boat speed

[Nemo]

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

[Mark Halliday]

Are the revs rising from 3/4 to WOT?

[Nemo]

Mark

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

Graham

[Wayne Rogers]

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.

[Rogue Wave]

Is this the same at all levels of trim?

[Mark Halliday]

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?

[Nemo]

Its a 2 stroke!

[Wayne Rogers]

Carb or Injected 2 Stroke

[jwalker]

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 ¾ to full throttle but not much. The wee diagrams below may help you understand why.

At ¾ 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 ¼ 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 ¾ 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.

[Nemo]

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.

[Biggles]

Nemo

See if you can get your hands on the next pitch prop.

This will satisfy your need to tinker and show you if all is okay.

[pellesmil]

hi!
Do you have a problem regarding getting full throttle movement on the carb. arm then you can do like i had to do on my yamaha C75.
I adjusted all carb (i have 3) the end stop at eatch carb was not correct, so i had to bend the metal arm on eatch carb. to make it stop at full trottle opening, i did that on eatch carb. seperatly.
I then syncronised the carb. to open correct and simultaniusly both at closed and full power.
What i then realised was that the lever did not reach far enough to give full opening, and as you can see i had to shim with tape to measure the distanse needed. (see pics)
I then built that up with epoxy adesive that i later grinded down to correct dim.
When the idle are adjusted at the carb arm the lever are supposed to bearly come in contact with the roller. (se pics for idle)
Good luck

I use the Clymer b784 repair handbook it is "ok" is the offisial shop manual coveres the most but i´m not impressed, take a look around before you by one for yourself

Pellesmil