1986 90hp Mercury won't accellerate or maintain speed?

The above motor is on a used boat a neighbor bought, it came from an estate sale and had been sitting for about 6 months. It starts right up and will move along fine so long as you apply power slowly and don't go over about 3/4 throttle. I had the carbs rebuilt at a dealer, a new water pump and fuel pump installed at the same time, and it had the water separator filter and all of the fuel hoses replaced. The same dealer also replaced the coils and plugs, as well as the switch box. There is no change. He's now into this motor for over $2200 and it still won't move the boat as it should. It's also got a brand new SS prop on it which was recommended to eliminate the possibility of it being a spun hub.

The dealer says the motor is tired?
I checked the compression and it's got 90 psi on all 6 cylinders?
What should I be seeing out of one of these? One private shop tells me he thinks I should be seeing closer to 140psi?
The motor don't knock, it starts right up and idles fine and will move the boat along to about 20 mph without any issues but any more than that and it bogs down or stumbles real bad. It will gain engine RPM only if you ever so slowly apply throttle.

It runs fine on a hose and ears, but won't run right on the water.
The boat is light, maybe 1600 lbs or so afloat and not water logged or overloaded.
The boat is set up the way it was from the dealer when new according to the son of the former owner, he said it ran fine when he was last on it years ago, but hadn't been on it in recent years. The boat has a stack of repair and maintenance history all from the same dealer. The prop installed was also the same pitch that was on it, and the same pitch that was installed on it in previous repairs over the years.

I was thinking it was a carb issue till I saw the compression readings?
That and the fact that I can nurse this thing up to speed and gradually apply power, it just won't tolerate being pushed very hard without stumbling.
When it stumbles, you have to return to idle and start over trying to accelerate.

Re: 1986 90hp Mercury won't accellerate or maintain speed?

Once compression drops under 100 psi, most motors develop starting problems. A lot of compression tester gauges are suspect, verify your readings with another gauge. Consistent #'s on all cyls is far more important than the actual #, of course the higher the compression the more the motor is capable of making greater HP, but the motor should run and carry load to WOT.

Carb idle is also suspected to be too lean, While accelerating test this by momentarily 'bumping' the choke/enrichener, if performance increases, fatten the idle mixture adjustment by turning each out (CCW) just the width of the screwdriver blade slot on the needle. Repeat acceleration test and adjustment to perfection.

DVA test the high speed output of the stator, a weak voltage here will limit WOT power and RPM. Complete instructions are well laid out in the Ignition Troubleshooting Guide available for free download on the Support pages of CDIElectronics

Do NOT assume the stator is bad and order one without testing to prove any fault. There is NO RETURN on electrical parts and waaaay too much money has already been spent without seeing a lot of improvement.

Re: 1986 90hp Mercury won't accellerate or maintain speed?

Your compression is fine. Do you have spark on all six cylinders?

Stumble on acceleration is typical of too lean idle mixture as Charlie cited. The bad high speed performance could be prop pitch or bad fuel pump or clogged fuel tank vent or antisiphon valve.

What is prop pitch? If you do not have spark on all cylinders, that would explain all your symptoms .

BTW get a new mechanic. The current one is replacing stuff w/o improvement. He is either ignorant or a crook.

Re: 1986 90hp Mercury won't accellerate or maintain speed?

There is spark on all 6 cylinders.
Bumping the enrichment switch make the engine sputter and almost die when running at speed.
The prop pitch is 17 on a 13 1/4" prop and an 18' open boat.
If anything it should need more pitch. The boat is light.
The tank is new, someone had just cut open the deck and replaced the tank and all the hoses with new before he bought it. The tank vents directly to the outside of the hull, there's a loop in the hose near the vent. The vent is clear, you can hear air rushing out when you fill the tank.
The stator hadn't crossed my mind but the boat does have a battery drain issue, if it sits over night the battery will be dead if left connected but I'm not sure if anyone pursued that problem yet.
In my opinion it needs a new motor, 90 psi sounds low to me. I rechecked the compression myself with a new digital gauge from Mac tools and got the following from top to bottom: 89-92-91-101-98-90. I ran it with the hood off, when it starts to act up, the carbs start to spit fuel out the front, it coughs and bogs then looses speed on the water. It rarely stalls completely but does on occasion. The two lower carbs spit more fuel than the top one. The carbs were supposedly new, he's got the old carbs, they were replaced by the last owner.
These carbs have red tamper paint sealing all the plugs and adjustment screws.
One thing I did notice is that if I manually reach back and open the throttles on each carb by pulling on the linkage while it's running down the river, the motor revs fine and the boat takes right off. If I use the throttle handle it bogs and carries on.
Could I be dealing with a timing issue? It seems to run best if I leave the throttle just past forward engagement and work the carb linkage by hand.

