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Old 06-02-2013, 11:42 PM   #1
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yamaha 90

Hi Guys
I'm running a 90 yam on a 5m humber rib and don't think its pulling as well as it could as it will only pull a 17 inch prop (standard ally yamaha prop.. New)
It reaches 5500 rpm on the tacho and sounds ok and pulls like a robbers dog.
I also tried both an 18'' and 19'' standard stainless prop but it won't pull full revs.
I measured the compressions cold and they were only 90psi, but equal on all three cylinders. I will however check with another gauge before getting the spanners out as I've had that one for about twenty years .
Anybody know what the compression pressure should be ? and if it is low as I suspect can I just have it rebored and fit new pistons etc ? or is it nickasil or anything tricky ?
I have been a car mechanic/garage proprietor all my working life so am fairly happy that I can rebuild it. I was thinking to give the ports a bit of a tidy up and fit some new reeds at the same time too ?
Once run in can I just do a flat out run and a plug chop to check the mixture and then main jet accordingly ? (like we used to with tuned cars in the old days !) I'd be grateful for any advice .

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Old 07-02-2013, 12:52 AM   #2
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Country: UK
Location: Portsmouth
Occupation: Marine Engineer
Interests: everything fast
Boat make: Phantom 28/phantom 18
Engines: 2x Mercury 3.0 225 optimax/yamaha 85
Cruising area: Solent

Join Date: Nov 2005
Location: Portsmouth
Posts: 1,329
Compression should be around
130psi+ the bores are steel so you can re bore if you need any info on the yam call me on 07702 958175 hope this helps rich

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Old 08-02-2013, 08:11 PM   #3
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Join Date: Mar 2010
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Good Thanks
I'll try it with a new compression gauge and if it then needs a strip down I'll ring you first.
Best regards
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