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Outboard Motor issue - Question for the mechanically minded


Kurt Barlow

Question

I have a Yam 2.5hp 4 stroke outboard - works perfectly on the test bench in the garage but always seems to have an issue on the tender. 

Went fishing on Saturday using the tender. Outboard started fine. Ran well in the test tank. Put on the tender - motored on low speed a bit and then gradually cranked up to full pelt. decelerated back to idle speed. When I accelerated again it stalled and kept stalling. It would run at idle speed so I pootled along.  I wondered if it was full starvation or running lean (i have heard this can be an issue with e10) so I pulled the choke out a bit but this made no difference.

Back at the yacht club put it on the test tank - ran fine accelerated up to full speed. Same when I got home. 

Plug looks a bit on the leanish side but i don't like fiddling with air / fuel mixes as I usually end up fouling the plug. 

May try running it on E5 which has a lower alcohol content so less oxygenated. 

Any  thoughts?

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E5 definitely or super unleaded or whatever.

The issue is under load, similar to the problem I have with my big engine at the mo. Need to tune carb under load as well as static. Check float, needle etc as any old fuel will gum it up . Check fuel pump flow rate and gumming up there too.

Iirc carb kits are quite cheap but once again tune for tank then tune under load.(adjustment screw).

I think e10 will fuck up any old 2 stroke without complete winterisation.

4 stroke might need the fuel mix changed just for that.

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2 hours ago, Kurt Barlow said:

Any  thoughts?

Is it water cooled?

I had one once that would overheat and seize up when hot. Not proper fucked, It just got really tight and only ran on tickover (generating less heat).

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17 minutes ago, XswampyX said:

Is it water cooled?

I had one once that would overheat and seize up when hot. Not proper fucked, It just got really tight and only ran on tickover (generating less heat).

Yes - water is running fine. I did wonder whether cold water might be a factor - sea water was about 12 Deg C. 

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1 hour ago, XswampyX said:

Pricey.

 

A mate wondered if the exhaust was getting submerged which affects the vacuum pump performance. This would explain why it works perfectly on the test tank but is problematic on the tender. Need to move some weight fwd and perhaps get a tiller extension pole so I can sit closer to the bow. 

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1 minute ago, XswampyX said:

Yeah, but you could use it as a trouble shooter. If it works then you at least know what it is.

The exhaust you would hear.

Did you?

 

I was so focused on the issue I didn't really consider that. I may have made the issue worse by leaning on the stern trying to work out the problem. The tender is an old skipper 14 dinghy hull so has a relatively low freeboard compared to the typical tender. see picture. 

 

If the weather is calm between Xmas and new year might take it out again. I have the Bison 55 as a back up 

skipper 14.jpg

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Added note,  my suspicion is the difference between the test tank and “real world” issues are the time it’s running.

In both cases it runs fine for a while but after continued use on the boat it starts to be starved of fuel.  It sounds like the tank tests are just a quick “up and down” the throttle.   You could test this by running it for longer and harder in the test tank?

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3 hours ago, Libspero said:

Hard to diagnose..  my 2 cents it sounds like fuel starvation,  so I would suspect the carb..  probably the float.

You could try this:


Or just get a new one for £40 and peace of mind:

https://www.ebay.co.uk/itm/333309794960

I  rinsed out the fuel tank a couple of times and put a new fuel filter in. 

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2 hours ago, Libspero said:

Added note,  my suspicion is the difference between the test tank and “real world” issues are the time it’s running.

In both cases it runs fine for a while but after continued use on the boat it starts to be starved of fuel.  It sounds like the tank tests are just a quick “up and down” the throttle.   You could test this by running it for longer and harder in the test tank?

At the yacht club I left the engine running in the tank  for 20 minutes in gear at about 50% throttle. I don't 'rev' the outboards as that is a fast track route to destroying them. I bring the power on gradually and then decelerate gradually. 

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3 minutes ago, Kurt Barlow said:

At the yacht club I left the engine running in the tank  for 20 minutes in gear at about 50% throttle. I don't 'rev' the outboards as that is a fast track route to destroying them. I bring the power on gradually and then decelerate gradually. 

As I said above, load conditions are different. In the tank its only pushing water around, in real life it also has to push the boat.
Also check the tank vent to see if thats restricting the fuel pressure.

