Loss of RPM's at WOT, Evinrude 90hp 1997

[Tbagley]

Great site, I've searched for an explaination but haven't been able to locate a potential fix. It's 1997, 90 hp 60degree engine. She seems to start and idle just fine, at some point in the past, which may have been after a carb job...I just don't recall... I would accelerate and get up to about 5000rpms then the engine by itself would settle back to 4500, without moving the throttle back. Possibly related, occasionally upon acceleration before the boat hits plane the engine would seem to hit a sweet spot for a moment and really accelerate the way I recall it use to a year or so ago, an then it would back off that. This motor use to easily hit the 5500 mark and these days 4500rpm is the norm. I had found potentially a couple of minor fuel/air leaks and have repaired/replaced what ever I found. I at first thought it may be have a fuel starvtion issue.

Is their a WOT adjustment?
Might this be a powerhead/coil issue?
Could this simple be a throttle adjustment?

If you all could help out that would be great!

 

[tashasdaddy]

Re: Loss of RPM's at WOT, 90hp 1997

Re: Loss of RPM's at WOT, 90hp 1997

i'm assuming you have a Johnson. it could be as simple as a streched throttle cable, not moving the linkage to it's limit. it could be a simple coil failing, not producing spark to one cylinder. could be clogging carb jet. could be loss of compression on a cylinder or 2. also could be the bushing in the throttle linkage, wearing.

the bushing, pictured in the slot, is a 2 piece bushing, the outer has been know to crack or what ever and come off, leaving the inner. this allow play in the throttle linkage. i know this from experience on 2 motors.

 

[Tbagley]

Re: Loss of RPM's at WOT, Evinrude 90hp 1997

Thanks for the reply, I'll check the bushing when I make it back home, but I don't believe thats the case, but I could be wrong. If it were a streched cable, how is one to identify that and how simple is the cure? Even back when it would get up to 5500 rpms, the throttle handle could be push farther forward approximately 2-3 inches where the engine wouldn't rev any higher and the throttle handle just seemed to have excessive forward play.

 

[Dhadley]

Re: Loss of RPM's at WOT, Evinrude 90hp 1997

That's not a 60* motor in that picture, don't be confused. There are a couple of things that could cause what you're describing. Perhaps the prop is venting which means the rpm will go up but you'll lose forward bite. The motor could be dropping a cylinder - coming and going.

 

[Tbagley]

Re: Loss of RPM's at WOT, Evinrude 90hp 1997

Oh believe me I knew that wasn't the same as my motor as I have stared at it for hours on end, but it clearly showed his point of the bushing.

What do you mean by dropping a cylinder?

 

[Dhadley]

Re: Loss of RPM's at WOT, Evinrude 90hp 1997

You may be losing spark to a cylinder intermittently, may have a partially clogged carb jet, may have cylinder damage. A good place to start is with a compression test, then check spark.

 

[Tbagley]

Re: Loss of RPM's at WOT, Evinrude 90hp 1997

Compression test it will be for that's an easy freebie, it's also most likely time for a carb overhaul as now that I've been thinking about it, I do believe I may have ran some ancient age, oh I mean old gas at some point quite a while ago. Thanks for the ideas.

The excessive forward play in the throttle handle has always bothered me some, could that be partly due to a streched or improperly set-up cable?

 

[tashasdaddy]

Re: Loss of RPM's at WOT, Evinrude 90hp 1997

yes.