My thundercat

[Roflhat]

Looks like i didn't gain any RPM with the exhaust mod. Sounds better and acceleration is considerably better but the top speed remained the same.
I had a fiddle with the timing while I was out today but advancing or retarding it from standard both seemed to make it slower. What I can say is that putting the ignition closer to 24* made the acceleration even better, and that the other way made it worse. Makes me wonder why it didn't help the top end though..

[Roflhat]

Couple pics of my new engine:



Just bought it yesterday and not yet arrived at the house. Unfortunately I'm off to uni for 3 months so won't be able to try it out for a while!
It's a Yamaha 70 which has been shortened and with a CES 'box on it. Just been rebuilt by a marine engineer, should be good to go. Came with an 11x21g yamaha cleaver. Need to get a tiller for it, going to try and adapt one of our old hydraulic steering systems to go on as a damper. Will be staying electric start for now until I can source parts to convert to pull start.

[lukewhiting]

Nice! What would you be doing with the tohatsu??

[Roflhat]

Originally Posted by lukewhiting

Nice! What would you be doing with the tohatsu??

Think I might have to be selling it unfortunately! Yamaha still needs a lot of parts to make it work on a thundercat.

[Roflhat]

engine arrived today:






prop is absolutely mint as well

[Roflhat]

Got the wingplates off, had to remove the powerhead to get the bolts out...
Tiller is on, although the cables aren't connected.
The throttle cable on the engine is pull to open, whereas the tiller is push to open. Think I read somewhere about reversing a gear inside the tiller to fix this? Anyone know? I also appear to be missing a bracket of some sort to hold the cables in place, going to google it and see what I can find.
Looking at the engine more closely it looks like it is a genuine CES, or 90% CES parts, Here's a couple photos of what I mean about the cables.

[Roflhat]

Got myself a set of 90 carbs, bought the reed chest plate as well just in case the throttle linkage doesn't work. The roller for the throttle cam is on the bottom carb of the 70 and the middle on the 90. Photo of them on the 90:

[Neuner]

Saw you are running NGK plugs. They are good but I don't think they have as far of a reach into the combustion chamber as some of the others. Also, not sure what yours requires but you might try the ones without the Interference Suppression Resistor to open your gaps.

[1eyedjim]

I would want to check how's he space there is at tdc before sticking longer reach plugs in. Lad I know did it on a Jetski and blew his engine in minutes

[Neuner]

Originally Posted by 1eyedjim

I would want to check how's he space there is at tdc before sticking longer reach plugs in. Lad I know did it on a Jetski and blew his engine in minutes

Everytime I've compared NGK plugs to other plugs that are made OEM (stock) for the engine, they have been shorter. I'm not saying to look for longer plugs but that NGK tends to always run shorter than others even when built to specs.

If your friend ran longer plugs without measuring, then he is a fool.

[Roflhat]

Yeh you have to be careful with the spark plugs.
for tohatsus with standard heads you can run BR8ES, the E is extended so it reaches further in. But with a shaved head for more compression you need to be running BR8 standard length or else you risk having problems.
Need to check the 70 still to see which ones will work best. I usually run the Iridium plugs. You can check for interference by putting the plugs in and slowly turning the engine over by hand, then check if the pistons have hit them.

[Neuner]

Majority of the time I see the profs running a simple copper plug without internal resistors. You can have a larger gap and get a larger, hotter spark but if you don't have a coil that can handle it, it will fry.

In a different application, I ran a better coil, copper plugs and a greatly increased gap. Made enough of an improvement to note. Also improved my efficiency.

Just trying to help if you hadn't already gone down that route yet.

[Roflhat]

i've not looked into it much, sounds interesting though

[Neuner]

Just remember, it only takes three components, Gas, Air & Spark and boom you got fire. Looks like you put a lot of work and time into two of them. I think the third is your next step. The bigger and hotter the spark the better and quicker the boom.

[Roflhat]

Not sure I completely agree with you there. A quicker spark is less than desirable unless you're trying to advance your timing, which you can do by adjusting a linkage anyway.

[Neuner]

Originally Posted by Roflhat

Not sure I completely agree with you there. A quicker spark is less than desirable unless you're trying to advance your timing, which you can do by adjusting a linkage anyway.

Not sure you understand or how you got timing out of that. Not quicker, but BIGGER

[Roflhat]

Originally Posted by Neuner

Not sure you understand or how you got timing out of that. Not quicker, but BIGGER

Sorry I misunderstood!
yeh I'll look into it for sure. I've got uprated - nakuru - coils (from a motorbike) and iridium plugs on it on the moment. So dyou reckon an increased plug gap would increase power?

Edit: looked into it, and apparently increased spark plug gap gives greater power except usually at WOT, when you may even want less gap to prevent misfire?

[Neuner]

Originally Posted by Roflhat

Sorry I misunderstood!
yeh I'll look into it for sure. I've got uprated - nakuru - coils (from a motorbike) and iridium plugs on it on the moment. So dyou reckon an increased plug gap would increase power?

It will to a certain extent. Haven't found anyone that has provided an equation or rules of application. More of try and test in gap steps starting at 1-5/1000 from where you are at. There are general guides that should get you started. Would guess diving into the performance motorbike world as a start? If you go too far, not sure what happens with your setup. Most likely misfires with your coils unable to produce an adequate voltage for the wider gap.

Getting a larger gap which is closer to the center of the gas/air mixture is what you want. This is why I was saying that I found the NGK plugs to be shorter than others when you compare them for the exact same application. In my testing, I found that a wider gapped NGK was equivalent to a normal gapped Champion because the Champion extended further into the chamber.

[A1an]

Originally Posted by Roflhat

So dyou reckon an increased plug gap would increase power?

Its going to make about the same difference to the boats overall performance as emptying your bowels before climbing aboard.

[Roflhat]

Originally Posted by A1an

Its going to make about the same difference to the boats overall performance as emptying your bowels before climbing aboard.

That's the conclusion I'd come to to be honest! haha