1995 Yamaha C85 fuel consumption has increased.

[Michael B]

My Yamaha has 1200hrs and runs great. I noticed thou my fuel consumption has changed from a average of 3.23 mpg to 2.76. Not sure why.
1st I checked compression and noticed in the last 100hrs it went from 115,115,115 to 111,110,110. Could this be a problem?

2nd I thought maybe the thermostat could be stuck and the engine was running cool. I check the temp with a gun around the thermostat housing and it was 100degress. The stat is 122degree's. should it come up to that at idle on a hose muff? I'm changing the stat now so I'm hoping it runs warmer and maybe more efficient.

What do you guys think? Thank You

[robert graham]

Maybe bad fuel pump diaphragm causing raw fuel to be sucked into that cylinder....check it and replace if required.

[Michael B]

If that was the case would it still run normal? I might just replace it for preventive maintenance. I go off shore a lot.

[99yam40]

how did the stat look when you pulled it out?
was it stuck open?
did you test it to see what temp it opened up at?

Could be carboned up rings causing loss of compression.
use a shock treatment of yamaha ring free or Yamaha internal engine cleaner

[boscoe99]

MPH did not get changed to knots did it? 2.76 knots is very close to 3.23 MPH.

[robert graham]

Are you sure there's no water standing in the bottom of the bilge area?....from rain or leaks?

[Michael B]

how did the stat look when you pulled it out?
was it stuck open?
did you test it to see what temp it opened up at?

Could be carboned up rings causing loss of compression.
use a shock treatment of yamaha ring free or Yamaha internal engine cleaner

If it was stuck open then I would know. It wasn't. I tested it and it opens at 120.
I've used ring free in damn near every gallon of gas. I would think A shock treatment might be a good idea. I would think the compression wouldnt be so even if I had a carbon problem. Thanks for your input.

[Michael B]

MPH did not get changed to knots did it? 2.76 knots is very close to 3.23 MPH.

Both are knots.

[Michael B]

Are you sure there's no water standing in the bottom of the bilge area?....from rain or leaks?

No water in the boat. Thanks

[Michael B]

Maybe bad fuel pump diaphragm causing raw fuel to be sucked into that cylinder....check it and replace if required.

What cylinder? Can you explain this more? Thank You

[panasonic]

I believe Robert is thinking you have fuel pump that is built right on to one of your carbs. If so , a bad diaphragm can dump extra fuel into that particular carb. I don't know if this is your set up or does it have a separate fuel pump?

How do the plugs look, are they black from running rich?

Have you tried a different compression gauge...maybe the Guage is just reading low.

[Michael B]

One fuel pump and three carbs. Plugs are fine. I'm sure the compression gauge is fine too. Thanks

[panasonic]

I have to ask....is the bottom of the boat clean?

[rodbolt17]

when was the last time you had a metcal done on to compression gauge?
your looking at about 4 psi difference.
were both tests done at the same temp and baro pressure?
are all fuel calculations done at the same temp,baro and current conditions?

[99yam40]

What cylinder? Can you explain this more? Thank You

I am fairly sure all 3 cylinder Yam motors use the crankcase pulse of #3 cylinder to power the fuel pump

also running at different RPM or loading of the boat will cause different fuel usage.
different wave/water conditions will change usage too
did you change props or damage the prop someway?