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Old 20 February 2018, 12:51   #1
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Yamaha 2-stroke 50Hp Tuning

Hi

Getting fast for summer in Sweden!

My to do list:

1.135#Jets
2.Boysen Reeds
3.Exhaust mod?
4.Velocity Stacks
5.Cut rev-limiter
6.Weld Crankshaft?
7.Shaving Cylinderheads?
8.Mill Cylinderhead?
9.Remove Autolube.
10.Unsure of porting?
11 More tips??


I am tuning my Yamaha 2-stroker 50hp for my Thundercat.
I have red about sevrel tuning tips but still have a few questions?
How much can I cut off from the exhaustmanifold ? Is there a risk cutting off too much. As I have learned a 2-stroke engine must have some resistens? Maybe there is enough resistens inside motor ?

Why should I weld my Crankshaft if I cut off rev-limiter?

How much can I remove from the cylinderhead and how much shaving on cylinders?

Porting? only exhaust?

Grateful for all tips.
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Old 21 February 2018, 16:35   #2
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Exhaust mod is loud, only really helps if you are running over-hub exhaust props. If you are then the acceleration is far better.

The rev limiter should be a grey wire looping out of the CDI box, you can cut it or it might have a connection on it you can remove.
I think on the Yamaha 50hp the crank is very strong, I've not seen one but I've heard you can get away without welding the crank. On the tohatsu 50 and Yamaha 70 you need to weld the crank if you're running high RPM or else it rotates on the journals and the timing will go off. You only need a small weld to hold it in place. You can check out my thread for more details on this kind of thing - http://www.rib.net/forum/f50/my-thundercat-61912.html

Not sure on the Yam50 cylinder head but on the tohatsu you remove 1mm and that increases compression around 25-30psi.
Take it easy on the porting, just clean them up and remove any casting imperfections. If you can find the homologation sheet for the engine you can use that as a guide

You can balance the crankshaft and flywheel, quite expensive because the crank is press-together so it has to be taken apart, balanced then re-assembled. But the engine runs very smooth.

Most of the speed comes from good propellers, a 14/15p is good for all round use and 18p cleaver or chopper for high speed use.

Hydrotec in the US make some nice performance parts for the Yamahas

For velocity stacks - https://www.facebook.com/forgemarine/ (my site)
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Old 24 February 2018, 16:24   #3
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Thank you for your answer. Really helps alot.

I have now started working with my engine. Wondering if I should use my Reed Stops as in picture nr.2 or leave them out as in picture nr.1?
I have disassebled autolube and will run on premixed 1:50.I will probably make some kind of tray with a gasket for sealing, insted of leaving oilpump on motor.
I have looked at Hydro Techs homepage and was thinking of buying a High Perf Head Kit. But probably I can mill the original one instead? If I do mill the original one 1 mm do you think I also have to shave the cylinderheads?

Is this what I should put on for extra speed?

https://www.ebay.com/itm/Signature-S...JaXSLC&vxp=mtr



Cheers Fredrik
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Old 25 February 2018, 03:37   #4
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You don't need the reed stops, I removed them on my tohatsu 50.
I'm not sure how much you can take off the cylinder head, best to do a squish test. I would guess 1mm but it might well be different on the yamaha 50's, you don't want the pistons hitting the head!
Have you got any photos of the exhaust tuner? Can't remember what the Yam 50 one looks like.

Those signature props are made by Ron Hill, I would avoid them if possible. Very poorly made, usually 1-2 inches of pitch off and unbalanced across the blades. I'm having some propellers made just now which will be perfect for high speed on thundercats, will be heat treated, blueprinted and balanced. Should be ready in the next couple months. If you can't wait then try and find a good used Mercury or Winrace cleaver.

cheers,
Seumas
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Old 25 February 2018, 17:45   #5
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Hi

No Reed stops then on my two stage Boysen. Did you use sandpaper 300grid and 600 grid before installing new reeds? I have rubber on cage underneeth the reeds and think they will seal without using sandpaper?
You can see HydroTech exhaustmanifold and original in the photos. Probably I will buy HydroTech High Performance Head and modify original Exhaust manifold.
Will you get your propellers before summer? I now only have one propeller Trophy Sport 15 pitch.After I tryed my propeller and registrated revs/min perhaps you could give me a hint what pitch I shoud go for on a bigger one?

Cheers..
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Old 25 February 2018, 17:50   #6
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Looking for lifewests like Formula LifeJacket B/210 or simular.
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Old 26 February 2018, 08:25   #7
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I didn't use sandpapaer, everything was in good condition.
Not sure exactly what the difference is between the hydrotec and standard head, i think for their 70hp phase kit they use a replica of the 70ces head. Let me know how you get on with your exhaust, I'm looking at designing a billet one for my 70hp at the moment. May end up using welded stainless as the billet is very expensive!

Yes props should be ready by summer I hope, will need some testing before I put them for sale too. Trophy sport should be a good prop, for surf and rough water anyway but won't be very fast top speed. Unless you have a very heavy crew + setup an 18" pitch is usually ideal for a high speed prop on a thundercat

I use old grabner racing lifejackets, not the prettiest but they have a big collar and good back support

cheers
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Old 27 February 2018, 14:30   #8
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Anyone knows about milling cylinderheads?

How much is it possible or recomended to mill my Cylinderhead? Yamaha 2-stroker HMHO.
I was thinking of buying a Yamaha High Perf Head Kit from Hydro Tech Marine.But it seems like they simply have milled down a original one? I was thinking I will do the same. As I am unsure how much I can mill my head Please help.
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Old 04 March 2018, 06:13   #9
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Complete takedown.

