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1985 M 39 DC gas tank selector

Posted: Mon Jul 29, 2019 4:34 pm
by kimbo
Hello All,

Still learning the system of my M39DC. When fueling up I always have equal amounts of gas into both tanks. I noticed that the port tank is reading 3/4 full and the starboard tank is 1/16 full or less yesterday. The tank under the bed looks like one big tank but with two opening for gas on deck I would assume two tanks below. I can tell the boat has a slight list to port I am assuming from the port gas tank. There are three gas valves in the engine compartment with all three inline and open. So, question is to my fellow Big M's, what am I missing? Does the port tank free flow to the starboard? Could the fuel pickup in the port tank be faulty or some other item that I am missing? Thanks in advance. Photo of gas valve is hopefully attached.

Kim

Re: 1985 M 39 DC gas tank selector

Posted: Mon Jul 29, 2019 4:57 pm
by Harryb
From the picture it shows you have selected to fuel any engine from any tank.I run mine with the middle valve closed so as to run fuel from port tank to port engine and starboard to starboard only

Re: 1985 M 39 DC gas tank selector

Posted: Mon Jul 29, 2019 9:18 pm
by jralbert
I was going to say the same thing.

Another factor at play here might be that the engines are burning fuel at different rates - that would account for fuel gauge differences.

Also, list may be due to water tank - is it on the port side? At 8lbs / gal times x gals, that could be weighty enough to cause a little list.

Re: 1985 M 39 DC gas tank selector

Posted: Tue Jul 30, 2019 6:31 am
by kimbo
My fear is that the pickup on the port tank is not working and if I ignore this fat dumb and happy, I might run out of gas. I also thought there might be some other valving or tank selector that I was unaware of at the helm. Guess my next trouble shooting project will be to close the starboard valve, start her up and see what happens.

Thanks for your replies,

Kim

Re: 1985 M 39 DC gas tank selector

Posted: Tue Jul 30, 2019 8:36 am
by SunbeamIV
On my 32, I noticed that the generator can only take suction on the starboard tank. That would also explain the tanks not going down evenly.

Re: 1985 M 39 DC gas tank selector

Posted: Tue Jul 30, 2019 10:12 am
by Fastjeff
In RVs, the intake pipe for the generator is higher in the tank so the generator ruins out of gas long before the truck does.

Could that be related to this issue?

Jeff

Re: 1985 M 39 DC gas tank selector

Posted: Tue Jul 30, 2019 3:34 pm
by Harryb
More than likely different resistance in the port and starboard supply lines resulted in the one side being favored. Now you have to find out why. There"s been lots of posts on here about cleaning the tanks and anti-siphon valves and servicing the water separators.