My Thundercat - Page 4

roflhat · 17-12-2015, 12:54 PM

Got home a couple days ago, stripped the engine down. Looks like one of the centre main bearings on the crank has had it, which is apparently quite a common problem on the Yam 70. So I've now bought another 2 complete powerheads, hopefully one has a decent crank in it. Was also given a 60hp head, unfortunately turned out to be a merc or mariner, non-removable cylinder head. Was looking into having the bearing replaced, maybe have the crank dynamically balanced while it's being done, but its pricey.
Put the back half of the powerhead together, TSR head is on, all new gaskets and seals going on. Just doing welded crank and TSR head for now. You can get pre-ported liners for £145 each, maybe build up one of the other power heads with mod parts

You can see the difference in head shape here, this is the original CES head

roflhat · 17-12-2015, 01:02 PM

This is a side project I'm working on for Uni, modifying a surface piercing propeller. I sent the propeller to a company who scanned it and created a point cloud. It's based on an 11x18p Mercury Cleaver. Got the IGES file back a few days ago, here's a couple screenshots from Rhino

I've bought a 13 spline RUBEX hub which I'm going to use as a reference to model around, I'll start modifying the design hopefully towards the end of January

roflhat · 23-12-2015, 08:20 PM

3 scrap cranks later... I've sent the original crank away to have all the main bearings replaced, dynamically balanced, conrods and pistons weight matched and then welded.

Decided to use one of the other blocks to try the 90 carbs on. Had to countersink the top screws on the bottom 2 carbs. Took the 165 main jets out of the 70 carbs and put them in the 90 carbs.

The throttle and choke linkages from the 70 carbs all seem to work on the 90 ones.

The throttle cam isn't perfect, rubs on the metal part of the carbs a wee bit, going to wait until it's back together and see how it goes.
Think I'm going to have to cut a hole in the tray for the bigger bowls on the carbs.

Also put the CCM reeds on this one, removed the stops as well.

roflhat · 28-12-2015, 01:12 PM

Got this from America, Yamaha 15G Semi-Cleaver. Should make a good all round/surf prop

roflhat · 13-01-2016, 09:08 AM

Another addition to the collection, 10x19 overhub Cleaver

Here's the collection minus 2x Solas 15p props

roflhat · 09-03-2016, 08:58 PM

home for a few days, started putting the engine back together.
Got the crank back from balancing + new bearings + welding

Got everything put back together, 90 carbs are on - had to cut the tray to accomodate the deeper bowls.

Going to get the floorboards back in tomorrow and try get the engine going

roflhat · 15-03-2016, 09:17 PM

Designed some stacks, I've ordered a set and depending how they go I may start selling sets.

They are quite short as there isn't too much room with the 90hp carbs.

I've also designed stacks for tohatsu 50's

roflhat · 05-04-2016, 08:40 PM

Anyone got timing link lengths for a yamaha 70? Or if someone with a yam 70 could take photos of the timing links so I can get an idea of the lengths that would be much appreciated

roflhat · 14-04-2016, 01:00 PM

Went out for a spin today after adjusting the mixture to help with low down. Ran a lot better, topped out at 54.7 knots with th 21g cleaver. Took the main jets out and replaced them with the 160's. Using small drill bits for reference:
165 = 1.65mm
160 = 1.85mm

So I'd guess the 160's have maybe been drilled out at some point, they were in the 90 carbs which I got from a racing guy. Still need to try it again with these jets. Also need to smooth the intake on the reed block, there is a slight step between the carbs and the reed block.

roflhat · 14-04-2016, 06:03 PM

ran terribly with the 1.85mm jets... Idle has dropped about 200rpm, acceleration seems much better but doesn't go. Also found one of the carb gaskets was torn, I think it was probably while I was removing the carb as the engine was running fine. I replaced it when I swapped the jets over
I guess I need around 1.7mm or 1.75mm jets?

roflhat · 11-05-2016, 07:45 PM

Put the 70 carbs back on to try get some better acceleration

The 50hp stacks fit on these too. Also painted the cowl, getting some custom stickers made

Also got another prop, 11x18p Merc Cleaver but the newer kind. Absolutely mind condish

roflhat · 16-05-2016, 09:23 AM

Ran terribly with the stock carbs yesterday, going to order 3x carb rebuild kits and take it from there. Took about 40 pulls to start and would die every time I put it into gear. Getting around 120psi with the TSR head, less than I was expecting but hard to judge anything by static compression.

roflhat · 25-06-2016, 01:11 PM

Got the stickers on, looks alright IMO

Also got a new prop, 16p Radice in brass, set a world speed record in 1979 on a Yam 55

Engine is running properly, the gasket on the carb bowl on the bottom carb wasn't seating properly and one of the spark plug tips had come undone. Still dropping onto 2 cylinders occasionally so need to have another look but it's running really well, finally! Dropped it down to 10mm off the transom and tried some of the smaller props to check acceleration.

