Yamah F2.5 ob only runs with the choke on...

A blocked jet, possibly sticking float valve.

If you're capable of servicing it, take the carb off, take it to a motorbike shop (a backstreet one, rather than one with a flash carpeted showroom) and ask them to clean it with some compressed air. It will save you hours of faffing with carb cleaner and puffing through it.
 
Likely to be a partial blockage somewhere in the fuel system often the result of gunge developing during storage. Thought to be more common since the inclusion of ethanol in petrol.

No big deal just take the carb bowl off and see if you have a good supply of fuel running in when you turn the tap on. If not clean the fuel line. If yes clean the main jet by blowing through it. Best to take it out but don't sweat it if you find it hard to move, blowing through with a piece of pipe works.

It is a common problem when OBs don't get used. Expect to have to clean it again in an hour when a bit of gunge from the tank comes through the line.

NB On some engines the carb float can be fitted upside down. This is a bad thing to do.
 
My usual method which works an absolute treat is to remove the carburettor, don't buy any of these fancy carb cleaners just remove the jets and the float bowl. Place the entire contents in a pan fill with 6 parts water and 1 part lemon juice or distiller vinegar and simmer for 20 minutes it will remove every spec of vanish and grime. Rinse parts in clean water, dry and reassemble. I have fixed over 30 outboards using this method over the years
 
Cruless said:
My usual method which works an absolute treat is to remove the carburettor, don't buy any of these fancy carb cleaners just remove the jets and the float bowl. Place the entire contents in a pan fill with 6 parts water and 1 part lemon juice or distiller vinegar and simmer for 20 minutes it will remove every spec of vanish and grime. Rinse parts in clean water, dry and reassemble. I have fixed over 30 outboards using this method over the years
Click to expand...
If you're over Lewes (Brighton) way any time, I have an ultrasonic cleaner which is made for this part of the job.
 
Hi, you either have a blocked pilot jet or blocked venture transition ports. A little tip when cleaning a carb is read up on how and why each part of the carb works. Once you understand its childs play. I could try to explain but is easier to google pilot jets and carb transition ports
 
I have found that a blast to high revs then cover the intake this will sometimes clear the blockage, you will need to remove the air filter first ! ( And if your hand get sucked into the engine you can de coke it while your there ! Only joking with the last bit )
 
Thanks all.
Am in Dorset, so a bit far to travel to Lewis but thanks for the offer.
Will try the simmer in vinegar/lemon juice method over the weekend I think.
 
Make sure the carburettor is fully immersed in the pan. Lemon juice or distiller vinegar ( not malt vinegar!)
20 mins stir occasionally and serve. Then hide the pan from the missus as it may smell a little!
 
Cruless said:
My usual method which works an absolute treat is to remove the carburettor, don't buy any of these fancy carb cleaners just remove the jets and the float bowl. Place the entire contents in a pan fill with 6 parts water and 1 part lemon juice or distiller vinegar and simmer for 20 minutes it will remove every spec of vanish and grime. Rinse parts in clean water, dry and reassemble. I have fixed over 30 outboards using this method over the years
Click to expand...

Worked a treat...

Many thanks
 
I came across this thread because I had exactly the same problem with a Yam 2.5 I was servicing. I just wanted to say thanks; 20 mins simmer in the solution did the trick!
 
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