Anti-Siphon Valve - hard starting issue

My 2000 Mercruiser 5.0L with 2bbl carburetor tends to be hard starting if the boat has sat for several days or longer. I'm fairly certain this is a fuel delivery issue and not spark related. If you squirt a little fuel in the carburetor after setting the engine fires immediately. I don't do this regularly but have squirted in fuel just for troubleshooting. Once started for the first time it runs just fine and starts easily after turning it off. This issue has existed the last few seasons and I didn't give it much thought but now it's really bugging me.

My 32 gallon fuel tank is below the floor and sets lower than the fuel filter and engine. When the boat sets for a while I don't smell any excess gas fumes and I don't see leaks anywhere. When the key is in the on position I can hear the fuel pump spin up but it takes several repeated tries cranking the engine until it finally fires.

Could this be the anti-siphon valve? My understanding is this valve prevents an auto-siphon condition from the tank to the engine so that the carb doesn't flood out with fuel when the motor isn't running. But, I also thought this acts like a check valve and keeps the fuel line loaded and prevents fuel from draining back to the tank. Is this correct?

Please let me know if you have other ideas to check into.

KJ

I had the same issue with my 5.0. If it sat for a couple days i would have to crank and throttle bump it several times to get it to fire. When it did it was all good. I also had that if i ran it for awhile when i stoped and tried to re start it i was getting a vapor lock. I put a glass see through filter on it and saw that the fuel was rolling back to the tank.. I found that i did not have a anti siphon valve. I put a anti siphon valve at the tank and problem fixed..

Duhh502: My 4.3 LX had the same problem, so I jumped the engine harness at the oil pressure switch to cause the elec. fuel pump to immediately start at key on vs. waiting 'till the cranking built enough oil pressure to permit fuel pump start. This has worked fine, but I know I'm defeating a safety feature. I live with the risk, 'cause I observe my gages, and shut off the key if the engine kills, etc.

Buuuut, I like the idea of the anti-siphon valve so I can set the switch back to spec. So, where did you find this valve?

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kevinj said:

My 2000 Mercruiser 5.0L with 2bbl carburetor tends to be hard starting if the boat has sat for several days or longer. I'm fairly certain this is a fuel delivery issue and not spark related. If you squirt a little fuel in the carburetor after setting the engine fires immediately. I don't do this regularly but have squirted in fuel just for troubleshooting. Once started for the first time it runs just fine and starts easily after turning it off. This issue has existed the last few seasons and I didn't give it much thought but now it's really bugging me.

My 32 gallon fuel tank is below the floor and sets lower than the fuel filter and engine. When the boat sets for a while I don't smell any excess gas fumes and I don't see leaks anywhere. When the key is in the on position I can hear the fuel pump spin up but it takes several repeated tries cranking the engine until it finally fires.
On a carbureted engine, I'm thinking that the electric fuel pump should not be operating until the starter motor is engaged.
During starter motor operation, a start-by-pass circuit operates the fuel pump until engine oil pressure is achieved.

Could this be the anti-siphon valve? My understanding is this valve prevents an auto-siphon condition from the tank to the engine so that the carb doesn't flood out with fuel when the motor isn't running.
No.... the anti-siphon valve is a requirement for gasoline engines in the event of a fuel line becoming severed, thus creating a siphon.
The mild resistance from the Anti-Siphon valve prevents a siphon from occurring, yet allows the fuel pump's suction to over-come the resistance.


But, I also thought this acts like a check valve and keeps the fuel line loaded and prevents fuel from draining back to the tank. Is this correct?
Well, in a sense.... yes! The fuel pump itself is also equipped with check valves.

Please let me know if you have other ideas to check into.
It may be possible that your carburetor fuel bowl is becoming evaporated or is leaking fuel into the intake manifold.
If so, the carburetor's low fuel bowl level is not allowing for any accelerator pump action, or low speed fuel metering during cold starts.
Once the start-by-pass circuit is energized, the pump fills the fuel bowl and all is good.

KJ

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harringtondav said:

Duhh502: My 4.3 LX had the same problem, so I jumped the engine harness at the oil pressure switch to cause the elec. fuel pump to immediately start at key on vs. waiting 'till the cranking built enough oil pressure to permit fuel pump start.
This is illegal according to USCG regs.
To do this properly, a momentary helm switch can be installed. It MUST be a momentary switch! In other words, a switch that CANNOT be left in the "ON" position.

This has worked fine, but I know I'm defeating a safety feature. I live with the risk, 'cause I observe my gages, and shut off the key if the engine kills, etc.
Yes.... a very important safety feature!

