2004 70hp yamaha 2 stroke knocks on quick deceleration

[aussie_boater]

hi im looking at a 2nd hand yamaha 70hp 2004

upon looking it over i noticed that when i gave it a good rev (3000-4000rpm approx) on deceleration it would give a couple knocks...

nothing huge just enough for me to notice when i was holding onto the motor or i wouldn't have noticed.

the seller claimed it only had 90 hours on the motor and it seems in fair good condition with compression being 115, 110, 105 psi. (cold)

if the motor has only 90 hours should the compression be more??? like 140psi i was thinking???


please help

 

[aussie_boater]

Re: 2004 70hp yamaha 2 stroke knocks on quick deceleration

is something wrong with it or is this normal for yamaha motors?

 

[Bamaman1]

Re: 2004 70hp yamaha 2 stroke knocks on quick deceleration

I would suggest an in water demonstration including a full throttle run (after warmup)--looking to see how the engine runs at full throttle (approximately 5500 rpms.)

If there's still any knocking, you might have the engine look at by a real marine mechanic--at the seller's expense. Knocks can indicate problems with connecting rods on the crankshaft. It's just not normal.

The 90 hp Yammie has a reputation for being very bullet proof--great motor.

 

[jeffersondodson561]

Re: 2004 70hp yamaha 2 stroke knocks on quick deceleration

Timing first. But due to your compression results I would do a decarb of the engine. I have heard on this site that Seafoam works well, I personally have not used it butI know OMC Engine Tuner and Ringfree/Quickleen works well.

 

[robert graham]

Re: 2004 70hp yamaha 2 stroke knocks on quick deceleration

hi im looking at a 2nd hand yamaha 70hp 2004

upon looking it over i noticed that when i gave it a good rev (3000-4000rpm approx) on deceleration it would give a couple knocks...

nothing huge just enough for me to notice when i was holding onto the motor or i wouldn't have noticed.

the seller claimed it only had 90 hours on the motor and it seems in fair good condition with compression being 115, 110, 105 psi. (cold)

if the motor has only 90 hours should the compression be more??? like 140psi i was thinking???


please help

If you're revving motor to 3-4K RPM's out of the water and on the muffs, maybe not a good idea....I believe about 1500 RPM's is considered a max on the muffs. Take the boat out on the river for a real sea trial and you may not hear that "knocking"....hopefully. It's the only way to really check the motor's health and performance. Good Luck!

 

[aussie_boater]

Re: 2004 70hp yamaha 2 stroke knocks on quick deceleration

hey thanks for the feedback guys, got it mounted now jsut need to find a yamaha controller box that has a push accelerator and not pull =(

 

[aussie_boater]

Re: 2004 70hp yamaha 2 stroke knocks on quick deceleration

ok i took the motor out for a test run.

warmed it up for a good 10-15 mins at the boat ramp.
took it out held it at about 4k for a good 5 mins then took her to about 5200-5500 for a couple mins.

the knock wasn't present during the run at all, in fact it completely went.

she ran like a dream for the first 15-20 litres of juice.

on the way home however she just slowed and died almost, running at idle and that's it.

so i assumed it was plugs as i put a little oil in the fuel tank as well until i'm 100% sure the oil mixer is working.
cleaned the plugs with no difference.

i let it idle a while and it seemed to return to normal and went like a dream again, wierd,
it continued to stop about 3 times more on the way back but then returning to normal.

i did a compression test on the boat at home.....

now its saying that the 2nd cylinder has 0psi with the others still showing above 100

have i got a stuck ring or something??

will this seafoam stuff you guys talk of unstick a valve if that is what it is??

 

[99yam40]

Re: 2004 70hp yamaha 2 stroke knocks on quick deceleration

Sounds like you toasted a piston due to lean fuel/air mix as it was only on one cylinder or just over heating.
Either way fuel additives will not help at this point.
need to take apart to see what happened

 

[aussie_boater]

Re: 2004 70hp yamaha 2 stroke knocks on quick deceleration

wow i hope not, is there any way that its a sticky ring??

if the ring was "fried" wouldn't it still show some compression even 5?? on this the needle doesn't even move a tiny bit.

 

[99yam40]

Re: 2004 70hp yamaha 2 stroke knocks on quick deceleration

sorry but stuck ring is out, May have a hole in the piston if it is truly 0, or the piston is not moving up and down like it should

pull the plug and look into the cylinder with a flashlight to see what you can see

 

[aussie_boater]

Re: 2004 70hp yamaha 2 stroke knocks on quick deceleration

yes piston is moving, i will look with a torch, thanks.

no valves on 2 stroke outboards?

 

[aussie_boater]

Re: 2004 70hp yamaha 2 stroke knocks on quick deceleration

went out and did another check today with the torch, couldn't see any damage to the piston so did another compression test,


from 0psi a couple days ago to 95psi again.....

definitely a sticky ring, ran it on the earmuffs a couple minutes and then you could hear when the ring stuck again and the motor begun to miss badly again.

does anyone from Australia know the equivalent of OMC Engine Tuner and Ringfree/Quickleen please??

 

[aussie_boater]

Re: 2004 70hp yamaha 2 stroke knocks on quick deceleration

SOLVED

put about quater of a can of CRC / WD40 in the cycinder and put the plug back in then turned it over a few times and repeated 2 times,

let it sit a couple hours

took it out not long ago ran like a dream.

weirdest thing ever.

compression when warm was up to 140 on the cylinder that had 0 a couple days ago.

weirdest thing ever.


thanks everyone for all your help =)

 

[99yam40]

Re: 2004 70hp yamaha 2 stroke knocks on quick deceleration

never seen a stuck ring cause 0 compression before,
good luck in the long term for your new used motor