My thundercat - Page 3

lukewhiting · 14 June 2014, 18:30

Looks great!!!

spartacus · 14 June 2014, 20:51

Looking good.

I'm up last two weeks of July... If I see you I'll race you. No doubt you'll win... but only because I'm going half throttle!

Roflhat · 15 June 2014, 18:49

Cheers,

Sounds good to me, you never know - if I end up upside down you could win!

Had a look at the carbs this morning, lots of measuring and testing later this is the end result -

Maybe not a "bolt on" modification after all...

Got everything back on the engine, a lot of pulling over (not easy with this compression!), and IT RUNS!

A wee bit lumpy just now, needs tweeking to get it running right. Tried to find the size of the main jet by seeing which drill bits would go through the hole. 1.3mm will go through but 1.4 won't. I assume this means it has 135 mains?

IMG_2552.mp4 Video by Roflhat | Photobucket

Lots of smoke due to the 2-stroke oil we used during assembly.

Locozodiac · 15 June 2014, 19:09

On that engine all main jets are not same size, upper and middle should be same, lower bit less. Don't fiddle with jets on a standard engine.

Happy Boating

Roflhat · 15 June 2014, 23:26

Quote:

Originally Posted by Locozodiac

On that engine all main jets are not same size, upper and middle should be same, lower bit less. Don't fiddle with jets on a standard engine.

Happy Boating

Alright thanks for that. I had to switch the top and bottom carbs over because the throttle cam on the tohatsu is on the top carb, yamaha was on the bottom.
It's not a standard engine.

Cheers

Roflhat · 17 June 2014, 14:42

The barb I was waiting for finally arrived so I got my fuel pump setup.

Made up a plate so I could mount the pump using two bolts instead of relying on one.

Got the boat launched to try it out and immediately had problems. The ground wire for one of the coils had snapped. So 15 minutes later that was fixed, took it out for a blast, was very slow getting on the plane, the all of a sudden the power is just there and it takes off. Couldn't really open it out, bit too windy. Managed 48 knots, not a personal best but not bad for the conditions. Highest RPM I saw on the tach was around 6100, so hopefully there's plenty more in it.

Idling very low just now, need to sort that out first then see how it goes. I'm hoping the acceleration bogging problem can be fixed by changing the carb jets?

A1an · 17 June 2014, 17:34

Did you set up the timing?

Roflhat · 17 June 2014, 17:59

Quote:

Originally Posted by A1an

Did you set up the timing?

I've not done that yet, do you think that could be the problem?

A1an · 17 June 2014, 18:13

Possible.

I'm just wondering how you will do it, didn't you cut the ring gear off the flywheel? Timing marks are usually on it.

Roflhat · 17 June 2014, 19:59

Quote:

Originally Posted by A1an

Possible.

I'm just wondering how you will do it, didn't you cut the ring gear off the flywheel? Timing marks are usually on it.

Yeh I cut it off. I'll look into this, it's not something I've done before. Is it a matter of adjusting it, testing it, adjusting it? Whats the best way to go about doing it?

Locozodiac · 17 June 2014, 20:16

There's not only flwheel timing advance but carb sync, both works hand in hand. If that is not a modified engine, pult back carb jets as factory delivered. Did not understood the part where you have switched carbs, was that on a Tohatsu or Yam engine ?

At 6100 rpm you alone and lightly loaded sib you're over revving engine, max WOT rpn should be 6 K, worst is have more throttle to go, get a higher pitch size prop to lower rpm or risk engine doing KABUM at full open throttle, that said if being a factory standard engine.

Happy Boating

Roflhat · 17 June 2014, 22:02

Yeh was reading up on a couple things online, looks pretty complex. Some folk were using carb sync air flow meters, don't think we have any of those.
I've cut the ring gear off the flywheel, not sure how I'm going to do the timing, will have a look at it tomorrow and see.

It is a modified engine, the details are in this thread. The engine is a tohatsu 50 and the carbs are from a yamaha 70.
Yeh the rev limiter is at 5850, but it's been removed. Expecting it to rev above 7k, maybe 8.