Buuuut, I like the idea of the anti-siphon valve so I can set the switch back to spec. So, where did you find this valve?
The anti-siphon valve will be located at the top of the fuel tank's dip tube or pick-up tube fitting..... typically a 90* fitting.
It will be disguised as a normal but long barbed fitting. [/QUOTE]

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The valve! First, the thing actually looks almost exactly like a normal gas fitting and those I have seen are located on the top of the fuel tank. AND THEY CAN GET PLUGGED with poop collected from the fuel tank. Do a google to see what the look like, you may already have one on your tank.

And there are other problems with antisiphon valves! For instance they can only prevent siphoning over specified heights. I worried about the height problem on my boat which I later learned did not even have them. A couple of years ago I reran a new fuel line so that it was above the tank all the way to the engine’s fuel filter located just above the engine. Now if that line ever develops a leak it cannot siphon.

RicardoMarine said:

On a carbureted engine, I'm thinking that the electric fuel pump should not be operating until the starter motor is engaged.
During starter motor operation, a start-by-pass circuit operates the fuel pump until engine oil pressure is achieved.

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You are correct and I mis-spoke in my original post. Once the starter engages and I return the key to the ON position after a no-start I can hear the fuel pump keep running for a few seconds.

If the fuel pump has check valves then they should be preventing fuel from flowing back into the tank. Hmmmm......

kevinj said:

If the fuel pump has check valves then they should be preventing fuel from flowing back into the tank. Hmmmm......

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Whether electric rotary vane style, electric piston style or mechanical diaphragm style, a fuel pump will usually have check valves in them.
Not so much for back-flow prevention, but for pumping operation.

Duhh502 said:

I had the same issue with my 5.0. .....I put a glass see through filter on it and saw that the fuel was rolling back to the tank.. I found that i did not have a anti siphon valve. I put a anti siphon valve at the tank and problem fixed..

Click to expand...

Good to know.

My boat is a Crownline which tends to have pretty good build quality and reputation. I would think this builder would follow NMMA and USCG guidelines and use an anti-siphon valve on the fuel tank. I have a nice access port to get to the fuel tank and I'll take a look at the pickup tube to see if there is a valve in place. I may just order a new one and replace it just to eliminate one possibility. The valves are fairly cheap (less than $20) and since it's 18 years old it wouldn't be a bad idea to replace it anyway.

I would guess the valve is 3/8" NPT x 3/8" hose barb. But, I'll double check before ordering because I'm also not sure if it's a straight fitting or 90*.
KJ

RicardoMarine said:

Whether electric rotary vane style, electric piston style or mechanical diaphragm style, a fuel pump will usually have check valves in them.
Not so much for back-flow prevention, but for pumping operation.

Click to expand...

Okay, thanks for the explanation. Mine is a low pressure in-line electric fuel pump like this one: http://www.marineengine.com/newparts/part_details.php?pnum=MER861155A+3

Once the lines are primed and the boat is running I have no issues so I tend to think the pump is working fine and delivering the necessary fuel to the carb.

Good idea Rick on the momentary switch. I can figure how to install a push button or return rocker switch and use it a a 'prime' button. Release it and crank.

Have you gone through the carb possible fuel strainer needs replaced? The fuel in the float bowl evaporates but should quickly fill up if you cycle the key a couple of times or crank on the starter with a standard pump.

kimcrwbr1 said:

Have you gone through the carb possible fuel strainer needs replaced? The fuel in the float bowl evaporates but should quickly fill up if you cycle the key a couple of times or crank on the starter with a standard pump.

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I haven't really started the troubleshooting process. Right now I'm gathering ideas of places to look for issues. Thanks for the suggestion, I'll put it on the list of possible culprits.

KJ

New here but reading through this thread it is obvious there is a fundamental misunderstanding of anti-siphon valves. First an anti-siphon valve is not required unless fuel can be accidentally siphoned out of the tank into the engine intake manifold. The USCG and ABYC standards only require an antisiphon valve if the fuel pickup on the tank is higher than than the fuel inlet on the engine. The fuel inlet is usually the inlet on the fuel pump. So you do not need one if the tank is lower than the engine. It is normal for the fuel to flow back into the tank if the tank is lower than the engine. The fuel pump is required by the USCG to be on the engine or with in twelves inches of the fuel inlet on the engine. The principle is that the pump sucks fuel (bad pun) to the engine rather than pushing it as is done on most cars these days. This is so the fuel line is not under pressure. If a leak develops then the engine just dies.

Yes anti-siphon valves do get clogged. They have a small ball that is held in place by a spring. The ball stops the fuel from flowing. When the pump starts up it develops enough negative pressure to pull the ball away from the hole and fuel flows. This orifice is easily blocked by gunk in the tank. even a grain of sand will do it. The solution is usually, take the valve out, blow high pressure air through it, and put it back in. If you aren't required to have one get rid of it. They are almost always located at the top of the tank where the fuel is extracted from the pickup tube. If you can't access this part of the tank then the boat manufacture did it wrong. The law requires all tank fittings to be accessible.

