets get started. Everything runs pretty good after putting a ton of work into this boat. However, a few things need to be tweaked. First of all when wakeboarding ect. I launch the boat at full throttle but I only hit @ 3200 rpms on launch, it takes a while to get up to speed especially the more people on the boat. Since everyting runs good my first though is to upgrade to an electronic ignition. My questions are 1)will this increase the rpms on the hole shot 2) will this increase my overall horse power and 3) would I be able to push a larger prop for better performance. My next question is the prop. I am currently running a 14.25x19pitch 3 blade. Would a bigger prop say 14.5x19piitch be a better option or should I look into a 4 blade or lower pitch and what size? Could the increased power from an electronic ignition solve my problem by itself? big thanks to all those who reply.

 

[Boatin Bob]

Re: slow hole shot behind 120 merc I/O

Before going to electronic ignition, when was the last time a tune up was done? Plugs, points, condenser, rotor and cap. If after a tune up your RPM's are still too low then I would go to a smaller pitch prop, as a rule of thumb the RPM's will change about 200 for every inch of pitch so if you go from a 19 down to a 15 your WOT RPM's could go up to 4000. I'm not sure what the book recommends for WOT RPM's for that engine but suspect it should be at 4200. I don't think the electronic ignition is going to give you too much in performance but easier starting and smoother running.

 

[JB]

Re: slow hole shot behind 120 merc I/O

Moving to I/O

 

[sabastianunf]

Re: slow hole shot behind 120 merc I/O

Engine currently starts and runs good. If plugs, points, condenser and rotor are fouled wouldn't I have problems with starting and rough idle rather than lack of power? What would be the easiest/cheapest way to get a little more power and higher Rpm from this engine?

 

[Mercathode]

Re: slow hole shot behind 120 merc I/O

Sabastian<br />I am going to say you are well over proped for this set up. I would say 15" prop would be a good start. With a 18ft cuddy you are also quite underpowered. I don't think the ignition change will give you even a noticeable increase. The 120 mercruiser are good motors they are actually 2.5 liters and were designed for the Chevy 2 back in the early 60's but most opted for the 6 cyl. So GM began selling them as commercial/marine engines. This engine just does not offer a lot of room for performance with such a large but with exception to the prop change.

 

[cuzner]

Re: slow hole shot behind 120 merc I/O

Hey Sabastian,1st you should be looking to a 17p prop, a larger p will make your hole shot worse,not better a 17p will also lower your top end speed.Trim tabs will also help you get out of the hole. I think you might be expecting too much from a 120hp engine.If you want to keep your 19p prop try adding Tabs,It may give you the hole shot you want,chances are you'll still have to go to a 17p prop though.Good luck.<br /><br /><br /> Jim

 

[Bondo]

Re: slow hole shot behind 120 merc I/O

What would be the easiest/cheapest way to get a little more power and higher Rpm from this engine?

It Ain't Going to Happen........

 

[cobra 3.0]

Re: slow hole shot behind 120 merc I/O

I have a 17" pitch prop on my 3.0 4 cyl. 140 hp. and it's a 16 ft boat. You'll definitely need the 15" pitch just to get it up to its normal WOT range. (The 19" pitch is a "dog" on my boat if there are more than two people aboard.)<br /><br />I put on the pertronix ignition II and Flame thrower II coil with accel high performance 8mm wires and I cannot see any improvement in horsepower on my boat. It may run a little smoother, but even that is debatable.<br /><br />Also, a hydrofoil or trim tabs would also help get it on plane in 1/3 the time...but do invest in the shorter 15" pitch prop first.

 

[FORMULA1]

Re: slow hole shot behind 120 merc I/O

Trim tabs.....