3.9hp Cresent Outboard - Carb tuning

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long_ben

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Before anyone suggests its applications as an anchor... I know... its a terrible old outboard which I won't find spares for and probably does belong at the bottom of the ocean.... thing is... its what I have for now, I paid peanuts for it, and it starts and runs well enough that it'll do very nicely until I manage to source something better that I can afford. I have so many more pressing tasks to deal with (and a set of oars) so it's a long way down the list.

What I'm wondering is, bearing in mind the above, does anyone have any experience tuning the carbs on these. When it starts, the prop is running. Is this normal? It revs up fine but at idle the prop doesn't stop. I've tuned chainsaws before but I'm at a bit of a loss to work out how to sort this... or whether it is sortable. As is, I can just shut the engine off when I need to but I'd like it if I could make the prop stop at idle. Must be possible... I guess I'm just missing something. It's a bing carb... I'll stick some pictures in and hopefully someone can offer me some advice...

Thanks all...

Ben

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I have the 5 hp version in Volvo Penta Clothing ( VP 51)

Your 3.9 has a fixed drive , which is why the prop is always turning . (Mine has forward neutral and reverse gears )

The idle mixture adjustment is the slotted screw above where it says BING the idle jet is the slotted hexagonal head next to it . ( screw it out , keeping the revs dow as low as possible util its too rich, then screw it in a little at a time until it falters, the back out again to find the sweetest spot. Readjust the idle speed

Take out the idle jet carefully and don't loose the tiny fibre washer which might come out with it or stay in the hole. Clean the idle jet with a fine , soft, copper wire.

Take care bits dont fall out if you remove the bowl to inspect the main jet ( IIRC the needle valve might fall out if you are nor careful)

I have the owners manual ... which I did scan and store on Photobucket many years ago ... but we all know what a load of rat bags Photobucket is now in the hands of. I will see what I can do later
 
Okay... thanks. That makes sense. I thought the BING screw was the idle mixture as it seemed to be behaving the way I would expect a 2 stroke to. I guess I'll keep having a play and see how well I can get things to sit. Its likely worth my while stripping and cleaning the whole carb anyhow as I'm not sure how long it had sat for before I got it.

So... with the fixed drive, am I not going to be able to get it to stop then? Is it just a case of adjusting it down to a point where there is as little movement as possible?
 
long_ben said:
Okay... thanks. That makes sense. I thought the BING screw was the idle mixture as it seemed to be behaving the way I would expect a 2 stroke to. I guess I'll keep having a play and see how well I can get things to sit. Its likely worth my while stripping and cleaning the whole carb anyhow as I'm not sure how long it had sat for before I got it.

So... with the fixed drive, am I not going to be able to get it to stop then? Is it just a case of adjusting it down to a point where there is as little movement as possible?
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Thats right. Seagulls and good few other small outboards were like that .... At least it has a stop button, Seagulls dont but you get used to judging when to shut the fuel off
 
Okay... cool. Looks like it’s actually in pretty good shape then... Once I manage to repair the ropey old dinghy that came with the (apparently not so) ropey old outboard, I’ll get it out and see how it all goes... thanks again for your help... :-)
 
long_ben said:
Okay... cool. Looks like it’s actually in pretty good shape then... Once I manage to repair the ropey old dinghy that came with the (apparently not so) ropey old outboard, I’ll get it out and see how it all goes... thanks again for your help... :)
Click to expand...
See if you can do anything with the images here. If you copy them into Paint and resize them to about 120% they are readable

V Penta 39 51

(Includes the German pages because someone wanted the German version)
 
:)
I bought one of these for £10 about 15 years ago. I had to get another fuel tank as the original was rusted out, then the plastic cover would'nt fit.

Great little air cooled engine running on 100/1 mixture. More power and much lighter then Yamaha/Marina 2.5 but noisy and got quite hot. No difference when running 50/1 but more smelly
 
I would not run it on 100:1

My manual says 30:1 if using VP oil other wise 50:1 but I guess that's a bit of marketing nonsense

50:1 is a safe mix for most 2 stroke outboards from the mid 1960s onwards

I did wonder whether to use an air cooled engine oil but never tried it

I was also going to try a hotter plug ( such as an NGK B6HS) as I used to have problems with plug fouling but after about 5 or 6 troublesome years I pensioned it off in favour of a 6 hp Evinrude. Uses a lot more fuel but is still a reliable engine 35 years later.
 
I've got one, too! It goes really well - could get my Tinker Tramp on the plane. The weak part is the alloy cooling water tube that runs up inside the leg. It corrodes in salt water and the water all leaks out before it gets to the exhaust manifold. The motor runs vey hot, then. I fixed mine by sleeving the tube with heatshrink.
 
I'd heard about the water cooling tube corroding. Might try stripping the leg down whilst I wait for the dinghy repair bits to come and make an examination of the situation.
 
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