Help ! Mercury 90 surges mid-rpms in rough water

[Deas]

This is a strange problem I have searched the posts and found similar cases but nobody finished the posts stating what it actually turned out to be...I have given a lot of detail here to explain what happens...<br /><br />The boat starts fine every time. It will purr like a kitten and run perfect for ever until I run it in 3-4+ foot waves for a while. Then it starts surging. In rough water I will run about 3500-4000rpm for the perfect speed for the waves. The rpm will suddenly drop from 4000 to 2500 (I call this stage 1 of the problem). The boat will drop off plane, so I punch it and RPM will go up to WOT 5000 or so, then I slow back to 4000 to avoid launching off the waves and it surges down to 2500 or so again, and on and on. <br /><br />After about half an hour of this it may reach what I call "stage 2" where it wont ever go past 2500rpm, I can punch it to WOT and it will go from idle to about 2200-2500 and hold and hold but no more and will not get on plane at that RPM. In a rare case it will get on plane, then it will power up and run at full speed OK as long as I do not reduce throttle down below 4500-5000 rpm. <br /><br />It will always start fine, under both stages of the problem. On the next day water is calm again or even moderate chop and the motor will run fine at all rpm ranges all day.<br /><br />This is a 98 Mercury 90HP with low hours, well serviced over the years. It has a water separating fuel filter, no water detected, and I replaced the filter element too. I replaced the switch box that did not fix the problem. I do not have a dealer in my town here so your help would be greatly appreciated.<br /><br />Cheers

 

[daza1]

Re: Help ! Mercury 90 surges mid-rpms in rough water

Gidday m8, is your 90hp 3 or 4 cylinder?<br />I have a 115hp 4 cyl, which surges a little between certain revs, which is due to it swapping (in lower rev range) between 2 & 4 cylinders. So you can troll at low revs with just half the motor running effectively, which creates a surge in the rev range & speed.However my motor is due for its 1st service next week and i think the carbs may need tweeking., which may also be the case for you?? I,d look at carbs, fuel, filters etc,,, + check how much oil is going through premix etc,, i assume its a 2 stroke?? <br />Hope this helps

 

[Deas]

Re: Help ! Mercury 90 surges mid-rpms in rough water

It is a 3cyl 2-stroke. The carbs were all done about 1 month ago and the filter is new.

 

[mccrea]

Re: Help ! Mercury 90 surges mid-rpms in rough water

im having the same problem with a 135 (1992)carburated and it can be just at the dock not ocean waves and if i load it in fwd with 2400 rpm it runs fine but at full throttle it stays in the 2400 range. not fuel or carbs i suspect it is the high rev stator box as plugs are getting spark and just cant do high rev at load. with no load in neutral i get 5000 rpm no problem. sorry if this interupts your post but im checking old posts first before posting/ mike

 

[fdfairln]

Re: Help ! Mercury 90 surges mid-rpms in rough water

I have a similar problem. My 3 cylinder 1990 mercury 90HP runs fine at idle and WOT. It will run fine for a time at any mid range throttle position, but occasionally with a throttle position which is giving me 4000 RPM, the engine surges between 3000-4000 RPM. It will continue to do this as long as I am willing to allow it to continue. Changing the throttle position will stop the surging ie WOT or idle, but the problem will eventually return if I try to run at 4000 RPM. So far, it is an intermittent nuisance, but I am growing weary of it. This motor will run at WOT all day long without a hiccup, but I don't like to listen to the noise or burn all that gas. Since I purchased the boat 3 years ago, I have accomplished the following:

1. Overhauled all three carbs.
2. Changed the spark plugs.
3. Replaced the fuel filter.
4. Replaced all fuel lines from the tank all the way to the carbs.
5. Ran several cans of sea foam through the engine.

 

[Traceyh]

Re: Help ! Mercury 90 surges mid-rpms in rough water

It is a 3cyl 2-stroke. The carbs were all done about 1 month ago and the filter is new.

Did this problem occur with the carbs being redone? It sounds like a high float level allowing to much gas in the carb bowl or a faulty float needle ans seat not sealing properly, then getting "splashed " up through the main jet flooding the cylinder.

