1998 Force 75 vibration

[1uz4x4]

This motor is on a sun tracker party deck 21. Just got the boat last week. Took it out for the first time today, everything ran great in the yard on muffs, new prop, a 12 x 10.5 the old one was a bit chewed. She runs fine at idle and on takeoff, the problem occurs at midrange. It feels good at takeoff then at around 3000 rpm I feel a vibration and notice a change in the amount of air in the water the motor is pushing. Its not overeving, runs at 5000rpm and goeas up to 5250 rpm if i trim it up a little. If I stop the boat and take off again, same thing,lots of power out of the hole, then air and vibration.
I also have a 15ft Terry bass boat with a Johnson 75 Stinger, I know its not going to run like the small boat, but feel like something is not right.
Thanks in advance for any help.

 

[1uz4x4]

Re: 1998 Force 75 vibration

Well I took the boat again today and tried to pay a little closer attention to how the 75 was acting. I'm thinking it might not be a prop problem. She may has a miss at mid and upper RPM. Any suggestions to what I need to look at first, gonna check the plugs tomorrow and see how the look.

 

[steelespike]

Re: 1998 Force 75 vibration

I'll assume this is a pontoon.
on a pontoon trim is used mostly to find a sweet spot with the least ventilation(not cavitation thats another issue)
Weight placement is used to make the boats trim best.
It doesn't seem likely that the rpm would go up if it skips.
Pontoons do have ventilation issues The water is so turbulent the prop will sometimes suck air.(ventilate)
This can be severe actually slowing the boat as rpm rises or minor with some growling and a slight increase in rpm.
Let us know if it is a pontoon and your wot rpm and gps speed.

 

[1uz4x4]

Re: 1998 Force 75 vibration

The boat is a 21 ft aluminum v-hull.A tad under 5000 RPM at WOT. The speed, I'm guessing is between 15-18 mph. Not good weather here today, or I would take it out with GPS, plus it's the wifes B-day.

 

[steelespike]

Re: 1998 Force 75 vibration

Ok We'll wait for a light load wot run with rpm and gps speed.remind us of the prop size. as you might have guessed your seriously under powered.
The anti vent plate just above the prop should be about even with the bottom of the transom.
Some pictures will help.
You can raise the motor to a point just before venting becomes a problem.
Many people tolerate a little occasional venting if it allows maximum speed.

 

[1uz4x4]

Re: 1998 Force 75 vibration

Ok, I know, not the best trailing job ever done but had three tired kids a dog and two dead striped bass that someone left rotting on the launch. The kids were laughing at me trying not to puke. 96deg. in Alabama, not a good smell. Anyway, got a few pics. prop is a 12 x10.5

 

[steelespike]

Re: 1998 Force 75 vibration

Good job but still a little hard to tell. With the motor on a pod I think you can probably raise it.
With the set back you may need to move it up to be ideal.
Usually you should be able to see the plate at least partially at speed.
It is possible you could gain 1 or 2 mph with a slight gain in hole shot.
and mpg.

 

[1uz4x4]

Re: 1998 Force 75 vibration

Do you think that would cause the vibration,gaining a couple miles per hour would be great, I just don't want to tear anything up by not running it at a proper setting if that's what you think the problem is. I only paid $800 for it so spending a little more on it to make it dependable won't hurt as bad.
Where should I look to find info on doing a good tune-up.
And by the way, I think ya'll are great for being here to help.

 

[1uz4x4]

Re: 1998 Force 75 vibration

Checked the plugs today, they were NGK BUHW,should be NGK BUZHW, did some checking the Z is for a resistor in the plug to cut out RFI that does funny stuff to the CDI. Gonna try the correct plugs and see if that may be my problem.Also gonna rebuild the fuel pump, and maybe the carb this week, doesn't look like the fuel filter under the cowling has been changed in a while.

 

[steelespike]

Re: 1998 Force 75 vibration

You should try to find a factory service manual maybe on cd or the real thing. Ebay is a good place to start looking.
Also post Force repair questions on the Force forum.You'll get better,quicker help there.
being sure the motor is at the best height is just a way to get a little more performance.
It may already be at a good height.if the trim is touchy.
Have someone carefully look over the back to see if they can see the plate.
Maybe pictures.
Is the pod clear of the water at speed?
By the way i think the gear case/prop on your motor may be Mercury parts.
You may find that the Force prop numbers are the same number as the Merc props.
Your better off than those with the Force gear case as props may be limited.
It is important that we get a lightly loaded wot rpm and speed by gps.
I know it does run slow but does it seem to get up on the water as it should
at that speed.
You might check of water intrusion in the flotation foam under the floor.
While your under powered it shouldn't struggle with light to moderate loads.

 

[1uz4x4]

Re: 1998 Force 75 vibration

Taking it out this weekend, will get GPS speeds and get someone to drive while out check things out in the back. I'll get some pics and maybe some video.

 

[1uz4x4]

Re: 1998 Force 75 vibration

Ran the boat the other day and checked the timing at WOT, 33deg advance at 4200 rpms. Talked to the local boat Guru here,Mr. Hudson, he told me to try champion plugs, back the timing up 3 deg.
He also wanted to make sure I was pitched right for my boat, might be loading up to cause the miss. I'm running a 12 x 10.5 now, can go to a 8pitch, would that bring rpms up if it's not a motor problem.

 

[jestor68]

Re: 1998 Force 75 vibration

Those boats did not come with a bracket. The outboard versions had a regular transom. That looks like an I/O to outboard conversion, executed poorly.

You may need to do some fabricating(or have it done) to extend the center portion of the boat bottom out to the lower edge of the bracket to get clean water to the prop.

 

[1uz4x4]

Re: 1998 Force 75 vibration

This is the way it came from the factory, may be a bad design but It must have worked at some point. Go Snakes!

 

[jestor68]

Re: 1998 Force 75 vibration

This is what the factory outboard version looks like.

 

[1uz4x4]

Re: 1998 Force 75 vibration

That looks like a 02 model. Mine is a 98 This is how the 98 ,99 look.