2002 40hp Mercury 2 stroke knocking noise

[Chamelion]

Re: 2002 40hp Mercury 2 stroke knocking noise

OK.. Bit of an update here.

I decarbed her the other day, so today I set about following the workshop manual and syncing the carbs. Idle was still rough, but sounded a little different. I also put the mixture screws on the carbs back to the 'default' setting 1.25 turns from seated and played with them a bit until I got what "sounded" like the smoothest idle.

I took it out on the water and here is what I noticed - some are the same as before, some different:

a) Still cannot get it to make the sound on muffs, but can in the water as before, in both fwd, reverse and neutral.
b) The sound now is way more noticeable in reverse and neutral. However in FWD it's much harder for me to find the right rev range to cause the sound under way. If I throttle down below about half it'll rattle quite a bit as I slow down, but it's definitely changed. It's almost like the sound is now louder, but more restricted in terms of RPM range, if that makes sense.
c) The rattle/knock/banging sound will occur for a few seconds, then the motor will 'cough' and the sound will stop for a few moments before starting back up, before another cough... rinse and repeat. The motor ONLY coughs at less than half throttle and whilst its rattling. If I have it at a 'smooth' RPM there is no rattle and no 'cough'.
d) I listened to the motor with a stethoscope that I purchased. It did appear to be in the bottom half somewhere. It was still quite hard to pinpoint.
e) The motor will idle more or less just as well on the bottom cyl only as it will on the top two cyls only.
f) Removing bottom plug makes engine chug pretty hard and it will sometimes stall if I leave it like that too long. Removing either the middle or top plugs causes either a very minute drop in audible RPM or nothing will happen at all.
g) It's definitely more smoky than I think it should be. In fact, after shutting the engine off 'smoke' will continue to come out of the exhaust that's just under the cowling for a number of seconds.

All of that said, it still felt to run fine at cruise speeds and WOT. Got onto the plane nicely and stayed there.

At this point I'm holding out hope that it's something not too expensive.... - I'll see about getting it looked at by a mechanic later this week.

Does any of this give anyone some fresh ideas? :-/

Cheers,

Matt.

 

[Maxz695]

Re: 2002 40hp Mercury 2 stroke knocking noise

From what I,m reading when removing the #3 cy the #2 and #1 cylinders should at least keep the motor running. when leaving #3 in it appears that it,s firing correctly and allowing the motor to continue running. Are the plugs fairly new? Check coil grounds for cyl #1 and #2 check coils for cracks. Heres a page that might help troubleshoot it Tricks to Testing with Minimal Test Equipment

 

[Chamelion]

Re: 2002 40hp Mercury 2 stroke knocking noise

Yep, plugs are very new. Bought them right at the start hoping to solve the problem with the cheapest replaceable component... Didn't work.

 

[Chamelion]

Re: 2002 40hp Mercury 2 stroke knocking noise

It's off to the mechanic tomorrow. He theorised that it sounds like it might be a broken reed valve getting sucked in further and breaking a chunk off a piston skirt as apparently that can cause RPM/load dependant noises, but of course that's just a very rough guess based on a quick phone conversation.

 

[Maxz695]

Re: 2002 40hp Mercury 2 stroke knocking noise

Reed valves have reed stops to keep them from hitting the crankshaft. Unless one broke off it wouldn,t be hitting the crank or piston and it would severely effect the intake of fuel.View attachment 174172 This is a reed block off a 40 HP you can see the metal stops that allow the reeds to open enough for fuel intake but not to excced that point

 

[Chamelion]

Re: 2002 40hp Mercury 2 stroke knocking noise

Well, no good news. It's not going to be something simple. I'm looking at $250~ to have the motor cracked open to fully diagnose it. $1000 if I want it put back together with no repairs done. If it is what he thinks it is it'll be anywhere from $1500-$2500.

He's going to put it in the water tomorrow so he can get a better listen to the noise (couldn't get enough load on it in the test tank to get the full sound)... But at this stage my options are pretty thin. I don't think it's worth $1000 to pull it apart if I'm only going to have it put back together without repairs. I don't have the money to afford even the minimum re-build. And the boat isn't worth half of what I paid to sell with a shat engine..

His best guess at this stage is one of the centre main crankshaft bearings and also possibly an issue with one or more end-caps being slightly out of alignment (pretty sure that's the part he mentioned).

All I can say at this point is as much as I wish I had have had a mechanic look at it before I bought it and you do assume certain risks as a buyer, I'd feel downright terrible if I sold someone a boat that just so happened to have a shat engine. I need to ask the bloke who sold it to me some more questions to try and work out why the engine has been opened before/what was done to it. But last time I contacted him I wasn't given a very warm reception

Sucks... Only option might be to sell it to someone else as is and take a $3000+ loss or run it until the engine grenades itself and either HOPE I can get a loan to buy a new motor or again, sell it at that point with a shat motor and take a huge loss. Either way and no matter what I do, I stand to lose in the realm of $2000-$3000 (which I don't have).

Thanks again for all of your help. I'll keep this updated as things progress.

 

[Faztbullet]

Re: 2002 40hp Mercury 2 stroke knocking noise

I been sort of reading /watching your post.
(1) the reeds on this engine cannot even come close to contacting crank so thats out...
(2) a broken reed will cause idling and running problem and a popping noise thru carb which you haven't mentioned so that out...
(3) Broken skirt will cause piston to slap all the time under a load.....
(4) If rod end cap where out of alignment it would have already failed due to needle bearings spalling...
If it was in my shop I myself would check the following in this order: (A) Flywheel for being loose or keyway partially sheared..(B) index flywheel and check timing a all 3 cylinders..

