Do I need a prop swap for my1994 90hp Force?

Guys, hoping you knowledgeable types can point me in the right direction!!

Im in Australia and have a mid 90's 90HP Force on my fishing boat. The motor runs very well and the local mech has given it a full bill of health.

Now, the motor is on a 5.3m boat that has a reasonably deep V at 19 degrees. Its a heavy well constructed boat and handles our choppy seas very well.

Now my query is this:
At full throttle, Ive never seen more than 4100rpm on the tachometer, and this equated to 50km/hr on the speedo. Can I assume that this Force should be closer to 5000rpm at full throttle? Or will higher revs come at a cost to durability?

Most of the time, this is ample due to conditions, but in summer we get some days where the ocean is like glass and I feel like Im missing out on fishing time by not having a higher top speed!!

My question is, should I be exploring a prop swap to give me more revs and potentially more top end speed?

As it is, the craft doesn't have any issues getting up on plane quickly and at reasonable pace so I feel I have some scope to improve my speed. I also have the choice of adding a foil on the motor if I loose a bit of low down power in the quest for say an extra 10km/hr in the top end.

Ive attached pix of the hull, and prop to hopefully give you guys a bit of an idea or my current setup.

The prop has mercury written on it, does this mean that my Force motor uses the same prop as a merc or is it still unique to Force only?

I'm hoping you guys can answer my Force specific questions because these motors aren't very common here in Australia so have a very little info locally.

Lastly, if we come to the conclusion that a prop swap would be good, what do you guys suggest and maybe a link to a place to get one from?

Atm , the prop is Aluminum, Ive seen the stainless steel ones, what should I go for? My initial thought was to stay with Aluminum cos they are lighter, cheaper and easier to repair if I give it a nick .
Does the extra cost for a S/steel offer that much more performance than the correctly sized equivalent in Aluminum?

many thanks.

ale

Re: Do I need a prop swap for my1994 90hp Force?

You have an early 90s Force with most likely Mercury ignition. It has the Force dual exhaust lower unit but the Mercury type transom mount.

4100 RPM at full throttle is a tad low. The engine should be running somewhere in the 4500-5500 range. However, you are currently using a 17 pitch Mercury prop. The Force has the small gearcase and only Merc. props from small gearcase engines (90 HP might be one) will fit it. Since the lower unit is a dual exhaust, you MUST use thru-hub exhaust props.

Dropping down to a 15 pitch prop will give you about 400 RPM more BUT----You may actually sacrifice a bit of top speed. The same prop in stainless will give 200 RPM more.

Before you go messing with props, examine the engine health. A 5.3 meter boat is only 17 feet long. Unless she is a garbage scow I would expect a little more than 34 MPH (50kph) and a 17 pitch prop should be just about right.

Mess around with engine height on the transom. The anti-ventilation plate can often be one inch above the bottom of the vee and depending upon hull, sometimes more. Check the hull for excess weight. Be certain the compression ratio is high (145 is excellent) and equal on all cylinders. Check that the carbs are opening fully and be certain that timing is fully advanced at wide open throttle.

Re: Do I need a prop swap for my1994 90hp Force?

Thanks for the reply mate, much appreciated.

In regards to the health of the motor:

I havnt comp tested it, but the mechanic did when he serviced it, he said it was very even on all 3 pots, and I was surprised when he said it was quite acceptable at just over 100psi , I thought that was lowish?

I will comp test it myself and post results.

I will also double check throttle, and know that it has a tiny bit more in the linkages but I didn't think it would be worth 5-600rpm.

How do I check the timing at full throttle?

Re: Do I need a prop swap for my1994 90hp Force?

The comp with a good guage should be about 150#
His guage might be old/piece of crap??? It might read different.
As long as all 3 read about the same it should be ok.
Your trimming the motor as you drive?
How heavy is your boat(more pics),and how much stuff do you have on board??
2 coolers 5kids a dog and a full tank of gas a couple of fat chicks???
Saying too much load will slow you way down.

You sure the tach is right?
Top post is a post by FrankA, linc and sinc. Make sure the linkage is right.
Dynamic timing is done at WOT. Your not getting wot so it can't really be done.
Static timing is all you can do for now.
Stay with aluminum props. Stainless is pretty expensive and if you happen to hit something you can damage the shafts and more.
Alum. gives a lot easier.

Re: Do I need a prop swap for my1994 90hp Force?

Well, forget the prop swap, comp test reads 135, 145, 85 from the top down . Seems I have a bigger issue right?

Whats my next step? can it be anything other than piston blow by causing the low reading?

Re: Do I need a prop swap for my1994 90hp Force?

Sure.
Blown head gasket is the easiest.

Remove the head, carefully so as not to break any bolts off.

Examine head and gasket, pistons and walls.

Re: Do I need a prop swap for my1994 90hp Force?

Easiest, sure, but how likely/common is it? It certainly isn't visibly leaking to the outside. I've filled that chamber with power tune, let it sit over night and Il start it tomorrow. Maybe some magic will happen and it was just a stuck ring but I doubt it.

What should I pull off first, the head or the side plate to get a good diagnosis?