Cheers

Roflhat · 19 June 2014, 18:21

Had it out on the water today, not ideal conditions, still a bit windy.
I soldered the main jets and drilled them out to 130, running much better now. Checked the plugs after the run and they're still wet so either go leaner or change the mixture screws. Going to try the mixture first.
Got 53.7 knots with my 18p cleaver, up around 6300rpm.
Then tried the new (to me) 20p cleaver, only getting 47.9 knots at 6000rpm.
Need to lift the motor up some more and trim in.

Think a nose cone with LWP is on the cards...

henryfreston · 19 June 2014, 19:17

Really interesting thread looks like you are doing an awesome job on everything, when it's done it's going to be a brute! Are these standard mods that any one would make if they were thinking of racing, or are you going a bit further? Saying that for racing the engines must have to be all pretty standard for the race to be fair?

With the engine revving higher is there much risk of damage or will it just decrease the outboards life/wear quicker?

Henry

Roflhat · 19 June 2014, 20:57

thanks very much. Yes hopefully I'll be close to the world record, although it won't be official because you have to have raced in a UIM event with that setup.
People don't usually do so much to the engines, although several of these are common modifications and are allowed in the standard close as well, such as welding the crank - more of a strengthening mod than performance.

The racing is done in classes, standard, blueprint and modified. The rules are fairly strict to make it more upto driver skill than differences in engine/hull. Although emphasis is put on boat setup as well. The tohatsus require some strengthening, solid mounts for example, whereas an equivelent yamaha 50 is strong enough from factory.

Hopefully nothing gets damaged, although if a piston cracks I'll have an excuse to go oversize! Engine life will be slightly decreased, but saying that a standard motor could last a lifetime. If this one blows up I'll be looking for a yamaha 70CES.

henryfreston · 20 June 2014, 15:58

I suppose the reason they use yamaha over tohatsu is the weight?

Roflhat · 20 June 2014, 19:20

Quote:

Originally Posted by henryfreston

I suppose the reason they use yamaha over tohatsu is the weight?

No they are both almost exactly the same weight. They use yamahas because the factory parts are stronger, you have to "race prep" a tohatsu to make it strong enough.
I've also found that yamaha parts are much cheaper than tohatsu.

spartacus · 20 June 2014, 19:43

Quote:

Originally Posted by Roflhat

I've also found that yamaha parts are much cheaper than tohatsu.

It would be cheaper running a Princess gin palace than keeping a Twatsoo going!

Roflhat · 21 June 2014, 19:02

Quote:

Originally Posted by spartacus

It would be cheaper running a Princess gin palace than keeping a Twatsoo going!

Hahaa true that!

Decided to open up the reed block, apparently it's a bit of a strangle point on these 3cyls.
Because I've opened up the reed block some of the reed cage was visible while the throttle butterflys were open in the carbs. I marked on in red pen and opened them up a bit.

After that spent a lot of time carefully opening out the reed cages themselves. It's just a plastic coating over metal, so I removed the plastic from the openings. The centre brace was reduced from 5.3mm to 4mm, sides from 4.6mm to 3.9mm. The distance from the base of the cage was 26.4mm, now 25.7mm. Smoothed the rest of it out for better airflow. One on the right is opened up -

Roflhat · 21 June 2014, 19:10

Had a propeller/setup day today. Launched and recovered the boat about 10 times.
Best speed was with 20p cleaver at 51.2 knots at 6000rpm.
Next I tried lifting the engine out some more, from 40mm to 53mm, the idea being to get more RPM. I ended up getting the same RPM as at 40mm with each of the propellers but with slower speeds...

Tried the Merc chopper out for the first time since modding the engine. It really pulls hard! Accelerates a lot better than the cleaver and with not much loss of top end. Got 49.6 knots at 53mm up getting 6500rpm. Then dropped the engine back down, to 38mm and got 49.7 knots at 6100rpm.

Again not ideal conditions for top speed, bit too much breeze so I moved the fuel tank and kit bag up front, probably about 30~35kg. I'm 71kg. Still not getting the revs or speeds I'd like but not sure where to go next.
The engine is running better than before, acceleration is good, idle is fine, timing is bang on where it should be (for a standard engine anyway).