Wish me good luck..

What kind of gasket or sealent do you use here?

Fredrik
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Old 04 March 2018, 14:19   #10
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Quote:
Originally Posted by heffak View Post
Anyone knows about milling cylinderheads?

How much is it possible or recomended to mill my Cylinderhead? Yamaha 2-stroker HMHO.
I was thinking of buying a Yamaha High Perf Head Kit from Hydro Tech Marine.But it seems like they simply have milled down a original one? I was thinking I will do the same. As I am unsure how much I can mill my head Please help.
You are correct in that hydrotec do modify new original castings for their performance head. However as well as milling the head, they lathe out the combustion chambers to allow piston clearance.

If you are going to have a go at tuning it yourself, you have to be careful how much you machine off before you encounter piston clearance issues.
I never messed with the 50 but I know you could mill off 1mm on the 90hp before you needed to worry about clearance. Best thing to do is use solder or plasticine to measure your stock clearance then work put what you can remove with a basic skim.
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Old 04 March 2018, 14:25   #11
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Also I used to make my own performance exhaust using hydrotecs photos as a reference (the website is unchanged since 2002 or earlier!)
They use stock Yamaha tuners cut down and then their own fabrication welded on. You can pretty accurately measure their dimensions from the photo and with a tuner in front of you and then make a replica if you are handy with an AC/DC tig. It used to take me 1/2 hr to make one.

When I worked at Yamaha dealer we used to use Yamabond if we had any on the go for the crankcases. If not then any sealant we had to hand - wellseal, hylomar etc as Yamaha machine the cases so accurately.
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Old 05 March 2018, 04:03   #12
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When measuring piston head clearance with solder, make sure it's lead as the new lead free doesn't squash as easily and will give a false reading, also use 1mm diameter solder and insert trough the plug hole above both sides of the piston above the gudgeon pins to prevent rocking. (just a tip guys) be careful not to insert it into transfere ports. I suggest setting to tdc then 15 degree back, to insert solder (also don't drop it inside use 1 length looped so you can feed it in easily).
I'm not sure about the std squish but I wouldn't run closer than 0.8mm.
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Old 05 March 2018, 12:38   #13
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Quote:
Originally Posted by phantom 19 View Post
Also I used to make my own performance exhaust using hydrotecs photos as a reference (the website is unchanged since 2002 or earlier!)
They use stock Yamaha tuners cut down and then their own fabrication welded on. You can pretty accurately measure their dimensions from the photo and with a tuner in front of you and then make a replica if you are handy with an AC/DC tig. It used to take me 1/2 hr to make one.

When I worked at Yamaha dealer we used to use Yamabond if we had any on the go for the crankcases. If not then any sealant we had to hand - wellseal, hylomar etc as Yamaha machine the cases so accurately.
Any photos of the exhausts you made? Did you do any work with the Yamaha 70hp's? I'm looking at making a new exhaust tuner for my CES. The one on isn't bad but theres a bit of a step from the flange to the tuner which can't be good for flow. Want to stick pretty close to stock for the class rules
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Old 05 March 2018, 17:56   #14
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It’s a long time since I last made one. I did have a nice polished alloy yam 90 one I bought from DB Marine years ago (they specialised in XR2s etc), that was a tubular cone type, you had to ditch the outer muffler, not sure who made it but it was very nice.

The ones I made were the standard tuner, cut off half way and then tig welded exactly how hydro tec do it. I may have some photos somewhere. I spent a long time flowing the exhaust through the mid block and midsection on the 90s, they have a few odd bits of casting which form restrictions.
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Old 06 March 2018, 07:57   #15
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I was thinking of designing one and either having it made using CNC or maybe casting the tuner section, although I suspect it will be easier and cheaper to spend time welding one together. Was going to have a new flange cut so I could test back to back and see the results

How much shorter roughly were the hydrotec ones to standard?
Here is the original one from my CES, I had it roughly replicated in stainless but I'm not 100% happy with it. Photo on the bottom is sort of what I'm thinking for a replacement

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Old 06 March 2018, 16:20   #16
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Milling heads.

Hi

Thank you.

Today I have made larger airintake on hood nr.1
Probably I will make my own exhaust in stainless with what you can see on picture nr.2.

About milling my heads. I have mesured them sevral times and as I can see they are exactly the same. Left one from my 50 hp Yamaha and the right one from a 40hp Yamaha. I was thinking of milling 1mm and I will also lathe out new squishbands. When Im finished the top of the head will look the same but depth of combustion chamber will be 1mm more shallow.

Am I on right track? Really dont know if this is right?Anyone knows how much hydrotech does mill on their heads?

1mm?
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Old 07 March 2018, 03:17   #17
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Loving your work!
What was your compression before? I would suspect the Hydrotec heads don't use massive compression as the kits are aimed at the US market with lower octane fuel
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Old 07 March 2018, 10:04   #18
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Compresion

I had more or less 120 PSI on all 3 118 124 119,

Have not yet milled my head. But probably I will take off 1mm. If anyone have done this on a yamaha 40 or 50 hp please tell me how much to start with.

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Old 29 March 2018, 15:24   #19
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Porting & Exhaust

Porting and my own exhaustmod.Classic 2-stroke funnel to create underpressure that hopfully will drag out more exhaust fumes. Im looking for new 70hp Yamaha carburators to fit my 50hp Yamaha. Is there anyone who knows what carburators I should look for? Any special model or year of Yamaha Carburators?
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Old 29 March 2018, 15:30   #20
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Exhaustmod&Porting

New mods.
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