Old Merc 7.5 / 9.9 clunking & banging @ WOT?

Jjmkerr

Cadet
Joined
Jan 29, 2012
Messages
7
Hi all! First time to the forum and was hopping i Someone could help me solve this mistery.
I just bought a 79 merc 7.5 for me and my young son to go fishing. Anyway, I put it on a drum and it Runs great at idle and half throttle, but at wot upper unit starts funking/banging - like a clunk that physically shakes the motor about 2-3 per second. It appears to becoming from the top of the motor, and it actually kicks the motor a bit with the same force as if some lightly thumped their hand against it. It happens in nutral and in forward (didn't try revers). I don't think it's related to the gear shifter (I think I'd recognize that type of clunk) but is it possible it's a bad case of detonation? Or any ideas?

One last clue - I grew up with an old merc 9.9 (est ~1982) it was a great motor that never let me down up until someone walked off with it. Anyway, i seam to remember in later years that it started to develop a internal rattle at wot that sounded that similar to what this 7.5 is doing, and it seemed to get more noticeable as the years went on. (or perhaps I jutted noticed it more as I became older and more aware of engines, etc) The issue I'm experiencing with the 7.5 feels the same except it's really noticeable (louder and with a physical kick to it like someone's actually wacking the upper motor with their hand). It's enough that even a inexperienced non mechanical person would know something's not right. Only think I can think of is detonation, but with the type of sound and kick it's a bit scary to think about whats happening in the motor - had me worried when i was running it without the cowel on.

Anyway, Presuming I'm experiencing the same issues I had with the 9.9, just to a greater extent with this 7.5 would lead me to believe that others exp merc owners must have experienced this too. So any one out there have any experience with this issue, or any ideas?

Any and all help much appreciated!!
 

impatico

Petty Officer 2nd Class
Joined
May 24, 2010
Messages
184
Re: Old Merc 7.5 / 9.9 clunking & banging @ WOT?

pull the plugs and see what they look like, they will tell you if there is a ignition problem. There are many charts online to tell u what is happening based on what the plug looks like....
 

Jjmkerr

Cadet
Joined
Jan 29, 2012
Messages
7
Re: Old Merc 7.5 / 9.9 clunking & banging @ WOT?

Thanks for the Reply!
I checked the plugs when i bought it and they looked clean but new. I asked the guy said he just put the new plugs in at the end of the season, so they don't have any miles on them yet, so they're not telling us much at this time. Any other ideas or suggestions?
 

phebus1

Petty Officer 3rd Class
Joined
Aug 19, 2011
Messages
85
Re: Old Merc 7.5 / 9.9 clunking & banging @ WOT?

My guess is that it had a partial seizure at one point and you have a ring stuck to a piston resulting in piston slap.
 

M9.9

Petty Officer 2nd Class
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Dec 4, 2011
Messages
152
Re: Old Merc 7.5 / 9.9 clunking & banging @ WOT?

Hi Jjmkerr, you may wish to be sure the plug wires are on the right plugs. Good luck.
 

Jjmkerr

Cadet
Joined
Jan 29, 2012
Messages
7
Re: Old Merc 7.5 / 9.9 clunking & banging @ WOT?

The wire to the top plug won?t reach the bottom plug, so that can?t be it. Any other ideas?
 

phebus1

Petty Officer 3rd Class
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Aug 19, 2011
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85
Re: Old Merc 7.5 / 9.9 clunking & banging @ WOT?

The first thing I would do is a compression test.
 

impatico

Petty Officer 2nd Class
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May 24, 2010
Messages
184
Re: Old Merc 7.5 / 9.9 clunking & banging @ WOT?

It is really hard to diagnose without actually hearing the sound, does it sound like metal on metal? or does it sound like a miss? If it is piston slap then that is caused by a worn out main bearing. Is the sounds consistant? is it in rhythm? or random? does your RPM drop a little when it happens?

http://www.youtube.com/watch?v=07XutAWsr3c
 

Jjmkerr

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Joined
Jan 29, 2012
Messages
7
Re: Old Merc 7.5 / 9.9 clunking & banging @ WOT?

I'll see if I can find some time this weekend to compression test it and let you know.
As for the sound, it's sounds like there is one or two lead marbles kicking around in the motor. no rhythm, not consistent, it's a random banging/ping around 2-3 times per second at wot. You can hear the sound staring at around 70% throttle, at which point it sounds like a bit of gravel in the motor, and it gets more noticeable, powerful and faster as it approach 100% throttle, at which point it sounds like marbles pining around. The sound's not pitch, whinny, or tinny - All i can say is it's sound like lead marbles kicking around. With the cowl off at wot you think one's going to come flying out of the motor any second. It's something physical, and definitely affects the RPM's.
the engine idle's smoothly and consistently, and throttles up smoothly (for an old motor), with consistently climbing RPM's until it hit's ~70% at which time it's more of a rattling, like there is some gravel in the motor, and as you throttle up more it becomes more apparent, up to wot where is sound has escalates into the sound of small lead marbles start kicking around - as it throttles up you can also feel the impact more, and the rate of impacts get's faster. at wot you can feel each bang through the handle, as it does this you can definitely feel the rpm's drop. I'm guessing at full out the rpm's are 4600 (just guess) and I'd imagine the rpm gauge (if it had one) dropping 100-200 for each bang - I can just picture an rpm gauge just jumping up and down at wot.
If the weather get's better i may take it out again and see if i can get some sound clips of it, but might not be able to do that for a couple of weeks, given the snow we're getting up here right now, but i'll try to get the compression tests results this weekend.
So does the extra info help? trigger any ideas?