 

[191Seanymphstriper]

Re: Help ! Mercury 90 surges mid-rpms in rough water

Ok! Update! I purchased new plugs today for my 3 cyclinder 90 hp mariner and i tried to buy 3 new coils and the dealer only had 2! I replaced 2 coils and 3 plugs and cleaned the wires! Put her back togeather and she ran better then she ever had! Prob do to the wires being dirty. So im not sure if it was the plugs or the coil packs but at 30 bucks a coil I said im changing all 3. So when the other one that is on order comes in I will replace that as well.... The motor was acting like it was a fuel prob at first. It was bogging out at 3k or 4K rpm Then when backing off at 3K it would come up to 3500 or 4K rpm but if you gave it gas to go past 4K rpm she would die out and bog down to 3K or less etc...... My recomendation to anyone having this prob with this motor is! Make sure you have a fuel water seperater hooked up. Then on the side of the carb bowls there are big screws to drain the bowls! Drain them! Not the screws with the springs on them. The lower larger screws! Then hook up a differnt external smaller tank and differnt bulb. try that! If it still presits... Change the coils and plugs. Cheap enough and next time i will buy new wires as well. I still might order them! But she runs great! And better then ever. I hope my experience helps someone out there! Good dat!!!!!!!!!!!!!!!!!!!!!

 

[191Seanymphstriper]

Re: Help ! Mercury 90 surges mid-rpms in rough water

Update problom came back! Found it was the diafram in the fuel pump. $3 fix. Now it purs. Hope this helps

 

[191Seanymphstriper]

Re: Help ! Mercury 90 surges mid-rpms in rough water

This is how I found the problem...


Ok listen. Put the boat in the water on your trailer with all trailer tie downs still locked down at the transom and bow of the trailer locked. Back her into the water so the motor when tilted down is fully submerged and able to pump water. Or if you keep it docked just tie up real good. Make sure your parking break on the truck is on good! But in reverse the thrust is less so you wont have a issue!


Start the boat. Rev it up and get her in reverse at 1700 rpm. With your air breather off of all the carbs close each butterfly one by one with your hand by sticking your finger in the carb and close each one and see if it bogs down on each cyl while the boat is under load at 1700 rpm in reverse.


They each should bog down the motor!


If you close one butterfly and dont here a bog or takes longer then others to get boged thenn that cyl is either getting to much fuel so the cyl is flooded or barly sparking and working less causing your ruff idle.


Now this can mean a couple of things. If the fuel pump is conected to the cyl that takes longer to bog its the fuel pump. because whats happening is the diafram in the fuel pump is warn punctered etc.

And what will happen is that cyl will draw alot of fuel flooding it causing the ruff idle and low spark bc of the flooded plugs.

Just because the fuel pump is shot does not mean that the other cyl arent getting fuel and wont get fuel.


It just means 1 cyl is getting to much fuel....


Or that cyl that takes longer to bog or dosent bog at all may have a bad plug or failed plug, or a bad coil, or a bad wire that goes from the coil to the plug.


Now these are the quickest easyiest ways to fix and try to solve the issue. Do these techmecs before you start ripping the carbs off...


""""(Also when the fuel pump is removed from the engine, start the engine with all hoses disconnected etc just as you disconnected it. And give some throttle and let it come down in idle and put in nuetral. Now if it is the pump she should idle in nuetral because now the 1 cyl is not getting extra fuel and the fuel left in the carbs will even out and should idle for about 2 min to 5 min,,,,,)""""""

"""""(Now if she dosent idle without the pump in then its not the pump!)""""


Now if you change the difram in the fuel pump and you change the coil, plug, wire and you still have a issue with that cyl then take all the wires off the plugs and do a complete comp test and be sure you have good even comp. Anything within 15 psi of each other is still considered good.

So if its 115 120 110 105 its still good but will give you a little uneven ness but not enough to sound like a uneven idle or be an unevian idle.


So if you check compression and its all good..... Then take that carb yjay bogged slowly or not at all off and rebuild it and put it back on.... Then it should be fixed..... You may find debri etc or signs that moisture may have bean in it etc.

Let the carb soak excluding the float in (SEA FOAM) (a product to clean fuel systems) for about 3 hours as long as all plastic pieces are off yje carb. Then rebuild the carb!


Those are the only things that can cause the un even idle....

You now at this point after doing this have checked and repaired spark,fuel,air, carb all the things you need to make her run... If you don't know how it works you wont be able to fix it!


Unless your stater is no good which if it is that would be most unlikly and it wouldnt even start at all. Or a elec short which is all uncommon


Follow these steps and trust me youll find the issue...


Dont start drilling your carb etc etc ...... Just not a good idea with debri shavings etc..could cause more problems.. ( No offence to Daviet at all) (I Just wouldnt do it at all personally!)


Get a very small hook pick and a small strait pick and get it out... And again No offence to daviets advice at all but I just wouldnt do that personally! Daviet may be a very experienced mechanic for all we know and he may have a very steady hand!

You can do whatever or however you would like to get it out! But just be carfull if your drilling


And tell me what happens...


Also your phone number was edited out and you cant post your number directly on the thread. You can click on my screen name and send me a P.M (personal Message) with your number and i willl walk u threw it again!


Hope my advice and daviets advice helped!

 

[Bob_VT]

Re: Help ! Mercury 90 surges mid-rpms in rough water

WHY are we answering a 5 year old post ???

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