Just one other thing. Those little inline auto type filters, clear plastic or glass filters, are prohibited. They break easily, leak like mad, and will not pass another requirement. Fuel filters and pumps are required to be able to withstand a fire for 2 1/2 minutes. Auto filters cannot do that. Do not use them.

Now to your problem. If your engine is having a fuel starvation problem it's probably the pump. Mechanical fuel pumps on marine engines have two diaphragms inside them. This is so if the main diaphragm develops a leak, the fuel won't leak into the boat. But it also means that the hole reduces the ability of the pump to suck fuel. Generally speaking electric fuel pumps don't have this problem. And on fuel injected engines it is permissible to have the pump run for a few seconds to build up pressure for the injectors before you crank the engine. Anyway, check the pressure in the fuel line between the pump and the carb, or injectors if you have an injected engine. If it's much below 3 psi for a carbureted engine, you have a problem.

Thanks kimcrwbr1. Yes. This is a 22 yr old issue. My bud has a 350 mag with a mechanical pump, and the same Weber 4 bbl as mine (larger jets 'o course) and cranks and pumps like crazy on a first start. Mine is a 4.3 w/elect fuel pump which requires some level of engine oil pressure before the pressure switch closes and allows the elect fuel pump to start. So it takes at least 10-15 seconds of cranking to make that happen. That is why I jumped around my pressure switch. On key-on I can hear the pump start up, and stall out once the float needles close.

I always wondered about float bowl evap., but I rebuilt that carb this winter. After laying fallow for five months, there was enough fuel in those bowls to service the accel pump through many, many shots. So I don't understand why that Weber and engine are so stubborn.

So I bypassed the switch. I thought the switch was a safety feature to protect the engine if it was low on oil, and shut off the pump and eventually kill the engine. I've been told 'not so' many times, and the cut out switch stops fuel pressure after an engine kill, just like the mechanical pump does. So that is a feature I've restored.

My crank time isn't obnoxious, 30-40 seconds max. And isn't a problem with a good battery. So I'll live with it, and install that prime button if it really bothers me. More important for me is getting the accel pump charge, and throttle control idle position correct for the air temp. Warm or hot, not too much. Cool, a little more. Cold, and my boat is in dry dock, or heading there. Next day or two starts always fire right off.

Thanks Peter. I know how the anti-siphon valve works but was not aware they aren't required if the tank is below the engine. Also, the inline, see-through filter was mentioned by Duhh502 to troubleshoot his fuel system for troubleshooting.

This is a 2bbl carburetor 5.0L engine with the low pressure electric fuel pump Merc Part# 861155A3. The fuel filter (Mercuiser spin-on type) and fuel pump are mounted in OEM locations which are on the engine, so they meet USCG requirements.

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PeterDE said:

New here but reading through this thread it is obvious there is a fundamental misunderstanding of anti-siphon valves. First an anti-siphon valve is not required unless fuel can be accidentally siphoned out of the tank into the engine intake manifold. The USCG and ABYC standards only require an antisiphon valve if the fuel pickup on the tank is higher than than the fuel inlet on the engine.

Ah..... but there is a bit more to this!
We may see scenarios whereby a fuel line is routed into an area that is lower than the fuel tank's highest fuel elevation. This fuel line may rise again on it's way to the engine's fuel pump and eventual to the fuel delivery system.
However, let's get back to that area whereby the gasoline fuel line is lower than the tank's fuel level.
In this scenario, a severed fuel line (in a lower-than-fuel-tank-level scenario) could possibly begin an un-wanted "siphon" once the engine has been shut down.
Hence the reason for the Anti-Siphon valve!



The fuel inlet is usually the inlet on the fuel pump.
correct!

So you do not need one if the tank is lower than the engine.
See above!

It is normal for the fuel to flow back into the tank if the tank is lower than the engine. The fuel pump is required by the USCG to be on the engine or with in twelves inches of the fuel inlet on the engine. The principle is that the pump sucks fuel (bad pun) to the engine rather than pushing it as is done on most cars these days. This is so the fuel line is not under pressure. If a leak develops then the engine just dies.
Hmmmmm!

Yes anti-siphon valves do get clogged. They have a small ball that is held in place by a spring. The ball stops the fuel from flowing.
Not true. The mild spring pressure against the ball creates resistance. This resistance is enough to stop an unwanted siphon (in most cases), but is not enough to stop a fuel pump's suction from pulling fuel from the fuel tank.


When the pump starts up it develops enough negative pressure to pull the ball away from the hole and fuel flows.
Yes.

This orifice is easily blocked by gunk in the tank. even a grain of sand will do it. The solution is usually, take the valve out, blow high pressure air through it, and put it back in. If you aren't required to have one get rid of it. They are almost always located at the top of the tank where the fuel is extracted from the pickup tube. If you can't access this part of the tank then the boat manufacture did it wrong. The law requires all tank fittings to be accessible.

