Genuine Yamaha head gaskets are the only ones worth running. TSR use o rings which is a good concept but they're hopeless in practise, less than 10 hours on mine and water was getting into the head. TSR head is no good in my opinion. Have run the one piece, two piece, TSR head eventually got fed up and designed my own, with assistance from the legendary late Rolf Magne Sunde. I got 5 of them made, 1 for myself have sold 3 got one left.
https://forgemarine.com/performance/p/70billetuses standard head gasket and my own water cover gasket
You want around 170psi, two piece head is roughly 150psi, one piece upto 180psi.
90hp carbs do add some top end, the more modified your motor is the more you gain. But standard carbs aren't as restrictive as people say, Rolf managed 103mph with his 70ces on an F4 hull, setting the record at 101mph.
Tuner I got a stainless replica of the original made, ran well but not as smooth internally as I'd like. Then I got a squared off tuner made, billet. Worked really well possibly more top end than original but not race compliant. Then a replica of the original made in billet which is what I run now.
Tried a dyno once but melted the pistons. Was pushing timing too far, can't have more than 16 degrees on a thundercat, I know the F4 guys ran 21 on the two piece head and 18 odd on the one piece but myself and Rolf were discussing it and reckon the cdi has a curve which drops back down over 7k rpm. So if you're not hitting that then the timing is staying high not backing off, hence the melting pistons.
These are very strong engines, but difficult to extract a lot of performance from. Very easy to do the wrong thing and lose power. Rolf taught me a lot about them although I suspect merely a fraction of what he knew
