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Old 19 July 2007, 07:37   #1
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Country: UK - England
Town: bicester, Oxford
Boat name: mach1
Make: avon SR5.4
Length: 5m +
Engine: 90hp yamaha
Join Date: Jan 2005
Posts: 177
performance upgrades for Yamaha 90hp

As a man who has owned 2 rally cars, which were heavily modified, i was hoping someone would have ideas (trade secrets) on simple performance modifications, filters, timing, tuning etc ??

I know reliability has to be at the top of our priorty list, but with cars! simple mods do not often effect reliability and can sometimes improve economy.

But as this is a 2 stroke i hounestly have no idea where to start, or if its worth persueing

I have a Yamaha 90hp 2 stroke (commercial version) 1996 grey not blue case!

ANY HELPERS!
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Old 20 July 2007, 00:53   #2
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Country: UK - England
Town: Gosport
Boat name: April Lass
Make: Moody 31
Length: 9m +
Join Date: Aug 2001
Posts: 4,850
Ian, these engines have been used in 1.3L circuit racing. You might be better off asking the same question over on Boatmad or using some of the links on there to the offshore racing drivers association etc.

Pete
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Old 20 July 2007, 05:43   #3
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Country: UK - England
Town: bicester, Oxford
Boat name: mach1
Make: avon SR5.4
Length: 5m +
Engine: 90hp yamaha
Join Date: Jan 2005
Posts: 177
cheers pete

i spent years tinkering with cars now of have a floating toy i fancy getting my hands dirty again
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Old 28 July 2007, 11:59   #4
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Country: USA
Town: boston
Boat name: Miss bad 61
Make: Crapko, AKA Mako
Length: 5m +
Engine: OMC Mod50
Join Date: Apr 2006
Posts: 398
finger ports wake it Right up, dapendin on your hull, you may benefit by addin some timein to the exhaust port. more compression would also help.
are you turnin it passed the limiter?
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Old 30 July 2007, 08:26   #5
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Country: UK - England
Town: bicester, Oxford
Boat name: mach1
Make: avon SR5.4
Length: 5m +
Engine: 90hp yamaha
Join Date: Jan 2005
Posts: 177
hi fast fred

no its reaching WOT at 5100 with the stainless titan2 prop, so limiter is not even reached. the extra compression would involve a head skim, i guess reprofileing the valves again involves rebuild.

As the engine is excelent in every way i think i will avoid rebuild mods for now. what im after is simple mods, bigger jets on carbs, is it worth removing the air box and retuning all 3 carbs, ignition advance etc..

you spoke of timing on the exhaust port?? how is that adjusted???
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Old 31 July 2007, 06:53   #6
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Country: USA
Town: boston
Boat name: Miss bad 61
Make: Crapko, AKA Mako
Length: 5m +
Engine: OMC Mod50
Join Date: Apr 2006
Posts: 398
seems you would benefit most by jackin the motor up on the transome, this may or may not work on your boat it all depeneds ware it is now.

reed blocks are the limiting factor on the intake, thare is a lot to it.
i 'd work the set up first. your not usein your full 90hps yet.
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