Got 46 knots with the 14p vengance, acceleration is very good.
49.3 knots with the 10 3/8 x 19p overhub cleaver, acceleration isn't great because the exhaust isn't drilled. Great top end pull though
SOLAS 15p cupped ran ok but not as good as it did on the tohatsu, 46.5 knots with decent acceleration
The 16p Brass needs a different washer at the nut end, the hub is slightly different.
Best of the bunch was definitely the 15G Yamaha, 47.6 knots with incredible acceleration and unreal grip through the turns. Think I'll get this one worked

roflhat · 11-08-2016, 09:44 AM

Bit of an update, engine started dropping onto 2 cylinders, then onto 1. Took the plugs out and it was fouling badly (oil). So I'm now running 50:1 with a set of new plugs and it's running much happier now, plugs are perfect.
Engine is now completely standard, even got a standard airbox for it. Runs awesome, feels very smooth thanks to the balanced crank, much more so than the tohatsu at low RPM.

More props and more testing, got a 2 blade brass prop, 10 3/8 x 17p. Unfortunately first time out it spun the hub... so it will be getting sent off to get a new hub.

Best prop is still the 15G yamaha, now getting a consistent 50 knots with it. Really nice prop, although I'd still like a 16G for a bit more top end, the 70hp has no problem spinning up the 15p. Weather hasn't been too great but got out to little Bernera with it.

Also a few more props to try,
18p merc cleaver (another one)

a cut 15G yamaha, meant to be closer to 16p

And this funky looking thing, 9 3/4 x 17p 4 blade Ron Hill

roflhat · 01-11-2016, 08:32 PM

Engine has been running perfectly recently, got the 90hp carbs on too. 99kmph with the modified 15G Yam. Also ordered a new boat, brand new Ceasar F10 from South Africa, should be here around the end of the year. Took a while to decide on colour but in the end I went for all black with carbon nose cone -

(that's not it, but same hull)

Also sending the powerhead to BDK Race Engineering to get it ported and some reed block stuffers. They also might CNC new head chambers for the TSR head.

Tore the whole engine down, and I came across this -

the remains of an exhaust pipe..

So currently trying to have that replicated in stainless. Makes me wonder how fast the engine would go with a functional exhaust pipe!
Think I'll get the whole engine painted while it's apart, all satin black with grey decals, stay tuned...

roflhat · 24-12-2016, 07:54 PM

Bit of an update, powerhead is still away getting worked on. Got a new exhaust made, looks the goods and is very similar to the original shape.

Had some time while everything was away getting painted to give the trailer a coat, in black of course. With mudflaps too, going to replace the carpet with black.

Photo of all the props:

roflhat · 24-12-2016, 07:54 PM

The new hull is now finished and on it's way, should hopefully be here in February some time. There's a thunderbolt (5.3m) coming up here too, 90hp yamaha going on it.

Looks alright, but not as good as the F10 Ceasar

carbon nose cone and honeycomb boards

Got the engine put back together today, new mounts and exhaust plate gasket. Whole thing was done in satin black with custom decals

roflhat · 07-02-2017, 04:35 PM

Hull arrived yesterday, got it inflated and on the trailer and got the shell of the engine on. Just waiting on the powerhead now!

The boat looks absolutely amazng, very pleased with it. Love the carbon nose cone. The floorboards are unbelievably light, can easily hold them both with one hand, must be 6-7kg lighter than the old thundercat.

The transom on the Ceasar is very short, 570mm from the top of transom to bottom of hijackers compared to 640mm for the Thundercat. So I've set the engine up at 50mm above the transom just now which should be good for semi cleavers. Issue is I can only get the lower two bolts in, so I've designed a jacking plate which I'm hoping to have made.

The all black fuel tank in place:

It's got a raised section underneath which means it doesn't sit flat. I'm going to look into getting it cut off so it sits better, and also the filler cap isn't stainless, so that might want changing too.

roflhat · 12-04-2017, 05:06 PM

I designed a jack plate and had it cut using waterjet, came out really nice

Finally got the powerhead back after 5 months, in the end it got reed block stuffers, it's been ported, new 0.5mm oversize pistons and new rings, the block skimmed and the head worked.

First thing I did was get it torn apart and paint it, black of course

Everything now plugged in ready to go, had it running briefly on the muffs last night. Started second pull and sounds very healthy! Hopefully launch tomorrow and get it broken in. Also had the fuel tank cut so it can sit flat on the deck, and moved the rubber D rings so the straps would sit nicely across it. Changed the top out for an aluminium one (original was steel) and fitted a yamaha pick up. Filled up with super unleaded mixed at 40:1.

Went to fit a primer kit to the engine but the blank plugs are the wrong diameter, so new kit on order and I'll fit that when it gets here

roflhat · 16-04-2017, 06:53 PM

Took the boat out for the first time on Thursday, got the run in done with an 18p merc cleaver. Slowly built the speed up over a couple hours taking breaks to get heat cycles into the pistons.
Had 55.3 knots 2up but there was a fair bit of wind. Engine runs so well, unbelievable power all through the rev range! Engine was low down on the transom to load the motor up so still not getting all the revs from it.

Had it out again today, 1up got 57.8 knots. Boat feels really stable even at high speed, next step is to raise the motor a bit and get more RPM from it. Got a photo of it behind our new ranger