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Old 27 September 2004, 08:42   #1
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boat speed

After having a good afternoon on the Solent i have a question.

I managed to get some good conditions that allowed a couple of full throttle moments. The question is when going from 0 to 1/4 open i have very good power then going from 1/4 -3/4 still very good power but from 3/4- wide open throttle there is very little or no change in engine speed or boat speed.

1 Do i need to adjust my throtte cables?

2 could i be running the wrong prop?

3 Is there much change on other boats?

Thanks in antisipation

Regards

Graham
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Old 27 September 2004, 09:32   #2
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Are the revs rising from 3/4 to WOT?
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Old 27 September 2004, 09:37   #3
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Mark

I dont have a rev counter fitted but it does not sound like they increase!

Graham
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Old 27 September 2004, 09:40   #4
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When I ran a Mariner 90 Four Stroke I had the same thing,very little throttle responce.Now I have a Ficht 2 Stroke you can feel and hear the engine pick up right through the throttle range.Is yours a 2 Stroke carb or Four Stroke?? I think you will only get that complete power band if it's an injection engine.
The only other engine I have run was a 115 Mercury 2 Stroke and that had the same sluggishness when at the top end of the throttle and rev range.
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Old 27 September 2004, 10:42   #5
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Is this the same at all levels of trim?
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Old 27 September 2004, 11:42   #6
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Quote:
Originally Posted by Nemo
Mark

I dont have a rev counter fitted but it does not sound like they increase!

Graham
Your ear is as good as a rev counter anyday.

Firstly I would check if you are getting maximum throttle opening. Whip off the cover and see if the throttle is opening to the max. There may well be a stop at maximum opening, as well as the tickover stop.

If not, adjust the cables if you can.
If it is, then things could be more complicated.

As asked above, 2 stroke or 4?
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Old 27 September 2004, 12:32   #7
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Its a 2 stroke!
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Old 27 September 2004, 13:54   #8
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Carb or Injected 2 Stroke
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Old 27 September 2004, 15:34   #9
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Nemo, what you are finding is not far off what you should expect. If everything about the engine and boat setup is perfect you should expect some increase in performance from to full throttle but not much. The wee diagrams below may help you understand why.

At throttle the red area is the area masked by the throttle plate.

At full throttle the black area is still masked by the spindle.

Therefore, the last opening only gives the difference between these two areas.

The situation is actually worse than indicated by the diagram. If you imagine looking down the carb, the edge of the throttle plate would appear as a crescent (elliptical). At throttle part of the spindle is already masking the flow so the increase in flow to full throttle is small (represented by the hatched red patch). The second diagram illustrates this.

You can see that once the edge of the plate aligns with the spindle there is unlikely to be an increase in flow.

So, although the relationship between the throttle hand unit and the carb throttle plate is linear, the relationship to flow is not.

I hope this helps a bit.
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Old 27 September 2004, 16:08   #10
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Thanks for all the replies so far!!

I have been having a look tonight and it appears that i am getting WOT from the 3/4 to full. The reason seems to be the shapped plate between the throttle cable and the rod that connect to the carbs it reaches full at 3/4 still with more movement in both the hand control and the plate.

Has any body got a tuning guide that may help me further.

Thanks

Graham.
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