Yamaha OX66 EFI vibrates and rattles

AQUADJ

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I have a 1998 250 hp Yamaha OX66 EFI motor with about 800 hours. I've been hearing a slight noise that I haven't been able to pinpoint until today when it got much louder and started vibrating. According to my mechanic something let loose with the driveshaft or damper that will require him to almost completly disassemble the motor. I'm expecting his estimate in a day or so and I'm wondering what I should expect and if I should be considering a new motor.
 

AQUADJ

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Re: Yamaha OX66 EFI vibrates and rattles

AQUADJ said:
I have a 1998 250 hp Yamaha OX66 EFI motor with about 800 hours. I've been hearing a slight noise that I haven't been able to pinpoint until today when it got much louder and started vibrating. According to my mechanic something let loose with the driveshaft or damper that will require him to almost completly disassemble the motor. I'm expecting his estimate in a day or so and I'm wondering what I should expect and if I should be considering a new motor.
 

AQUADJ

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Re: Yamaha OX66 EFI vibrates and rattles

I just received the preliminary report from my mechanic. Compression in the top 4 cylinders was 115 and the bottom 2 was 105. When he disassembled part of the motor he found the main bearing let loose on the bottom of the motor and rattled around in the power head and damaged the fuel pump along with other stuff. He's now searching further to see if this power head is worth fixing, or if it should be replaced. He also mentioned the possibility of a complete new motor which worries me since this is one of a pair. Any comments?
 

Suncoug

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Re: Yamaha OX66 EFI vibrates and rattles

Thanks for the update. It sounds like you need a new powerhead...
 

rodbolt

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Re: Yamaha OX66 EFI vibrates and rattles

they are easy to rebuild and corse are everywhere, Joes marine has a few rebuildable cores in the back.
as long as the block is not broken and the crank and rods are ok its an easy rebuild.
 

AQUADJ

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Re: Yamaha OX66 EFI vibrates and rattles

I had a conversation with the mechanic who gave me my options. As I understand it he can attempt to rebuild this motor, but there is the risk that metal fragments are in the motor and will cause damage down the road. The alternative would be a new power head, oil pump, fuel pumps, oil filters/strainers, and some misc. parts for about $8,000.00 compared with a liitle more than 1/2 that for the rebuild. I'm not capable to do this work myself so I'm relying on a reliable mechanic and his advise. Any comments on wether the new vs. rebuild is worth the cost?
 

AQUADJ

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Re: Yamaha OX66 EFI vibrates and rattles

Thanks everyone for all the suggestions.
I met with the mechanic and owner today who made my decision pretty simple. After pulling the heads and seeing some metal fragments in the cylnders, I decided to replace the power head.
 

rodbolt

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Re: Yamaha OX66 EFI vibrates and rattles

why?
yjats what they make scrub brushes and ater for.
ya think fragments are left after machining? ya think we clean it everyday ?
I usually base my go/no go on rebuilds by the amount of corrosion in the cooling system.
if the crank and rods are good its cheap and easy to build a powerhead.
bore all cylinders,carefuly inspect the main bearings, always replace the bottom main. I always replace all pistons,pins and pin bearings, rod bearings may be reused if in good shape and placed back where they started life.
use NEW rodbolts.
fuel pumps are a good idea. the mikuni oil pumps,unless contaminated with trash or water, are generally goodfor 3-5K hours anyway. yamha will give 90 days warrenty on a new powerhead. many places, like southwest, will give 15 month warrenty on a reman powerhead. check with some of the iboats suppliers on avalibility and warrenty.
 

AQUADJ

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Re: Yamaha OX66 EFI vibrates and rattles

Rodbolt,
I wish I had your input sooner, I may have looked into the rebuild further. My mechanic discussed the rebuild vs. the new and I thought he was being honest in saying he's not a rebuild shop and doesn't typically buy rebiult powerheads, but never said it wasn't an option. It's not as though I have money to burn but I based my decision on thinking the new power head would have less chance of failure in the future. There were also some minor issues - the #4 piston had something marks from something banging around between it and the head that concerned me and I'm still not sure why the lower bearing let loose.
Thanks for your help and suggestions, they're always appreciated. And why can't you work in NJ!
 

rodbolt

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Re: Yamaha OX66 EFI vibrates and rattles

the lower bearing has to accept radial as well as axial loads. its a ball bearing, ball bearings typically have shorter duty cycles thn roller bearings. that being said the normal cause is water injestion from somewhere then the engine sits and the bearing rusts some.
thats why I ALWAYS replace that bearing no matter why the block was apart.
most techs have never actually rebuilt a powerhead. there are only 2 shops in my area that will. the rest simply give you an option of replacing it or repowering.
but if the crank and rods are good its usually a simple job as long as you can find a competent machine shop. marks on the head can be smoothed with a die grinder and the piston would have been replaced anyway.
there is probably 5 rebuildable 3.1 OX 66 cores at joes marine that could be had for a few hundred or so.
if folks actually knew how simple a two stroke block was I would be out of a job.
hardest part about them is attention to detail and cleanliness at assembly.
takes me roughly 20 hours to strip,clean,inspect and reassemble a 3.1 powerhead and another 5 or so for removal nd instllation.
 
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