Re: 1986 90hp Mercury won't accellerate or maintain speed?

Sounds like a link and synch is in order. Also sounds like a decarb may bring up the compression. The fact that the compression is fairly even means the motor is servicable.

Does the throttle handle open the carbs all the way? If not that is definately a problem.

Re: 1986 90hp Mercury won't accellerate or maintain speed?

The carbs are fully open at full throttle on the stick, but the motor won't run right there with the stick all the way forward. It'll only accelerate if I leave the throttle handle just barely forward. At that point it runs really good, better than I would expect. The carbs all open the same amount and a flow meter reads the same on each carb.
The owner is getting frustrated with the boat, he's out looking for a new motor already. I got the impression that the next move will be to hang another motor on it.

I pretty much eliminated a carb issue since it will run fine working the throttle by hand at the motor. I'm still thinking that maybe a wire or advance problem. The fact that it draws the batter down also has me thinking now. I did check battery voltage and it barely maintains 12.3 volts running, but it never runs the battery down any further while running.
The motor is super clean, it could pass for new, but it's had far too many hands in it over the past year or so. I was comparing the old carbs with the new ones and they are different as well. The new carbs have little hose barbs on the port side where as the old one's did not. These are left unconnected for some reason? The old carbs didn't have these at all.

Re: 1986 90hp Mercury won't accellerate or maintain speed?

Those barbs may be for hoses to connect the venturis to the carb bowl covers on the the backdrag carbs. I recommend you check link and synch to make sure max timing is set properly as is idle pickup timing.

Re: 1986 90hp Mercury won't accellerate or maintain speed?

I kinda ditto on the timing issue (Max advance timing out of wack).Needs to be set properly or you will trash this motor quick.Timing and carb sync is very important,also check to see if your recitifier is blown.Your battery should show charging when running (12.8 to 13 plus at higher rpms).Does this motor have the recitifier/voltage regulator or plain recitifier?

Re: 1986 90hp Mercury won't accellerate or maintain speed?

I have a 90 hp that did the same thing new switchboxes coils stator and recitifier I changed all of this keep doing the same thing brought it to a friend that works on outboards he changed the triger it runs great now.you do need a decarb to bring up compression on that motor .look at my thread need help with motor. It has a lot of info. Good luck

Re: 1986 90hp Mercury won't accellerate or maintain speed?

My neighbor who owns the boat made things easy tonight, he came home with a brand new 115 in a crate. The old motor is coming off and he's taking it to his 'mechanic' to have the new motor installed. If I pull the motor tomorrow, it's mine.
I have nothing that it fits but I guess its my problem to fix now, but it won't be on a boat after tomorrow. I'll run some sea foam through it before pulling it in the AM, and recheck the compression though. I can rig it up on a stand to mess with it here I guess after that.

The open barbs have me wondering as there are plugged holes in the carb bowl caps, which are different than the original carbs. The original carbs didn't have the plugged holes nor the barbs on the side of the carbs. I'm not even sure if the original carbs are bad, I may just reinstall those and see what it does.
I bought a Mercury spark tester a few years ago, I am able to check for fire on all 6 cylinders at one time without damage to the electronics, all cylinders have good spark, and the fact that it revs fine when I pull down on the throttle linkage by hand, with the lever at near idle speed, tells me it don't like the advance its seeing when the throttle opens by the cable. By pulling down on the throttle linkage, only the carbs are opening, the advance or stator don't move.
Is there a parts break down online for these carbs? I'm wondering now if they even belong on this motor? Something else that a truck mechanic buddy mentioned was that if the carbs are wrong or messed up, maybe the low compression is due to washed down cylinders?

