help modding a 40hp twin yammy

**serious3** · 01-29-2012

few pics of the boat! its about 13feet long and 7 feet wide and wieghs about 220lbs they are rated for up to 70hp

and for those that want more info
http://www.zapcat-racing.com/Home.aspx ( if i cant/shouldnt post links please feel free to remove it!)

**88workcar** · 01-29-2012

You need a cleaver and a low water pickup. ASAP!!!!!

**microbream** · 01-29-2012

Originally Posted by serious3

so how much would you remove then?

to give you an example i took 1mm (40 thou) of my twin cylinder, this increased the compression from 115 to 190psi. I run 95 octane at 30:1 and she runs hard!!

Two strokes are all about the squish clearance, my initial squish was 2.2mm, it is now 1.2mm, i could have got it down to say 0.8mm but it is risky when you take into consideration all the engine tolerances. IMO 200psi is as high as you want to go, this is what mx bikes have and run hapily on 95 octane.

seriously it is a quick cheap mod that you can feel.

look up these guys as well they know all about thundercats!
you will find many are running rons cleavers and topping 100kmh!

http://www.thundercatinflatable.com.au/forum/

**serious3** · 01-30-2012

Originally Posted by 88workcar

You need a cleaver and a low water pickup. ASAP!!!!!

your preaching to the converted! trouble is i reckon there are only maybe 15-20 low water pick ups in the country and the smallest would be on a 175merc, as for finding a small cleaver or semi cleaver forget it!!

**Ballistic** · 01-31-2012

Originally Posted by microbream

to give you an example i took 1mm (40 thou) of my twin cylinder, this increased the compression from 115 to 190psi. I run 95 octane at 30:1 and she runs hard!!

Two strokes are all about the squish clearance, my initial squish was 2.2mm, it is now 1.2mm, i could have got it down to say 0.8mm but it is risky when you take into consideration all the engine tolerances. IMO 200psi is as high as you want to go, this is what mx bikes have and run hapily on 95 octane.

seriously it is a quick cheap mod that you can feel.

look up these guys as well they know all about thundercats!
you will find many are running rons cleavers and topping 100kmh!

http://www.thundercatinflatable.com.au/forum/

Are you sure? 190 sounds a bit much for just 1mm off the head...

Serious3.. I know you don't wanna hear it.. but get a 3cyl Yamaha or Tohie and spend some money on that to get it modded. I think putting money into this engine is just throwing it away.
http://www.youtube.com/watch?v=ASnj4l3NliE

**serious3** · 01-31-2012

sensible head says do just that! BUT there is a little bit of me that gets a kick out of modding something like my twin, i can get the head skimmed for very little and lightening the flywheel and balancing can be done by me, besides that i'd love to see peoples faces when they cant shake me off! if i can get a s/s prop that i'm happy with then i reckon 52-55mph is possible with the few mods i can make and still keep it reliable!

**microbream** · 01-31-2012

Originally Posted by Ballistic

Are you sure? 190 sounds a bit much for just 1mm off the head...

Serious3.. I know you don't wanna hear it.. but get a 3cyl Yamaha or Tohie and spend some money on that to get it modded. I think putting money into this engine is just throwing it away.
http://www.youtube.com/watch?v=ASnj4l3NliE

yeh 185-190 is what it puts out, maybe i took off a tad more like 1.5mm
all depends on the cylinder stroke and bore to how much you will gain.

Probabaly the safest method is to measure the squish height and machine enough off the head to acheieve around 1mm, the comp you will get is the comp you will get
but remember both cylinders may not be identical so check both

**serious3** · 03-06-2012

well after some delay i took off what i could from the flywheel last night and it weighed in at aboult a pound!! seems small but should make a bit of a difference. picked up a ballistic prop as well, semi cleaver type 15p 10.25" dia. i should be out saturday morning if the weather holds so i'll post up some figures then!