Ive attached a pic of the throttle cam,and a video with the throttle lever at the control box all the way forward.I then can still open the throttle further at the carbs, the butterflies can go a little way further than my full throttle currently gives, how much tho?





Lastly, if I was to embark on a rebuild, are all the rings/bearings/pistons same as chrysler? Any links to a good place to get these parts? Also, which brand enigine manual would be best for all the torque settings and clearances?

many thanks

Re: Do I need a prop swap for my1994 90hp Force?



The easiest is probably pulling the head. Getting to the side plates can be tricky? Then you only see a small portion of the cylinder.Plus the screws don't wanna come out easy.
I took apart an 85 last week and it took about 2-3 hours to get the head off.3 bolts were siezed.
I used a hammer a small propane torch WD-40 and a BIG breaker bar.
Once I got the head off I found the damage in the pics.
This was from a running motor.Comp was 150# all3 The flywheel wouldn't come off so I was dismantelling to change cranks.

The bolt only have 20# of tourqe and should be easy to undo.
Even then 1 of the bolts, the head sheared off.
Luckily I had the adaptor plate off and I could just unscrew the whole head and what was left of the bolt just unscrewed.
Most siezures ar in the head.The threads don't sieze.It's in the space around the bolt in the head.


Parts can be found all over. If you need a piston. Wiseco is the best bet.
The factory manual is the only way to go.Then Clymer then Seloc.

Re: Do I need a prop swap for my1994 90hp Force?

an over night soak in power tune

the excellent:


the good:



the bad:

Re: Do I need a prop swap for my1994 90hp Force?

Top is good,the next 2 are bad.that leads me to believe there could be a bad gasket between the two bad ones???
Take the head off and LOOK!!!

REMEMBER THIS!!!!! There is a certain amount of tourqe on ALL the smaller bolts/screws it's never more than 20#
So if it doesn't come loose easy then you can break it.
Except the flywheel nut,95#

Re: Do I need a prop swap for my1994 90hp Force?

I probably shouldve re-arranged the photos, but the good pot is the centre one

Im still calling it for a broken ring, but hoping headgasket. We find out on the weekend

and yeh, I envision lots of broken and seized bolts if I enbark on a rebuild of sorts

Re: Do I need a prop swap for my1994 90hp Force?

The carbs are opening fully. It is possible to get them to go past fully open and start to close a bit so don't mess with them.

The bottom cylinder is low, and that is the source of your problem. It could be a bad head gasket, but these later Force engines had semi-keystone top rings which tended to break for no reason. Pull the head and look at the gasket, then the bottom piston. If it has a deformation at the rim, then the top ring broke and pushed out the top.

As long as the cylinder is not scored, that can be fixed without tearing down the engine.

Re: Do I need a prop swap for my1994 90hp Force?

wow, thanks a heap for some more encouragement!!
Ive never pulled appart a 2 stroke boat motor, but your saying you can just pull out one piston at a time without pulling the power head off and tearing the whole thing down?

Re: Do I need a prop swap for my1994 90hp Force?

Frank changes them without dismantelling the whole engine.
Since it's the bottom one then taking it apart will be a lot more difficult.
It's still a bunch of work.
I like taking the whole thing down and replacing all three sets of rings and honing all the cylinders.
Most cases everything can be re-used except the gaskets.

Tell ya what we can do!!!
You pay for roundtrip tickets for myself and Frank and put us up in a hotel and we'll come and fix it?
If Frank can't go? I'll do it myself.

Re: Do I need a prop swap for my1994 90hp Force?

Hahahaha, done deal, send me your address so I can mail you guys the tickets!!!

Seriously, many thanks for all your advice and answers guys . My confidence is growing that I can get myself back in the water fishing sooner rather than latter!!

Re: Do I need a prop swap for my1994 90hp Force?

Guys, reviving a thread from the past.

have access to a rapture s/steel prop, same as the one I have in regards to size and pitch.
Its 15 spline so will have to check what I have, but Im wondering, what kind of performance increase would I see with such an upgrade?

http://www.gumtree.com.au/s-ad/ipsw...ture-prop-17-pitch-stainless-steel/1021673184

Re: Do I need a prop swap for my1994 90hp Force?

Probably not much if any?
The 17 stainless is like a 19 al.
You'd need a 15s to match a 17A
The regular prop is a 13 X17 a 13 1/4 will drop the rpm's even more.
Contact Michigan Prop. send them all the info and they will put it in their prop selector and they will give you the right size.

Re: Do I need a prop swap for my1994 90hp Force?

Thanx jerry, always helpful and your opinion is shared by a few others I've consulted. Think il wait till the motor is run in , opened it up and seen what I have now.

Thanx mate

Re: Do I need a prop swap for my1994 90hp Force?

In relation to the original topic of this thread, the simple answer is YES, I did need a prop change, I now run a 15P in aluminium and Im a happy man.

If you have a day to spare for some light reading, here is the journey of how I got there!!:

http://forums.iboats.com/force-chrysler-outboards/down-under-rebuild-90hp-force-596028.html

many thanks guys, I owe you all a beer!!