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Author Topic: 120 Hp Force: Prop and jackplate Questions  (Read 12947 times)

J10B

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120 Hp Force: Prop and jackplate Questions
« on: August 27, 2005, 12:59:54 PM »

I have a 22p SS on my motor, and I am running 4900rpm.   The Book says WOT should be 5250, So i have lost about 350 rpm.  My best ever run was with a 200lb passenger little less than a half a tank and I saaw 55mph on the GPS. This was in April and about 65-70 degrees, glassy water.

Now I am getting maybe 47 at best. I had the motor raised a hole and it seemed to run much better.   
I am thinking I may need to drop a pitch in prop. Mines a Laser II SS 22p.
Please help here.  The motor sounds fine and runs well, but I know from experience that a loss of speed is a sign something is wrong.
Now I will give some of the loss to Humidity and Temp. But a 7-10MPH drop is not good.
Do I need to trade props?
Do I need a jackplate? <-reason for asking that, is that I moved it up a notch, and now Im throwing a 1-2 foot rooster tail and the boat seems to run MUCH smoother.

I know I have a small 17 foot boat, but 55mph is no slouch, considering I had a passenger and a ton of gear and gas.  Plus Im happy with it and would just like to get it going the fastest and best it can.
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bassadict69

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Re: 120 Hp Force: Prop and jackplate Questions
« Reply #1 on: August 29, 2005, 08:26:59 AM »

I also have a 17ft boat with a 120 Force. I am running a 22p Laser II prop also! My WOT rpm's are right at 5200 rpms with a speed of about 48 mph. That is on a heavier boat with a Deep V hull. It is a Pro Gator 170DV.

I do not know near enough about setting them up to make you any recommendations but thought MAYBE it might help to see another boat similar to your set-up.

My main problem is holeshot. I have been playing with hole size combos  on the PVS plugs but can't seem to get it right.
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j10

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Re: 120 Hp Force: Prop and jackplate Questions
« Reply #2 on: August 29, 2005, 06:13:17 PM »

Its J10B,
I ran the Red at 55 with my fat Bro and this was GPS.  Now Im lucky to get 50.  
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bassadict69

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Re: 120 Hp Force: Prop and jackplate Questions
« Reply #3 on: August 30, 2005, 10:45:52 AM »

It definately sounds like a problem somewhere! But unfortunately, I am not real mechanically inclined when it comes to outboards.

Have you checked the plugs & wires? Might do a compression check?


Come on guys...Although we don't have any "experts" on the board right now, I am sure someone else has some suggestions!
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LoonyToon

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Re: 120 Hp Force: Prop and jackplate Questions
« Reply #4 on: September 21, 2005, 09:17:19 PM »

 ??? :) I have the same QuickSilver, Laser II prop on the SS140 Skeeter i just bought. The boat is a 1994 and the motor was put on then. I run this boat at 5000-5300 rpms and get 63 mph on the GPS/LMS480M..At 5000 rpm I hit on clean water around 58-60 mph, and I don't know if I'm over revving it or not as I have NO specs on this motor. It is a Force/by Mercury. When Vaughn pulled the cover he stated this motor looked as though it just came down the assembly line. He ran a compression check on the 4 cylinders and said it was great, but I replaced the waterpump as this boat was garage stored the last 8 years, and the Chief Petty Officer i bought it from said he only had it out 5-8 times the last couple years.............Does your boat have a tilt/trim.when I trimmed mine all the way up I got the same rooster tail, better handling and speed increased
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LoonyToon

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Re: 120 Hp Force: Prop and jackplate Questions
« Reply #5 on: September 21, 2005, 09:26:51 PM »

 ;) ;) I just talked to Vaughn and he said to trim your motor up until you hear the exhaust-----he said it will be a different sound-----this should take you to 5000-5300 rpms. if not in front of your throttle there may be a screw and to turn it back(back it off) 1/2 turn and try again. If that doesn't work try a carburetor/fuel injector cleaner in your gas and have your linkage checked if you haven't done that already.  He also said as he told me when I replace my steering cable this winter that a 5 1/2 in Jack plate is what the doctor ordered as these motors were rated at their heads, not at the prop as todays are, and this will place the motor in cleaner water and should increase speed by 3-5 mph and drop rpms. I don't know outboards that well. He also said to check your filters, fuel and air as they may have debris in them........Have you tried the 827-1 Champion Plugs yet? I have and they worked great for me---got em at wally-world for 1/3 the marina price
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bass1cpr

