86' Mercury 90Hp Inline 6 only 2200RPM's at WOT??

[MrMikeTN]

I have a 1986 90hp inline 6 serial #0a935296 elpto.

The motor has always been what I call " Hard to start Cold" If it sets for more than a 2 weeks without running .
But feed it the Choke and a slight Throttle and crank it a minute and it starts.
Idles I think Very smooth, and stays smooth when in gear and under load.
When I got the motor I knew it started good, sounded good, but never would get the RPM's up where they should be when
your in Gear and past Half open Throttle or Wide open Throttle.
But it was still impressive how well it pushed the Small boat around that it was on.


I took the boat and had the complete trim unit,Motor,throttle/gearbox, and wiring harness removed and placed on my 30'Pontoon boat.
I also bought new fuel tank,fuel lines,filters, steering componets and throttle and drive cables and a Faria Tach at the same time and had them all installed. Plus had the spark plugs and wires changed, new water pump impeller, lower oil changed, carbs rebuilt/cleaned syncronized.

When I got it out on the lake it still had the same problem of not getting above what the Tach says is 2200-2350RPM's.
Now before this we never knew what the Rpms were due to there was no Tach.

I started to diagnose the problem by properly adjusting the throttle and drive cables that go into the Motor housing,
But the problem was not there.
I then Tested the compression , Very good on all 6, slightly above 120, no more than 124-125.
Changed the plugs again, changed the fuel filter again, and we took it out and did the final adjustments to the carbs so that it runs great, UNTILL you want to go above 2200-2400Rpms.
at about 50% thru the throttles travel (while in gear and under load) the Rpms dont climb any higher, even if you push the Throttle to WOT, it does almost very slightly kind of choke it or very very slightly seems to bog down when your at WOT to about 2125RPM's, If I pull the throttle back just a touch it goes up to 2200-2350RPM's.
If I run like this for more than 30 minutes at WOT the motor will suddenly drop Rpms to almost a fast idle and sounds like its out of fuel, If Pull the throttle back to netural and let it run for about 15 seconds and depress the "NETURAL button" and lightly fast idle the motor to about 1600Rpm's and then back to Netural Idle, I can push it right back into gear and go right back to 2200Rpms for maybe 30-45 minutes, I usually dont ever run it for more than 45min to an hour because thats about how long it takes me to get to my next fishing hole.
Now it will REV up past 4500RPMs and probably 5000 when your in netural and you give the throttle a Quick rap.
Not that i have a habbit of doing that but I wanted to see if it if it would.

I started reading and first tested the ignition, the stator, the trigger, rectifier, both switch boxes as the manual and trouble shooting guides from mercury and CDI prescribe.
Im gettin spark to all 6 cylinders, I have not used my FAKE Sparkplug with the adjuster on it to measure the length of the spark.
I have not done the DVA test. I AM going to build one Tomorrow, I have a Fluke meter Thats True RMS 186 i think.
Everything checked OK.. Excluding the DVA test...
But a friend (mechanic) suggested I inspect ALL the wiring very closely.
I didnt like the overall look of the stator, starting to have degraded jackets on the stators wires and the trigger also, So i figured might as well buy a Stator, Trigger and Rectifier and install those parts and maybe itll run faster like it should.
And I also swapped the Switch boxes with each other.
AND I have not replaced any of the coils.
yes we checked TDC and timed it afterwards, its set excatly where it was, and that agreed with what I read in the book.
SIDE NOTE:::Replacing the 2 SWITCH BOXES (Friggen very expensive) is why I didnt buy them.

WELL IT DIDNT DO ANYTHING TO IMPROVE MY PROBLEM. Runs slightly smoother, Not that it ran rough before, but still nothing above 2200Rpms.
I have been running it like this about 2-8 nights a month when we go out fishing. its been this way since late last year.
The Stator I bought IS USED,, But it came from a boat mechanic shop, and they tested it before selling it to me and said its in perfect working order. It was actually in the original box, looked perfectly new and in a locked cabinet that said Mercury Test Parts.
It was one of their "TESTERS", only been used for maybe 2-5 hours over the last 3 years. They no longer are working on the Older Mercs and did not need it anymore.

