85Hp Force, Got her going, RPM seems low at WOT

[jbwillis76]

Hello again all,

I finally got my 1986 85Hp Force going and to the lake. I just rebuilt the carbs, new fuel pump (valves as well), fuel lines, bulb, impeller, new plugs, link/sync, timing checked out and 135# on all three cylinders.

It turned out I was running on 1 good cylinder, one dead and one intermittantly (weak). Replaced CDI and Coils until I have good spark on all three now.

Anyway, took her to the lake (16' VIP fish/ski. pretty small boat) with 13/19 stainless prop. Over all I'm tickled with the way she performed considering all the work I've done to her the last couple months. Cold starts instantly, idles perfect, and runs to WOT with no missing. The only problem is WOT, according to the factory tach, is around 4000-4200 RPM. Now at WOT she purrs and sounds great and seems to be really scooting across the lake. I will GPS next time out but according to the factory speedo (I know not accurate) she's doing 42-43 MPH with me my wife and kid in the boat. If I didn't have a tach and know it should be turning around 5000+ I would think it's perfcet and not bother with anything else, but I know it should be turning more RPMs.

It planes out ok, but seems it should plane just a little faster. It has a Sport SE 300 foil (I know). Once on plane it revs to WOT pretty quickly. Trimming up while WOT doesn't change much, maybe 100 RPM and 1 MPH.

The plugs were put in brand new just before this trip to the lake and I pulled them once home and all look the same with pretty heavy carbon on them. I haven't decarbed but do have a can of Seafoam in the gas.

Any suggestions or should I just live with it.

Thanks for the help, James

 

[Frank Acampora]

Re: 85Hp Force, Got her going, RPM seems low at WOT

Not having full information, I needed to make several assumptions:
1. 2 to 1 gear ratio
2. 19 pitch propeller.
3. 10% prop slip. This is an average guess applied to fast deep vee boats.

Given these three assumptions, the theoretical speed at 4200 RPM would be about 38 MPH. Given about 10% slip, then the theoretical top speed at 4200RPM would be about 34 MPH. You are claiming 42 MPH. Thus: Something does not agree.

IF your engine is equipped with 2 to 1 gears and a 19 pitch prop, and assuming your speedometer is reasonably close, then you need to suspect that the tachometer is inaccurate. Note that most pitot tube (impact tube) speedometers are a bit inaccurate and a GPS is the best way to measure speed, however, yours should be close enough to tell you if the tachometer is in error.

If your engine is not equipped with 2 to 1 gears (1.72 to 1) or if the prop is not 19 pitch, plug in the numbers as shown below.

To check gearing, with the switch off, put the engine in gear, mark one prop blade, and turn the flywheel two complete turns. The blade will either turn one complete turn for 2 to 1 or about 1 1/4 turns for 1.72 to 1

To check theoretical top speed, take the engine RPM and divide by gear ratio. This will give you propeller RPM. Multiply prop RPM by pitch to get inches per minute through the water. Divide by 12 to get feet per minute. Divide by 88 to get MPH. (60 MPH --1 mile per minute--is 88 feet per second).

If you are questioning the 88 feet per second versus the feet per minute you figured, you would divide feet per minute by 60 to get feet per second, then divide by 88 feet per second then multiply by 60 (MPH) the two 60s cancel each other so just plug in 88 and get an actual MPH in two less steps.

 

[jbwillis76]

Re: 85Hp Force, Got her going, RPM seems low at WOT

Thanks Frank,

Yes, all the assumptions are correct.

2:1 gearing (checked)
19 pitch prop

I have no good reason to doubt the tach, but can't verify it either. I have seen the tach go to 5500 revving the engine before (won't do it again, but have seen it do it).

Don't know, may just live with it for now and verify speed with GPS next outing. Thanks Frank.

James

 

[john from md]

Re: 85Hp Force, Got her going, RPM seems low at WOT

I run a 13x19 aluminum prop and get 5000 RPM with a 16ft Bayliner and '85 85hp Force. You may be a bit too high on pitch with a stainless 13x19. I seem to remember that, when switching to stainless props, you should decrease your pitch by 2.

Regards,

John

 

[Frank Acampora]

Re: 85Hp Force, Got her going, RPM seems low at WOT

I missed that you had a stainless 13 X 19 prop. Cupping is a slight turning over of the trailing edge of the prop blade. This does not increasae the pitch of the prop and does not increase boat speed. What it does do is to increase the prop's "bite" in the water so it will ventilate less in turns or at high trim angles. because of this, the trim can be run further out or the engine can be run higher on the transom. Net effect is slightly higher boat speed with a cupped prop versus a regular prop of the same manufacturer.

NOW: the reason for this explanation. Stainless props normally run about 400 RPM higher than the same aluminum prop from the same manufacturer or brand and 2-4 MPH faster. This is because the stainless blades are thinner (usually) and easier to turn through the water. Cupping the blades will increase the resistance in the water. THUS: A cupped stainless 13 X 19 from Michigan will run about the same rpm as an uncupped 13 X 19 aluminum Michigan wheel. Boat speed will stlll be a little higher with the stainless though.

