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Old 09 June 2017, 11:18   #1
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Country: UK - Wales
Town: Cardigan
Boat name: Poor Life Choices
Make: Gemini
Length: 5m +
Engine: 50HP Tohatsu 2 Strok
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New Thundercat (loss of power WOT?)

Hi all

Just picked up a New...Newer, Thundercat and it's a Beaut.
Enjoyed playing with the Zapcat but saw a chance to upgrade to something a bit newer with more life left in it.

Took it out for a spin today and it does go nice BUT, at WOT the revs did drop for a few seconds then the revs would pick up again. First guess is fuel related, I've ordered new Fuel pump Diaphragm and gaskets so i can rule the pump out and the only other thing is I'm using the flexible fuel bladder, never used one before anyone had any issues with these in the past causing power issues at WOT? Wouldn't have thought so but ask anyway.

Only got it up to 35Kts vs my old Zapcats 37kts but it's completely stock engine so might look into doing some stuff to it at some point, i think it's set a little low on the back too but going to keep it as is for now as I might get better performance in the surf and that's where i want to play in for now!
Put a 4 blade on for tomorrow see how that performs, the three blade is pokey enough so see how much grunt it will have with that, expecting top end to drop off a bit too.

Also Idles pretty high, well compared to my Zapcat engine, could just turn it down I spose!
Anyone else have a higher idle, think my old one just is really low.
Happy Boating!
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Old 18 June 2017, 04:05   #2
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Country: UK - Scotland
Town: Isle of Lewis
Make: Ceasar Thundercat
Length: 4m +
Engine: Yamaha 70ces
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Nice looking boat, I think I remember seeing that for sale many years ago.
With the tohatsus the fuel pump is a weak point, they're not as good as the yamaha design and unfortuantely cast into the bottom carb so you can't simply change it. It's worth getting a pump rebuild kit once a year. The other thing is the coils, when they work they work alright but they're very unreliable. The zapcat racing guys use Tohatsu 70hp coils, no performance gain just reliability, you can use just about any coils, yamaha coils are good too.

Try setting the idle with the engine in gear, that way it shouldn't stall when you put it in gear.

There's a couple basic mods you can do to get a bit more power, remove the lighting coils from beneath the flywheel, skim the head 1mm, remove the air box and fit velocity stacks.
What size prop have you got on it just now? Changing the prop is where you'll get the speed from
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Old 18 June 2017, 05:59   #3
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Country: UK - Scotland
Town: Aberdeenshire
Boat name: Sula
Make: Ribcraft 4.8m
Length: 4m +
Engine: Tohatsu 60hp + aux
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Other fuel related issues, then check the fuel connector on the engine. Non genuine Tohatsu ones then the rubber 'o' ring can displace. Also the fuel primer bulb can deteriorate with age and the diaphragm can restrict flow. Replace with Quicksilver part.
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Old 18 June 2017, 11:23   #4
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Clearly bought the wrong one...should've bought mine and you'd be getting 47knts haha

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Old 20 June 2017, 22:42   #5
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Country: UK - Wales
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Fair one Luke but this one was 30 minutes down the road! Rebuilding fuel pump won't be a problem easy enough. Where do you get your velocity stacks from. Looked online but can't find any UK manufacturers.
Tried a 4 blade prop on it the other day in the surf and it was massive out there. Gripped awesome and didn't lose much top end either. Think the engine is sat a little low but had brilliant grip in the surf and it's where I like to play! Might try some new fuel lines in case it's the bulb. Doesn't feel old but for the sake of a few quid I'll get it replaced. I'll check the connection but I think it's okay.
Don't fancy skimming the head, probably well out my skill range and had all sorts of fun 're fitting the head on my last engine so happy to leave that as is.
Remove lighting coils can they be taken off whole and 're fitted or will they be cut from the stator which ones are they as there's 3 on there when the flywheel was off on the last boat.
If that lot doesn't improve it I'll try new coils in case. Probably just fork out for tohatsu 70 ones seeing as the electrical connections are good. Put nakaru ones on the last engine seeing as it needed new wiring anyway.
Probably leave the idle as is for now. Doesn't clunk into gear to harshly and won't fiddle with that unroll I have to strip the carbs and 're balance it again. Anyone put 70hp carbs on theirs? Looked into it briefly but can't find munch about it. Would that cause issues with fuel pumps as it's part of the tohatsu carb?
Long post! Cheers.
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Old 21 June 2017, 14:51   #6
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I can make you a set of velocity stacks - https://www.facebook.com/forgemarine/ (might have to scroll a bit)

The lighting coils can be unplugged, so if you ever wanted to fit them again you could do so. I've tried Yam 70 carbs on my tohatsu, it ran well but takes a lot of tuning to get them right. In the end I went back to the 50hp carbs with stacks. You need to open up the reed block and modify the throttle roller on the Yamaha carbs to fit the tohatsu
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Old 21 June 2017, 14:57   #7
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what make is the 4 blade prop?

I have the heads skimmed by a local engineering shop, its ~60, all you have to do is remove it and torque the bolts up in the right order when putting it back on
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Old 21 June 2017, 15:11   #8
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Country: UK - Wales
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Length: 5m +
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I've had the head off before just had to make 2 attempts and two new gaskets to make a seal. Had water from the cooling camber get I to the cylinder. Can the work be done by any metal worker or is it fairly specialised?
I'll have a look to see if there's anything on the 4 blade and get back to you. Picked it up with the boat so no real idea what it is.
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Old 21 June 2017, 15:15   #9
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Country: UK - Wales
Town: Cardigan
Boat name: Poor Life Choices
Make: Gemini
Length: 5m +
Engine: 50HP Tohatsu 2 Strok
MMSI: 235085232
Join Date: Feb 2011
Posts: 164
Had a look roflhat interested in the stacks how can I get them ordered?
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Old 21 June 2017, 15:34   #10
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Country: UK - Wales
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Boat name: Poor Life Choices
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Length: 5m +
Engine: 50HP Tohatsu 2 Strok
MMSI: 235085232
Join Date: Feb 2011
Posts: 164
Is there any performance gain in skimming the head? From what I can find it just makes it fit the block better after removal?
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