[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...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. 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. 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.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.Cheers

[daza1]

Gidday m8, is your 90hp 3 or 4 cylinder?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?? Hope this helps

[Deas]

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

[mccrea]

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