Quote:
Originally Posted by CHANCE1234
. . . i still think i'm buring to much fuel. i am terible with mechanical stuff so what are some quick easy things to imporove fuel burn, ie, plugs, timing, etc...
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1. Make sure engine height is correct. Top of the anti-ventilation plate should be out of the water when you're up on plane, so next time you're out have a buddy drive and take a look over the transom to see what's going on. With a motor hanging on the transom, this typically requires the it to be mounted with the AV plate at least an inch above the bottom. Some props with lots of cup like a Stiletto can run even higher, so one option would be to keep raising motor until it ventilates on sharp turns or has to be trimmed down all the way and then drop it down one hole. Less lower unit in the water = less drag = better mpg.
2. Make sure your tach is accurate. Some analog tachs can be off by several hundred rpm, and if it is, you'll end up picking the wrong prop! On the E-TEC's the digital I-Command gages get rpm from the engine computer, so no question on their accuracy. Don't know about carb'd Merc's - maybe have a dealer or mechanic with a known good tach or a strobe type tach check it out.
3. Find out the OPTIMUM WOT rpm for your motor and pick a prop that turns that with your max load, or at least the load you normally carry. This is a much narrower band than the "Recommended Operating Range", which is 4850-5850 on my E-TEC, but it's OPTIMUM range is 5300-5500, so I'm propped to turn 5400-5450 with max load. A Recommended Operating Range is published for all engines, although I've only seen Optimum rpm recommendations published for BRP motors. If you can't find a published optimum, you'll probably be close if you shoot for the middle of the range. (Or PM Big Shrimpin - he seems to know Merc's pretty well!) In general a motor that revs easily and ISN'T lugged down to a low WOT rpm by too big a prop will have better hole shot and will have better durability because the pistons will run cooler and cleaner with less carbon build up.
4. I'm not familiar with Merc's, but on the carb'd OMC/BRP motors, with the motor off, if you have someone advance the throttle at the helm while you watch the linkage, you'll see that the carb throttle plates barely move for the first 2/3's or so of the throttle travel . . . all it does is advance the spark! Then the throttle plates open almost 90 degrees during that last 1/3 of throttle travel! I used to cruise at 4500 rpm at 20 kts, and at about 4700 the motor got much louder (visualize the sound of a toilet flushing with all the gas pouring thru the carbs!) and seemed to put out a lot more power, so I think that's where the carbs were really starting to open up. You'd obviously like to be able to cruise at a throttle setting below that rapid-opening threshold. The service manual described a "Link & Sync" adjustment on the linkage where a roller on the carb throttle lever is supposed to line up with a mark on a cam that's connected to the spark advance lever. If this spark advance and throttle linkage isn't adjusted right, you'll lose performance, so this is worth checking, as is the basic timing at idle. The Merc service manual probably shows a similar adjustment.
5. The style of prop you run can make a difference in your gas mileage. It seems like 4B props or 3B props with a lot of blade area like the Merc Mirage or BRP Rebel that have a lot of stern lift will hang on plane better at low speed and give better mpg, although they may cost you a few mph on top end. The bass boat guys that want to run fast in flat water like bow lifting props to get as much hull out of the water as possible, but that's just the opposite of what you want on an offshore boat! And of course, for max mpg you'll want to run a stainless prop with thinner blades and less drag than an aluminum prop.
Hope this helps. Denny