SO: depending upon cupping or not and manufacturer, you would still use about the same pitch stainless or go up 2 inches in pitch.

But after all this, the reason for this post is: Check the back of your tach to see if there is a multi-setting switch. If it has the switch and if the tach is set to 4 cylinders instead of three, it will read low. --About 25% low, which is about what you are reading now.

BTW: An RPM of 5500 with the 2 to 1 gears and the 19 pitch prop will give a theoretical top speed of 44 MPH at 10% slip. Close to what you are reading now, considering that speedometers usually err on the high side. All things considered, it appears that your boat is currently propped correctly. If you want to water ski, it may pay to change the prop to a 17 pitch for those days when you need the hole shot. Wakeboarding and tubing do not need so much acceleration.

 

[NFA]

Re: 85Hp Force, Got her going, RPM seems low at WOT

That's some amazing advice Frank, well done. I'm upgrading to a 150 Optimax and will be looking at after market props when the finances allow it. Just a stock one on it now.

A great read, thanks!



SSS

 

[jbwillis76]

Re: 85Hp Force, Got her going, RPM seems low at WOT

Frank, you are the MAN!

The tach has the switch and is set on 4. I can sleep at night now knowing there is not an extra 1000-1500RPM I need to get out of the motor!

Now I change it to 3 and Idle RPM went from 1000 (4) to 2000 (3) and I cannot get the idle below 1500 on the tach or the engine will die.

Here's the deal, on 4 the idle RPM is real close but the higher RPMs are lower. On 3 the idle RPMs are off but does seem to jump up pretty high with a quick blip of the throttle. I change it to 2 and idle is 4500 so I know changing the switch is doing something at least.

The reason it is set to 4 is because idle is so far off on 3. This may be my first boat but I've been playing with two stroke jetskis long enough to know the idle RPM I had on 4 was pretty close to the 1000 RPM in neutral 700 in gear in water. The tach is a Faria from the factory (I assume) in 1987 (boats an '87).

Well at least I know I was right in thinking the engine sounded pretty wound out at WOT. Now for the next problem. The prop is a cupped 13x19 stainless. Frank I have absolutely NO holeshot. Takes what seems like forever to plane out, I'd say 150-200 feet from a dead stop. The motor isn't hesitating or missing, and I hate to say it's bogging because it isn't it just feels strained until it planes. Do you believe I'd feel a noticeable difference if I dropped down to a stainless 17 pitch prop?

I'm honestly use to much peppier and quicker jetskis and may just be expecting too much from this old Force but my buddies mid 80's 85hp Evenrude on a 17.5' bass boat with an unknown prop pops right out of the water.

Again, thanks for the help.

*EDIT* by the way I took it back out today and on 4 I'm getting 4100 RPM WOT and the speedo reads 40-42. If on 4 I'm low 25% on the RPMs that would mean it is really 5125, which sounds about right. Using your formula earlier using 5100RPMs and the 10% slip that gives me 41mph which is real close to what the speedo says. If I went with a 17 pitch I think I read it usually gains 400 RPM every two inches dropped which would put me right around 5500. That same formula gives me 39-40MPH, so not much drop in top end, would just like to know how much more holeshot it would give me?

James

 

[Frank Acampora]

Re: 85Hp Force, Got her going, RPM seems low at WOT

There are so many variables in props and boats, that it is difficult to say for certain what a drop in pitch will do. You will most likely gain about 400 RPM top end and the hole shot will be better, but how much better? difficult to say. Top end may drop more than you expect, too.

It is difficult to compare the Rude to the Force. The Rude is four cylinders with about 99 cu. in. The force is three cylinders with about 74 cu. in. The Rude uses a 14 inc prop with someplace around 17-19 pitch. I don't knoe the gearing but I believe it is at least 2 to 1. While it is true that horsepower is horsepower, the way the engine develops it and transfers it to the water is different and the Force just will not keep up.

Reason pitch change may not deliver expected results: It takes a certain amount of horsepower just to turn the prop through the water. This assumes no thrust. Just turning the blades. Try to turn them faster and it takes more horsepower. The engine only produces a certain amount so anything lost to turning the prop is no longer available to move the boat forward. This lost horsepower will show as less RPM than expected and lower top speed than expected. How much? Again, difficult to say.

THUS: For example, I myself use a 2 to 1 ratio gearcase on my 140 which originally came with a 1.72 to 1. I use a 19 pitch instead of a 17 and since I am turning the prop slower, more horsepower is available to move the boat. I see 3-5 MPH more at WOT. Now, this engine is on a 21 foot cuddy. I get 38 MPH top end. When I want to ski, I drop down to a 17 pitch. Pulls me out at 160 lbs on a slalom ski, with no problem with three people in the boat. More people in the boat so I can't really compare top speed. I have never used the 17 pitch when I was alone.

