My Thundercat


Thundercat guy
Apr 30, 2014
Isle of Lewis
Boat make
Yamaha 70ces
thought I'd introduce myself and make a thread showing off my boat.
It's a 2005 Thundercat with a 2006 Tohatsu 50 D2. Got above prop exhaust relief holes, aftermarket nakuru coils and leads, lighting coils removed and boyesen carbon reeds.

Got 3 different props for it at the moment, trying to get another.
15p SOLAS cupped, got 42 knots.

Next to it is an 18p Mercury chopper (10 3/8 x 18), good for 49 knots

And the most recent one, Mercury cleaver (11x18) 50 knots so far.

If anyone knows of a 20 or 22p cleaver for sale let me know!

I've recently bought a set of yamaha 70 carbs to go on it and will be modifying the exhaust tube as well (opening it up and cutting 25mm from the end).
Will hopefully get the crank welded so I can remove the rev limiter and also remove the starter ring gear from the flywheel.
Should give much better acceleration, hopefully as good with an 18p prop as the standard engine was with a 15p.
If I manage to get a 22p prop and all these mods done then I reckon 60 knots is maybe achievable

Thinking about getting the head skimmed this summer as well for more compression. Tried to measure what it is currently but the tester was useless so I've ordered a new one. From what I can tell its running around 140psi just now.
Apparently 1mm off the head raises it 30psi, anyone know what PSI is safe to run using normal pump fuel?
I have run 170psi on 98, but it will be arround 180/190 after the mods. With the compresion ratio i have, i can still run 98 fuel...
Don't go to lean and watch out with the timing.
And what squish do you have now? 0,9mm would be perfect.
But would also mean you can't shave the head without machining the squishband...

Know what you do before its to late.
Never had it, but i quess seizing your engine sucks

How many rpm with the 11x18?
I have a merc cleaver 11x22, bit to big... Only 6300 wit std engine.
Now with mods hopefully arround 7000 .
Also 4blade overhub 11x20 , not driven yet.

Nice speeds you achieved so far
Hey, thanks for the reply.

The head is totally stock right now, will have to check what compression it's running before I touch it. 170psi should be about right I think.

Getting 5850rpm with all the props (limiter is still on until I get the crank welded).

7000rpm with a 22p would be fairly moving!
Just realised I've not been updating this thread! quite a bit has been done.
this was a couple months ago:

Started taking apart my engine today, got it completely stripped down today and got a few things done.

Started with basically a stock engine - boyesen reeds, exhaust mod, lighting coils removed and nukaru coils.

Plan is
Get the crank welded
Get the crank/flywheel balanced
remove flywheel ring gear
get the head shaved 1mm
polish pistons, head, ports etc.
yamaha 70 carbs
D1 exhaust tuner
BR8 HIX plugs - Think these are correct for a shaved head?
Mikuni triple fuel pump (reliability not performance)

Here's the coils:

Here's the yam 70 carbs next to the standard D2 carbs:

They're the square bowl type. Very tight fit on the tohatsu! Had to shave a bolt to get them to squish together enough. Think these are automatic choke? Not sure how I'm going to the get that to work...

Another small problem - the 50hp carb bolts are too short, so I've had to order 90mm long ones.

I was speaking to fast fred before I started so I knew this one -

You have to use the link from the tohatsu 50 on the 70 carbs. The 70 ones aren't adjustable whereas the 50 ones are.

Another small job -

You can see the step at the back of the carbs, to take advantage of the extra diameter of the 70 carbs you have to open up the reed block.
Next the powerhead came off -

Continued to strip it down, first time taking an engine back this far -

Pistons before polishing -

After -

Started wtih 240 grit sand paper then 500 then 2000.

The cylinder head got the same treatment -

Had a go at some mild porting -

Think I'm going to open them up some. The top one here is stock, the middle is after I smoothed it. They had very rough edges when I started and they're already much smoother, but not much larger. Thoughts on this?

Got the flywheel done, weighed 3.42kg to begin with, 2.96kg after. Not much less but all the weight removed was from the outside edges so it has more effect.

Just used a grinder to cut almost to the centre part of the flywheel, the used a hacksaw to finish it. Just popped apart, simple as.

(the tape is to stop anything getting on the magnets)

Job done
Gave the block a clean -

Used the parts washer first with Jizer then pressure washed them

Going to get some primer, not sure what colour to go for yet.

Looking a bit empty back there -
Managed to snap two bolts today taking apart the engine mounts. Got the old exhaust out and the new one on. The original was 270mm long and 39mm wide, new one is 260mm long and 48mm wide. Apparently gives better top end.

