87 Merc 350 Performance

[Merc5.7]

Hey guys, I just bought a boat and I have been all over these forums for the past couple of days.

The boat:
1987 Sea Ray Sorrento S23 23ft

The motor:
87 MerCruiser 5.7 carbed, Alpha One Gen 1, 1.5 drive ratio

Now, I only paid 3 grand for the boat and a pretty nice trailer. So I'm not too deep in the hole. The boat itself if is fantastic shape, and everything works on it. Seems like it was well taken care of. Of course, I should not have judged the condition of the powertrain based on the condition of the boat. My problem is out on the lake with 4 people and 55 gallons of gas, I can't get the boat to plane out and rev over 2 grand. My first thought was the prop was too big, but it meets exact OEM specs. I'm now onto testing the cylinder compression.

Last night, I pulled all the plugs off, got through three cylinders and stopped cuz fam was leaving for a baseball game. From here on out, lets talk theory. My first two cylinders read a little over 120 each and the third showed 90, cold. Say all but one cylinder shows 120, could that really be the source of my issue? And I know 120 is well below spec of upwards of 170-190, but for a 30 year old engine is 120 really all that bad? Basically what I'm getting at is with all my cylinders showing 120 except one that shows 90, could this be the root of my steep loss of power?

Also of note is that the engine was cold, and that it turned over at least 10-15 times for each cylinder before the needle stopped rising. Is this a problem?

Thanks to anyone who takes the time to read this. Last time I buy a boat without going for a ride!

 

[garbageguy]

Hey guys, I just bought a boat and I have been all over these forums for the past couple of days.

... Last time I buy a boat without going for a ride!

That's good. Others should be along on your theory(ies) and engine-specific questions, but is the boat waterlogged? Have you checked the transom and stringers?

 

[Merc5.7]

My dad mentioned to me that that could be the problem. Is there any way to get a look at the stringers without ripping up the floorboards?

Thanks for the reply!

 

[harleyman1975]

Well start from the beginning. when doing comp test you open the throttle wide and crank over at least 5 revs. you do all 8 at same time and then compare numbers. next check the plug wires/firing order. I believe it is 18436572. How old is the fuel? Does it have a water/fuel separator? If so replace it with a new one and dump contents into a glass jar and inspect it...should be crystal clear not cloudy at all.

 

[Merc5.7]

Thanks, harleyman. I will finish the compression testing and post results. Wires look brand new. I'll make sure they're hooked up right when I get home. Guy said he just had the plugs and cap replaced. Although I can't trust a word he says at this point.

Where would I find a water/fuel separator?

Also of note is that it starts up and runs like a top in the driveway. Revs up past 2 grand in and out of gear, etc.

Thanks!

 

[tpenfield]

First of all . . .

:welcome:

Secondly . . . Who won the baseball game

Third . . . if you have only done compression on 3 cylinders, you can't make any judgements. You need all 8 to see the 'norm' and the 'outliers'. Then you could say, "gee this ain't so bad", or "gee this thing is hosed".

 

[Scott Danforth]

Check the DIY stickies for low WOT

 

[Merc5.7]

Thanks, everyone, for the feedback. I have completed compression testing and I figured out I was using the tester incorrectly.

My numbers are (looking down at motor facing transom):

1: 145 5: 145
2: 160 6: 160
3: 160 7: 160
4; 155 8: 160

I figure these numbers aren't too bad, and following the 10% rule, everything checks out. Just bought a replacement fuel/water separator/filter that I will be installing tonight. I will get back to you on the condition of the old one. The thing runs like a top in the driveway...

By the way, tpenfield, it was a local game. The Tri City Valleycats took them to the cleaners!