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Old 07-01-2014, 02:28 PM   #1
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Hole in cylinder wall of Yamaha 55hp

Hi,
Just wondering if anyone has ever seen a small hole on the cylinder wall, 1" from head, it's about 2mm diameter and perfect (not added later by someone).
I checked where the hole went to, it is connected to the exhaust port. It is present on one of my power heads on both cylinders but not on my other 2 power heads.
All Yamaha 55hp 2 cylinder power heads.
I have never seen this before. Engine has low compressions but run fine.
I have the head off ready to be skimmed to bring compressions up but im wondering how this hole to exhaust will effect compressions.
I'm guessing its stock but cant see the point of it.
Thanks in advance.
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Old 07-01-2014, 06:21 PM   #2
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Is the engine a pull start? Some engines have an exhaust relief to ease pulling.
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Old 07-01-2014, 06:32 PM   #3
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Country: England
Location: Salcombe, Devon
Occupation: Marine Engineering and Storage
Boat name: Puffin
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Quote:
Originally Posted by Hydrostream View Post
Is the engine a pull start? Some engines have an exhaust relief to ease pulling.
to be honest I has thought it was a exhaust relief but couldn't work out why as its an elect start however nothing to say this power head wasn't originally a manual start.

trying to up the compression, do you think it will influence my final result? got to get it up from 110psi.

off to the machinist tommoz to have 0.6mm taken off the head (maybe more if he thinks he can do the squish, weird shaped combustion chamber not domed like normal so he may not be able to). I want to bring compression ratio up to 8.0:1 - 10:1, these come stock as 6.5:1
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Old 08-01-2014, 04:19 PM   #4
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Quote:
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Is the engine a pull start? Some engines have an exhaust relief to ease pulling.
That's exactly it - the manual start blocks had a decompressionn hole drilled or you'd never pull it over by hand.
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Old 08-01-2014, 04:23 PM   #5
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Country: England
Location: Salcombe, Devon
Occupation: Marine Engineering and Storage
Boat name: Puffin
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Cruising area: Plymouth to Exmouth

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Quote:
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That's exactly it - the manual start blocks had a decompressionn hole drilled or you'd never pull it over by hand.
how does this effect compressions over all?

head is currently being skimmed, just wondering if I should have this passage filled as worried about compressions being lower than if closed.

need compressions to be sky high (165-180 psi = 8.5:1 - 10:1)as racing this engine.
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