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Old 16 October 2012, 07:16   #1
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What's occuring here?

Some of you may be aware that I have been having problems with my set up - Valiant DR750 and old Mercury 2 stroke XL 150hp.

A few days ago I was out with some divers and a lot of kit. The boat was bow down due to the weight distribution but since I wasn't going too far and not too bothered about speed I accepted the situation rather than bother trying to explain in french that they all had to ship aft. The seas were calm.

I started with the motor around the vertical and we got up to around 15 knots then the motor started hunting with RPM rising and falling some 800ish RPM. Thinking this may be the fuelling issue I have been experiencing I wasn't too surprised but nonetheless decided to play with the trim. I trimmed further forward and the hunting disappeared??? As an additional consequence I was unable to get more than 4800RPM at WOT. I was also still experiencing the same "fuelling" issue as before but on this occasion when I backed off on the throttle setting and the motor started to bog down or hesitate I was able to overcome this by using the fuel enrichener button.

Do I have a real problem with this motor or could it be simply over-propped with the high 5 17 pitch unit fitted?

Would really appreciate some help here thanx guys.
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Old 16 October 2012, 07:29   #2
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Country: UK - Scotland
Town: Girvan & Tayvallich
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Hi how you doing?
I had the same problem ditch the High five went back to ally your pitch is wrong to I think 19 would be better. Were you idling for long as the old coal burners don't like it I had two bouts of that this year but it cleared itself after 5/10 mins next time out no probs. In another problem similar to this it was the diaphram have you checked that. Hope this helps.
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Old 16 October 2012, 07:56   #3
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Hi Jambo, fine thanx apart from this issue. I wish I could get this sorted out once and for all. I am going to re-engine the boat to a four stroke but the guy interested in buying this "coal burner" (hehe!) is looking rather sideways at it at the moment understandably.

You think 19? I inherited the boat with a 15 and was thinking that was the direction to go again? If WOT was 4800 ish whe the motor was buried would this not imply over-propping?

I went to 17 prop as I was getting horrendous prop slip when almost empty (40 percent) and prop gods have suggested going back to 15 for heavy load conditions like this. I simply haven't got around to doing that yet as I was concentrating on what was perceived to be engine issues. Of course there may still be engine issues......

No hadn't been idling for long. Which diaphragm are you thinking of? Fuel pump?

Thanx - Ian
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Old 16 October 2012, 08:08   #4
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Country: UK - Scotland
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Hi ian. Re the prop I was not performing well with my mariner 150 hp V6 2 stroke and spoke to dealer who said the correct pitch would be 19 but maybe that was because of the load I am not loaded with divers and all their garb but i still think its the same engine. Why not speak to Turbo diesel on here he maybe able to help. Yes the diaphram was the fuel one within the block two summers ago I could statr idle but when asked to 'go' I would not get above 2500 revs just leveled out it was the 'D' worth a check.
Hope you get to the bottom soon.
Cheers
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Old 16 October 2012, 08:30   #5
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Ok thanx for that - yes TurboD was working with my other thread regarding the engine issue.

In this case I was wondering if it was related to that at all or merely an "amateur fumbling with handling" issue.
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