5.7 running issues

[Cjpainten]

89 Donzi ragazza
OMC 5.7
Cobra outdrive

Got this boat last year for 3k. Hull was in great shape, owner had bought a new boat and was done with trying to fix this one. Apparently the motor was rebuilt in 17' and had roughly 100 hours on it since the rebuild. I compression checked it and the motor seemed healthy, just had a little misfire so figured it wouldn't be too hard to fix especially with the box of new parts he gave me with the boat...

So I got the boat, put it in the water and took it for a spin. Not too bad, but still could tell it wasn't running 100%

Took it home, and got to installing the new parts he had...

Delco Voyager est Rochester 4 barrel
plugs/wires
fuel pump

Ran a little better, but still not 100%

Drained fuel tank and refilled with fresh gas

Little better but still not there.

Put boat away for the winter...

This spring, boat has been decent enough, only issue I'm having is a loss of power and trouble getting on plane after warm up. Also have a sheen coming out of exhaust. I left it alone fearing a more serious issue that would put it out of commission.

Today I decided to do a compression test (warm engine) and I thought I would share the results here.

(#1) 130
(#3) 130
(#5) 130
(#7) 120

(#2) 130
(#4) 120
(#6) 130
(#8) 130

Seems decent enough...

The spark plugs though, not so great (see attached pictures)

#7 plug loosened up resulting in oil coming down threads, also pushed in...

Let me know your thoughts on the current issue



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[alldodge]

Compression is low, should be 150 for new and 140 for worn. As you can see it will run at 139 and even at 100 but just needs more compression

RV15YC are for BBC motors, you should have either MR43T or RV8C plug
Timing 8 BTDC

Your running real rich, and compression has some to do with it but look closer at the carb. Which rochester was installed?

 

[Cjpainten]

The carb is the Sierra quadrajet

I got the mr43t in right now after the compression check.

Going to check timing right now

 

[alldodge]

Which model/part number Q-jet?

 

[Lou C]

When they rebuilt the engine, what was replaced specifically? Due to the low comp test results, I might re-adjust the hydraulic lifters, whoever did it might have gotten them too tight and that can keep the valves from closing all the way...and can lower compression....
Also who rebuilds an engine and puts plugs in that look like that? Is there water leaking in from the engine hatch? My boat has always been in salt water and I never had a set of AC Delco plugs look like those....
The sheen coming out of the exhaust....maybe the float is set too high or sticking....the plugs sure looked carbon fouled....a properly set up QJet will not foul plugs like that...is it burning oil?

 

[Cjpainten]

Base Timing was a little off, got it set correctly now seems a little better...

The carb is a Sierra 18-7615N

No water leaking from engine

I'm just going on what the previous owner told me. He didn't provide me with much information

 

[Cjpainten]

Just did a quick oil change and replaced the fuel/water separator.

Hope to test out the boat tomorrow weather permitting. Will get back to you with my findings

 

[Cjpainten]

Took the boat out and she runs much much better

Was most likely a timing / plug issue, but a fresh tank of gas and fuel / water separator didn't hurt