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Old 10 November 2011, 16:39   #31
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Originally Posted by Johnscubanut View Post
Not sure if you are reading the size right? The 11.75 may be the diameter of the prop? The pitch will be the second figure i.e 11.75 x 12, 13, 15,17 etc?
People, please correct me if I am wrong? Ta-John
Might be wrong but close enough. am i right in think ing that a 12" prop should rev the nuts of the engine?
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Old 10 November 2011, 16:45   #32
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Might be wrong but close enough. am i right in think ing that a 12" prop should rev the nuts of the engine?
I would have thought so.
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Old 10 November 2011, 16:49   #33
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Cool that's what i had thought but as its all new to me i needed to get a few more opinions! iF i get the rev counter to work at the weekend all should become clear!
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Old 19 December 2011, 03:30   #34
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well took a few weeks to get a working rev counter fitted and decent weather to try it out. Took it out yeaterday and max revs are just under 5,000rpm and max speed on phone GPS was 23.5knots so a bit better than before but not as much as I had hoped for.
The water was lumpy and it was an entertaining ride! Played with the trim to get max speed but did not see much differrence in the lumpy water. Would the next step be change prop to get more rpm?

current prop is 11.75 x 12.

Also annoyingly the engine started to miss fire so I came back in and it conked out in the Harbour. Managed to start it at home but only by changing the plugs, the old middle plug was slightly wet. Now the tick over is about 3,000 rpm! Any ideas what would cause this? Stuck carb float?

Cheers TomKat
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Old 19 December 2011, 03:41   #35
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have you put a compression gauge on the engine before?

what results did you see?

a 4.8m boat with a 55hp you are ball park at 24 knots i'd suggest. maybe could see 26/27 with a prop change and if the compression test is ok.

i would also inspect the floats since a plug is wet and balance the carbs.

cheers
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Old 19 December 2011, 04:55   #36
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was sold to me as having been tested but will check it out myself, been a while since I tested compression do you just test it on the started with the kill switch out or do you run on 2 cylinders with the compression guage in the other cylinder?
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Old 19 December 2011, 08:12   #37
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ideally you want the engine warm so run it up on muffs if you can before doing this.

if however you can't do that then fair enough as the readings will be lower-but it is more important they match-ish.

just leave the killcord off will be enough as your only cranking it for 2/3 seconds.

what i do is take all the plugs out and leave the killcord off and note the PSI reading. if there is a large discrepency then you have found your problem, if readings are the same and a decent reading PSI wise then you probably could try a diff prop.

as a word of warning, NEVER buy an engine without doing a compression test and witness it being done-even better if it is a gauge you know/trust.

i don't know what PSI your motor should have, if you have a manual then it probably is in there but you don't want more than 5psi or so a difference between your cylinders.

cheers
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Old 19 December 2011, 08:35   #38
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cool, will try tonight if dry outside!
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Old 19 December 2011, 08:44   #39
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pic of the boat being 'tested'!
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Old 19 December 2011, 08:46   #40
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