M151A2 Fuel Tank Vent Modification?

[Lou]

Considering a fuel tank vent modification to my M151A2 with fording kit and want some opinions on this. It has a Holley carb and if it sits after hot, pressure builds up in the fuel tank and pushes the needle off the seat and floods the engine when trying to restart. The fuel we receive today just builds up vapor pressure much too easily.

Obviously if the gas cap vent is set to open this issue does not occur. If the vent is left open with the top and doors on fumes and gas fill the cabin when off roading or making right turns. Fuel today is so volatile if the gas cap vent is closed, the tank will pop out from the pressure in short order.

The vent that is in the fuel lid has a one way valve, it lets air in the tank so fuel can come out. The valve prevents vapor and or fuel from entering the air cleaner, master cylinder and fuel pump vent.

Considering defeating the one way valve in the lid of the fuel tank, this way the tank will be vented to the air cleaner and pressure will not build up in the fuel tank with the gas cap vent closed. My only concern would be that fuel may find it’s way back into the vent system and put fuel in the air cleaner, master cylinder and fuel pump vent.

Guess this could be addressed by running a separate line from the fuel lid vent directly to the air cleaner lid. My top lid has a fitting that is blocked off and can use that. That way if fuel came back up the vent line it would dump in the air cleaner instead of getting into the master cylinder and fuel pump vent. Would leave the existing vent system intact that goes from the bottom of the air cleaner to the master cylinder and fuel pump vent, that way no fuel could find it's way in there.

So what are your thoughts on this?

[rickf]

Yep. Separate line would be best.

[raymond]

Rick

Wasn't this a topic of discussion some time ago. Someone was going to mod a tank vent for similar problems, I think..........

[Fil Bonica]

Are you running an emissions style or older style?
The emissions tank utilizes a closed system that is vented to the air filter properly.
Have one A2 that is fitted with correct valve on engine to allow fording along with plumbing to tank and master cylinder.
Does the Holley have a retutn line connected to tank?

[muttguru]

The Holley TM says the following....."The carburetor is vented through the float chamber to the air filter to provide for underwater operation."
There should be a separate line from the Holley to the air cleaner, which in turn has an adapter for that connection.
The adapter was installed on early air cleaners to partner with the Holley. IIRC, it wasn't fitted to the later-type air cleaner.





The TM doesn't say whether the valve in one-way or bi-directional but as I don't have a Holley or access to an early M151, I can't confirm either way. But I'm presuming it vents vapors from the carb float chamber into the air filter.

Do you have that vent-line and the connection at the air filter body installed already?

If it's a case of excessive volatility of the fuel in the tank, there aren't many options apart from keeping the fuel level lower than usual.
ken

[Lou]

Ken and Fil, thanks for your input, will answer your questions. Mutt does have a fording valve with non emissions system. The original system had the vent from the air cleaner going to the fuel pump vent, Master cylinder vent, and fuel tank one way valve vent.

The Holley does not have a return line so created one. In the photos below at the very bottom is a T. From the right is the fuel line from the pump, to the left is the return going back to the tank. A home made restrictor was made and installed in the return line side of the T so all the fuel would not go back in the tank, instead of the carb. The blue line is the Vent line for the Holley. It was at one time tied to the vent system for the master cylinder and fuel pump vent. This did not work out very well because when pressure built up in the fuel tank it pushed open the float bowl needle and dumped fuel into the master cylinder and air cleaner! The blue line now runs all the way down to the bottom of the vehicle below the suspension arm. If fuel leaks out it goes on the ground instead of the air cleaner or worse. The L fitting going into the air cleaner is left as it was, going to the fuel pump vent and on it's way back to the master cylinder.





A new line was added to the top of the air cleaner where an existing blank plug was removed. This line is at the top left of the photo below.



In the photo below starting at the left bottom. 1/4 line is fuel return going back to tank, 3/16 line above it is the vent line from the fuel tank lid. The way this was originally a Y was installed here and connected the vent from the tank to the master cylinder and forward to the L on the bottom of the air cleaner. The Y was removed and the vent line from the master cylinder now goes forward to the L at the air cleaner. A new line was run from the gas tank vent directly to the top of the air cleaner lid shown in the previous photo.



The one way valve has not been defeated in the tank lid yet. Was able to blow air with my mouth in the new vent line going to the top of the air cleaner and back to the tank vent. Heard air escaping at the mouth of the fuel tank below the fuel cap. Never noticed this before there is a second passage in the fuel filler neck. Look close at the top and you will see this. I had expected to have heard fuel bubbling instead of air escaping because I fill my tank all the way up the neck to where the chain connects to the fuel cap. So even though the tank lid looks lower then the fuel level in the neck, there is apparently enough space for air to reach the tank lid vent and the vent in the fuel neck. Below are pictures of the neck vent.

