Running slow now...

[tnurgeniram]

I just recently got my boat. It's a 16ft Crestliner with a 94' Evinrude 30hp. The previous owner was running a 16:1 fuel mixture. I thouht this sounded like a little too much oil. Long story short, I started running the correct mixture, 50:1. Now it seems like it's not getting to top RPM. I don't have a tach. Am I crazy?

 

[jtexas]

Re: Running slow now...

You know how, just before a motor runs out of gas, RPMs increase, as the air:fuel mixture gets leaner?

Your motor was running lean before, you've richened up the mixture to where it should be (air:fuel mixture, not fuelil).

At 16:1, 6% of the gasoline is actually oil, instead of the 2% it should be.

So if the carb is calibrated to deliver say, 14.1:1 air to premix with 2% being oil, you'd get 14.4:1 air:fuel. (I don't know what the actual calibration is, just pulled 14.1 out of the air for an example)

But with 6% oil, the air:fuel mixture is 15:1

So, less RPM. Without tach, hard to say whether it was overreved before, or lugging now. But your plugs should foul less. And the pistons will be less likely to melt.

 

[tnurgeniram]

Re: Running slow now...

You know how, just before a motor runs out of gas, RPMs increase, as the air:fuel mixture gets leaner?

Your motor was running lean before, you've richened up the mixture to where it should be (air:fuel mixture, not fuelil).

At 16:1, 6.25% of the gasoline is actually oil, instead of the 2% it should be.

So if the carb is calibrated to deliver say, 14.1:1 air to premix with 2% being oil, you'd get 14.4:1 air:fuel. (I don't know what the actual calibration is, just pulled 14.1 out of the air for an example)

But with 6 1/4% oil, the air:fuel mixture is 15:1

So, less RPM. Without tach, hard to say whether it was overreved before, or lugging now. But your plugs should foul less. And the pistons will be less likely to melt.

How hard is it to adjust the carb? Never done it before...

 

[jtexas]

Re: Running slow now...

You can adjust the idle mixture -- step by step:
http://forums.iboats.com/showthread.php?t=167352

That only needs to be done after the carb's been pulled for cleaning and rebuilding. Once it's set, it shouldn't require any adjustment.

The hi-speed jet is fixed, so there's no adjustment to be made.

At 16:1 gas-oil ratio you had more oil in the mix; that means less gasoline in the combustion chamber, and more air -- that makes it burn hotter than it was designed for.

Is it just a slight difference in top speed, or really noticeable? Did you change anything in the boat -- move a battery around, add more beer, bigger gas tank, anything like that?

 

[tnurgeniram]

Re: Running slow now...

You can adjust the idle mixture -- step by step:
http://forums.iboats.com/showthread.php?t=167352

That only needs to be done after the carb's been pulled for cleaning and rebuilding. Once it's set, it shouldn't require any adjustment.

The hi-speed jet is fixed, so there's no adjustment to be made.

At 16:1 gas-oil ratio you had more oil in the mix; that means less gasoline in the combustion chamber, and more air -- that makes it burn hotter than it was designed for.

Is it just a slight difference in top speed, or really noticeable? Did you change anything in the boat -- move a battery around, add more beer, bigger gas tank, anything like that?

The only thing I changed was the fuel mixture. Changing the fuel mixture forced me to adjust the throttle linkage and idle set screw. The guy I got the boat from had them adjusted to accommodate the jacked up fuel mixture.