Knock at Idle

[Tucker]

Gents,
Just bought boat below in sig. Yam tech doing engine survey and I noticed a low speed engine knock at idle. Ran like a raped ape at WOT, 5200 RPM, with bad props! Compression tested good. Carbs were loaded up with crap and I'm paying a bunch to rebuild 5 and replace one. PO did not run any fuel additive and tech said engines were heavy with carbon. I didn't pay to decarbon, I can do that. Any idea's what the knock could be? Thanks Guys!!

 

[richie rich]

Knocks aren't good...could be as simple as detonation, or could be a bad crank or rod bearing....if compression is good, that rules out piston rings......decarb will help the detonation...the rest is strictly mechanical......what are you paying for carb rebuilds? A new one may be cheaper than paying someones labor to rebuild.....and Yamy carbs do well in an ultrasonic bath when cleaning vs just a soak tank or spray cleaner....lots of hidden passages.

 

[BobP]

Get that fixed it will not fix itself and only get worse until ... you know what!
Get another yamaha tech opinion.

Or find another good running no knock used motor in the meantime, and don't take your time about it.

 

[Grog]

You don't really see detonation at idle when there isn't a load on the motor and the timing is pulled back. It may be a piece of carbon acting like a glow plug but you should see the same problem at WOT. A de-carb is cheap so do and see what happens but it's most likely a bearing.

 

[Tucker]

Thanks Guys, the PO had some hack mechanic work on the engines and ran the slow speed air jets in against the stops bending the needle. On the carb I'm replacing, the tip of the jet broke off in the seat and the tech could not remove it. The whole carb job including rebuilding the carbs and replacing the one that's FUBR is costing me about $2,000 and includes ultrasonic cleaning and throwing the boat over for testing and sycronizing. Yea, probably getting screwed, but I know I'll have good carbs set right (hopefully). The boat's 400 miles away, and the tech appears pretty good. Boat's being delivered next Wed. The knock isn't all that pronounced and can only be heard with the cowl off. Yea, hoping a decarb is going to make it go away but I'm checking costs on powerheads. Who has the best price and warranty? Anyone know of a tutorial on replacing a powerhead on these motors? The tech said the oil pump linkage was out of adjustment and both motors were getting over oiled. Who knows I might get lucky with the decarb.

Stay tuned guys, I'll report developments and I'm sure I'll need help with lots of issues. Have a great Thanksgiving!!

 

[sfc2113]

2 strokes should never knock and can only mean a bad piston or piston bearing that is going to leave you stranded at some point.

 

[BobP]

Suggest only "factory new powerhead".

 

[Robcat]

Could be wrist pin pound out,bring each piston to top dead center with spark plugs removed,use 1/4extension supply pressure if you get movement it's usually the piston worn out were the wrist pin is pressed in

 

[Tucker]

Guys, I've searched hundreds of posts on these big block 2-strokes and found that the only things that make these engines blow up is a snaffu with oil pump failing, the water pump failing, or a lean condition. In those cases the piston melts, ceases, or the crank blows a hole in the block. If the engines are oiling properly how can a freakin' wrist pin go bad? And, if a wrist pin or wrist pin brg goes bad, won't the compression in that cyl be low? Thanks gents, I'm depending on your experience!

 

[richie rich]

the compression measures the seal of the piston against the cylinder or how well a head gasket is holding up..(cylinder pressure)..a loose pin or bad bearing won't show that. The piston will still move up and down and the bearing will knock, then eventually fail in a bad way...so if compression is good...your rings are doing their job and so is the head gasket...( and the valves on a 4s)

Bearings aren't perfect...you can get a little debris in betwen the bearing surfaces and cause one to go....if the oil pump fails, you are correct, it will be catastrophic.

 

[Tucker]

Ok got it. Is this an a known problem with Yamaha 6-cyl motors of this vintage with 900hrs? I'm taking back all the good things I said about the tech. I called him this afternoon and asked him for his opinion, strickly off the record, and he ducked the question by saying it could be real serious or it could be simple as carbon build up; little pi$$ed about that. Now, if it's a wrist pin brg, and I could isolate it down to the cyl; isn't this a do-it-youself job?
I hear stories like this all the time with the Rudes & Johnny's but never the Yamaha's.

 

[richie rich]

I don't think its a Yamy thing...you would have heard it by now with all the motors out there....mine are 88's with less than 1000hrs and still no knocks or pings......a DIY job? depends on the DIY'er......if you've never been a gear head, maybe not so easy....if you've done engine work before, not so hard...thats for you to decide.....but if its one cylinder, one wrist pin is a lot cheaper than a whole new power head....but you'll only know after you've done your due diligence as mentioned above and finally take it apart as a last resort. Keep your ears and options open. Eliminate the obvious and cheaper solutions first....then go for broke.

