Deteriorating Performance

[iwombat]

I'm having a problem with slowly deteriorating RPMs at wot, as well as increasing problems holding idle.

We're working with a '77 85hp.

Over the summer I've been noticing a decrease in WOT by about 500rpms every 15 or so hours of use. I've also been having to adjust the idle up by increments of 50-100 at about the same interval in order to keep running. It's behaving like it's just wearing out, only really really fast. I've done regular decarbs to no noticeable effect, as well as checked link and sync and made sure the throttle is opening all the way. It's had a recent carb rebuild, and even clogged jets I have a hard time imagining would cause such a gradual decrease.

During this, the compression numbers have remained the same and while low are consistantly in the 88-91 range on what is typically a low-reading gauge.

Plugs are nice and toasty brown all the way across and all look the same.

The boat has foam blocks, and inspection reveals slightly damp, but not waterlogged foam. I can pretty safely rule waterlogging out.




Could this be a failing exhaust gasket maybe? I'm kinda stumped on this one.

 

[HighTrim]

Re: Deteriorating Performance

You're not dragging your anchor around are you, having it slowly build mud up on it?? lol

 

[iwombat]

Re: Deteriorating Performance

Hrm. Maybe I left those downrigger balls out after all.

 

[mikesea]

Re: Deteriorating Performance

I know you do alot of posting here,so you know all the answers I can think of.Are you saying that over time your losing rpm like 5000-4500-4000,so on.If so,do you have any creatures growing on your hull bottom.I know it happened to me once,didnt use the boat for about 10 days,no bottom paint,the barnicles ,though not big,caused a heck of a drag.Its happy hr.I'll grab a thinking cap.

 

[iwombat]

Re: Deteriorating Performance

It's a trailered boat, so no chance of critters unless they're invisible. Now, wouldn't that be a story!

It started out the year at a solid 5k rpm. It gradually slid down to 4.7k. Now, I've been messing with height and trim (no T&T) the whole time, so add that to the mix. I recently re-propped with one pitch smaller, both due to needing more RPMs and wanting to get an undinged prop to set it all up right. The new prop added a couple hundred RPM, so now I'm at 4.9k with a smaller prop. I've since set it back to the position I _was_ getting 5k with a bigger prop - still 4.9k with the smaller. Really though, the idle issue has me more concerned. Maybe it's finally just turned the corner.

I was already planning on either a rebuild or a repower this winter depending on what bargains I can find. I enjoy working on this stuff though, so $500 in parts and some bench time is looking like what I'll probably do. At least it'll keep me out of the bars (wishful thinking I know).

Still, there's a lot of summer and fall to get through, and I'd like to get a handle on what's really going on here.

 

[Molaker]

Re: Deteriorating Performance

Rewind it.

You don't by chance have a leak in the boat and are taking on a little water, boat getting heavier???

 

[iwombat]

Re: Deteriorating Performance

It's a riveted 30 year old aluminum boat. If it didn't have leaks I'd be surprised. Although, the leaks through the hull are pretty minimal after I did a thorough gluvit job this winter. It will take in water from a not-so-well sealed splash well. The auto bilge pump works very well and I manually run just to make sure, and no excessive water out the drain at the end of the day. Anyway, I can pretty safely rule out any weight problems with the boat itself. Even so, that wouldn't explain the idling issue.

 

[Molaker]

Re: Deteriorating Performance

Anyway, I can pretty safely rule out any weight problems with the boat itself. Even so, that wouldn't explain the idling issue.

You may be trying too hard to come up with a single cause for both WOT drop & idle problem - unless it is something like a stretching throttle cable or a wad of fishing line wrapped around the prop. You might want to concentrate on the idle problem. Lick it then move on. As for the motor just deteriorating over the season, all your tests so far are to the contrary.

I've also been having to adjust the idle up by increments of 50-100 at about the same interval in order to keep running.

Are you saying you're having to raise the idel RPM each time or is it slowing down gradually and you have to increase it again to get back where you were? Is it "in gear" idle or in neutral idle?

 

[iwombat]

Re: Deteriorating Performance

I have to raise it a bit to keep it from dying when transitioning from neutral and forward gear. It's still in the proper idle envelope, but I find it odd that the two problems are happening.

 

[Molaker]

Re: Deteriorating Performance

Hmmm. Now that's a boat of a different color...

What about the possibility of a timing change, flywheel, timing base slipping - something like that? A link & sync would not expose that, I don't think.

 

[iwombat]

Re: Deteriorating Performance

Possible the magnets have slid on the flywheel. If weights can slide on an automotive harmonic balancer, then why not.

I just wonder how I'd go about checking for that.

 

[Molaker]

Re: Deteriorating Performance

I don't think the magnets can slide. Work loose, yes, but not slide. But, it would certainly be possible to have sheared a Woodruff key and allow the flywheel to slip a little on the shaft. But, that's a fairly easy check - just pull the flywheel and check. While it's off, check the security and position of the timer base. Someone else might jump in here because it may be possible to check this with a timing light instead of pulling the flywheel.

 

[iwombat]

Re: Deteriorating Performance

Well, see that's just it. Timing is dead on perfect, otherwise I'd have pulled the flywheel long ago.

Still scratching head.

 

[itsaboattime]

Re: Deteriorating Performance

HMMMMMMMMM......I had an old 75 hp that developed a hairline crack in one of the flywheel magnets and it was enough to mess with the idle. I had to adjust it alot thinking it was the carbs. The crack also messed with my wot. Not the way you are describing, but the motor would sort of run just rough enough to not get full performance. The timing showed dead on with this problem.

I would check my magnets with a good magnifying glass.

 

[iwombat]

Re: Deteriorating Performance

a little bump

 

[Gary H NC]

Re: Deteriorating Performance

I'm interested what it is too....still watching the thread.

On mine i found 2 ground wires from the power pack to the mounting bolt on the coils were all rusted. Not sure but will find out when a splash it next time.

So far all my other tests have been good.Only thing i have not done is a carb rebuild...

Hope you find it...

 

[iwombat]

Re: Deteriorating Performance

- 500rpm/15hrs? What was I saying? I slipped a zero.

More like - 50rpm / 15hrs

So far I'm down about 400rpms give-or take (new prop complicates) over the course of the whole summer.

 

[Molaker]

Re: Deteriorating Performance

Well, heck. Back to my earlier posting. Just concentrate on the idle problem. The WOT problem is a bit too subtle to nail down. Besides, the idle problem may well resolve the WOT problem anyway.

 

[iwombat]

Re: Deteriorating Performance

Well, see that's just it. If I came at this motor fresh, there wouldn't be an idle problem. It's all still in spec.

I just know that a week from now I'll be bumping it up a tad to hold idle and so on.

It's gone from an idle setting of 600 (in gear) to about 750 now over the course of a few months. I consider 750 a bit high, but acceptable. It'll probably idle at 650 all day long, it just won't make the neutral->in gear shift at that setting.

At this point, solving the drop in WOT seems a much more approachable problem since that is actually outside good operational parameters.

 

[iwombat]

Re: Deteriorating Performance

I have a theory that maybe it's a small air leak that's getting worse. I'll take apart and re-clamp all the hoses and see what's what.