1998 force 75hp surging

[jbp84]

sunday i picked up my first boat i have ever owned, monday eager to hit the water i started the boat up it seemed like it was running some what rough thinking maybe i had choked it to much let it warm up then eased into gear, i noticed right then it seem to run a little rough thinking maybe it had water on it from where i washed it. well went wot trimed motor up went to 5k rpm 35mph i notice the tach waving around at 5k -5100 rpm convinced myself a set of plugs fuel filter and fresh gas would cure the problem. i got home pulled th eplugs notice a few had small specks of water i thought it was fuel drained tank fresh fuel. this afternoon started right up still seem a little rough eased into gear idling still same roughness passed marker went WOT went to 5100rpm 35-36 mph gps samething rpm waving around. i had added seafoam thinking well i run up the lake maybe it will straighten out when i passed the boat ramp it hicuped i immediately turned around got the the ramp put in N the motor was idling high then i was at loss of words and fuming thinking why did i buy a boat. please help guys i dont want my first boat to be my night mare. oh also if helps the edges of the prop is kind of rough if some may think my problem maybe here from pitched messed up
oh my new to me boat is a 1998 tracker 185 with the force 75

 

[TwoFish]

Re: 1998 force 75hp surging

I have the same motor as you and it can run a little rough at idle if the idle speed is a bit low or the idle mixture screw is a bit off. I thought I had water on the plugs once but it turned out to be unburnt fuel. I have attached a photo of the plugs. As I understand it water cleans the plugs. If you find a clean plug suspect water then. My plugs get a light brown sludge on them after sitting for a few days.

My suggestion would be to give the motor a good service so you know what condition it is in. Check compression , spark & tune the carbie. Only having one carbie on the motor trying to deliver fuel to 3 cylinders is a bit of a problem. To get the fuel mixture in the top cylinder running right the bottom cylinder can run a bit rich. At idle the bottom plug can oil up in a relatively short time causing it to run roughly. This design is never going to be good for trolling.

I was a bit disappointed with my motor when I first got it. I had similar problem to you. I have had it nearly 3 yrs and have learnt to service it myself. After fixing a couple of problems with the fuel system it now runs well and is very reliable. It will never be as quiet and fuel efficient as the new technology motor but it gets me to my fishing spots and I am able to work on it myself.

Post a photo of your prop. Small marks are not a problem but larger dents can cause the water to boil due to turbulence and make the prop slip. Most of these motors have 1.64 : 1 ratio gear box with a 13 tooth spline and a 10 ? inch diameter prop. Basically a 50 hp gearbox on a 75hp motor. Most 75 hp motors would have a higher ratio gear box 2.25 :1 and a larger prop 13 inch. The smaller prop spins faster and is better for high speed. At lower speed it does not have the surface area to get much grip on the water so you get a lot of slippage with a heavy boat. It took me a long time to work out why the boat had no bottom end performance. It would rev a lot when starting until you got to a speed where you could feel the prop bight and it would be OK from then on. I recently had the prop reworked. They added about 9 mm to the outside edge of the prop and gave it some cupping. It solved the problem & has completely changed how the boats preforms.

The recomended rev range for your motor is 2850 - 5200. So your prop is good!

Cylinder 1 left and no.3 cylinder right, in first photo.

Good Luck

Dean

 

[Cannondale]

Re: 1998 force 75hp surging

Dude, not to be a grammar Nazi or anything, but periods are your friend. Reading a wall of text with no punctuation for sentences is really tough.

 

[jbp84]

Re: 1998 force 75hp surging

Two fish my top plugs were clean the bottom was slightly dirty. Im going to get a vacuum gauge check integrity of the fuel system. Im sorry about punctuations I was going on a rant last night my first expectations from this boat was high seeing how good of shape it was in.

 

[TwoFish]

Re: 1998 force 75hp surging

The plugs in the photo had been in the motor for about 8 hours of average running with the old prop. Since changing the prop & retuning the motor the plugs run slightly cleaner. Possibly due to the combustion chamber running slightly hotter with the heavier load of the new prop. I?m only guessing here. I will check them this weekend and post some photos for you.