PS...make sure the zinc trim tab on the motor isn't loose and causing the knock at low speed/idle...had that happen and thought the motor was falling off...it was a lousy 10mm bolt that loosened and allowed the zinc to rattlle and even touch the prop tip when backing into the slip.

 

[seasick]

Ok got it. Is this an a known problem with Yamaha 6-cyl motors of this vintage with 900hrs? I'm taking back all the good things I said about the tech. I called him this afternoon and asked him for his opinion, strickly off the record, and he ducked the question by saying it could be real serious or it could be simple as carbon build up; little pi$$ed about that. Now, if it's a wrist pin brg, and I could isolate it down to the cyl; isn't this a do-it-youself job?
I hear stories like this all the time with the Rudes & Johnny's but never the Yamaha's.

The knock could be a lot of things. A wrist pin is just one. If a wrist pin goes bad, usually the piston is bad or the rod is bad. A crank bearing is a possibility but so is a loose flywheel or stator. Knocking in this case will be more noticible when the idle is rough.
By the way, compression could be fine even if the pin/rod/piston is bad.
Personally, I think you got taken by the mechanic, especially if the original issue was the knock. After you spend $2,000 will he guarantee that the knock is repaired?
Do I think you yourself can repair the power head? No...

 

[Tucker]

Thanks Guys,
How loose does the stator need to be? Is it something that can be checked by reaching under the flywheel and trying to shake it? Or, does the flywheel need to be removed and the (4) stator hold down bolts checked for correct torque?

 

[Doc Stressor]

I had a wrist pin go on a 140 hp Evinrude years ago. It had the same symptom. You could hear a knock at idle, but not if you increased the rpm. When we tore the engine down, part of the wrist pin had fallen out and gouged the cylinder below the ring area. I had the block re-bored and replaced all of the pistons, which were provided under a recall program. The problem was diagnosed before tear down using a method similar to what Robcat posted. Compression in the cylinder was OK.

Had a connecting rod bearing go on another engine. There was noticeable vibration and the rapping noise could be heard up to 4000 rpm.

As posted above, carbon build up will not give you a rapping sound. Just a rough idle and low compression from stuck rings.

We always like to believe that a knock is caused by a loose part. But that is hardly ever the case.

You can usually isolate a noise at idle by using a stethoscope or just a hose held up to one ear.

 

[Tucker]

Thanks doc, been using the hose to the ear thing for years. I thought I was the only one that did that. I've had outboards all my life and I'm an old guy. To have 2-failures like you just mention happen to one person is bad Karma or you keep your foot in the carborator. I'm hoping for the best and planning for the worst. Yea, I plan to do the wrist pin check. But I'm concerned that I'll miss the "give" when pressing down the piston. How much slop is considered worn out? I'm not sure I can detect say 10-thousands of an inch out of tolorance.

 

[Doc Stressor]

As I recall, the other 3 pistons did not move at all when pressed with the rod. The bad one was obvious.

I''ve actually blown up 3 power heads over the years on big motors and one on a kicker. I'm also an old guy.

Back in the 70s and early 80s, when they began reformulating fuel on an annual basis, 2-stroke outboards couldn't keep up. Big 2-strokes were on borrowed time after about 500 hr. The TCW3 oil standard solved some of the problems, but they continue today.

 

[freddy063]

Explain your knock? I had a trouble on my port motor, start hard, rock and knock at low speed, checked motor all over, did not find anything wrong. Turned out being a bushing in the upper shaft seized on the drive shaft. A 40 buck part. Explain your knocking. When outboards run, the shaft turns the water pump in the lower unit. A bushing keeps the shaft right. Well it started to seize up and at first loaded down the motor at low speeds, until it locked up totally and the motor would not turn over. I never thought it was a trouble until it did not start, then I was worried.

 

[raisinkane]

Tucker, Does your Perko gas tank cap/vent assembley have the revised up dated raised collars under them to prevent water from entering the tank,thus causing problems.It was a bad design on all Tigercat hulls as they have that recessed area around fuel fills. Perko Part cat. no.054DPRBLK Accessory Mounting Ring 2 Required

 

[Tucker]

Boat comes up Wed afternoon. I'm in fresh water here and going to spend the first day washing down everything in vinegar and paint the bottom. Though no high heat alarms went off, I know the cooling system needs flushing on both motors. I'm filling the drum with vinegar and let the motors idle for a while. I'll have a better description of the knock then. Raisin, not a clue about the fill design; I'll take note when the boat comes up. Are you taIking about just the cap or the whole fill? I can say the carbs where really screwed up; a sure indication the boat had bad fuel. That's why I spending the money to get them right. If I need a new power head, I'll know the carbs are good. I'll post some pics when she arrives. Thanks for hanging in there guys.