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My brother and I raced a 90 HP Yamaha on a 16' Tandem Deck Phantom powerboat back between 1979 and 1981. We were sponsored by Brimlow's Garage and Marine Centre, a Yamaha Main Agent in Poulton Le Fylde, Blackpool, Lancashire, England, UK. Mr John Brimlow acquired some tuning advice from Yamaha at the time which was to increase the compression and increase the exhaust ports Top, Middle but not the bottom one, there may have been some re-jetting of the carburettors also. We won many powerboat races, North-West Powerboat Championship on three consecutive years, Newcastle Grand Prix, and North-West Ski Racing Championship on three consecutive years to name some of the successes, the engine was reliable and quick. I am now setting up another boat at this moment, a 14' Bristol racing boat / 90 HP Short Shaft conversion combination. I will run it with the gearbox a certain height as we did on the 16’ Tandem Deck Phantom Hull, high but still able to pick up cooling water through the normal intakes. Would it be possible to expand on the tuning of the engine, advise the exhaust port duration opening in degrees, and suggest any surface propellers, we ran 20 & 22 QSS Chopper and an OMC SST 21 pitch propellers on the 16' Tandem Deck Phantom Hull, however this boat is lighter and more streamlined. I may construct three expansion chambers exiting the water jacket with silencers fitted. My brother started this conversion in 1980 but found the sound to much as he only used a straight three into one exhaust pipe and trumpet tailpipe. Any advice or comments would be greatly appreciated. I now have a 75HP, 90HP, 90HP Modified Powerhead with +45 Thou Wiseco Pistons, a decked cylinder block and cylinder head so the compression has increased, 80HP leg assembly & 70HP leg assembly that I intend to cannibalise to build My engine. Is the 70 HP gear casing and gears the same as the 90HP gear casing and gears? Are the 70HP driveshafts and 90HP drive shafts the same dimensions and do you make and supply 15" short shaft drive shafts that fit the 90HP powerhead and gearbox so all I will have to do is shorten the internal components, cut reduce and weld up the leg section. I have four power trim and tilts units at the present. Do you sell fibre reads valves for the 90 HP carburettors (Yamaha 60-90 Hp Racing Boyesen Reeds - 333R)? What is the maximum PSI you recommend We went up to 180 Pounds per Square Inch, but it was difficult to start with the original starter motor so that this was then reduced to a lower value of Pounds per Square Inch? I intend to use racing spark plugs and a ratio of fuel to two stroke lubrication oil of 25 to 1 to create an improved seal between piston, piston rings and cylinder bores and lubrication. I will use performance, racing spark plugs. This may not be as much as a problem with a modified exhaust system. give Me your thoughts. Regards Kevin P Cross Incorporated Engineer MIET kevinpaulcross1@gmail.com
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