So now to my QUESTIONS...
Could it be the Power Boxes that are giving me this problem??
Or maybe I need a Brand New Never seen daylight Stator? Maybe the High Side of this Stator is bad also????
Have I done the correct steps in diagnosing and fixing this problem? Excluding the DVA meter test that has not been done, nor measuring the length of Spark coming out of the plugs.

What do I need to do to fix this motor right so it will preform correctly.

i cant be too far from having it fixed,, or am I way off base??

Thanks for ANY help in advance!!!
Mike Jenkins

 

[Laddies]

Re: 86' Mercury 90Hp Inline 6 only 2200RPM's at WOT??

You need to test the system with a DVA at idle and at WOT to see if one of the high speed windings in the stator is bad, the place you bought the stator obviously don't know how to check the system or the would not need to switch parts to find out whats wrong with one.

 

[MrMikeTN]

Re: 86' Mercury 90Hp Inline 6 only 2200RPM's at WOT??

Am I correctly thinking that for me to test at WOT Ill have to take it to the lake, Probably leave it tied down to the trailer and back down the boat ramp, sumberge the motor foot a little deeper than it would normally be so It will get PLENTY of water???
This is not a problem, I live within 3/4 of a mile from a huge public boat ramp.And my trailer is all but new and Way heavier duty than I need and it has a very good tiedown system.
Also I do NOT want to test in gear or ever have the motor above 1200Rpms when im hooked up to my Water hoses and Muff..Correct???

I plan on doing this DVA or Fast Peak Voltage testing either tomorrow night or maybe sunday night before as I go out fishing,, Just depends on the weather.

 

[Chris1956]

Re: 86' Mercury 90Hp Inline 6 only 2200RPM's at WOT??

OK, you took it off a small boat and put her on a 30' toon? Did you change the prop? What prop do you have on her? If the pitch is too great, she will likely only go a little faster than idle.

 

[MrMikeTN]

Re: 86' Mercury 90Hp Inline 6 only 2200RPM's at WOT??

it was on a 17foot fiberglass ski/cruising boat,
It was doing the excat same RPM issue on that boat.
So I didnt equate the prop to be the problem.
The previous owner said he never had any problems from it except a little hard to start sometimes,
I had been out with him in years past and that motor would make that boat fly like hell.
THEN 2 yrs ago when he took it out of the garage and put it in the lake it just started not going to its full rpms.
He is also the original owner since 1987 when he bought the motor NEW and had it installed on the 17' boat.
It has a stainless prop that came with it and is still on it.
The prop info : QuickSilver 48 72762 A5 19P...... 3 blades,, looks to be maybe 12" or 13"

Since I put the Merc on My 30' Ptoon I have also added a generator, 10'Canvas cabin walls, and alot of other weight and things that should slow the boat down.
But fully loaded and into a 4mph head wind, going up the Tn River my GPS and Speedometer show my boats doing 13-14mph at 2100Rpms the way it is.
I thought about getting a 4 blade prop, but not untill I solve the RPM issue with the motor.

 

[Laddies]

Re: 86' Mercury 90Hp Inline 6 only 2200RPM's at WOT??

You should have no problem doing the DVA test in the river on the trailer just back it in far enough for the water pump to work, warm it up put it in gear and have someone advance the throttle while you do the test as the RPMs increase the high speed windings should increase voltage as the low speeds will decrease the voltage.

 

[narvlebenoit]

Re: 86' Mercury 90Hp Inline 6 only 2200RPM's at WOT??

I have a 90 to did the same thing with RPMs changed new switchboxes coils stator and rectorfire and did the same thing brought it to a localmech. changed the trigger know it runs at 4800 rpm with a 19 pitch prop on a 17 foot starcraft.

 

[MrMikeTN]

Re: 86' Mercury 90Hp Inline 6 only 2200RPM's at WOT??

