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Old 20 February 2018, 11:51   #1
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Country: Sweden
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Make: Gemeni
Length: 4m +
Engine: 50hp yamaha
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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, 15:35   #2
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Make: Ceasar Thundercat
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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 - My thundercat

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, 15:24   #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, 02:37   #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, 16:45   #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, 16:50   #6
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Looking for lifewests like Formula LifeJacket B/210 or simular.
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Old 26 February 2018, 07:25   #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, 13:30   #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, 05:13   #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, 13:19   #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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