5hp 661 BS Long shaft air cooled 1979 problems

[mudhopper]

Hi,

I have a 5hp 661 BS air cooled Yamaha which seemed fine when i bought it a few months back. Now I find that the engine will not run at high revs for any length of time. Also cuts out at idle after ten minutes or so.
Stripped carburetor, cleaned and replaced. Fuel pump had some crud in it which I have removed but not sure how to test it is drawing properly. Seems fine tbh.
Seems to be a fuel problem. I have a long line on it to the fuel tank - is that likely to be a problem?
When I kept it on high revs last time, I pumped the fuel through with the primer bulb and it did not like that at all and stalled with a backfire, smoke coming back out through the air intake (air cooled).
Great starter and runs fine (until it just stops) or unless it is kept at very high revs for a few minutes. Seems to be starved of fuel but not sure why?
Any thoughts? Not much to the engine really, which is why I am at a bit of a loss now.
Thanks.

 

[tommarvin]

Do you have the vent open on the gas tank,?
You could add a clear inline fuel filter, and see if any gas is going through ?
Maybe shorten the fuel line if you can ? Little motors don't pump fuel that hard.do this last.

 

[mudhopper]

Yeah the vent is open, I will try shortening the fuel line but i don't think that is all that is wrong with it. Starts running erratically at high revs then just stops. It won't start again immediately either. I thought maybe it could be the coil but it acts like a fuel problem. It is 100:1 but the identical Mariner model runs on 50:1 so i wondered if if the mixture is leaner than it is used to, but not convinced that would cause it to cut out at high revs either.

 

[99yam40]

if pumping primer bulb makes it stall then the needle and seat are not closing off when the bowl is filled up and the float tries to shut off the in comeing fuel

 

[tommarvin]

Better clean/rebuild the carb,again, for some reason it's always harder than it looks.
You-tube has a few good videos, watch them until you feel comfortable.

 

[mudhopper]

I have stripped and rebuilt the carbs of outboards many times and I am pretty sure that there is nothing wrong with the carb or the float or the needle or the jets or any other part of it. The whole assembly could not be simpler on this little engine.

Just found this:

Re: Lean Sneeze...Common Causes ?

Hi Ray. It sounds like the ignition is ok, so I wold check the carb. Even though it was recently rebuilt, they can get gummed up pretty quickly. I would clean/inspect/rebuild the carb just to be sure. If you think it's the fuel pump there are a few tests you can do to check and see if it is working properly. Start by squeezing the primer bulb it should get and stay firm. If not, the the pump may be bad. If the motor picks up rpms when you manually squeeze the bulb, then the pump may also be bad. Another test is to brielfy disconnec the fuel line that leads from the the pump to the carb. If you see fuel pulsating out, the the pump is ok. Better still install one of the litle inline fuel filters in the pic below inline between the fuel pump and the carb and it will not only filter the fuel, but give you visual confirmation of fuel delivery. Here are some good carb links too. If it is the pump. No worries there's a link below where you can get them for about $20 and they install in about 5 min.

http://forums.iboats.com/showthread.php?t=167352

http://www.duckworksmagazine.com/04/s/columns/max/25/index.cfm

http://www.aircraftspruce.com/catalog/eppages/mikunifuelpump1.php

I have a weak fuel pump I think. Will check by disconnecting line to carb and see if it pumps properly. I did wonder about it not firming up properly when I pump the primer bulb - well not always...maybe it is on its way out?

 

[99yam40]

on most outboard If the diaphragm on pump leaks fuel into the crankcase it will cause an over rich condition when you pump the primer bulb.
But I know very little about this small motor, so I could be wrong

But you said you have cleaned and inspected the pump

 

[mudhopper]

I cleaned the carb and took the front of the pump and took some crud out by the filter. Today I took the whole pump apart and put it back together but when i went to turn the engine over it appears to have more or less seized up since yesterday. Fuel mixture was fine so no idea why that is.
Anyway, I am buying a new engine, bored of this game. Spares or repairs.