Evinrude 90 Hp spl. LOST COMPRESSION PLEASE HELP!!

[kobx]

I just bought my first boat. A 98 Aquasport 175 w/ 98 Evinrude 90hp SPL. I checked the compression on it, and all cylinders read 120-125 psi. The water test went flawless, so I made the jump and bought her.

The motor had a 15" prop on it, with some damage to it. At WOT, I was running around 5000-5500. When I brought the prop in for a refurb, I was talked into a 17" prop. I figured no big deal because I have read others running with a 17" on the same package.

So I took the boat out today, and she ran awesome, except my tachs were reading 6500-7000. I figured WTF? How could that be? If anything my RPMs should of gone down with a more aggressive prop, right? The way the tach was bouncing/floating around, the fact that it would drop whenever I hit the trim (but I certainly didn't hear the motor dropping), and the fact that the motor didn't sound like it was screaming, I chalked it up to a inaccurate tach.

Well, after about a total of 20minutes of WOT (broken up by "no wake zones") I went to pick the wife up at the ramp. After she got in, I noticed the motor wasn't running right at idle. It was all jerky/sort of sputtery, and wouldn't go over 3k at WOT. To make a long story short, I checked the plugs, and one of them was closed shut (no gap at all). I thought I found the problem, except when I checked the compression in that cylinder, it now only reads 90psi.....:facepalm: As an FYI, I did hear something "drop" in the motor when I was idleing up to dock to pick the wife up. Almost sounding like a screw fell or something.

So, why would my RPMs be higher with a more aggressive prop, and what could of came in contact with the plug to close the gap, and what is the corelation to loss of compression.


Thanks for you time,
Ken

 

[Bosunsmate]

Re: Evinrude 90 Hp spl. LOST COMPRESSION PLEASE HELP!!

Hi Ken,
ive no idea why your rpm increased with the increased pitch prop.
In regards to the sparkplug damage, try putting another cylinder to top dead centre, feel it there with a screwdriver and see how it shouldnt have any movement (or virtually any movement) when you push with the screwdriver.
Try that again with the ill cylinder, see if you get a different amount of movement.
Put a bit of cloth on the screwdriver to protect the piston
You are checking the wristpin and main bearings by doing this

 

[metriccrescentwrench]

Re: Evinrude 90 Hp spl. LOST COMPRESSION PLEASE HELP!!

Just a guess, but I'd think that the two problems are unrelated. I'd suspect a charging system (regulator/rectifier) problem caused the erratic tach reading, that will make the tach act exactly as you described. I can't believe for an instant that a 90 with a 17 pitch prop could hit 7000 rpm.

As far as the piston hitting a spark plug and the loss of compression, well, unfortunately that problem sounds like the real deal and you've got good advice on where to begin with that.

 

[V153]

Re: Evinrude 90 Hp spl. LOST COMPRESSION PLEASE HELP!!

You've got bigger problems than a tach/rectifier "isuue". Sounds to me like the motor ate a ring, or worse mebbe connecting rod bearing. When yer plug got schmushed it was likely due to a broken piece of ring/bearing getting pinched between it'n the piston. Wouldn't by any chance be #4 cyl would it? (That's where mine always take a dump ...)

Not to sound patronizing, but it's been my experience that when a motor starts runnin like crap? It's best to shut it off'n find out why. 'Fore ya blow it up ...

 

[pn]

Re: Evinrude 90 Hp spl. LOST COMPRESSION PLEASE HELP!!

here's my guesses, spun hub on the new prop which led to over rev/heat on that cylinder melted electrode. scored walls and rings

 

[kobx]

Re: Evinrude 90 Hp spl. LOST COMPRESSION PLEASE HELP!!

Thanks for the responses, guys.

Don't think it was a spun hub causing high revs, I never did hear the motor scream like you would think. Plus it was running great. Whatever the "event" was happened during idle.

I tried the screwdriver check, and didn't notice any remarkable difference between the bad piston and a good one. I did notice a nice shiny witness mark on the piston head, though. Not sure what number cylinder, but its the upper starboard one. Time to start tearing her down.

Here is another question, is it worth having a reputable guy do a rebuild, or just buy a rebuilt power head and swap them out? (assuming this is the end result). I'm tempted to lean towards having the guy do, simply because I'll be working with him as its done, and making sure its done correctly. Thoughts?

 

[V153]

Re: Evinrude 90 Hp spl. LOST COMPRESSION PLEASE HELP!!

That would be the #1 cyl. Oddly enough I've never had a failure in #1.

Pull the head & see what's what. If'n the head & piston are all dinged up then yes, something very hard passed through there.

Remove the inspection cover on the side of the block'n see if'n the rings are still intact. Which I doubt.

It's also entirely possible something came through the intake & caused the damage. Broken reed, loose reed screw, loose screw/bolt from somewhere under the cowl, really big bumble bee ..., etc. Won't really know til ya pull it apart.