have a question about cruise throttle.

[jonnymarquis]

I have a 1977 70 hp evinrude. on the throttle cam assy. there is a mark that says start then there is a mark closer to the top that doesn't say anything. is that where it needs to be for "cruise". i do notice that WOT is past that mark and as you pull it back it closes the throttle valve some but does not make the spark advance go back. so am i right about that mark?

also i was told that on outboards that it doesn't matter if your WOT or putting 5mph that they use the same amount of fuel. and that the rule of thumb is 10% of hp is fuel consumption per hour. like a 70 hp burns 7g per hour.

now the question i have is does it burn 7g at 5mph and 35mph? or just at the higher end.

with the price at the pump going up just curious how to pinch a penny.

thank you guys!

 

[QC]

Re: have a question about cruise throttle.

I moved this over to non-Repair OB Discussion., We try and keep the Boating section free of engine topics .. . Thanks.

I am not clear on the notch in the control so won;t comment on that. Definitely lower fuel rate at 5 MPH than 35 . . . Probably less than 10%. You shoudl poke around at BoatTest.com just to look over some data. It will be hard to find anything that will diectly correlate on an older carb'd 2 stroke, but you can see some of the trends in fuel use vs. speed at similar power levels.

 

[steelespike]

Re: have a question about cruise throttle.

The actual throttle posaitioon for most mpg will vary based on the ability of the boat to get on plane and stay easily.Actual best cruise speed could be just on plane to as much as 1000 rpm higher.
gph at wot is roughly 10% of the hp.Whether pushing a house boat or a light speed boat.
MPG is achieved with speed and prudent throttle application.
As you reduce throttle gph goes down as well.
Best practical crusing speed is the fastest speed with the least throttle/gph.
Only way to accurately tell the most efficient setup is with a flow meter.
And the difference may be in tenths of a gallon.
Lowest gph may not deliver best practical mpg.
It is possible to get realtime mpg tying a gps and flowmeter together.
Results could be surprising.

 

[JimS123]

Re: have a question about cruise throttle.

All too many people have only 2 settings on their throttle. They idle in a no-wake zone, but go WOT at all other times. Partly because some people just don't know any better, partly because some people have underpowered boats, and partly because some people feel the need for speed everywhere and at all times.

OMC's OB motor owner's manuals of years past referrred to what they called "cruise throttle". If you read the explanation, it was meant to conserve fuel without losing a whole lot of speed. The written procedure was to power up at WOT, then throttle back until you heard a slight reduction in the engine rpm. That represented an optimum between speed and gas consumption.

We are much more sophisticated today. Just look your boat up on Boatest.com, or get the mfgr's test data, and run the rpm at max mpg!