1998 Force 120

[Cantrn1135]

Hello all I am looking for some info, I have a 1998 force motor by mercury marine. The engine runs good, I am going to do a compression check on all the cylinders, and was wondering what the proper compression is on all four. Thanks for everyone's help in advance.

 

[Jiggz]

Re: 1998 Force 120

The compression number is actually very subjective depending on the type and calibration of the compression gauge. The rule of thumb is not to have a difference of more than 10 psi between readings and the range can be as low as 110 and as high as 150 psi!

 

[Cantrn1135]

Re: 1998 Force 120

Well after looking closely at the power head it's had numbers wrote on the block by each cylinder 165 on the first then on the remaining three 175 I have not performed the compression test yet but those readings seem really high if those are from a previous test

 

[I`mNotMe]

Re: 1998 Force 120

Yes. Compression numbers is a very subjective area. Especially if PSI was done on cold vrs hot engine. And, if previous owners replaced factory pistons with different dome shape pistons.

Those written down numbers (re: 165 - 175) seem high to me as well.

I have a '96 Force 120 and its factory specs is 135-150-150-145. If someone replaced its factory pistons, then its new PSI of outer cylinders would be closer to 140-150 range. If wondering, my Force 120 numbers are NOW much different than factory specs but are still above 120s. (which is good to me).

Note: Details of factory '95/'96: http://www.marinemechanic.com/merc/...rine/outboard/sportjet120-compressiontest.PDF

For other Force 120 engines, I've read many patterns in the 140s/150s is great. Above 125 PSI is good. And, being within 10%-15% "variance of factory" specs is great as well. I'd use the same numbers for your '88 Force 120 as well.

If compression is too high, perhaps its time for a very long de-carb task? Thus, helping to remove unwanted carb residue - which can +/- influence compression numbers as well.

Hope this helps...

 

[Cantrn1135]

Re: 1998 Force 120

What's the best way to do a de-carb? Is this as simple as running some sea-foam through the engine ?

 

[jerryjerry05]

Re: 1998 Force 120

NO!!!! 165 is NOT too high.
These motors starting with the 95(I think) the comp should be about 165 #1 and 2/3 about 10# higher and #4 the same as 1.
Someone just posted a service bulletin on this just a couple of days ago.Couldn't find the link.
Before the 95?? The comp needed to be all the same for all the pistons.

 

[Cantrn1135]

Re: 1998 Force 120

So your saying I should not worry if I get the same readings when I perform the compression test? After further examination I found a date and it looks like this test was performed 5/21/2012. So I' m thinking I should come up with the same results. I will post the results of my test. Thanks again for everyones help.

 

[I`mNotMe]

Re: 1998 Force 120

What's the best way to do a de-carb? Is this as simple as running some sea-foam through the engine ?

Yes.

Lots of decarb videos within You-Tube Search.

re: outboard motor decarb - YouTube

If wondering, most folks use Sea Foam "Engine Treatment" product. Some folks use other products made by mercury or Yamaha. Not too sure if I'd use WD-40 as decarb agent. Surf You-Tube videos and apply their "most common" method.

Hope this helps.

 

[Cantrn1135]

Re: 1998 Force 120

Okay results of the compression test first cylinder 1-155, cylinder 2-160, cylinder 3-170, cylinder 4-165. Now the research I have found says that the compression ratio for this motor should be 140,150,150,140. So now it seems I have a decard to do on this motor. My question is will this harm my motor if I keep using it like this? Can I just mix the seafoam in with the Oil and gas? I tried to research this method of decarding but I could not find a step by step method. Thanks again for the help guys.

 

[jerryjerry05]

Re: 1998 Force 120

NOTHING wrong with those numbers.Every gauge will give different readings.
Doing a de-carb is just part of normal maint.
I use Mercury's Powertune. Or OMC's Engine Tuner.
I tried Seafoam as a decarb and it did NOTHING!!
Follow the directions on the PT or ET.
It can be added to your fuel.
Just remember the more gunk you add to your fuel is more gunk in your fuel.
If it's set up right and you do a decarb every year or so. You won't need gunk.
Make sure the air/fuel screws are set right and you don't troll excessively.