The rattle/knock/banging sound will occur for a few seconds, then the motor will 'cough' and the sound will stop for a few moments before starting back up, before another cough... rinse and repeat. The motor ONLY coughs at less than half throttle and whilst its rattling. If I have it at a 'smooth' RPM there is no rattle and no 'cough'.

This is the reason why I would look at ignition a a bad CDM module or trigger can cause timing problems. Swap #3 CDM with #1 and see if the result stays the same. Also if this is on a aluminum boat it may be nothing but a harmonics issue which I have seen several times with the big triple motors..

 

[CharlieB]

Re: 2002 40hp Mercury 2 stroke knocking noise

Fazt and I often agree on what to check first.

Check timing on all three cyls.

Remove the spark plugs and turn the motor by hand and scribe a mark on the flywheel for each cyl at Top Dead Center.

Crank the motor and check timing on each cyl, each mark should appear at the same place. If any are off then the trigger becomes suspect.

The 'cough' at low to mid throttle is most likely one or more carb idle mixture set just a bit too lean, try opening each just the bladewidth of the screwdriver blade slot and retest.

Lastly, carb sync is critical to smooth operation, be very careful adjusting carb linkage to ensure it does not change during tightening the set screws.

 

[Chamelion]

Re: 2002 40hp Mercury 2 stroke knocking noise

I been sort of reading /watching your post.
(1) the reeds on this engine cannot even come close to contacting crank so thats out...
(2) a broken reed will cause idling and running problem and a popping noise thru carb which you haven't mentioned so that out...
(3) Broken skirt will cause piston to slap all the time under a load.....
(4) If rod end cap where out of alignment it would have already failed due to needle bearings spalling...
If it was in my shop I myself would check the following in this order: (A) Flywheel for being loose or keyway partially sheared..(B) index flywheel and check timing a all 3 cylinders..

This is the reason why I would look at ignition a a bad CDM module or trigger can cause timing problems. Swap #3 CDM with #1 and see if the result stays the same. Also if this is on a aluminum boat it may be nothing but a harmonics issue which I have seen several times with the big triple motors..

Fazt and I often agree on what to check first.

Check timing on all three cyls.

Remove the spark plugs and turn the motor by hand and scribe a mark on the flywheel for each cyl at Top Dead Center.

Crank the motor and check timing on each cyl, each mark should appear at the same place. If any are off then the trigger becomes suspect.

The 'cough' at low to mid throttle is most likely one or more carb idle mixture set just a bit too lean, try opening each just the bladewidth of the screwdriver blade slot and retest.

Lastly, carb sync is critical to smooth operation, be very careful adjusting carb linkage to ensure it does not change during tightening the set screws.

Thanks fellas. I'll look into those things.. I might have to buy a timing light.

He didn't re-mention the reed valves.. ruled that out I believe. At this stage his best guess is a centre main crankshaft bearing.. Does this still sound plausible to you? Or can you see a reason to rule it out, even partially?

Someone posted on my youtube video suggesting it's the gearbox bearings and someone else watched my video and suggested that it's "back lash" (I'm not really sure what they mean). Plausible?

I think I'll go down to the ramp with the mechanic tomorrow when he puts in the water for a better listen to the noise. That way I can look at actually recreating the noise exactly for him so I am absolutely certain he's hearing the same thing.

 

[Maxz695]

Re: 2002 40hp Mercury 2 stroke knocking noise

If the bushing on the crankshaft rusted away it is posible for the crank to drop even a few thosands of an inch making the rods off center and rub the crank making a ticking noise. On my rebuild I heard the ticking after warm up. and it got louder. Good guess took it back down and adjusted the endcap shims and prestor chango the ticking stopped. Just some info food for thought.

 

[Chamelion]

Re: 2002 40hp Mercury 2 stroke knocking noise

Turns out he had someone come in with the same motor today for a service, so we ran that one in the tank and then mine to compare... I'm not sure what the GO is, but mine refuses to make the same sound at the same volume in a tank when compared to out on the water. That is absolutely baffling. That said, there was certainly a common noise between the two. It sounds like my noise, but quieter. So I have a couple of theories to throw around:

a) There is something different about being on the water VS in the tank. MAYBE the difference in the hull. Perhaps somehow the sound is absorbed/drowned out a bit with the hull out of the water with the hull allowed to vibrate itself and dissipate some sort of harmonic issue between the motor and the hull. - I'm not even sure that makes sense, but neither does the sound occurring only with the hull floating on the water.
This may tie in with what Faztbullet suggested

Also if this is on a aluminum boat it may be nothing but a harmonics issue which I have seen several times with the big triple motors..

b) I've got two different noises occurring that sound similar. One being common to the series and not to worry about and the other louder and perhaps more concerning. I guess it's possible there could be a second noise originating from the drive shaft or related components? In saying that, the noise we replicated in neutral in the flushing bin between the two motors sounds the same as mind out on the water, just with less volume. So I'm thinking this may be less likely. Unless again, the noises between N and FWD/R are actually independent of each other.

Certainly has the mechanic fairly stumped, so I don't feel so bad. He's going to put it on the water tomorrow. I think I'll go along to ensure we fully replicate the sound.

 

[jbcurt00]

Flbunfield

You flagged a post in this topic, but only sent a flag notice instead of posting your question about similar knocking.

This topic has been inactive for over 4yrs. Not likely to get a reply.

Start a topic about your motor and ITS knocking and the huys hanging around the Mercury forum will help you assess and/or repair it.

Closed