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2 minutes ago, Kurt Barlow said:

At the yacht club I left the engine running in the tank  for 20 minutes in gear at about 50% throttle. I don't 'rev' the outboards as that is a fast track route to destroying them. I bring the power on gradually and then decelerate gradually. 

20 minutes sounds plenty..    often people put it in a test tank for a few seconds and think "yep,  that's working fine".

I like your thinking with the fuel tank and filter..  definitely a possibility.

If that doesn't solve it,  personally I would just try a new carb.  They are pretty cheap and seem to account for the majority of phantom outboard problems.   

I've no better advice/suggestions to offer I'm afraid.

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2 minutes ago, Libspero said:

20 minutes sounds plenty..    often people put it in a test tank for a few seconds and think "yep,  that's working fine".

I like your thinking with the fuel tank and filter..  definitely a possibility.

If that doesn't solve it,  personally I would just try a new carb.  They are pretty cheap and seem to account for the majority of phantom outboard problems.   

I've no better advice/suggestions to offer I'm afraid.

A mate of mine has a spare carb for his outboard. He said its easier to just swap over if you get an issue out on the water than fiddling around trying to resolve the blockage. 

I bought an electric outboard as I'm getting a deeper water mooring further away from the club. I am impressed with it although range will be a lot more limited with one 85 AH battery. However not an issue for shore to yacht trips which are too short and probably aren't good for the OB. 

Longer term I may buy a suzuki long shaft 2.5HP which are about 4kg lighter than the Yam. As a long shaft it would be a better back up to my 8hp main OB on the yacht which sits in a well. The Yam 2.5 will work (just)but you have to stand on the stern and much of the power is lost by prop wash hitting the well - not propelling the yacht. 

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11 minutes ago, Chewing Grass said:

Has it had old fuel left in it for 6 months or more? If so it may be corrosion issues within the carb.

I stripped down and cleaned out the carb a few weeks before the trip. New gaskets put on. Since then I have tightened up the carb screws that attach it to the engine body. Wondered if I had air seeping in. 

Fresh fuel. Less than a week old. 

I am going to try using E5 - less alcohol so a lower % of oxygen in the fuel. The other option give that fuel Swampy suggested a go but £4 a Litre?????

Edited by Kurt Barlow
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17 minutes ago, belfastchild said:

As I said above, load conditions are different. In the tank its only pushing water around, in real life it also has to push the boat.
Also check the tank vent to see if thats restricting the fuel pressure.

That wouldn't cause the engine to stall though 

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3 minutes ago, Kurt Barlow said:

I stripped down and cleaned out the carb a few weeks before the trip. New gaskets put on. Since then I have tightened up the carb screws that attach it to the engine body. Wondered if I had air seeping in.

I have a service manual for my engine. I followed it to the letter.
Ending up with exactly the same conditions as you.

Went on to forums and top comment was 'dont follow the service manual, turn the mixture screw another 1 3/4 turns'

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11 minutes ago, Kurt Barlow said:

Longer term I may buy a suzuki long shaft 2.5HP which are about 4kg lighter than the Yam.

Ive a nearly 20 year old suzuki dt4 2 stroke backup. Great wee engine, need to change the main bearing this year but like all outboards, has its quirks, it seems to throw prop shear pins quite a lot (I keep a supply on the boat).

Best thing about it is it can be serviced in the kitchen ;-)

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26 minutes ago, belfastchild said:

I have a service manual for my engine. I followed it to the letter.
Ending up with exactly the same conditions as you.

Went on to forums and top comment was 'dont follow the service manual, turn the mixture screw another 1 3/4 turns'

Thanks. I am going to have a little play with the fuel - air mix later. 

 

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Chewing Grass

Read your first post and an air leak sprang to mind which would be more noticeable when hot and off choke.

Edit: or the mix as posted above.

Edited by Chewing Grass
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21 minutes ago, Kurt Barlow said:

A mate of mine has a spare carb for his outboard. He said its easier to just swap over if you get an issue out on the water than fiddling around trying to resolve the blockage. 

Not so much help,  but by way of an anecdotal,   my father has a 4hp Honda.

Similar problems to yours a while back (unreliable,  cutting out randomly under load) etc.

He spent months cleaning, tweaking and fettling his carb..   all to no avail.   

In the end he just bought a new one,  bolted it on,  and it worked perfectly out of the box..  never looked back.

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