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Old 17 June 2014, 14:16   #51
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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
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Old 17 June 2014, 16:02   #52
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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
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Old 19 June 2014, 12:21   #53
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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...
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Old 19 June 2014, 13:17   #54
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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
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Old 19 June 2014, 14:57   #55
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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.
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Old 20 June 2014, 09:58   #56
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I suppose the reason they use yamaha over tohatsu is the weight?
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Old 20 June 2014, 13:20   #57
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Quote:
Originally Posted by henryfreston View Post
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.
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Old 20 June 2014, 13:43   #58
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Originally Posted by Roflhat View Post
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!
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Old 21 June 2014, 13:02   #59
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Quote:
Originally Posted by spartacus View Post
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 -
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Old 21 June 2014, 13:10   #60
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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).
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