motor knocking

F

Feelin Froggy

New Member
Aug 6, 2013
29
Havre de Grace MD
Boat Info
1983 270 Sundancer
1975 Fiber Form 19' Runabaout
Engines
7.4 Merc
Need some help. Have 83 270 Sundancere w/ 7.4 Mercruiser. Bravo 2 outdrive. Have owned the boat about 2 months. Was told that the motor only had about 60-80 hours on it. Looks like it was just installed. While running the boat near WOT (4000 rpm's) the motor started to bog down. Rpm's dropped down to about 3500 rpm's without touching throttle. Acted like there was water in gas. This occurred about 2-3 times. I slowed the boat down and slowly raised rpm's again. Motor seemed to recover and ran for another 5 minutes with no issues. Then the motor made a loud bang like a backfire which was followed by a knocking sound. Wife was in the cabin and she said it smelled like burning rubber down below. Stopped boat, opened motor compartment,
no issue with belt, checked things over, nothing jumping out. Checked oil, still golden in color. Started motor again. Knockingnoise continues. Increases with Rpm's wether in gear or not. Motor still runs at 3500-3700 rpm's with no noticeable performance issues other than the knock. No increase in temperature, no change in oil pressure. Looking for advice on how to start troubleshooting where the issue stems from. I was planning to start by pulling each plug wire to see if I can determine which cylinder might be the issue. Any advice appreciated. Thanks
 
Feelin Froggy said:
Need some help. Have 83 270 Sundancere w/ 7.4 Mercruiser. Bravo 2 outdrive. Have owned the boat about 2 months. Was told that the motor only had about 60-80 hours on it. Looks like it was just installed. While running the boat near WOT (4000 rpm's) the motor started to bog down. Rpm's dropped down to about 3500 rpm's without touching throttle. Acted like there was water in gas. This occurred about 2-3 times. I slowed the boat down and slowly raised rpm's again. Motor seemed to recover and ran for another 5 minutes with no issues. Then the motor made a loud bang like a backfire which was followed by a knocking sound. Wife was in the cabin and she said it smelled like burning rubber down below. Stopped boat, opened motor compartment,
no issue with belt, checked things over, nothing jumping out. Checked oil, still golden in color. Started motor again. Knockingnoise continues. Increases with Rpm's wether in gear or not. Motor still runs at 3500-3700 rpm's with no noticeable performance issues other than the knock. No increase in temperature, no change in oil pressure. Looking for advice on how to start troubleshooting where the issue stems from. I was planning to start by pulling each plug wire to see if I can determine which cylinder might be the issue. Any advice appreciated. Thanks
Click to expand...
I have had knocking motor also, Rebuilt motor and then blew up the outdrive. Before doing a lot of engine work pull the outdrive and run the engine on the hose to see if the knock goes away.
 
The clutch is in the outdrive, so you are still spinning parts in there whether you are in gear or not. But if you can definitively isolate the noise to the engine, and not to the outdrive/transfer shaft, then removing the outdrive doesn't buy you a lot.
 
I would start by taking the rocker covers off, check for bent push rods or worse a dropped valve :(
I have seen a small block snap the top off a piston at the oil ring groove once.... Sounds similar.....It would run fine apart from a knock as the piston and skirt smack together. You should be able to put a wire down the plug holes while some one turns it over by hand and check the pistons aren't hanging up near the head. Very rare but worth a check...... if it happened at WOT you can be sure it's going to be nasty:(
 
If you smell rubber burning you blew out the coupler on the flywheel.


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Is there anyway to check if it is the coupler, short of pulling the motor? I'm also wondering if this a common weak link. If so is there any sort of upgrade I should look into since I'm assuming I'm going to be pulling it apart.
 
Not sure how to prove that it's the coupler. But that's what seems to be the problem.

Check the alignment of the gimbal to the coupler. If not aligned properly you will eat up couplers and gimbal bearings.


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Feelin Froggy said:
Is there anyway to check if it is the coupler, short of pulling the motor? I'm also wondering if this a common weak link. If so is there any sort of upgrade I should look into since I'm assuming I'm going to be pulling it apart.
Click to expand...

I do know that on our 88 22 Pachanga the original coupler on the 454 had a circular bolt pattern that attached near the center of the flywheel.
When the engine had a rebuild, bore and bunch of performance work done I upgraded to a coupler with a triangular attach plate.
This is the type of couplers we ran on our race boat too with no issues with wear, alignment or failure.
 
If you smelled that awful burnt rubber smell, it's a safe bet it blew. I was on my buddie's Cobalt 22 and he was thrashing on the 502 Mag. The coupler (he called it something else) blew, and it was loud, nasty, and horrible.
 
Thanks everyone. General consensus is the coupler. I'll start looking there. One last question. In its current state, I can still run the boat. Would I be ok to baby this to move the boat. I have no trailer and marina where I plan on keeping it this winter is an hour away. Thoughts
 
Well, I'm thinking that if you've blown the coupler, (and I might be wrong) that means that your input shaft is probably moving around quite a bit. Not sure I'd like the idea of going a mile or two with my shaft banging away at the gimble bearing/housing, even at the lowest speed possible. Dunno, but I'd be looking for my buddies to tow me over...
 
just run it on one engine and take it very slowly.
 
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