I have a Brand New still in The BOX Original OEM Trigger assembly.
The one thats on the motor now supposedly tested good last year when I had a mechanic buddy of mine doing the testing while I was reading the directions to him.
Its wasnt very hard to change that one out.
Im seriously thinking about just buying 2 new switchboxes, new Stator, and 6 coils,.
Im still going to do the DVA tests before I do anything else. I want to know what is wrong with it first.
Then change all those parts out for new ones so that I eliminate any old parts causing a new one to go bad.
Its been very frustrating.

 

[Faztbullet]

Re: 86' Mercury 90Hp Inline 6 only 2200RPM's at WOT??

Sounds like one side of the stator has failed on high speed side...Easy way to test is with timing light and check and see if spark drop on a switchbox when it hits that rpm as that about where internal rectifier flips to high speed side..

 

[muskiemike12]

Re: 86' Mercury 90Hp Inline 6 only 2200RPM's at WOT??

You should be running a different prop also. A 9 pitch BlackMax with the extra cupping would be about right for that much load. Could give you 2000 rpm's by switching. What pin setting do you have your tach set on?

 

[MrMikeTN]

Re: 86' Mercury 90Hp Inline 6 only 2200RPM's at WOT??

Im not sure about the PIN setting. I had it installed. We tested the accuracy of the TACH by using a handheld Tach a few times and it seems to be right on.
If the problem was an issue of the PROP pitch or size then I wouldnt think the motor would of had the excat same problem on the original boat that it came off of, that boat was only 16 foot long and weighed MUCH less.

 

[MrMikeTN]

Re: 86' Mercury 90Hp Inline 6 only 2200RPM's at WOT??

Well I went ahead and bought a new stator, new rectifier, 2 new switch boxes, new trigger, 6 new plugs, 6 new plug wires, i know the coils are good, took each wire off the motor and tested continuity, tested all wires I possibly could with a Megohmmeter, cleaned the ends, tightend each one back down, retested continuity, triple checked wiring diagrams against how it was wired and what 2 different manuals say the wiring should be, installed all the parts I bought, and I purchased a fuel pump rebuild kit just incase, AND My DVA adapter FINALLY came in late Saturday, lol , I installed all the new parts THURSDAY AND FRIDAY !!, go figure!!
lol I KNOW i PROBABLY should of waited untill my DVA adapter got here and tested all the old parts, but I cant see how ALL new parts can really hurt, Hopefully I wont have to use My DVA unit due to the Motor Has NO REASON to not go above 2100-2200 rpms with all the new parts, fresh filters, fuel lines,... but I am going to use the DVA to test when I put the boat in the water, and then as I have a buddy drive it. Maybe if there is another problem ILL SEE IT using the DVA and not fry one or more of my new parts. Ill let yall know Monday or Sunday night if it runs above the WOT of 2200 rpms.
If it dosent i think ill use it as a backstop for my .338 Lapua..

 

[MrMikeTN]

Re: 86' Mercury 90Hp Inline 6 only 2200RPM's at WOT??

Well, it was ONE of the switch boxes. BUT i still changed everything. the motor Now runs like a MONSTER.
I think im gonna have to drop the motor 2" more into the water, between 4000 and 4500 rpms it almost sounds as if it not in the water enough unless i trim it almost all the way down.. Never had a pontoon over 30foot that will pull a nice HOLESHOT, this thing gets up and goes!! cant get over 4600 rpms but im sure thats due to the prop, and it moves this big pontoon faster than I really ever need to be moving at 4500-4600rpms just fine.
Especially untill I figure out how to lower it 2" more. Looks like Ill have to refabricate the rear a little and drill new holes in the transom.
Or maybe a different prop would solve the problem and I wont have to lower it.
Im going to borrow a prop with less pitch and give it a try.

 

[mr 88]

Re: 86' Mercury 90Hp Inline 6 only 2200RPM's at WOT??

When your boat is sitting still in the water where is your cavitation plate in relation to the water line.With a "normal" hull you would line up the bottom of the boat with the cavitation plate. If fairly even you should not have to lower it any more.The biggest concern is that your water inlets are drawing water and if the tell tale is working you will be good to go, as long as you have water streaming out.Some will raise the motor as much as possible to cut down on the drag caused by the lower unit,but you have to make sure the inlets are drawing H2o or you will fry her.

