Fuel burn question.

[CHANCE1234]

GFS, I'm not trying to get 10 mpg. I'm trying to determine what the problem is. Thanks bushwhacker and McGillicuddy, excellent info of your experiences. That is what I was looking for. Also thanks for the link. I will do those tests and check the ohms on the gauge.

[CHANCE1234]

So I pulled the sender and tested while hooked up to the gauge still and it works fine. The gauge reads about half right now. With the sender out I manually sounded the tank with a stick and sure enough it is only half full. The pump I was using is by no means fast, so what would cause fuel coming out the vent at half tank and burping out the fill. I guess I will try to fill it super slow next time but any thoughts out there or experience with this? Thanks

[erebus]

Originally Posted by CHANCE1234

so what would cause fuel coming out the vent at half tank and burping out the fill.

Are you absolutely 100% positive that your vent line runs uphill from the top of the tank to the vent fitting?
Any sags, kinks, pinches or loops can fill with gas and give you a premature burp.
And I mean any. Just a tiny low spot can collect liquid, and if you've had to squeeze the hose at all (i.e. make it not round) to fit through anywhere or have pinched it with something, it's pretty much game over.
Same goes for the fill.

[gofastsandman]

Originally Posted by CHANCE1234

So I pulled the sender and tested while hooked up to the gauge still and it works fine. The gauge reads about half right now. With the sender out I manually sounded the tank with a stick and sure enough it is only half full. The pump I was using is by no means fast, so what would cause fuel coming out the vent at half tank and burping out the fill. I guess I will try to fill it super slow next time but any thoughts out there or experience with this? Thanks

I have the same issue. I have to fill very, very slowly or the pump will kick and stop. The vent line may be too small in diameter for the flow. I recently went to a different station where the boat was tilted forward a good bit. I could pump a bit faster and didn`t get the burp til full.

Now your thread got me thinking. Dangerous, I know. If this e 10 is swelling the inside of delivery lines, would it not do the same to vent lines? Alcohol is a strong solvent and I avoid e10 like the plague. The station on Blue Heron and Old Dixie also has rec 90 now.

Is the vent line also foamed in? What I cant figure out is why the vent burps at all before full.

Cheers,
GFS

[CHANCE1234]

thanks for the reply guys. the fill is dead vertical. i fill through the top of the seat on the console and it goes straight into the tank. there are no sags in the vent line, i fixed that prior to sealing the deck up. i have filled it to 3/4 before with no burping. i'll just try to fill it slow next time and see how that goes. i have emailed the builder to see if he can offer any intel. i'll let you know what i find at fillup at a different station.

[Bushwacker]

I think Zach is right about gas in the vent line restricting the vent. If there isn't a positive slope from the tank all the way to the vent outlet, you could have fuel trapped in there. Another potential issue is where the vent is located on the tank relative to how the boat is sitting in the water or on the trailer. The fuel fill on my Seafari is on the starboard side a couple feet from the transom. The tank vent is near the front of the tank a couple inches in from the port side. It's designed so that if I'm refueling at a marina the boat the boat will be heeled to starboard and down at the stern with my weight in the back, the vent is at the high spot on the tank.

However when boat is on trailer at a gas station, the front of the tank is lower than the back, so the vent will spit gas before the tank is full. When I get home and raise the front of trailer, it will typically take about another 5 gallons to get the tank completely full.

Regarding your fuel gage, those wiper type sending units can be notoriously inaccurate because if you don't use the boat frequently, oxidation can build up on the resistance coil and wiper and change the resistance and gage reading! I've noticed mine often does not show full until I've burned off enough gas for the float to start bouncing around to clean off the wiper surfaces, and of course by then it's not full anymore, but it usually reads higher than it did when I first started out with a full tank! A Weema sending unit like Gillie has should eliminate that problem.

One thing I did when I first got the boat was to calibrate the fuel gage. I was running premix then, so it was a big help to know about how much it would take so I could add the proper amount of oil if refueling at a marina. The calibrated gage was accurate enough that I could usually predict within a couple of gallons how much it was gonna take! Here's how to calibrate your gage, starting with an empty tank:

1. Put a piece of white vinyl tape across gage. If it's just a simple 90 degree sweep gage, maybe use two pieces of tape, above and below needle, so you can put reference marks at each end of needle. A fine Sharpie marker works good and the ink doesn't fade.
2. Pull the sender, measure the inside height/depth of the tank, and then make sure the float is adjusted so it just touches bottom of tank at one end of it's travel, and that it's also at the other end of it's travel when it touches the top of the tank.
3. Hook up battery charger so battery is at same voltage it'll be at when motor is running.
4. With the sender still out of tank but connected to the gage and ground, turn on power to the gage and mark Empty and Full positions of needle when sender is at the extremes of its travel. Then install sender and recheck your E mark.
5. Put boat on a level surface and use trailer jack to get it level fore & aft.
6. I then filled the tank in 6 gallon increments, since I was running the 50:1, 1 pint/6 gallons oil mix, and I had 3 portable tanks. Wait for gas to quit sloshing around and needle to stabilize and then mark gage after each increment. If not running premix, maybe 5 gal. jugs would be more convenient.

Although the E-TEC's I-Command gage system tracks fuel usage and is usually accurate within a gallon or two, I did notice on one occasion on a long trip that the gallons used number had stopped increasing so I had to reset the gage to get it working again. Since running out of gas can ruin your day, I like to have an independent measurement to keep the electronics honest!

Here's another tip that makes it easy to drain the tank. Most Racor type filter heads have more than one outlet, so just install a male outboard fuel line fitting like those in portable tanks in one of the outlets. I made up a 3/8" drain hose with a female fuel connector in one end, a primer bulb about 6" away, and then about 6' of fuel line at the primer bulb outlet. I use my boat as my emergency fuel supply for my vehicles during hurricane season, so if we have a storm that knocks out power to the gas stations, it's easy to just run that hose out the bilge drain hole into a 5 gal jug to get gas to keep the cars or a generator running without the hassle of trying to find a gas station with power when everyone else is trying to do the same!

[CHANCE1234]

bushwacker, what a great post. Thank you. after over thinking this i have come to the hypothesis that when i lower the trailer onto the truck that the fwd end of the tank is lower than the back end. this would cause the vent to collect fuel before the sender sees the same amount. my vent is on the fwd 1/4 of the tank the fill is aft of that by about a foot or so and the sender is 11 inches fwd of the aft end of the tank. i may be able to solve this by just getting a 4 inch raised hitch vice the 1 1/2 inch i have now. i have the wema brand sender and it works fine when i pulled it yesterday. i'll fill it today and try to monitor cruising RPMs and distance traveled when i head out tomorrow. 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... i will definetly do that gauge cal you talked about later as well. thanks guys.

[FossilMan]

Hi Elijah,
I replied to your email yesterday. Just to go over the points I mentioned. If it gurgles from the fill, then it's not venting properly. If it spurts and gurgles from the vent, then the vent is under or near the fuel level in the tank when filling. Next time you go to fill up, fill up as usual and stop when it spurts fuel from the vent. Go ahead and disconnect from the truck and jack it up a bit. Continue filling. If it doesn't spurt any more, then you have isolated the problem. You may have to jack up again if it's really sitting low in the front. You can use a raised hitch or you may be able to tweak your trailer a bit so the bow sits a little higher. Let me know what the outcome is.

Tony

[CHANCE1234]

Thanks Tony. I did jut that at the pump today and it filled up perfectly. I guess I'm a little paranoid. Thanks for all the info guys

[Bushwacker]

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...

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