Low rpms under load

woodster · Jan 9, 2012

chiefaponte said:
Finally got to enjoy the boat with my family. As soon as we started to motor from the dock, I noticed that in gear the engine would rev down regardless of how much throttle was applied. Reading the Moyer marine blog I read about cleaning the carburetor ( done).

They also spoke of a fuel tank vent valve. I have never seen one on my boat. It makes sense for the need to vent the tank while the fuel pump takes a suction. Where is this vent?

Please consider that while in neutral, the engine revs just fine up and down the rpm curve.

Please advise

get down by the fuel tank and look for hoses...you should have one to fill the tank..one to feed the fuel pump...and at least one more ...that will be your vent hose ...follow it till you find the other end and you shoule find a chrome or brass fitting on that end...it may be clogged....and you may find one more hose if you have a fuel return from the carb...i am assuming you have an a4 engine....hope this helps...

regards

woody

ewoden1 · Jan 10, 2012

chiefaponte said:
Thanks woody.
I had my carburetor ultrasonically cleaned and had to fix my float. So I am thinking that this may be a starvation issue

Vacuum leak? When replacing carb after cleaning, what shape was gasket in? Torqued down well?

BobM · Jan 11, 2012

Vent

chiefaponte said:
I am leaning towards fuel starvation. I never seen a vent only fuel tank

The vent is usually visible on the side of the cockpit under where your legs would end up. It projects out about a half inch and has a hole in it. Maybe a mud wasp got into yours.

ewoden1 · Jan 11, 2012

Vent

BobM said:
The vent is usually visible on the side of the cockpit under where your legs would end up. It projects out about a half inch and has a hole in it. Maybe a mud wasp got into yours.

My vent is on starboard side on outside of cockpit . However I run a diesel, my tank is below and forward of the engine amidships. Same place for your gasoline setup? Looking at the manual for the 8.0C no mention of a vent valve.

You know this I am sure, but you can also eliminate vacuum leak possibility by spritzing a little carb cleaner on the carb intake junction and observing engine idel speed. If leaking, then engine rpm will rise when spritzed.

If fuel starved is next, then what about gunk on tack pickup tube? Probably biggest problem with diesels versus gas but hey another place to check.

BobM · Jan 12, 2012

Actually...low RPM under load is also a symptom of a blocked exhaust elbow. It may be time for a new one.

jrowan · Feb 2, 2012

You may already be aware of this, but I have to post that my previous boat a 1980 S 2 8.5 did the same thing with a Yanmar diesel. It would be very slow to rev under load. The problem I found after a huge investment of time & headache was a fuel strainer installed by S 2 at the bottom of the fuel pickup tube in the fuel tank. U have to actually remove the tube from the tank and look through it. If this piece of rolled up screen is in there it blocks fuel flow & starves the engine, until it eventually stalls. Sounds like U have the same symptoms I did. My engine barely got any fuel at all. It ran like a champ after the crappy screen was removed. Try this & also look for a clogged fuel filter. Good luck.