1989 mercruiser 5.7 running 2500 rpm wot

[chris0061]

Just got this boat out on the water and the most I could get out of it was 2500 rpm. Running a 1989 25 foot Larson San Marino cruiser with a fresh 17 pitch prop. when I got it the flame arrestor was really cloged with burnt oil I assume, It was thick and black so I cleaned it and sprayed the carb with sea foam and put new water seperator on it and fired it up on muffs and man the black smoke rolled out and you could see the carbon in the exhaust water running down the drive way. it cleared up for the most part and started running really smooth on the muffs. Started it up a few days later and the same thing. Started up one more time a few days later and it wouldent quit with the carbon burning. Took it out today and it idled great and I thought I heard the four barrel kick in on the quadrajet but it would not get past 2500 rpm no matter how I trimed it and it shook a little. Pulled it out and the hub was just coated in black looked like oil. Got it home and the oil was still at the same level. Flame arrestor was still clean. Any idea were I should start?

 

[Starcraft5834]

Boat history? U just buy it..engine compression numbers ok? Plugs. Wires in good shape?

 

[chris0061]

I did just buy it. not done any compression tests yet. I gues thats the starting point?

 

[Starcraft5834]

yep, for sure.... will tell you plenty...if your compression is ok, most likely timing/plugs/wiring,, if compression bad,, that's not good...

 

[MikDee]

It sounds very rich to me? I would go over the carburetor real good, especially the float level that might be leaking extra gas into the carb.

 

[GA_Boater]

Make sure the choke is pulling off. A closed choke or partially closed choke contributes to rich running.

 

[bruceb58]

After you check the compression, do a full tuneup including plugs, wires, rotor, cap and fuel filters. Pointless to troubleshoot until you have done all those things

 

[philipp10]

I have to disagree with the above posters. Don't start throwing all kinds of parts at it. Do the compression check and if it is good, look closely at the choke and then possibly the float. It is more than likely fuel related and IMO, you are always better to try and "diagnose" BEFORE throwing possibly un-necessary parts at the issue.

 

[chris0061]

Ok I love this site. I'll be digging into it Friday starting with pulling all the plugs and doing the compression test. Will update Friday.....

 

[acdc96]

Sounds like it's running very rich. How was it starting up?

 

[chris0061]

It took a little cranking to first get it running then after it was warmed up just a bump of the key. I got at it tonight. Pulling the plugs number 5 was very loos and had the most gas on it. 1,3,5, and 7 had carbon build up. The other side looked like they were firing good. Nice and dry. Took the cap off and there has been moisture in there and all the landing points on the cap had little piles with points on them. The wires looked original to the boat. Kind of flimsy in my mind. The control on the carb I could hear and feel the diafram popular every time I pushed it in and let it come back out. The compression going 1 threw 8 as follows 160,170,180,174,180,173,175,170. What do you think? I have a thunderbolt IV ignition with a good cap rotor distributor and coil just to throw that out there.

 

[NHGuy]

I'm not sure I understand what you are saying about the carb, but it has a pump in it that will cycle when the throttle is moved. So that could be just fine.
Your compression numbers are OK.
Definitely use a new cap & rotor. Are you saying you have new ones? Anyway get a good set of marine wires, and get or use an original equipment cap and rotor along with fresh spark plugs. I like the NGK plugs. I bought a set of stainless core wires about 4 years ago, they are awesome. Recommend something like that. And maybe get some wire separators to keep em apart, that keeps down cross firing which sounds like a miss.
Change the spin on fuel filter, check & clean out the carb inlet filter. change the oil in the motor and the drive and try it out!

 

[chris0061]

Oil already changed. Water fuel separator changed. Lower unit serviced with new bellows bearing and trim switches. Will get the parts for the tune up tomorrow and see how it runs before I mess with the carb.

 

[chris0061]

Update. Got new plugs, wires, cap, rotor, sensor in and set the timing and sounds smells and runs like a new boat on the muffs. No more carbon blowing out after the first start. I assume that is what was left in before the tuneup. River test tomorrow. .....

 

[chris0061]

Bruce you was right on track. Thanks