VIDEO LINK INCLUDED. What causes this knocking? I'm at my wits end

[makrellen]

It was suggested to me to post on this particular forum, so here goes.

A couple of years ago I bought an older 14" boat with a 30 HP Yamaha 2-stroke on it (1998 model) and I've never been able to get it running right. I rebuilt and synced the carbs and set ignition timing according to the service manual specs. Also installed a new impeller. It has great and even compression on all 3 cyls (120 psi). I just replaced the diaphragm in the fuel pump and went for a test ride immediately thereafter and had it running great for 15 minutes and thought I had cracked the puzzle. Which in turn made me comfortable about bringing the family out for a ride. And then it happened again: After running great for half a minute it did what it's always done: Engine loses power and when trying to apply throttle again, the engine starts shaking/knocking heavily when throttle is applied. If in neutral and using the fast idle theres's heavy knocking every half second or so with the engine visibly jumping to one side. I can run it fairly when just above idle (but still kinda rough like I'm very near to having it knock/vibrate), and as soon as more throttle is applied, the knocking/shaking starts again. And it's not like I can "push through it" with the throttle lever - it's as if the lever becomes hard to move because of the strong vibrations. It sounds really unhealthy so naturally I always ease back immediately.

What puzzles me the most is that I have had it running great, but now it's horrible again. I'm running premix until I'm sure the autolube is working, so it might be running on a pretty rich mixture but I can't see that being the reason. Next time I'm with the boat I will drain the gear oil to rule out a dog clutch issue, but I don't think that's it seeing as I can also make it **** using the fast idle.

I could really use advice on how to proceed. I don't have a trailer that's road legal - otherwise I'd have a mechanic look at it and naturally I'd rather not shell out about a thousand bucks for a trailer (trailers for this specific boat are costly), before I know that it will run.

Here's a short clip on Youtube of what the issue sounds/looks like: https://www.youtube.com/watch?v=MM4Jz2FDaF0

Thanks very much in advance.

[99yam40]

you say you set timing according to manual, but have you used a timing light to see what the timing is actually doing at idle and then when advancing the throttle?
Timing light can also show if each spark plug is getting spark(light flashes) when that problem shows up if you hook it up to the different plug wires one at a time.

If one or more plugs stop sparking , you know which ones and chase that down.

does this motor have an alarm for over heat and low oil level that sounds a warning buzzer along will keep it from running at RPM above idle?
do you know if the alarm works?

[robert graham]

Maybe put a new set of spark plugs in motor, run it until it starts missing/ vibrating/ whatever, turn motor off and remove and carefully inspect each plug for indications of the issue…the sound sorta reminds me of a marginally fouled spark plug which acts up the worst at higher RPM ‘s…

[makrellen]

Thanks for responding, guys.

I have not used a timing light. I set the timing using a TDC gauge (meticulously), but I will consider checking with a timing light. But it's not an option checking timing on the water. How would I go about checking timing when increasing the revs - just let the prop spin in free air?

The engine does have heat and low oil alarms, but I don't think they work - at least not for oil. I have run the engine with the oil tank empty (on premix) but there was no alarm. Also, this issue is also present when the engine is cold, so I don't think overheat is a problem. Also there are no signs on the engine that it has ever run hot.

I put new plugs in when I got the engine and it had this behavior back then also. It does oil the plugs somewhat because of running both premix and the autolube system, but even after having cleaned the plugs it still has this crazy vibration. Could one of the plugs be bad even when sparking and therefore hard to troubleshoot?

[99yam40]

fix the alarm so it can tell you when that kind of problem shows up

[robert graham]

The double oiling could be fouling one or more of your plugs….

[jjimbo113]

Remember that the jets in the carb are set up for a certain fuel/oil ratio adding too much oil or in your case double oil will cause a lean mixture

[makrellen]

Thing is, that I've had it running very well on a few occassions, which confuses me. But I'll try switching to regular gas now, that I've seen the autolube system seems to work. I'll also check timing with a timing light. And try to measure voltages from the coils/CDI. And fix the alarm. I'll report back when I've had a chance to look at it.

[makrellen]

Quick question: The fuel lines are quite old on this outboard, although they still seem fine. Is it possible that this violent shaking can be caused by vacuum leaks? I don't think so, seeing as it also happens when I've primed the carbs full of fuel, but it never hurts to ask those more knowledgeable.