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Old 28 September 2005, 04:58   #1
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Yam 55 down on power

As you might have noticed from my previous post "worth repairing?" my Yamaha 55 suffered a terminal fault recently. I did however manage to find another 55 on Ebay with a knackered gearbox for 100 and swapped the powerheads over.

I've set the timing, overhauled the carbs, fitted new plugs, checked the compression (both cyinders in the 140-145psi) etc etc.

Both plugs are the perfect tan colour so the mixtures right but it just hasnt got as much power my old 55 had. It doesnt accelerate as well and is about 5kts down on top speed. Top engine speed is now 4900rpm not 5500rpm as it used to be.

Is there something I'm missing? I'm sure they never came out of the factory with so much variation in output.

Anyone got any suggestions?
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Old 28 September 2005, 05:57   #2
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Is the throttle opening fully, that would explain the loss of 600 revs and slow acceleration.

Pete
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Old 28 September 2005, 06:01   #3
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Craig,

when you say you swapped the powerheads over what do you mean exactly:

1. Undressed the old, failed powerhead and transferred everything onto the "new" powerhead.

2. Lifted the "new" powerhead lock, stock and barrel and swapped over.

If it's option two then you've swapped ignition components from the "new" engine?

Have you checked the temp that the "new" head is running at?

Did you decoke the "new" head or just transfer it straight across?

Did you compare engine numbers and the year of manufacture of the powerheads.

Sorry for all the questions but the more info we have the better the chances that we might collectively spot the problem.

Good luck
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Old 28 September 2005, 06:50   #4
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Quote:
Originally Posted by stan_deezy
Craig,

when you say you swapped the powerheads over what do you mean exactly:

1. Undressed the old, failed powerhead and transferred everything onto the "new" powerhead.

2. Lifted the "new" powerhead lock, stock and barrel and swapped over.

If it's option two then you've swapped ignition components from the "new" engine?

Have you checked the temp that the "new" head is running at?

Did you decoke the "new" head or just transfer it straight across?

Did you compare engine numbers and the year of manufacture of the powerheads.

Sorry for all the questions but the more info we have the better the chances that we might collectively spot the problem.

Good luck

Lock stock and barrel, allthough I did swap some components: Ignition coils, carbs, starter motor, thermostat and fuel pump since these were all in better condition on the old head.

Running temp is around 130F (54C), wich is about 10F (5C) warmer than the old. The thermostat in these is about 60C so taking in to account the gauge error that seems OK

new engine is 3 yrs younger but most of the parts including the carbs are the same.

Didnt de-coke, had a look in both cylinders through the spark plug holes and they look clean enough, could be worth a try though.
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Old 28 September 2005, 06:53   #5
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Quote:
Originally Posted by Pete7
Is the throttle opening fully, that would explain the loss of 600 revs and slow acceleration.

Pete
The butterfly is opening to within about 5 degrees of horizontal. Theres no adjustment for them since they're opened by a cam connected to the throtle/timing controler. This is probably my number one suspision but I cant see how to improve it.
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Old 28 September 2005, 12:36   #6
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Craig I just re-read your original post and you say the plugs are tan?

That's great in a car but suggests you may be running slightly weak in a two stroke. Brown and with a "damp sheen" is what you really want.
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Old 28 September 2005, 13:30   #7
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Quote:
Originally Posted by stan_deezy
Craig I just re-read your original post and you say the plugs are tan?

That's great in a car but suggests you may be running slightly weak in a two stroke. Brown and with a "damp sheen" is what you really want.
electrode brown, electrode surround tan - brown, and black around the end of the metal case!
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Old 28 September 2005, 19:18   #8
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are you using a different prop?
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Old 04 October 2005, 08:41   #9
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Spare Parts

I have a Yam 60 with electrical problem when its warm, would you consider selling the spare electronics you have, I can collect
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Old 06 October 2005, 18:00   #10
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Quote:
Originally Posted by rama
are you using a different prop?
Same prop!

Have adjusted the butterflies by using thinner gaskets!

Ran it for 4 hours up the Menai today, still down on power, poor acceleration & top speed. plugs still the right colour, engine still smooth, its driving me mad, what can it be???????????????????
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