New Thundercat (loss of power WOT?)

[watersidemat]

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!

[Roflhat]

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

[spartacus]

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.

[lukewhiting]

Clearly bought the wrong one...should've bought mine and you'd be getting 47knts haha

[watersidemat]

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.

[Roflhat]

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

[Roflhat]

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

[watersidemat]

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.

[watersidemat]

Had a look roflhat interested in the stacks how can I get them ordered?

[watersidemat]

Is there any performance gain in skimming the head? From what I can find it just makes it fit the block better after removal?

[Roflhat]

Any machine shop with a mill should be able to do it, or anywhere which does car engines. Skimming the head is sometimes done to get it flat again, so the gasket will seal better. But removing 1mm will increase compression by 20-30psi, you'll feel the extra power! It's mostly low down grunt you gain

Send me a message on facebook, easier than through here

[watersidemat]

Someone was asking about the prop. Couldn't find the pitch but it's a power tech prop if that helps?

[watersidemat]

[watersidemat]

Shout out to roflhat for the velocity stacks. Look great in orange!

[watersidemat]

Any advice on removing lighting coils?

[Roflhat]

That looks really smart!
For the lighting coils you remove the flywheel and remove 2 of the units on the stator,
here is a photo of how it should look after you've taken them off


Just make sure you do the flywheel up nice and tight afterwards!

[watersidemat]

So it's the 2 coils that are wire not wrapped in the cloth type stuff. Did you just cut the wires and have you unplugged anything from the cdi? Did you get any performance gain from this? Cheers.

[Roflhat]

Yeh the two copper wrapped coils can go. From memory they unplug but maybe I cut the wires, was quite a while ago now. Was very easy to do, don't have to touch the CDI or anything. The performance gains probably aren't that great, but removing them reduces electromagnetic drag on the engine, so slightly better acceleration and top speed

[watersidemat]

I get no luck with these engines lol.

Took for a spin today and still not hitting that 100 percent top end it's still dying off getting worse up to the point it starts running on 2 cylinders.

Had to pull the choke out literally about 3mm to get it to rev up under load, very odd.

Pit some nakaru coils in, they work great on the last zapcat but not sure if they are good here, I don't think my problems are ignition related though. Does the wires I'm using have an effect? They are a bit thicker(17amp) than the wires coming from the cdi box. Have thinner ones on the zapcat, can this make a difference?

New fuel pump so it's not that and I'll get a new file line and bulb to rule that out.

Think engine is mounted a touch low but should still get better performance under load than it is.

Going to strip and clean the carbs again in case some water got in. Lots of gunk in the carb throat I noticed the milky gum that must be oil and water mix.

Might try the standard 50 coils and the air box back on for next run just to check it's back to normal stock.

[watersidemat]

One of the wires fell off the top coil so that explains losing one of the cylinders. But still not getting max power and holding it at WOT.