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Re: 120 Hp Force: Prop and jackplate Questions
« Reply #6 on: September 21, 2005, 09:29:59 PM »

Did you raise the motor after you ran the 55 Mph?
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J10B

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Re: 120 Hp Force: Prop and jackplate Questions
« Reply #7 on: September 28, 2005, 06:50:59 PM »

Nope.  Which is what lead me to believe something was wrong with the prop.  I also called Mecury a few months back about a transom trim switch and they said the 98+ are rated at the prop.
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LoonyToon

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Re: 120 Hp Force: Prop and jackplate Questions
« Reply #8 on: September 29, 2005, 12:57:11 PM »

J10B,, Is your boat equipted with a Tilt/Trim Gauge. If not as your motoring on the throttle keep raising the motor until you hear a distinct different sound. It should be like the exhaust just got louder and a differnent pitch to the sound. As this occurs you are dead center of your pitch and tilt and the prop/motor should be doing 5300 RPM or there about. That will be the optimum in your WOT. in both RPM's and Speed..If you have done this and it has not improved your performance, try a gasoline additive to clean your carbs without having to take them off, check your fuel filter, air cleaners etc, and replace the plugs with new if you haven't done this already...If you don't have the Tilt/Trim on the throttle or on the motor I recently saw one new on e-bay for a Force 120 H.P........I hope this helps and gets what you need out of her, but 55 MPH isn't bad
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bass1cpr

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Re: 120 Hp Force: Prop and jackplate Questions
« Reply #9 on: September 29, 2005, 08:36:42 PM »

     J10B if you drop a pitch in prop you will loose more speed, but have a beter hole shot. I read your first post again and it sounded like you raised the motor. It may sound better but you may have went past the best prop to pad measurement for the boat and prop. A non surfacing prop to close to the surface will definatley have to much slip but the RPM's should also go up strange. The 350 rpm loss due to Higher heat and humidity is some what normal in the summer. Cool damp air really has a positive affect on two cycles. IT cold be in the linkage and throttle stop or spark advance. Just a little bit out. I'd be patient and see how it runs as the weather gets cooler unless you know someone that is good at tweaking the motor.
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LoonyToon

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Re: 120 Hp Force: Prop and jackplate Questions
« Reply #10 on: September 29, 2005, 11:27:37 PM »

 ::) ??? While we have been on this subject , it has occured to me......Does anyone know the specs on the RPM's of this motor.........Operating and WOT........if this motor was new?
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bassadict69

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Re: 120 Hp Force: Prop and jackplate Questions
« Reply #11 on: September 30, 2005, 08:56:01 AM »

Mine is a 99 120hp Force. I want to say WOT is 5400-5600 rpms
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bass1cpr

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Re: 120 Hp Force: Prop and jackplate Questions
« Reply #12 on: September 30, 2005, 11:56:44 AM »

Max Rpm's should be 5250 to 5500. About normal for this size motor.
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J10B

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Re: 120 Hp Force: Prop and jackplate Questions
« Reply #13 on: October 01, 2005, 09:52:26 PM »

Heres the problem: In April with my fat ass brother 14 gallons+ of gas, tons of gear and glassy water= 55MPH on the GPS.

Recently: my much lighter wife about half the gas and gear+ 46-47 GPS

Raised the motor Saw about 50mph.  RPm was 4900-5050
The last two instances were on chop.

Mercury says 5250 in the book.  But 5300-5400 wont hurt. 

I DO NOT think the weather could account for that MUCH of a loss.   I'd give summer about 2-3 mph but not almost 10.  I do not even think I was trimmed all the way on the 55mph run.  That along with glassy water makes me think I would have been even faster.  Which worries me even more.
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bass1cpr

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Re: 120 Hp Force: Prop and jackplate Questions
« Reply #14 on: October 02, 2005, 12:30:44 PM »

    Glass slick water is usually slower than a chop. Aeration of the hull is less with slick water causing it to stick to the surface.
    Is this boat stored outside or inside? Have you ever ran a compresssion check on this motor? Lots of things to be done to narrow down what the problems may be.
    Boats aren't anything like cars when figuring out performace issues. 47 to 48 mph and 55 mph aren't ten MPH apart. 
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J10B

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Re: 120 Hp Force: Prop and jackplate Questions
« Reply #15 on: October 05, 2005, 10:12:30 PM »

Well I can add  ;D  I was thinking along the line of glassy water is slower than chop, so if I got 47 in chop it "could" be 45 on glass.

