1994 5.7 mag mercruiser

Joined
Jan 5, 2017
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23
1986 chaparral 235xlc w/5.7 mag engine. Was out on water today all day, engine running good no problems. Had a skier go down on a boat ahead of me and another boat nearby make for a tight passage in between so I cut back on throttle and make a pass between the skier and pontoon boat close by. After making a safe pass and in the clear to open up throttle again had a loud pop almost like backfire type noise when I gave it gas then a loud ticking from engine even at idle. Have a 20hp kicker on back so shut her down and trolled back to dock. When back at dock tried staring her up again, and she did but tick is still there. Any help on what this may be? I'm trying to be as descriptive as possible, and will try to upload a vid if possible of sound to help.

Here is a little info on the boat since I got it in April. Had some old gas I siphoned as much as possible out(all but maybe the last 1/2 gal. Or so). Put 20+ gal. Of fresh gas in it, from the 3 times we've had it out this year and ran 2 bottles of seafoam through it. I've been having to dump fuel/water separator several time as well as putting a new one on. Changed spark plugs and cleaned carb with carb cleaner( just spray, did not actually "boil" carb seemed to be running OK with just some spray) have had a bit of water in the fuel thus the reason for the frequent dumps and change of filter. Please any help would be appreciated, I guess I should mention oil level is good and i didn't lose any after the pop. Oil pressure was also OK under load and so was temp.

I'm just praying I didn't throw a rod or something, please help šŸ˜¢šŸ˜–
 

NHGuy

Captain
Joined
May 21, 2009
Messages
3,631
I bet you had to chop the throttle when the skier fell in front of you. If you did there's a possibility that water got in the exhaust tract during that sudden stop.
You should do a visual inspection and a compression test.
 

NHGuy

Captain
Joined
May 21, 2009
Messages
3,631
Look for signs of water around the joint between the exhaust manifold and the attached riser. Also look where the exhaust hoses connect to the metal castings.
When removing the spark plugs keep them in order and check for damaged or overly clean condition, which indicates water in the combustion chamber.
 

Bondo

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Apr 17, 2002
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71,137
I've been having to dump fuel/water separator several time as well as putting a new one on.
have had a bit of water in the fuel thus the reason for the frequent dumps and change of filter.

Ayuh,.... Dumpin' the filter, 'n puttin' it back on, just puts the water, 'n crud into both sides of the filter, 'n onto the fuel pump, 'n carb,....
and cleaned carb with carb cleaner( just spray, did not actually "boil" carb seemed to be running OK with just some spray)

'n sprayin' stuff around on the carb does little to no good, at all,...
The water, 'n crud plugs the internal passages,... hence a crab Rebuild to clean it up,...

Here's My synopsis of what happened,....
You've been fightin' fuel delivery issues,...
It's Finally runnin' decent, 'n ya go for a cruise,...
It has been, 'n is still runnin' way to lean, but you don't know that,....
Durin' the cruise, the motor has been consumin' itself with detonation,...
The valves, 'n piston tops at Hot, Too Hot, from detonation, caused by the lean condition,...
These "Hot Spots" in the cylinders don't cool evenly, 'n when ya hauled back on the throttle, it allowed rapid coolin' of the motor,... damage was done,...
Then after ya cleared the hazard, ya throttled up, but again, it's still runnin' lean, so something Popped,.... from detonation,...
the Pop ya heard,...

I hope I'm wrong, but I'd start with a compression, 'n leak-down test,...
I'm guessin' yer gonna find a low pressure hole, or 2,...
The leak-down test can tell ya whether it's valves, or pistons,...

'n don't forget to fix the fuel delivery system,....
 
Joined
Jan 5, 2017
Messages
23
So after getting a chance to inspect the engine last night I found what I am hoping was the problem. When I gave the engine a tune up there was one spark plug that was a real beast to get in. Turns out in that process I cross threaded the plug and it blew right out, so it was a mistake on my part. Anyway I re-taped hole blew it out and replaced plug with another new one. Boat fired right up no more tick, no nothing :) Actually sounds better and running smoother than ever before. Hopefully that's all problem was and no other damage was done, only way to find out is take it to the lake for a run. Most likely will be out for fathers day will report back, hopefully with good news then. only thing I've got to do now is go exchange my prop, bought a used 14 1/4 x 17 from a parts place here called kendor. They said that the used stainless I bought I could exchange if it didn't work out, which it was only turning 3800-4000rpm on full throttle should be up around 4200-4600 from what I have found on this engine. Does anyone know otherwise? The 17 pitch is ok, but think im going to go down to a 13 3/4 diam. anyone have suggestions? Thanks a bunch! I would have been at a loss without the help on here.
 

Scott06

Admiral
Joined
Apr 20, 2014
Messages
6,820
What was the original prop pitch ? Seems awfully low wot rpm for that engine in that boat. Would verify the tach first as I would expect a 17" pitch to be screaming high on rpms not good for much except pulling skiiers
 
Joined
Jan 5, 2017
Messages
23
Unsure of original prop size and pitch. When only myself and buddy in boat I can get the 4200 when trimmed up, put our wives and 4 kids on boat then I can only get 3800-4000. From what I have found I should be turning 4200-4600rpm at WOT. when I do the prop calculator I get various sizes and pitches ranging from 13 3/4- 15 diam. and anywhere from 14-19pitch. mind also boat sat for about 3 years with the reman engine and until last weekend carb was still touchy and had a hard time getting to go WOT without it stalling out. was thinking I should probably pull carb and rebuild, as stated before carb spray only does so much. will probably give her another good run this weekend depending on weather, then decide if im going to pull and rebuild carb. I think it's inevitable at this point.
 
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