Now, you already have a 2 to 1 ratio gearcase so you have no choice. If you can borrow a 17 pitch to test first, do so. If you simply WANT a 17 pitch, look for a cheap one on the auctions

 

[john from md]

Re: 85Hp Force, Got her going, RPM seems low at WOT

If you want to get up faster, you can add a set of Smart Tabs for about $125 and take off that hydrafoil. This will also give you better control in turns and the boat won't feel like it wants to roll over.

Do a search for smart tabs and you will find many happy owners.

Regards,

John

 

[NYBo]

Re: 85Hp Force, Got her going, RPM seems low at WOT

Doesn't this motor require a tach that reads 20-pole alternator output? I know my '88 motor does. These tachs do NOT have a setting for number of cylinders. Further, I don't think Bayliners of that vintage with 85's came with tachs from the factory. The factory gauges were the square-faced "Star Wars" type.

 

[john from md]

Re: 85Hp Force, Got her going, RPM seems low at WOT

Some of the old Bayliners did have instruments. Mine is an '85 and has a factory tach and speedometer. The tach requirement is for a 20 pole stator.

John

 

[NYBo]

Re: 85Hp Force, Got her going, RPM seems low at WOT

Some of the old Bayliners did have instruments. Mine is an '85 and has a factory tach and speedometer. The tach requirement is for a 20 pole stator.

John

I know by '88, a tach was only included standard with the 125.

 

[Frank Acampora]

Re: 85Hp Force, Got her going, RPM seems low at WOT

Yeah, NYBo, you got me! I was thinking old Chrysler with the 12 pole alternator when I wrote about the switch in back. HOWEVER: There are some tachs out there for combination 12 pole AND 20 pole alternators. I still would be incorrect though, because on these tachs, the "3" setting is for 12 pole and the "4" is for 20 pole.

JB: Given the error, and given the tach is only correct at low or high speed, it is time to double check the wiring. It should be getting signal from the purple engine wire. I do not have experience with Faria instruments but it is conceivable that the previous owner read instructions incorrectly and wired it differently than the engine requires.

You would be surprised at modifications previous owners do to these engines. Nothing surprises me anymore. The CLASSIC was an engine I bought as junk. When I tore it apart, the previous owner had replaced one set of rod bearings with plain babbit bearings. I'll bet the engine ran less than five minutes after that. Ruined the bearing, rod, crankpin, and cracked the block. So I would not be surprised if you had the incorrect tach OR it is wired incorrectly.

I would go online to Faria and,

1. with the model number of the tach, ask if it is the correct tach for the engine
2. Ask for wiring instructions.

Faria has a name in quality instruments. I would be willing to bet that their customer service would be very helpful.

 

[jbwillis76]

Re: 85Hp Force, Got her going, RPM seems low at WOT

Thanks for all the input.

OK, Boat is a 1987 VIP, motor is an 86 Force 85. Instruments are all Faria and look like they are original to the boat. Won't guarantee they are but they all look from that era, and from getting under the console the holes are definitely factory cut.

Tach is fed with the purple wire, plugged into the "source" post on the tach. Back of tach has a turn switch marked 1,2,3,4. It is on 4. Purple wire runs to motor terminal switch board marked tach and then to the bottom post on the rectifier.

Now I worry there's more in the engine, but swear it is giving all it has at WOT. It has to be by the way the engine sounds and runs, carbs opening, link and synced, plugs all look the same, etc, etc. But in the back of my mind I bet the tach is working and it really is 4100RPM...... Will call Faria or try new tach I guess.

Now I haven't been back to the water but if I told you why I think it planed slowly I think you would all laugh at me.............Had trim all the way out. Told you this was first boat. Bet if I trimmed in it would plane normally.

Now I did post a prop question in the prop forum and was told to raise motor one hole. I will say I can trim all the way out and never cavitate/ventilate the prop unless turning sharply at decent speed. WOT, straight away, and I can go all the way up and never cavitate. At this point I don't think I will switch props unless it is truly causing low WOT RPMS, considering the only choice out there is aluminum.

James

 

[mrdoorguy]

Re: 85Hp Force, Got her going, RPM seems low at WOT

hello i can not figure out how to write a new post. help please??

 

[jbwillis76]

Re: 85Hp Force, Got her going, RPM seems low at WOT

hello i can not figure out how to write a new post. help please??

Go back to the "Force/Chrysler Forum" page, or any for that matter. The very first threads/posts will say "sticky" to the left of them. Right above the first threads you will see a button that says "Create New Post". Click it and fill in the title and write your post. Since you were able to reply to this one you are signed in and should be able to start a new one. If you don't see the Create New Post button you will have to email a tech I guess. Good luck.

James

 

[mrdoorguy]

Re: 85Hp Force, Got her going, RPM seems low at WOT

thanks james, got it figured out.!!

 

[NYBo]

Re: 85Hp Force, Got her going, RPM seems low at WOT

Faria no longer makes a tach compatible with our motors (last FAQ on list, also check troubleshooting guide linked in another FAQ).

(BTW, I obviously had Bayliner on the brain and missed the actual brand of your boat. Sorry!)