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I got an e-mail asking about 70CES Yamahas.
To-day the UIM T850 rules allows aftermarket pistons and stock 70 hp yamaha parts.
Motors running in T850 in Europe now are generally slower than in the late 1980's and early 1990's.
Motor first appeared in the mid 1980s.
Most of the special CES parts, pistons, head and tuner are NLA.
The Last 70 CES 1990 version motor was 70 hp @ 6400 rpm. NOT THAT SPECIAL! Equal performance to the late 1980's OMC Formula E motor.
The problem with the record book with the 70CES was poor inspections in Europe and very mod motors getting passed.
HERE IS A THREAD FROM 2012 THAT COVERS THE SUBJECT.
Long but interesting read.
http://www.boatracingfacts.com/forums/s ... aha-70-ces

Since 2012 Matt and I have learned more about the twin cylinder T750 Yamaha and Yamaha built Mariners.
There are many different components and combinations than we thought.
There are several different power heads and two different engine displacements.
The early ones and the latest ones are 2/3's of the 90 hp and use the same parts [sleeve, piston and rod]
The Yamaha 60C and Mariner 60E models are 6F0 blocks with larger displacement and separate tuner and exhaust cover.
The Yamaha 60D is a 6F1 block with larger carbs, different porting and a very large steel tuner with no exhaust cover.
These motors are very popular in the world as a commercial outboard. There is an ENDURO series sold world wide which has different models depending on where it is sold.

THIS IS THE RAREST MODEL, THE 6F1....60DE
https://i.ebayimg.com/00/s/NzY4WDEwMjQ= ... F/$_86.JPG

http://www.boatmad.com/forum/attachment ... 1393443763

AND YOU CAN BUY A NEW SHORT SHAFT
https://i.ebayimg.com/00/s/NjAwWDQ1MA== ... V/$_20.JPG

Don,
The image of the block and tuner above is a true 6F1 block? If so, it looks like the 6F1 is more similar to the 760cc 60E/55B block.

I'm also not sure how a tuner like that would help since each piston has its own (different length) run down the side of the block and don't combine until they both open to the same extra wide expansion chamber.

From my experience so far, these engines are built quite well and great to work on but it'll be tough to catch the OMC 2cyls in power without block mods..

Matt, the current T750 racers are VERY good and VERY FAST. When the class started a few years ago the good T750 boats ran 54 to 55 mph. Jake had the top T750 and ran 57 mph. There is a video of Jake racing showing the speed. Eric's VooDoo T750 was a low 50 mph boat. Since then we have feedback from non T750 racers. Their lake boats are running 50 to 55 mph with stock non race prep motors. On the race oval to be a top 3 boat to-day in T750 you need 59 mph top end with acceleration, a good start, good driving and luck. The 3 OMC boats are running 59 to 62 mph is my guess. You need 2 to 3 mph to pass, so 59 mph is the goal. Matt you are going at it the right way. The largest gains are set up, prop and DRIVING. Once you maximize them. The engine prep may get you 1 to 2 mph.
There is no question in my view that the OMC race prep motor to the other T750 race prep motors will have a 5 to 7 hp advantage. BUT, the 50 Tohatsu, Pro 50 Yamaha, 4 stroke Mercury and the twin Yamaha/Mariner like yours have BETTER gear cases with less drag and better boat handling. The 50D Tohatsu currently holds the kilo record at 66 mph. That motor is turning a 22 pitch prop. Our 50D Tohatsu turns an 18 pitch prop and runs 57 mph. It cannot turn a 22 pitch period. There has to be some strong Tohatsu motors in Britain. I am working on finding one.

Agreed...set up is key..over the past two years we have gained the most from small set up changes and props coupled with jesse's improved driving to extract the most out of the set up. Jesse being light gives us some advantage out of the corners as he's up on the pad pretty quick. I can remember that days of running 54, excited to run 57, never dreaming of the days we'd break 60. This didn't happen overnight we have spent lots of hours testing and tweaking as well as listening and being open with other racers that have offered advice.

Matt has done an amazing job in a short period of time and his driving is pretty impressive considering he has the single cable steering, i think the yamaha and mariner have the ability to run the same as the omcs if not quicker especially the yamaha, proping the omc has it's advantage that's for sure due to availability

Paul, thanks for all the advise. And kind words. All the T750 folks as well and Don and Rick have all helped tremendously. A certain T750 team has helped so much! And I had an eye opening session of testing with them where I got to compare my thrown together rig to their winning rig by driving the same props on both boats. I'm certain now that the OMC'S keep making torque and power past 6500 rpm where the Yamaha falls off earlier. They might be 700 or 800 rpm apart.

I can forsee seeing my top speeds going up by 2 or 3mph with jetting and tuner and boat renovations.. But I doubt I'll gain that top speed *and* get anywhere near the torque that I felt with the OMC.. You guys are currently 4mph faster with more torque too!

All that having been said I'm sticking with Yamaha for now. I want to see how far I can go with this engine and boat. It has been too much fun.

If I could find a spec sheet to blueprint the block, I'd be pretty stoked.

Matt, a few thoughts:
- its the gear ratio and the 7000 to 7300 rpm limit that gives the OMC more punch.
- are you running stock metal reeds and the stock reed stops? Yamaha controls horsepower and RPM with their reed cages on motors where they use the same block assembly for different horsepower motors. Try 2 stage reeds, no reed stops. up the main jet size by 3, move the timing up 2 degrees and run 50 to one shell 91 with Evinrude 100% synthetic 2 stroke oil. If you have 150 compression with your head you could? shave it to get 160. Run that tuner you got from me this week.
- make a second trim stop on the motor so the trim does not go all the way in.....The yamaha gear case works better and is quicker thru the turn when it is not buried in the water.....get use to the boat being looser in the turns.....you will have less chance of hooking.....and the boat will have a faster exit speed which allows you to run MORE pitch without loosing acceleration out of the turn.

Too be a very good driver in smaller horsepower classes like T750 your must have the ability to run the boat at WOT all the time.Never back out of the throttle because it takes time for these smaller horsepower motors to wine up. You need to have confidence in yourself and confidence that the boat will recover when it gets out of shape. Kevi had raced the Tohatsu a few times and was comfortable racing the boat. He was sitting with Todd and I talking about T750 and the speed difference with T850. We told Kevi to never lift in the next heat and try not to trim down for the turns. Just drive the boat thru the turn even when it gets out of shape. He finished second in that heat and was only nervous entering the first turn. The turn itself set the boat and slowed it down.
Todd did exactly the same thing at Waterford one year when he raced Thom's T750 boat in T850 and had top 3 finishes.