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Old 01-05-2016, 05:10 PM   #1
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Yamaha 90 AETO

Hi Chaps
I have a Yamaha 90 on my rib that has low compressions.
I understand it has conventional steel liners so I intend to have it rebored and rebuild it.
A google search comes up with a few US engine and tuning parts suppliers.
(Hydrotec for one) but their parts lists don't seem to show the AETO as an engine code. Are US engines different to UK ones ?, will the bits fit or are there any UK engine and tuning parts suppliers for this engine ?
It seems that Wiseco pistons are the way to go but I did meet a very knowledgeable chap at the thundercat event in Saundersfoot last year (or the year before, we were helping with safety cover) that recommended Mahle pistons ?
He was something to do with the organisation of the event, very knowledgable but I've lost his details !
I'm thinking rebore and pistons ,bearings, reeds and jets and a clean up of the ports. I still want it to idle normally though.
Any advice greatly appreciated.

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Old 02-05-2016, 11:07 AM   #2
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Hi yes usa engine codes are different from the uk.

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Old 07-05-2016, 05:45 PM   #3
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Thanks for that Ady
The parts supplier in the states that I emailed said that the powerhead internals are the same but the carbs etc vary.
I've had a couple of very useful PMs from forum members so I'll be following them up when I get back from holiday in a week.
Going to recheck the compressions with a new gauge before I get the spanners going though !
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Old 12-06-2016, 05:59 PM   #4
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Hi again chaps
After a couple of useful conversations with boatmad members I've got the bits together for my engine rebuild.
I've got new oversize genuine Yamaha pistons off ebay as they were cheaper than importing them from the states, But................
the part number is 688 11636 81, they are second oversize, I have matching rings.
I was going to use the old gudgeon/wrist pins as they are perfect but the new piston takes a shorter pin.
I'm assuming they are old stock or something, dimensionally the pistons look like they will fit, skirt cut outs and piston height look right as does the crown shape for the squish.
Does anyone know if they will be ok ? particularly will they hang together on a slightly modified engine ?
I haven't had the block bored yet so I can still go with a different piston if needed.
Thanks in advance for any advice !
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Old 21-08-2016, 07:03 PM   #5
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I had it rebored and imported all the bits from the states plus a hydrotec phase kit (head,reeds,jets,exhaust) I can confirm that US engine internals are exactly the same as UK market ones.
Cleaned up the block to match the the ports in the liners,and generally cleaned up any rough bits and screwed it all back together with a carb clean and new water and fuel pumps,thermostat etc..
Ran it in for five hours then a further five hours of sympathetic use and it now happily pulls a 19'' prop when before it would only manage a 17''
I haven't checked the compressions yet but it feels and sounds a lot better.
Its on a 5m humber destroyer rib which is quite heavy, the engine is on a jack plate with a four inch setback and the vent plate is about two inches up from the bottom of the hull.
The 17'' prop was a standard alloy yam one ,the 19'' is a basic looking stainless solas, no cupping or vent holes,both are new and unmarked.
Any advice for a better prop please and should I run it higher ??
water pressure is 15psi at WOT , flat out RPM is 5500.
I'm thinking 19'' laser 11 or is there something better.
Many thanks for all the previous advice too !
(I know it'll never be fast but its fun and seaworthy as hell)
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