2002 40hp Mercury 2 stroke knocking noise

[Chamelion]

I bought this boat about a month ago and it's been making a concerning noise.. I didn't notice it when I test drove the boat, but I was probably so excited I simply didn't pay enough attention.

Here is a clip I made yesterday.

(Please excuse my aussie accent, I hope you can understand me )
http://www.youtube.com/watch?v=0xtHwZUPivU&feature=plcp

It mostly does it when I'm just below planing speed and putting some load on the motor. However, as the clip shows it'll also do it in neutral as well. It doesn't do it at idle and it doesn't do it when I'm at cruising speed or WOT.
Whilst on MUFFS today I held a screwdriver to my ear and against the engine beside each of the cyls (beside spark plugs) and there doesn't appear to be any foul sounding noises. But again, the knocking noise doesn't happen on muffs regardless.

I followed this guide and there was no play whatsoever - http://www.youtube.com/watch?v=bgfgbpvOFrQ

I have been running 50-1 Valvoline and Mercury 2 stroke oil. Plugs look healthy, though I replaced them today just to be sure and will water test the motor with them in tomorrow.

Compression is 125, 125 and 123 (can't recall which cyls spat out which readings).

Engine starts fine and top speed is about as expected, just shy of 30 knots. Boat is an aluminium 13.5' with full floor, front and rear casting decks, bow mount electric, battery that runs the sounder/nav lights and a deep cycle for the bow mount. The prop is well worn, so I expect some more top end speed once it's replaced. Hole shot is rather excellent.

There is another sound that happens within a certain rev range right before WOT. It's almost like someone has suddenly put a hole in the exhaust.. hard to explain, but the motor becomes noticeably louder before going quieter again once I get closer to WOT.

Any and all help greatly appreciated. Unfortunately I can't really afford to pay a mechanic to look at it at the moment.

Cheers from down under.

Matt.

ps: Here is another clip I took where I removed plug leads to make sure it was running on all cyls.. Was hard to tell actually. - http://www.youtube.com/watch?v=hk0aon9HZaY&feature=plcp

EDIT: New video http://www.youtube.com/watch?v=iNl3ofJwqqY

 

[Chamelion]

Re: 2002 40hp Mercury 2 stroke knocking noise

Haven't got the tacho yet... Still a couple of weeks away I'd imagine.

It is pumping plenty of water both on muffs and out on the water.

Took it for a run today for a few hours... Notes:

Happens in neutral and reverse at medium revs. At idle it's fine.
In FWD it doesn't happen until I back off the revs. I can reliably power up, back off a bit to make the noise occur, power up a little and it goes away, drop back and it comes back again. If I drop revs until the noise starts I can hold it in that throttle position and the noise will stay. Go faster again it it goes away, slow right down and it goes away.
At about 3/4 throttle the engine becomes noticeably louder and at the same time you can feel a surge in power (just the powerband coming in to play?) - There is no rattle or clunking at this point though.

Any ideas, anyone?

Had someone on another forum suggest it may be detonation ie: timing is out... I'm wondering if it's a loose exhaust or maybe the main drive shaft down the leg has thrown a bush or something?

 

[Chamelion]

Re: 2002 40hp Mercury 2 stroke knocking noise

http://www.themarinedoctor.com/cgi-bin/YaBB.pl?num=1314065520/0

Sounds like a similar problem... Clip sounds about right.

 

[Chamelion]

Re: 2002 40hp Mercury 2 stroke knocking noise

Update: Just ran it on muffs.. No noise at all. Blocked the through hub exhaust with my hand (firmly) and gave it a few revs and bingo, rattle was there.

 

[Dave1027]

Re: 2002 40hp Mercury 2 stroke knocking noise

Kind of sounds like a loose connecting rod or spun bearing. You could try spinning the prop in neutral and all gears to see if you have a clutch dog problem or shift out of adjustment. Maybe take the exhaust plate off and push the pistons in and out to see if you have a loose one.

 

[Maxz695]

Re: 2002 40hp Mercury 2 stroke knocking noise

could be connecting rod bearings worn out hopfully the crank isn,t wiped yet. also the piston wrist pins as well should be checked for excessive play when dismantled.

 

[Chamelion]

Re: 2002 40hp Mercury 2 stroke knocking noise

Thank you for the replies.

What really baffles me is that the noise only occures when this is pressure against the exhaust. Either water or my hand.

 

[Maxz695]

Re: 2002 40hp Mercury 2 stroke knocking noise

Have you tryed seeing how much play is in the upper endcap bearing? It may be possible to get the endcap out and replace the bearing if that is the cause without a dismantle

 

[Chamelion]

Re: 2002 40hp Mercury 2 stroke knocking noise

Sorry, I'm not as clued in on outboards as I suspect that I am about to be.. The upper endcap bearing.. Is that on the top of the main drive shaft just underneath the powerhead? (will have a look in my workshop manual)

How might I test it?

Thank you

Matt.

 

[Maxz695]

Re: 2002 40hp Mercury 2 stroke knocking noise

It is on the top of the crankshaft. try wiggling the flywheel and see if there is substantial movement back and forth. I,m just trying to come up with a reason other than a total teardown that may help or fix the problem. If the upper bearing is worn out it would cause a binding of the rod bearings. This would wear the crank journals down in time. I,m hoping you just started hearing the noise just recently and caught it before any real harm was done. Using less than a 50:1 ratio will have inadiquate lubrication and the parts will start to fail. Also the upper and lower oil fittings sometimes become clogged or the hose brittle and cracked which also causes damage to the bearings from lack of lubrication. You may be ok if the damage was limited. Continuing to run this engine with knocking will definitely result in catotrophic failure

 

[Chamelion]

Re: 2002 40hp Mercury 2 stroke knocking noise

I've still got the cover on over the top of the fly wheel.. Reaching underneath and trying to find movement from multiple angles turned up nothing interesting. Can't feel any play at all.

