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Old 08 September 2008, 09:48   #11
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does it 'hit a wall' at 4000 rpm?
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Old 08 September 2008, 15:06   #12
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Maybe the tacho is wrong and its hitting the revlimiter??
if you changed the prop pitch and the RPM and speed are exactly the same then thats really wierd Are the props all 13.25 diameter? are they all ali? and are they all cupped the same?
material and cupping changes aparent pitch AFAIK.
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Old 08 September 2008, 16:39   #13
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Is the engine compression good on all cylinders? Even accounting for the extra weight of your Yamaha 2-stroke you should see some difference with an extra 30 horses onboard. 90hp Yamaha (20" shaft) weighs in at 119kg and is generally regarded as one of the best engines for power-to-weight ratio (2-stroke).

What about the outboard set-up. Is it off-set or raised on the transom? Anti-cavitation plate should be level with the hull.
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Old 08 September 2008, 18:34   #14
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Did your mate clean the carbs?
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Old 09 September 2008, 03:18   #15
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Hi All

Yes it seems to hit that wall at around 4000 revs. Both of the props tried are 13.25 in dia.

The chap came out and checked all the relevant bits etc (not that i know what is relevant) but he is sound and genuine. It all fits fine on the boat central to the transom and the caviation plate is in exactly the right place.

Have no idea what to do but am gutted as i thought it would be a fair bit quicker than the rude but its about the same and cant help but feel something is clearly not right.

Forgot to add that compression is fine on all cylinders.

Any more thoughts.

Regards
Jeff
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Old 09 September 2008, 04:52   #16
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Quote:
Originally Posted by Jeff J View Post
Hi All

Yes it seems to hit that wall at around 4000 revs.

Forgot to add that compression is fine on all cylinders.

Is your trim gauge working correctly?

Might be the rev limiter kicking in because it thinks you're trimmed out too far or still in neutral. (Not sure if your engine has a limiter though)
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Old 09 September 2008, 05:01   #17
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well perhaps 4000 rpm is on the rev limiter and the tacho or the rev limiter are wrong?
maybe its worth either checking the tacho calibration or trying a 19"?
also check you are getting full throttle and the ignition advance is working properly.
if the carbs are clean does squeezing the primer bulb make any difference? Ie fuel starvation.
I am sure NOS4r2 will have some more ideas?
did you check the cupping on the prop? its possible i think to replace a plain 17 with a cupped 15 and get the same apparent pitch? someone will correct me if i am wrong!!

if all else fails put the rude back on and i will take the yam off your hands
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Old 09 September 2008, 17:27   #18
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Hi thanks again for your thoughts.

Can you explain to me about the rev limiter what it does and how it works etc please.

If it was indeed something to do with this would disconnecting the tacko from the engine show a change in performance.
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Old 09 September 2008, 17:55   #19
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I dont know how the rev limiter works NOS4R2 probably will, i guess it stops the sparks or retards the ignition timing. worth a try disconnecting the tacho, if its driven off the coils i suppose it could affect the spark quality and hence possibly RPM.
even so if your engine hits a wall at 4000 rpm with a 17inch prop @28mph, if it only gets 4000rpm with a 15" then that should only be about 24.5 mph.
theres something strange going on there! you shouldnt have the same speed and rpm with 2 different props.
I dont know for sure but i would have expected that engine and boat combination to be OK with a 19" or even a 21" prop, and would have expected to get 30-35 knots.
Most people on here with 5.4 m seariders get around 36-38kts with a 19" pitch i dont know how the drag on your boat would compare though, but obviously it should be a heck of a lot quicker than the engine you took off.
I would double check tacho calibration, and consider a 19" to try.
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Old 09 September 2008, 18:50   #20
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Should just cut the sparks at the limit of the revs. Not sure where it is on the Yam 90 though without sticking my head under the hood.It won't be tacho dependant though.
Have a look round the saddle and unplug the wires from the trim gauge or any microswitches you can see then try again-but really this needs to go into a workshop to sort it out. It's not really something that can be diagnosed without seeing it.
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