Thundercat not going as fast as expected - help!!

[Aron50tohatsu]

Hello, i am new to forums but thought i would give it a chance!

I have a 2023 Aquarius Thundercat with a Tohatsu M50D2 shortshaft with new racing reeds and intake. Prop is a winrace 19 pitch cleaver and the over rev safety is not plugged in. And the engine is lifted 45mm from the top of the transom. The thing is… it Will only reach 50 knots exactly 1up and 42 knots 2up, is there something i am missing or is it just normal becouse i haven’t ported and modified it more?

I thought i would be able to reach over 50 knots 2up? And what would be necessary in that case to make more power and gain more speed?

 

[roflhat]

I think your speed is ok for a standard engine. When you are 2up try and keep all the weight as far back in the boat as you can and play with the trim to get it just right. Worth skimming the head, you can take 1mm off the standard head gives 30psi more compression and more power

 

[Aron50tohatsu]

Okay! Will take 1mm off and maybe blueprint it! And is it safe to not have the overrev safety or is it necessary to weld the crank??

 

[roflhat]

I would weld the crank before you cut the limiter, the crank is pressed together and if the press lets go then you have big problems

 

[Aron50tohatsu]

Okay thanks alot!

 

[Cornishpowerboater]

I got 58mph 2 up out of my 50 on a ceasar f11 with very minimal mods boyesen reeds, welded crank, and some velocity stacks with a forge marine v2 18 pitch
luckily your talking to the right guy as roflhat is the guy behind forge marine
my engine was about 25-30 mm up from bottom of hijacker valves and was running 2nd trim pin up with a fastcats trim pin spacer so basically adds another half a hole of trim

 

[Aron50tohatsu]

Hello again! Just wanting to make sure becouse now I’ts time to shave 1mm off the head, is there ANYTHING else I have to think about or could I just put I back and be good to go? Thinking about carbs, jets, squish, sparkplugs, timing etc!

 

[Aron50tohatsu]

Also! My boat does a lot of bouncing as soon as I go get up to speed which makes me think it’s slowing me down! Any way to fix that? Is it possible to upload a video?

 

[Aron50tohatsu]

Also! My boat does a lot of bouncing as soon as I go get up to speed which makes me think it’s slowing me down! Any way to fix that? Is it possible to upload a video?

The only way to get around it is to have 3-4m/s wind and go along with it when it’s little chop on the surface, then it won’t bounce…

 

[roflhat]

Hello again! Just wanting to make sure becouse now I’ts time to shave 1mm off the head, is there ANYTHING else I have to think about or could I just put I back and be good to go? Thinking about carbs, jets, squish, sparkplugs, timing etc!

You should be good to go as the Tohatsu is jetted a little rich from factory anyway so it should run better with the skimmed head. Check what plugs you have but as long as they don't have an "E" in the number you'll be fine. E is for extended thread which you can run on the standard head for a bit of extra power, but with the skimmed head they will touch the pistons. Not many folk run E plugs though so again you should be fine. Should be B8HS10 or BR8HS10 or similar.

Timing I wouldn't touch, doesnt make a lot of difference on the Tohatsu and standard works fine.

Also! My boat does a lot of bouncing as soon as I go get up to speed which makes me think it’s slowing me down! Any way to fix that? Is it possible to upload a video?

Try trimming her in a touch if you can or shift some weight forward. Getting her running absolutely flat will unlock a lot of speed, these boats are all about propeller and setup

 

[Aron50tohatsu]

Ok! Will try when the ice melts here and do you think that oversize pistons could help get some power? Already got reeds, velocitystacks and high flow intake from fastcats. Maybe a d1 exhaust tuner? I would really like to mount 70 carbs but the problem is nobody sells stuff like that in Sweden… So then my only option is to buy brand new from fastcats… too expensive… Any way to get around the nearly 600$ for a pair of carbs? Or put it like this, what modifications are the most necessary, i WILL shave the head. Will it help to get oversized pistons? And will it help noticeably with getting 70hp carbs? Exhaust tuner? And/or should I port the head like you did with yours?

 

[roflhat]

I wouldn't use oversize pistons unless you need to, the increase in cc is barely worth it. Then if you need to rebore in future you get one less rebuild. D1 exhaust tuner gives more top end, it's shorter and wider.
I would use the standard carbs, to get any benefit from the 70hp carbs you need to have a highly modified engine and even then the difference is small and you will lose a lot of bottom end.

In terms of porting I would keep it close to standard, just clean them all up and make sure everything is smooth and flows as well as it can. Can usually gain a lot of performance just by doing that. In order of the things I would do

Weld crank
Clean ports whilst the engine is apart anyway
Carbon reeds
Shave head 1mm
Velocity stacks
D1 exhaust

Then if you want more
Low water pickup on gearbox
Balance crank and flywheel (expensive)
Proper porting job
70hp carbs

 

[Aron50tohatsu]

Great! Will do the basic stuff and se what results will be made! Thank you for your help!! Much appreciated!

 

[Aron50tohatsu]

Hi! Started to dissasemble the engine today! Got everything surrounding the engine off and then took of the 6 bolts from underneath but the powerlead won’t budge… is there some bolt I’m missing or is it just hard seized on there??

 

[Aron50tohatsu]

Also, is it possible for you, roflhat, to maybe send some dimensions of a exhaust tuner that I could weld together, can’t seem to find one available online…

 

[Aron50tohatsu]

And are these reeds bad? It is carbonfibre reeds.

 

[roflhat]

It’s been a while since I did a tohatsu but I’m pretty sure it is just those 6 bolts. The powerhead can be tricky to come off especially if it’s been on for a few years!

Yeh I’ll have a look at my files, I’m away this week but remind me next week I’ll have a look. It should also be on the tohatsu homologation sheets if you can find them.

Reeds should be fine as long as they’re not cracked, they don’t tend to cause many issues

 

[Aron50tohatsu]

Got the head off with some elbow grease… Yes! Please have a look at the files!

 

[Aron50tohatsu]

Hello again! Did you find some of the specs for the exhaust tuner?

 

[roflhat]

sent you a message