The requirement of back pressure on the exhaust has to be the key to this? I've been searching the net for hours and haven't found a single lead.

 

[Maxz695]

Re: 2002 40hp Mercury 2 stroke knocking noise

putting your hand over the exhaust is like putting your hand on your forehead and trying to see if one can move the other (Force against Force) Doing this you will feel the prusure in the upper part of your back and neck.The same type of pressure is being put to the connecting rod bearings when you cover the exhaust.This would put presure in the opposite direction which allows the bearings to slap back and forth against the journals. rather than just rotate freely under the timing of the engine. I wasn,t able to turn up the Volume on the videos as My PC is in the same room as my GF (Sleeping) anyway after hearing it it seems 2 of the 3 cylinders are not firing correctly. The engine should die down considerably when the spark plug wire is removed. Several factors canbe the reason for this, weather electrical or mechanichal being rings or cylinder walls but you say compression is good on all. The knocking may be a result of not fully firing correctly over a prolonged period of time. Pull the plug and attach it to the coil wire and hold it about 1/4 inch from a ground and see if there is a bright blue arhcing spark. Do this on all 3 cylinders. If not it may have loose coil wire bad plugs a bad switchbox, coil. Getting the engine to fire properly will help in diagnosing the problem

 

[Chamelion]

Re: 2002 40hp Mercury 2 stroke knocking noise

oh dear.. that sounds prohibitively expensive.

 

[Maxz695]

Re: 2002 40hp Mercury 2 stroke knocking noise

I,ve edited the post re read please trying the spark test and checking the coil wires are inserted properly into the coils fully. As well as check for loose wires and grounds. You did appear to have some spark on the upper two coil wires which I think can rule out the trigger. After revisiting the video The two upper plugs look like there NG. You seem to have plenty of sprk to number 2 not so sure about the top coil I couldn,t see the spark in the video. Try 2 new plugs in the upper 2 cylinders and retest the engine. It is not recommended to remove the wires from the plugs when running as it can and may already have damaged the switch box

 

[Chamelion]

Re: 2002 40hp Mercury 2 stroke knocking noise

Appeared to get spark across to my pliers (and into my hand heh) on all 3.

It is strange.. removing the bottom plug made the engine chug, middle and top made no difference at all at idle, but if I gave it a few revs you could hear a little difference.. A slightly longer gap between air/fuel detonations, as one would expect.

 

[Maxz695]

Re: 2002 40hp Mercury 2 stroke knocking noise

By removing the wire from the plug it should be running on 2 cylinders and increase tremendously in RPM when replaced. This is why I,m thinking bad plugs or a lack of proper voltage to the plug through the switchbox the coil or the wire to the two malfunctioning cylinders. Machanical wouldn,t seem to be an issue if the comopression is that good.

 

[Chamelion]

Re: 2002 40hp Mercury 2 stroke knocking noise

A long shot I'm sure.. but couldn't be some sort of preignition could it? Possible for an engine to have preignition to the point where the spark plug doesnt even need to spark for the air and fuel to ignite? The bottom cyl keeping the pistons moving and at least one of the top two cyls kicking along mostly on pre ignition... at idle the RPMs are low enough for the sound of the engine running to not audibly change and only noticing it a bit at higher revs... I don't know, bit of a hair in the wind on that one. Just got the thoughts racing..

The only clips I can find on youtube that sound the same both ended up being blamed on the hub. But as mine does it in neutral that would surely have to be ruled out?

 

[Chamelion]

Re: 2002 40hp Mercury 2 stroke knocking noise

Back again, adding more thoughts.

Wondering if it might be the driveshaft bearing?

 

[Maxz695]

Re: 2002 40hp Mercury 2 stroke knocking noise

It is a possibility I had also thought it might be something rubbing against the flywheel but seemes to be more metalic. The only way I know to test that would be drop the lower and clamp a hose to the water tube. Run the engine without the LU. You still hve two cylinders that are not firing correctly. That needs to be addressed foremost. if you unplug both top plugs when running then put the top on to see if it increase. then remove it try again on second plug see what happens. To me it sounds like the top cylinder is the worst of em. Maybe a broken ring or again electrical issue. The more this thing runs the more damage is gonna be caused. Try to trouble shot as best you can on limited running time. i,m watching Nascar. I don,t know what you paid for it and probably bought it as is. The dude litterally gave you the shaft. There is no easy fix for the problem but streps can be taken to see if it,s even worth fixing. Depending on casts. Take the bottom plug out and switch it with the top plug and try the test again removing the wire and see if the bottom cylinder acts the same as it did on the first test That will tell you if the plug on top is good or bad

 

[Chamelion]

Re: 2002 40hp Mercury 2 stroke knocking noise

Mechanic and I went through eliminating all of the simple stuff and everything we could diagnose from outside the motor. It's definitely coming from within the powerhead and preliminary educated guess points to the crank shaft and/or crankshaft bearings... No way to know without pulling it down and having a look.

Compression re-tested as OK. Timing is OK. Plugs are getting spark.

edit: Finally got onto the previous owner. He watched the clip and confirmed that the noise has been there as long as he's owned the boat. He said it doesn't sound any worse than he remembers. He said they had it looked at eons ago and the diagnosis came back with nothing noticeable wrong (not sure how far they dove into the problem and he can't recall). He said that he was advised to just see how it goes as the motor was otherwise running great, which mirrors now.

Not sure what to do.