I was told in an outboard maintenance course to only remove or fit sparkplugs in aluminum head when it is cold. The aluminum expands at a different rate to the metal of the plugs. Fitting plugs to a hot engine will cause them to be over tight when the engine cools.

My boat does 36 mph with a 14 inch pitch prop on it. Different boat but a similar size. I think the performance you are getting is about right. 75 hp is not a big engine and it takes a lot of power to push a boat through the water. Working backwards from the figures you provided I think you are probably running a 14 inch pitch prop.

As far as the motor surging goes is it the motor that sounds like it is surging or the taco showing the revs changing? I ask this because I suddenly gained about 400 rpm at one stage. The taco needle would fluctuate with out the sound of the motor changing. The problem tuned out to be interference on the signal wire to the taco. Corrosion in one of the plugs on the main wiring loom was causing extra signal pulses to get to the taco making it read high. This was shortly after it bought the boat. I thought the extra revs were due to fresh fuel & giving the motor a good run after it sitting around with little use before I bought it. It took me a while to work out what was really happening. It may be worth listening to the sound of the motor next time or using a different taco to check what is happening.

 

[jbp84]

Re: 1998 force 75hp surging

You can hear it through the motor going down the lake and see it on the tach

 

[jbp84]

Re: 1998 force 75hp surging

Guys made some head way I attached vacuum gauge before pump showed 0-1in vaccum. I checked pressure after pump before my filter showed 4psi idling I noticed reving it didn't change much,early today had my dad run to local mercury dealer get me fuel pump rebuild he felt like if it never been rebuilt nows a good time. So I changed gasket and teflon seals inside I put it back on pump bulb had 4 psi idling this time I reved it was going up to 7-10psi. After this I ran a clear line from the filter to the carb here's were I got stumped again. In the glass filter there's always a small air pocket I noticed air would mainly circulate around the exit port of the filter I watched the inlet of the carb and never saw and air enter the carb but still feel like it should be sealed tight any takers at this?

 

[pnwboat]

Re: 1998 force 75hp surging

I have a clear in line fuel filter between the fuel pump and the carbs. At idle I always have a small air pocket too. Once I get it out on the water and run the motor above 3000 RPMs for a few minutes, the air pocket disappears.

 

[jbp84]

Re: 1998 force 75hp surging

Update went to lake this weekend with gf she don't like going fast on water so we cruised across lake 2500-2800rpm boat felt like it was skipping as if a plug wire was jumping fire or bad coil pack. So I took to a local guy had him take a look at it he could not find anything wrong with he said supposedly sometime fuel pumps don't have enough strength to suction fuel passed the drain back valve in the pick up so he took it out. I asked about the skip from 2500-2800 he says supposedly that's probably the transistion of the reed valves starting to open up. Me I was thinking wth kind of answer is that I feel like this thing should run some what smooth please help fellas my next move finding a tech that lives on water who will go and drive the boat!

 

[TwoFish]

Re: 1998 force 75hp surging

What did the mechanic check when he looked at the motor? Compression? Spark?

It sounds like a fuel problem but there is no point pulling things apart until you have covered the basics. You need to find out if the compression is OK and that you are getting a good spark. With a compression gauge & an inline spark tester you can eliminate a couple of possible causes without having to tear anything apart.

My engine runs smoothly throughout the rev range.

If you are going to start working on the motor a workshop manual is a must.

I was very lucky that my local mechanic knew these motors inside out. This gave me time to learn about the motor before working on it myself. It?s a pity you have to start working on it from day one. The upside is that these motors are very basic and when you get it sorted you will feel more confidence in the motor than if you were having it serviced at a shop.

As this is your first boat I just wanted to make sure you knew that these motors run on 50:1 premix fuel. Are you mixing oil into the fuel?


Good Luck