It hit 55 3 weeks before it dropped. It had just come out of storage, and been tuned.  Now I hadnt run the gas out of the lines.  I am thinking this may have a lot to do with it.  If I went and gummed up the carbs with the oil+fuel mix........even WOT wouldnt do a lot if I gummed them up really good.

Keep in mind the day after I raised the motor I run UP rive into the wind and showed 49.7 on the GPS.  I am SO confused.
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bass1cpr

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Re: 120 Hp Force: Prop and jackplate Questions
« Reply #16 on: October 05, 2005, 11:44:55 PM »

IF you've added anything equipment wise to the load or moved things around weight distribution can make a big difference. Have you dinged the prop any even small nicks in the blade will cause more slippage.
    What you have lost isn't that significant it could even be in the linkage two cycles vibrate and it's easy for adjustments to change a little. Throttle stops timing advancements it only takes a little to make a difference.
     Running into the wind will give the hull more lift. So it sort of counters the resistance you think you'd be getting. Going against the current has more of a negative effect than wind. If the boats been sitting outside it could hae extra water sitting in the bottom of the hull, or the moisture could have already been sucked up into the wood. Liek I say tons of things could be the culprit.
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LoonyToon

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Re: 120 Hp Force: Prop and jackplate Questions
« Reply #17 on: October 06, 2005, 08:07:10 PM »

 :) :) :) I just recieved a reply from the Motor Doctor for In-Fisherman on the Force 120 H.P. Motor.........States, RPM's 5000-5500, Plugs LV76 Champions..........Suggests doing Compression check on all cylinders after warm-up, and make sure there is no big fluctuations between cylinders.........Check Tilt-Trim and make sure it is operating properly and raising the motor to it's limits allowing the prop to produce it optimum performance............I hope this helps, along with the other suggestions you recieved........................
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LoonyToon

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Re: 120 Hp Force: Prop and jackplate Questions
« Reply #18 on: October 06, 2005, 08:14:14 PM »

 ??? ??? ??? The more I read this , the more confused I get.Are you sure your talking about a water craft and not a Harley...My Harley's ran great in cooler weather, but seemed to get sluggish when the weather got hotter..........I hope these motor weren't made in Milwaukee, but they probably were, so that explains why they run better in cooler climates :roll2: ~roflmao, and I know what your saying ~xyz ~xyz, but ~read will come out of this I guarantee ya...............
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bass1cpr

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Re: 120 Hp Force: Prop and jackplate Questions
« Reply #19 on: October 06, 2005, 08:47:18 PM »

    All motors run better in cooler weather. Cooler air is thicker and denser.
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J10B

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Re: 120 Hp Force: Prop and jackplate Questions
« Reply #20 on: October 09, 2005, 09:38:13 AM »

I do not think a headwind will make you go faster,  yes it will give you lift but that will be offset by the wind speed.
Take a direct 7knot HW, it will slow you. A 7Kt Tailwind may not give you 7.7mph but it will speed you up.

I think my problem is a mixture of jack plate need and my failing to run the engine out of gas leaving the 50:1 mix in the lines and carbs.  Im pretty sure if I get a jackplate and clean the carbs I'll see a major increase in speed and RPM.
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Robwall

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Re: 120 Hp Force: Prop and jackplate Questions
« Reply #21 on: June 12, 2019, 07:17:40 AM »

What is the best size jack plate for a 17 ft nitro with a 120 force?
Also what is the best prop for the same boat?
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Mike Cork

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Re: 120 Hp Force: Prop and jackplate Questions
« Reply #22 on: June 12, 2019, 12:23:46 PM »

What is the best size jack plate for a 17 ft nitro with a 120 force?
Also what is the best prop for the same boat?

Robwall this thread is pretty old and I don't think many are watching it. If you go to this board and post it, you'll get some folks that might have your exact same rig and be able to help.

Just click this link to get to the Nitro board, then start a new thread with your questions :-*
Nitro Boats
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