 

[narvlebenoit]

Re: 86' Mercury 90Hp Inline 6 only 2200RPM's at WOT??

Glad to see you got it MrMike I love the sound of the inline six their great old motors just make sure shes pumping water and I think a 17 pitch prop will do it.

 

[MrMikeTN]

Re: 86' Mercury 90Hp Inline 6 only 2200RPM's at WOT??

if you were to lay a straight edge across the bottom of the toons it crosses the outboard 1/4" below the top of the prop.
The cavatation plate on the motor sits just under water when I have a full load of gas (18gal port tank, 10gal starboard tank) , also back there I have 2 - 57lb batteries. The motor gets plenty of water and "pees" a very strong stream fast after starting the motor and while running at any speed.
When Im moving , the faster I go the deeper the trough becomes around and under the cavatation plate on the motor as the water comes out from under the transom, so technically the cavation plates level is under the level of the water, but a void, valley, or trough forms and unless I keep the trim all the way down i can hear it start to suck air thru the prop above 3800 rpms, when this happens the rpms do not increase or jump , its not sucking enough air to slow the boat down or race the rpms. But I really dont want it sucking any air,, RIGHT?
My thoughts were to use a "REVERSE" perminate Jack plate maybe and lower the motor 3" - 4" , this would also help with my full UP tilt problem, I cant 100% tilt the motor up or it hits the bench seat that covers the fuel tanks, Batteries, and the access area to the motor from onboard. The jack plates I have seen "EXTEND" or relocate the motor a few inches reward and could easily lower it also.

I have no idea what would be needed to modify the transom to stop the valley effect, so I figure just lower the motor..
Possibly a slightly less pitched prop?? or one of those 4 blade props for larger pontoon boats?

Took it out all yesterday evening and last night untill 05:30 this morning, Ran it flat out WOT up river for 1.5 hrs non stop, most of time water was smooth to light wind chop, had to cross some big wakes and waves 5 or 6 times and the rpms never really jumped and I didnt hear it cavitate wildly, just a little cavatation on the big 2 - 3 foot swells/wakes/waves. This Riveria Cruser pontoon is so heavy it just plows right thru.
AND HONESTLY ,, I do 80% of my boating at night when I go Catfishing, I got no business running 24-26mph at night.

And I know what you mean by the deep sound from an inline 6 Mercury.I love that sound too, and if you got the rpms wound up it gets quiet enough to carry on a conversation.
I got into one of the big marina's about 20 min before a huge crowd of boats did about 45min before dark, got some ice, checked the motor over, and when I touched the key and it fired up HEADS TURNED, 3 smaller pontoons (2 with larger HP motors than mine) left the Marina and went down river 5 minutes before I did and I had no problem catching and passing them within 15 minutes, 2 of them stopped where I stopped to fish to ask me what size and year Mercury I had and about my lighting system.

If anyone has an idea/advice about lowering the motor or to not lower it please advise.
And some firsthand knowledge on prop possibilities would be appreciated too.
ive read alot of the online prop sales and manufacturers info, but they all promise everything like a used car salesman.

 

[narvlebenoit]

Re: 86' Mercury 90Hp Inline 6 only 2200RPM's at WOT??

MrMike I have a 150 johnson on a 18 foot startos and it was cavatating took the prop to a prop shop here in Louisiana and he put some cup in my prop it help a lot only coast 30 dollars.

 

[MrMikeTN]

Re: 86' Mercury 90Hp Inline 6 only 2200RPM's at WOT??

Adding cup.. hmmmm never had any experience with customizing a prop? Might be a option.. Thanks!!

 

[MrMikeTN]

Re: 86' Mercury 90Hp Inline 6 only 2200RPM's at WOT??

I hoping to find a Prop Dealer within 50 miles that will let me try out a few props before I buy. Ive read that alot of them will just take a full deposit and let you try it out for a day to see which one works best.
I really been reading and studying about a stainless 4 blade 12.5" or 13" Dia. 15 or 17 pitch prop. It sounds like it has all the benefits Im looking for, I just dont know enough about the best pitch for my boat to raise the WOT RPM's a little and possibly decrease any small cavatation issues I may have.