Re: 1986 90hp Mercury won't accellerate or maintain speed?

I sure would suspect the rectifier. A shorted diode can cause a high load on the charge coil and affect the ignition circuit performance. You mentioned the battery going dead overnight if connected. A shorted rectifier can cause this. It may even warm up the flywheel until the battery goes flat! Try running it with the rectifier disconnected and the charge coil left open circuit...

Re: 1986 90hp Mercury won't accellerate or maintain speed?

I had a problem with my high speed wires in the stator on my '85 90hp Merc. Symptom was that it would simply not get above 2500 rpm. Once changed, the motor has run flawlessly. Once you find a mechanic who is willing to do some diagnostic work, you might check into that.

Re: 1986 90hp Mercury won't accellerate or maintain speed?

schematic said:

I sure would suspect the rectifier. A shorted diode can cause a high load on the charge coil and affect the ignition circuit performance. You mentioned the battery going dead overnight if connected. A shorted rectifier can cause this. It may even warm up the flywheel until the battery goes flat! Try running it with the rectifier disconnected and the charge coil left open circuit...

Click to expand...

Disconnecting the rectifier takes the draw off the battery, but it don't run any different, but I'm only running it on the ears now. The boat got a brand new motor yesterday.

It revs to the moon if I let it, it just won't take throttle if I don't baby it. The motor tops out around 5400rpm at wot if I work the throttle by hand when on the water. On the ears I wouldn't even try for WOT.
I took the second set of carb apart and it looks like their a combination of new and used parts.
The carbs on it now have new bodies, but the bowl covers are old. It looks like they took the new carbs and swapped the bowl covers on two carbs and left the top carb alone. I pulled the second set of carbs completely apart and found plenty of green brass and ethanol damage.
I ordered three carb kits and plan to start with putting the original carbs back on the motor. I have no idea what is on it now, or why anyone would swap parts between new and old carbs after spending big bucks for new carbs.
After seeing some of the things his 'mechanic' did, I changed my mind. He shouldn't be fired, he should be shot. I got a bag of spare parts, what bothered me was why I had a new impeller and kit box, but no gaskets. I pulled the lower unit, the guy used the new housing, gaskets and plate, yet put the old impeller back in????
He told the neighbor it wasn't worn out yet and still had some use left.
If that wasn't bad enough, he used blue RTV on everything, including the lower unit to mid section. I didn't even know they still made that crap.

He installed a new prop, but never cut all the wound up fish line off the shaft and he used red Locktite on the prop nut and reused a tab washer with all the tabs broke off.
Now I remember why I never took my boat or motor to anyone for service.

Something that has me wondering now is if the carbs on it now are even for a 90hp?
Also, why one odd carb? (He only swapped the bowl covers on two carbs).

Re: 1986 90hp Mercury won't accellerate or maintain speed?

I have a 1986 115hp that has barbs on carbs with hoses hooked up to them. I have not really looked at were they go but it sounds to me that the carbs need to be set up under a load you cannot tune that engine on a garden hose it has to have a load. Mine had problems simular to yours I had to regasket the intake side of engine and left boat on trailer to tune carbs.

Re: 1986 90hp Mercury won't accellerate or maintain speed?

Hello and I am new to this site. I have been following this thread regarding the 90 hp Merc and the inability to "open up".
I was just made a gift of a boat and trailer with a 90 Hp merc.3 cylinder engine ( I so far assume that it is around a 1989/1990 year model). The original owner had not used it for many years because of its inability to accelerate to higher rpms.
As best as I can learn is that this problem just happened and after the repair bill exceeded $1,000.00 the boat was left in the garage for many years.
One report was that the boat was beached for a lunch break and then forgotten. The surf beat the boat up and soaked the engine. Supposedly the engine problems started after that. I dont believe that I can get more history than that. Other comments were that it would start easily, run smoothly to a slightly higher RPM but would falter when the throttle was opened up.
I wont be able to get the year and serial # for a week or so so I have held up trying to buy a maint manual. Is there a preferred manual that i should
seek? And doesnt this sound very similar to the recent 1986 90 hp thread from vintageglass?
Frej