Yanmar Single Cylinder Engine Running Rough

[FlimFlamII]

I have an 8hp Yanmar. It was running fine and then after about five minutes the rpm's began to drop off then come back up then drop back off. When I applied more throttle the rpm gauge showed increased rpm's but no there was no additional power.
This same thing happened about six weeks ago and then all of the sudden it began to run fine...

Thanks,

 

[Benny17441]

90% of problems are fuel related. It sounds like fuel starvation. Replace your fuel filters, they could be clogged. If that does not do it disconnect the fuel line and blow back into the tank to make sure the tank's pick up tube screen is not clogged.

 

[Bill Roosa]

What engine work have you done lately?
As Benny posted could be fuel starvation due to clogged filters. Could also be a clogged vent on the fuel tank or air in the lines. The latter being likely if you recently did any fuel line work.

 

[FlimFlamII]

Thank you!

 

[jssailem]

FlimFlam, With a boat name like "Aphrodite" what is not to love....
How did you get a Swedish designed Danish built boat on lake Coeur d'Alene?
Plus 1 for Benny. That would be the direction I would explore.

 

[FlimFlamII]

Filters replaced, blew back on the line from the fuel tank. The vent for the fuel tank is inside a stanchion so as a quick alternative, I opened the cap on the top of the tank. Used the fuel manual lift pump to try and bleed any air and still have the same problem. Could it be that after opening all the lines and replacing the filters I just still have some air left in the lines?

Thanks,

 

[FlimFlamII]

jssailem,

The boat was purchased new from Denmark back in 1980 and then delivered to Cda. The first owner, a good friend, owned it until he passed away back in 2004 Unfortunately, he didn't really care much for upkeep and when he got sick it sat unused for about six years... Lots of issues... The restoration has been slow but fun.

 

[Sailm8]

I had a 1gm with a similar problem. I pulled the fuel pick up tube out of the tank and found a load of crap in it. Cleaned it out and all was well.

 

[FlimFlamII]

I'll give that a try as well.

Thanks!

 

[Benny17441]

Check the condition of the fuel. Follow the fuel lines to try and detect any leaks, a spot of fuel on a line connector is a place where air will get in when the engine is shutoff.

 

[jssailem]

FF unfortunately that is nature of boats. For awhile they are all important. Then like girlfriends (sometimes) they loose there shine. That's the time to either marry them or set them free.

I have a boat like that. I'm readying it for some extended sailing in the Pacific NW. Sold my Montgomery 15 to a guy up in your neighborhood. So if you see a M15 on the lake say Hi to Stan.

Weak connections can let air into the lines. When your pumping you may get mostly fuel. When they sit the air can get in and if given enough time replace the fuel in the line.

Sailm8 made a good point. As fuel degrades you can get a goo (asphaltine? ) form from the microbes. It can plug up the lines. I have found it in the bends and connections of the fuel lines. Pull/blow it out. I found the
Biobor Diesel Biocide JF to help.

 

[FlimFlamII]

Question - I've cracked open one of the screw heads and begun using the manual lift pump. When I do this should i see/hear any bubbles coming from around the head of the screw or should it be a solid stream of fuel with no bubbles or noise. Is this an indication there is still air in the line?

Thanks,

 

[jssailem]

You want solid fuel. Bubbles means air. Keep pumping.

 

[FlimFlamII]

It does appear it was a fuel starvation issue. After taking the steps mentioned above it's running much better. Thanks for all the suggestions.