350 gas use

[handball]

Twin merc 350 engines 260 h.p.in a 30 sea ray inboard.Starboard engine burns 8 gal hr @3000 rpms . port engine seems to burn clost to 16 an hour at the same rpms.Last owner claims to have had similar problems and had yard install new carb.on the port engine, it had no affect on m.p.g. Both engines do not use oil only about 1/2 quart for a 300 mile trip.Both engines start easily and run beautifully. Spark plugs on both engines show no fouling and have a nice light color.Is it possible that port is opposite rotation and that is a factor in m.p.g.?<br />Bill

 

[flashback]

Re: 350 gas use

don,t know much about how much they should use, but thats a heck of difference for two engines that are in good tune, try swapping the fuel meters and see if the problem follows. good luck

 

[Walt T]

Re: 350 gas use

Do a compression check for starters. Verify props okay and both are same pitch and no damage. Don't laugh, it's happened. You could have a bearing starting to seize on one of the outdrives.

 

[akriverrat]

Re: 350 gas use

when my 4.3 had only 5 degrees total advance (long story) it burned 10-12 gph. got that fixed and got it down to 6-6.5 gph.

 

[johnkc]

Re: 350 gas use

also verify that it is getting up to temperature

 

[inlet]

Re: 350 gas use

How are you measuring fuel consumption?<br />I also run a pair of 350's in a searay. 305,12ft. beam. 10 to 12 gph sounds about right for those revs. With two tanks is it possible that one tank is feeding the other via cross over lines?

 

[handball]

Re: 350 gas use

Thanks for the comeback guys.<br />My boat is a straight inboard model.<br />Each engine has its own tank and there is no crossover system to change over the tanks.<br />The way I have come up with the fuel consumption is as follows;I filled both tanks ran three hours at cruise then refilled both tanks . I then did the same thing on a trip up the hudson three more times, each time I filled up the port engines tank took more gas then the starboard.<br /><br />Bill

 

[flashback]

Re: 350 gas use

Well Bill, I'm no expert by a long shot, but two identical engines in good tune should not be that far apart in fuel consumption unless the port engine is haveing to work harder than the starboard... I think I would start checking the tranny, shaft, and cutlass bearings. something is making that engine labor.......good luck...fuel tanks not leaking into bilge is it? one other thing, you might try checking the fuel pump on the port engine, if it is bad it maybe dumping raw gas into the engine...just a thought.....

 

[navigator336]

Re: 350 gas use

A 100% difference in fuel consumption is huge. What's the difference in rpm's between the engines when you go from neutral to forward? This will tell you a little about the shafts, cutlass bearings and trannies. You could have a diver look at the props, get the dia and pitch numbers and spin the props looking for smooth easy rotation.

 

[navigator336]

Re: 350 gas use

What is the difference in WOT rpm's? It should be huge too. What do the plugs look like immediately following a WOT run? Have you tried swapping coils and/or carbs? What was the diff. in fuel consumption for the previous owner (100%???). Are the vac secondaries sticking open a little? The replaced carb might have different jets, etc.

 

[handball]

Re: 350 gas use

Thanks for the comebacks guys. <br />This winter I pulled the port prop and shaft as the boat is out of the water . Took the shaft to a local place as I suspected it to be slightly bent.The fellow found about .005 bend and removed it.I also changed the cutless bearing as it looked quite worn.Removing the old bearing was a bit of a hassel but I ended up taking a hacksaw and making a cut down the side.Installing the new bearing was easier then expected as it was not a tight press fit.<br /><br />One place I will definately check is the fuel pump, as it is the type that if the diaphram is going it runs the gas back to the carb. thru a clear hose.This I can do in my yard before the boat goes back in the water.<br /><br />There is no difference in w.o.t. between the two engines.Also when the boat was in the water surveyed, the engines were run w.o.t. for a short time and the surveyor backed off the throttle of one engine to see how many rpm's the other engine would loose.He said if the engine left at full throttle only lost say 50 rpm's it was in good shape. He repeated that for both engines with good results.In fact he was so intent on this that he ran over what looked like a long 2x4, thats one of the reasons I had the shaft checked for straightness.<br /><br />Bill

 

[Bhamil]

Re: 350 gas use

Dieselwalt; you can't test starters for compression. Seriously, I believe DW is on the right track here. The load on these engines isn't anywhere equal, one is straining up a storm. I would try to resolve the drive line problem and I'll bet you find the problem. Now, if one of those engines was running a little goofy...

 

[mabbott]

Re: 350 gas use

Have you verified you are running each engine the same rpm the tach's are representing? Do you have a sync gauge on your boat? Do you have access to a photo tach that reads the mechanical revolution per minute on the harmonic dampner?<br /><br />Is each propellor at the same pitch and cup? Have it checked.<br /><br />Check the compression and points/dwell and timing.<br /><br />If the compression check is the same in each engine I would swap the carb's and see if the problem moves to the starboard side.<br /><br />Just some ideas.

 

[handball]

Re: 350 gas use

Mabbott,<br />I do have a sync meter and use it to set the engines at cruise, you can also hear when they are out of sync.<br /><br />The previous owner had same gas problem and put a rebuilt carb on the port engine ,he saw no change.<br /><br />I like some of your other sugestions such as timing, and as soon as the boat is back in the water this spring I will check it out .<br /><br />Another thing I will look at is if there is any gas in the return line to the carb. from the fuel pump.<br /><br />Thanks for the help,<br /><br />Bill