Also opened up the exhaust ports some more (top one is from yesterday, middle is towards the end of today)

Did some polishing and cleaning up on the intake/ front half of crank case

Then started on the back half of the crank case. You can see the casting imperfections here -

Removed -

Smoothed off the sharp edges -

Still needs to be polished -

I tried to keep the work done equal for all cylinders
Bit of an update.
Crank has been welded. Before -

After -

Reed block has been opened up -

Really nice job done on it, very pleased.
The final ports look like this -

I polished this bit too, the cover for the exhausts -
Had 1mm removed from the cylinder head, was 33.6mm, now 32.6mm. Should increase compression by 30psi. Was at 140psi before the rebuild.
Again a really nice job done on it. I polished up the head as well as the pistons. The photos don't really show up how shiny they are.

Also drilled and tapped a hole for the pulse feed for the new fuel pump, a Mikuni triple outlet from a 3cyl jetski. More of a reliability upgrade than performance - the Tohatsu one isn't too great.
Started putting everything back together, crank in, case halves together, everything torqued up. Block ready to go -

Primer -

And Paint -

Painted the recoil cover as well -
Spent today getting the engine together and back on.

Using a fuel pump from a yamaha 30 just now, waiting for a fuel barb for the mikuni pump.
Having problems with the carbs and the throttle setup from the tohatsu not wanting to work together. The way the linkage on the yam carbs is means they're at almost wide open throttle when the tohatsu tiller is at idle.
Took off the carbs to compare, not quite sure how to sort this?

The roller on the top LHS of the carb is the problem.
I've tried taking the assembly from the 50 carb over to the 70 but it's much shorter and won't work. Can't see an obvious solution other than welding the 50 setup onto the 70 one?
Had a look at the carbs this morning, lots of measuring and testing later this is the end result -

Maybe not a "bolt on" modification after all...

Got everything back on the engine, a lot of pulling over (not easy with this compression!), and IT RUNS!

A wee bit lumpy just now, needs tweeking to get it running right. Tried to find the size of the main jet by seeing which drill bits would go through the hole. 1.3mm will go through but 1.4 won't. I assume this means it has 135 mains?

Lots of smoke due to the 2-stroke oil we used during assembly.
The barb I was waiting for finally arrived so I got my fuel pump setup.

Made up a plate so I could mount the pump using two bolts instead of relying on one.

Got the boat launched to try it out and immediately had problems. The ground wire for one of the coils had snapped. So 15 minutes later that was fixed, took it out for a blast, was very slow getting on the plane, the all of a sudden the power is just there and it takes off. Couldn't really open it out, bit too windy. Managed 48 knots, not a personal best but not bad for the conditions. Highest RPM I saw on the tach was around 6100, so hopefully there's plenty more in it.

Idling very low just now, need to sort that out first then see how it goes. I'm hoping the acceleration bogging problem can be fixed by changing the carb jets?
Had it out on the water today, not ideal conditions, still a bit windy.
I soldered the main jets and drilled them out to 130, running much better now. Checked the plugs after the run and they're still wet so either go leaner or change the mixture screws. Going to try the mixture first.
Got 53.7 knots with my 18p cleaver, up around 6300rpm.
Then tried the new (to me) 20p cleaver, only getting 47.9 knots at 6000rpm.
Need to lift the motor up some more and trim in.

Think a nose cone with LWP is on the cards...
Decided to open up the reed block, apparently it's a bit of a strangle point on these 3cyls.
Because I've opened up the reed block some of the reed cage was visible while the throttle butterflys were open in the carbs. I marked on in red pen and opened them up a bit.

After that spent a lot of time carefully opening out the reed cages themselves. It's just a plastic coating over metal, so I removed the plastic from the openings. The centre brace was reduced from 5.3mm to 4mm, sides from 4.6mm to 3.9mm. The distance from the base of the cage was 26.4mm, now 25.7mm. Smoothed the rest of it out for better airflow. One on the right is opened up -
Had a propeller/setup day today. Launched and recovered the boat about 10 times.
Best speed was with 20p cleaver at 51.2 knots at 6000rpm.
Next I tried lifting the engine out some more, from 40mm to 53mm, the idea being to get more RPM. I ended up getting the same RPM as at 40mm with each of the propellers but with slower speeds...

Tried the Merc chopper out for the first time since modding the engine. It really pulls hard! Accelerates a lot better than the cleaver and with not much loss of top end. Got 49.6 knots at 53mm up getting 6500rpm. Then dropped the engine back down, to 38mm and got 49.7 knots at 6100rpm.

Again not ideal conditions for top speed, bit too much breeze so I moved the fuel tank and kit bag up front, probably about 30~35kg. I'm 71kg. Still not getting the revs or speeds I'd like but not sure where to go next.
The engine is running better than before, acceleration is good, idle is fine, timing is bang on where it should be (for a standard engine anyway).