So, it appears my concern of fuel coming back up through my added vent line, with a defeated tank vent 1 way valve valve are unfounded. This would be hard to occur also because the air cleaner top lid is way higher then the fuel tank lid. I tried sucking through this line and sure enough the one way valve prevents air from exiting the tank this way. The final step will be defeating the one way tank vent valve. Then the vent in the fuel cap can remain closed and air can get in the tank with the new added line which will be 2-way meaning pressure will not be allowed to build up in the tank and air can get in the tank to allow fuel to come out. Hope this helps someone in the future.

[rickf]

I am a bit confused by the line configuration on the inlet of the carb. I see the inlet goes into a Tee fitting with a line coming out of the other side of that Tee, Where does that other line go? There was no need for a vapor bypass on that setup and if that Tee is a standard Tee the hole for the bypass is WAY too big. If it has been running that way great but it is not optimal. You will have very little fuel pressure at the carb itself if it is what it looks like.

[Lou]

I am a bit confused by the line configuration on the inlet of the carb. I see the inlet goes into a Tee fitting with a line coming out of the other side of that Tee, Where does that other line go? There was no need for a vapor bypass on that setup and if that Tee is a standard Tee the hole for the bypass is WAY too big. If it has been running that way great but it is not optimal. You will have very little fuel pressure at the carb itself if it is what it looks like.

In my description I mentioned a restriction was installed on the T on the fuel return going back to the tank. It is a copper disc with a small hole drilled in it, think it was a 3/16 hole. This way there is plenty of fuel pressure for the carb. Been running fine like this since 2006. Maybe I am old school but liked the idea of a return to prevent vapor lock. Unintended benefit is fuel is constantly being recirculated and cleaned by the fuel filters and helps to prevent ethanol phase separation.

[rickf]

Aha, I didn't see the part about the restriction. It is a bit large but as long as it runs good then it is fine. The phase separation would be pretty much taken care of just by the sloshing in the tank but once the moisture hits 7% it drops out like a rock no matter what you do. That is a major issue in boats since they are surrounded by moisture and alcohol sucks it out of the air. So what do people do? They fall for marketing and add "Drygas" to their gas tank. What is drygas you ask? It is alcohol!!!! That was what worked and worked quite well back in the old days of leaded gas when there were no kinds of hygroscopic additives already added at the refinery. NOW, You already have a lot more alcohol added then you would ever have put in with your little bottle of Drygas. but you are adding even more to it and making matters worse. Got to love marketing.

[Lou]

Aha, I didn't see the part about the restriction. It is a bit large but as long as it runs good then it is fine. The phase separation would be pretty much taken care of just by the sloshing in the tank but once the moisture hits 7% it drops out like a rock no matter what you do. That is a major issue in boats since they are surrounded by moisture and alcohol sucks it out of the air. So what do people do? They fall for marketing and add "Drygas" to their gas tank. What is drygas you ask? It is alcohol!!!! That was what worked and worked quite well back in the old days of leaded gas when there were no kinds of hygroscopic additives already added at the refinery. NOW, You already have a lot more alcohol added then you would ever have put in with your little bottle of Drygas. but you are adding even more to it and making matters worse. Got to love marketing.

The hole may be smaller then 3/16, it was done so many years ago cannot remember. I know there is enough pressure that the plastic added fuel filter expands and contracts with the fuel pump pulses. Tried every single brand and they all do this, so keep a spare on board and change them often. I have no idea how the plastic filter can keep doing that and not get fatigue cracks. if you have never seen a filter do this will make a video.

Agree 100% on the drygas and ethanol. This is a very big problem for me as we have a inflatable dingy with a 15 HP 4 stroke Honda motor. 1st issue is it is a 2 stage carb with 2 sets of jets. The idle jet emulsifier tube has holes slightly larger then human hair, not kidding about this. Had to purchase 3 sets of carb cleaning kits to find one that had piano wire thin enough to clear those jets. The crap that comes out is green, like the brass tubes were under salt water!

If the motor sits for 2-3 weeks the ethanol plugs up those jets and the motor will not idle. At first thought this was a moisture issue in the fuel. Installed an external 2 micron water separating filter with drain. Same crap. The best fuel treatment found out the hard way was the crc brand.

So sadly after years of trouble the only way to stop this from occurring is to open the drain screw on the carb after each use. Just running the motor until the carb runs empty will not work, it still leaves fuel in the carb, it has to be drained all the way. I am not alone with this as all of us with small 4 stroke motors are faced with this. Hell, even have to do this on my small Honda generator. The new Honda generators have a special fuel valve now that empty's the carb to prevent this from happening. I am counting the days when fuel injection becomes the norm on small motors.

Own a get ski with fuel injection since 2002, no issues at all. As you know fuel injection is a closed system like a car and moisture does not enter the fuel system as easily. No moisture no ethanol issues. You have no idea how much I hate ethanol!