My thundercat

[Roflhat]

Originally Posted by lukewhiting

Got a first judge of top speed the other day. Raced a jetski that topped out at 55mph on his speedo & I must of been doing a good 3-5mph faster with two up & 20ltrs of feul.

Do you recon take the outer ring off the flywheel will make much difference?

That's pretty good going, that prop you've got must be a nice one. Is your rev limiter still intact?

It doesn't make that much difference to top speed, maybe a tiny bit. Mostly acceleration, helps when you're jumping waves and when it's choppy, especially running bigger pitch props.

[lukewhiting]

Originally Posted by Roflhat

That's pretty good going, that prop you've got must be a nice one. Is your rev limiter still intact?



It doesn't make that much difference to top speed, maybe a tiny bit. Mostly acceleration, helps when you're jumping waves and when it's choppy, especially running bigger pitch props.

I like it! Getting reconditioned soon so I can find out what pitch it is then. Yeah I believe everything is race ready so not really changed much.

Might have to give it a go then!

[Peter_C]

If I may, I would recommend continuing to polish the head, and the pistons until they are a mirror. This will help keep the carbon from sticking, which can cause pre-detonation. Use a 2-3 inch polishing pad on a straight or angle die grinder with some polish. Should only take a few minutes on each one. Wash them really well after.

That is a major undertaking for a first time tear down. Organization is the key to success, and as long as you reverse your disassembly procedure you will do just fine

[Roflhat]

Originally Posted by Peter_C

If I may, I would recommend continuing to polish the head, and the pistons until they are a mirror. This will help keep the carbon from sticking, which can cause pre-detonation. Use a 2-3 inch polishing pad on a straight or angle die grinder with some polish. Should only take a few minutes on each one. Wash them really well after.

That is a major undertaking for a first time tear down. Organization is the key to success, and as long as you reverse your disassembly procedure you will do just fine

thanks, all advice much appreciated! Feedback is always good, I'm trying to do it right first time so the engine lasts.
I'll do that on tuesday, away tomorrow. The crank and head are going over to town tomorrow to get done.

I've taken hundreds of photos as I go along, hopefully I can figure it out! My dads giving me a hand as well.

[Roflhat]

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

[Roflhat]

Bit of an update.
Crank has been welded. Before -



After -



Reef block has been opened up -


Really nice job done on it, very pleased.

[Roflhat]

The final ports look like this -




I polished this bit too, the cover for the exhausts -

[Roflhat]

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.

[Roflhat]

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 -

[Roflhat]

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?