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Old 13 March 2009, 19:47   #1
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Honda 50 revving problem.

Hi guys, i wonder if anyone can help.

I have a fairly old circa 1998 Honda four stroke 50hp that ihave been using on a 6.5m aluminium boat. Its developed a problem where when its running at wot something gives and the revs suddenly shoot up untill it appears to hit a rev limiter.

The first time it happened my reaction was to go back to idle straight away. I have to admit that i thought the driveshaft had parted or the prop had fallen off! However on subsequent occasions i have noticed that it is still actually turning the prop and the loss in speed is only marginal if left to carry on. I guess from engine tone (guage is goosed) that it increases by about 4-500 revs.

Any ideas?

Cheers, Caledonia.
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Old 13 March 2009, 21:43   #2
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Any prop damage? Trimmed out too far?
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Old 14 March 2009, 03:40   #3
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Is it running ok other than that? Had one of these that would run on 2 cylinders and then with out any warning would start to run on all 3 and the revs would shoot up. It was also quite old and high hours.
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Old 14 March 2009, 03:48   #4
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Might be worth checking that the rubber bush in the centre of the prop is still intact, although if it had spun, i'd be surprised if it would grip and drive the prop again. Maybe worth a quick check tho

Regards Robbie
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Old 14 March 2009, 05:34   #5
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Thanks for the replies guys. The prop is new, i don't think its the trim because i would rcognise that sound.

I did think about the rubber in the hub but came to the same conclussion, it wouldn't slip then stick again.

The performance isn't as it was when its new but i don't think its dropping a cylinder because its quite smooth running and most of the original power is there.

If i take it out now it will be fine for a time, then it will happen, only at WOT, i'll throttle back, then up to WOT and it'll be fine again for a time. I can't predict when it'll happen.

Frustrating because the engine runs ok, in an old outboard sort of way!
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Old 17 March 2009, 05:37   #6
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Quote:
Originally Posted by Caledonia View Post
Thanks for the replies guys. The prop is new, i don't think its the trim because i would rcognise that sound.

I did think about the rubber in the hub but came to the same conclussion, it wouldn't slip then stick again.

The performance isn't as it was when its new but i don't think its dropping a cylinder because its quite smooth running and most of the original power is there.

If i take it out now it will be fine for a time, then it will happen, only at WOT, i'll throttle back, then up to WOT and it'll be fine again for a time. I can't predict when it'll happen.

Frustrating because the engine runs ok, in an old outboard sort of way!
i dont think you will be dropping a cylinder that would cause massive power loss.
Did it do it with your old prop? i think from your description it can only be either the prop ventilating or a rubber hub slipping, as i guess any steel shaftsplines or gears slipping would have totaly wrecked themselves by now and make a very loud noise.
perhaps you could try trimming in futher if its fixed trim or dropping the engine a hole? or your original prop back on?
I assume you dont gain any speed when the revs climb?
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Old 17 March 2009, 07:10   #7
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I got the rubber letting go scenario on an old (1972) Johnsorude 25 - but it wasn't a "let go / grip - it was more of a gradual decline as the speed dropped off over the space of an hour or so to the point that it wouldn't get the boat on the plane & we did the last half hour of the trip back at displacement speed....

The way to check the rubber hub is to look for extruded rubber round the joins between the propbody & the hub. - if it's slipping then there will be heat generated that will melt a thin layer of the rubber & squeeze it out the ends. Assuming the hub is still fully painted, the paint will have lifted at the squirt joint.


Assuming your "normal" performance isn't as good as it used to be, are your coil(s) on the brink of a total failure? They usually break down & stop the spark rather than improve, but if there is dampness that is being evaporated in one as the engine warms up? On a similar theme check your HT leads for damage - I found one on the Merc that had literally snapped & was conducting through one of the copper strands that made the cable.... Alternatively grot in the carb that maybe as you are bouncing along at WOT gets in the way, giving you a slightly leaner mix - you back off, the suction is less & it falls away, "resetting" it?

I do wonder based on what you say if the "spurts" you are getting are actually the engine finding it's natural rythm again?

Is it a step change in revs, or does it wind itself up slowly(ish).
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Old 17 March 2009, 08:59   #8
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Sounds like ventilation to me. Does it happen in a straight line or when going round a corner?
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Old 20 March 2009, 05:04   #9
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Thanks for the ideas guys.

Its a defiante step-change in revs and really quite a significant rise, I think about 500rpm that take it to revs higher than ever before and, by ear, far to high.

We did wonder if it was ventilation but decided that it probably wasn't because it couldn't be induced by a tight turn or rougher water or anything like that.

I haven't checked the prop rubber yet but i'll do that, i'm thinking its quite unlikely that is could slip then grip again many times.

Any more suggestions would be gratefully recieved.

Cheers
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Old 20 March 2009, 05:09   #10
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The bushing failed on our Honda 50 prop after a couple of years. We put it down to some corrosion weakening the housing where the rubber bush is fitted. It was an intermittent problem and only happened under load. A new prop solved it, so may be worth trying to borrrow a prop if poss.
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