jet sizes on modded yam 50hp w/ 70 carbs

Hi,

My name is Victor, I'm from Finland. And 18 yrs old and I am very interested of race boats.

I have a Yam 40/50hp two-smoker on a 15feet v-hull boat (Marino Swing, check out the MarinoSwingCup on google, awesome boat racing here in Finland).

Im tuning the engine now under the winter, and the block is getting ready (higher ports). It will also have lowered head, tuner, CCMS reeds etc.

I will put 70hp carbs on it, and my question is what jet sizes I should start from with the 70 carbs. Especially the ide jet size wonders me, the main jet I will seek for the best one in the spring. Any suggestions on jet sizes?

Thank you for any help,

Happy new year,

/Victor


Picture of the block, not yet finished, only raw grinding done this far.
Attachment 59033

Check out my thread, I built a modified tohatsu 50 with yammy 70 carbs. I ended up with 130 main jets, I think I could go leaner though. I know of another 2 guys who put 70 carbs on 50's and they had 140 and 130 jets, but they both said they could go leaner.

start with 145's

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Originally Posted by Fast Fred

start with 145's

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Okay, what about the idle jets?

never did change them low speeds, just worked the screw, sinc and link them up nice, so they are all in time, start at 1 1/2 turns out.

What about the roller shaft on the lowest carburettor, do I need to change the damper shaft to make the carbs work?

the cam may have to change, the wedge that pushes the roller, but that should be all. the 50hp link is used on the 70hp carbs when on a 50hp.

finger ports..... get some

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Originally Posted by Fast Fred

finger ports..... get some

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I have 145's in my 70 carbs mounted on a 40hp and they are on the edge of lean. Finger ported motor.

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Originally Posted by proprider01us

I have 145's in my 70 carbs mounted on a 40hp and they are on the edge of lean. Finger ported motor.

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I am wondering if these motors are reacting the same as my 70 hp yamaha. When I got a 70CES head and mounted it on my 70 hp yamaha I was advised to run 90 hp carbs. The motor ran lean. I thought the motor was getting too much air and not enough fuel due to the larger carbs. So I put larger jets in the stock carbs. Other than a very high idle 1200 rpm the motor, has run perfect and untouched since 1996. So maybe? you just need larger jets in the original carbs??

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Originally Posted by proprider01us

I have 145's in my 70 carbs mounted on a 40hp and they are on the edge of lean. Finger ported motor.

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I think I start out with 150 just to be sure, better rich than lean when breaking in the engine.

What else have done to your engine?

What makes the yammi 40hp tick :cool:

Lightened flywheel, welded crank, HydroTec high compression head, coated pistons, Fiber reeds with massaged cages, custom tuner pipe....http://youtu.be/uqRzw5Tihu8

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Originally Posted by vStrandell

I think I start out with 150 just to be sure, better rich than lean when breaking in the engine.

What else have done to your engine?

What makes the yammi 40hp tick :cool:

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Any suggestions regardning compression? Mine has only 115psi even with 1mm lowered head (newer one, center plug). Is it enough to give some Powerr? :cool: Or should it be more around 150? The crank rods on this engine does not feel strong at all, very tiny ones, I wonder why yamaha put so small ones in... So is 115 enough? I would prefer lower comressions, since the engine should last long, as I use every day troughout the summer.

I get 180psi out of mine, but don't use it daily..

First start-up yesterday http://youtu.be/wbrnvExB7oM

Porting your engine will also affect your compression

Yes, I suppose that is why it has only 115 now, I raised the exhaust port 1.5mm.