As for how it sounds, i found one clip on youtube where i detected the sound (or really close) Check out this clip at the 0-9 seconds, and at 34-35 second mark - that's the sound, except it happens is i approach wot, and is more violet in my motor - likely because mine is doing it a wot.
http://www.youtube.com/watch?v=KJcuTihZZFU&feature=related
 

impatico

Petty Officer 2nd Class
Joined
May 24, 2010
Messages
184
Re: Old Merc 7.5 / 9.9 clunking & banging @ WOT?

if it was piston slap it would be in rhythm. sounds like detonation to me, how old is the gas? might be an ignition problem, pull the plugs and see if they have strong spark? one might be just sitting in there. ignitions can be fine at an idle but fail under load at higher RPM's
 

M9.9

Petty Officer 2nd Class
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Dec 4, 2011
Messages
152
Re: Old Merc 7.5 / 9.9 clunking & banging @ WOT?

Hi OP and all, Dunno?, but sure seems that demo motor is missing and missing in your engine's case at high rpm may convert into a clunk or rattle depending on whether it's not firing on a plug on each cycle or now and then? I know my 40 did the same thing during warm up but after reaching it's operating temp it smoothed out. I attributed the cold-miss due to oil/gas wetting the cold plugs through it's cold choke operation. Assuming the bearings are OK, you may, if youv'e not already, check the timing and timing advance/retard for correct operation. Good luck.
 

jbjennings

Captain
Joined
Jul 18, 2007
Messages
3,903
Re: Old Merc 7.5 / 9.9 clunking & banging @ WOT?

SOunds like the recoil starter pawls contacting the flywheel to me. Look at the pawls and see if there is wear on the tips or on the flywheel.
JBJ
 

impatico

Petty Officer 2nd Class
Joined
May 24, 2010
Messages
184
Re: Old Merc 7.5 / 9.9 clunking & banging @ WOT?

SOunds like the recoil starter pawls contacting the flywheel to me. Look at the pawls and see if there is wear on the tips or on the flywheel.
JBJ

Good thought!!
 

Mi duckdown

Commander
Joined
Apr 14, 2007
Messages
2,575
Re: Old Merc 7.5 / 9.9 clunking & banging @ WOT?

Jj. first OFF you should not be running the motor @ wot in neutral gear or foward in a barrell or on muffs. You have NO prop resistance.. You will blow that head.
Put it on the a boat then test again.
 

Jjmkerr

Cadet
Joined
Jan 29, 2012
Messages
7
Re: Old Merc 7.5 / 9.9 clunking & banging @ WOT?

Yes, the gas and oil was fresh. I’ve already ordered some spark testers to test the spark when I do the compression test, and will let you know, though I’m not sure that it’ll rule out detonation, but might give us an idea if it’s missing.

I let the engine warm up for 3-4 minutes before I started to play with the throttle a bit, and then attempted WOT, though it was pretty cold out (+3 or their about), but the engine should have been warm enough – the water output was lukewarm

Recoil start paws – that’s an interesting idea that I’ll check into, though as optimistic as I’d like to be, the sound is more substantial then the typical ticking some report with that type of issue. I don’t have a repair manual, but will see if I can open the top up and take a look. On the subject can anyone recommend a good repair manual (Seloc, Clymer, etc?) or anywhere that I can download a repair manual (not a owners manual)

Lastly, as for wot in a bucket – I agree there are real risks associated bucket testing at wot, but believe the risk is manageable with a small engine by limiting WOT to a couple seconds ‘and’ using a deceit size container (large plastic storage bin).
In general, I certainly think the risk is justifiable when ‘initially’ assessing a used ‘small’ motor out of season with a 'deceit' size container.
In retrospect, hindsight being 20/20, in the absence of being able to test the engine on the water, I wouldn’t buy a used motor without doing a bucket test in a decent container including a shot at wot.
Anyway, I need to know now if I have an issue now, so I can try to repair it while I still have some spare time. I’ve got a two year old and another one due next month, so I won’t have the spare time in the spring to sort it out. Risk, like most things in life, is relative ;)
I’d rather test and run the risk of killing the motor now, in which case I’d just buy another, then end up on the water this summer with a dead motor (and a 45 min row to show and a two year old screaming he wants his mommy)
 
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