Just one other thing. Those little inline auto type filters, clear plastic or glass filters, are prohibited.
Clarification:
The glass see-thru filters are not legal in either the suction or pressure side of a gasoline fuel line.
The metal in-line filters (if installed within the suction side) are legal, but are stupid because the small square area of filtration media is a poor choice.

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kevinj said:

Thanks Peter. I know how the anti-siphon valve works but was not aware they aren't required if the tank is below the engine.
See above!

Click to expand...

.

You could put a wire with a diode from the start circuit directly to the pump. When you hit the starter it provides power to the pump and after it starts the diode will block the power from backfeeding the starter?

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harringtondav said:

.................... Mine is a 4.3 w/elect fuel pump which requires some level of engine oil pressure before the pressure switch closes and allows the elect fuel pump to start.
This is where the "start-by-pass" circuit is to provide fuel pump power and operation until oil pressure is achieved.

So it takes at least 10-15 seconds of cranking to make that happen. That is why I jumped around my pressure switch. On key-on I can hear the pump start up, and stall out once the float needles close.
The helm located momentary switch allows us to primed what may be a dry or somewhat dry carburetor prior to any starter motor operation.
Hit the momentary switch prior to starter motor operation, and an engine should fire up in far less than 10-15 seconds.



I always wondered about float bowl evap., but I rebuilt that carb this winter. After laying fallow for five months, there was enough fuel in those bowls to service the accel pump through many, many shots. So I don't understand why that Weber and engine are so stubborn.

So I bypassed the switch. I thought the switch was a safety feature to protect the engine if it was low on oil, and shut off the pump and eventually kill the engine. I've been told 'not so' many times, and the cut out switch stops fuel pressure after an engine kill, just like the mechanical pump does. So that is a feature I've restored.
Yes..... the N/O Low Oil Pressure Switch is there to protect against a fuel line issue in the event that the engine stalls.
Engine Stalls = No Oil Pressure.
No Oil Pressure = NO fuel pump operation.
Imagine a fuel line rupture and a pump that does not shut down!

My crank time isn't obnoxious, 30-40 seconds max. And isn't a problem with a good battery.
I would suggest that 30-40 seconds of cranking is excessive. In fact, even 10 seconds is approaching excessive.
Cranking for that duration shortens starter motor life and it certainly doesn't help extend battery life.

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kimcrwbr1 said:

You could put a wire with a diode from the start circuit directly to the pump. When you hit the starter it provides power to the pump and after it starts the diode will block the power from backfeeding the starter?
Kim, that may work with diodes that can handle the load.

Also, let's not forget the older starter motors with the "R" terminals.

However, the "start-by-pass" circuit has been used successfully for years and years. It was first used as to circumvent the ignition's ballast resistor during cranking. In other words, that circuit provided full 12 v to the ignition coil during cranking, then allowed it to return to resisted voltage........ cars, trucks and boats.

Later as Marine electric fuel pumps came on board, this same circuit was used for powering the fuel pump's relay prior to oil pressure.

If it's nice and round, and if it rolls smoothly and with little friction, I see no reason to re-invent the wheel!

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Okay......so I'm feeling like a dufus. I was talking with the guys at the local boat shop and this issue came up. His first question was "well, are you choking the motor when you try?" His quick explanation made me realize I've never been using the correct sequence when starting it cold. I've always just moved the lever forward (in neutral) thinking that would engage the choke. Nope.....2 full sweeps to WOT, then bring it back to about 10 o'clock. I had always assumed since it had an electric fuel pump that it should have a start sequence similar to a fuel injected engine. Bad assumption! Guess I should have read the manual from beginning to end.

Thank you for the explanations of fuel pump operation. Now I feel like I have a much better handle on the complete system.

Here's an example of what I should have been doing:
https://youtu.be/lTKFC6mWKqU

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kevinj said:

Okay......so I'm feeling like a dufus. I was talking with the guys at the local boat shop and this issue came up. His first question was "well, are you choking the motor when you try?"
When I hear a mechanic call an Engine a Motor....... I question his expertise. These are Engines, NOT motors!


His quick explanation made me realize I've never been using the correct sequence when starting it cold. I've always just moved the lever forward (in neutral) thinking that would engage the choke. Nope.....2 full sweeps to WOT, then bring it back to about 10 o'clock. I had always assumed since it had an electric fuel pump that it should have a start sequence similar to a fuel injected engine. Bad assumption! Guess I should have read the manual from beginning to end.

Think of the carburetor's electric choke as an Electric "UN-Choke!"
By default, the Helix operates the choke plate linkage closing the chimney off, causing an enriched fuel mixture.
As the Helix heating element produces "heat", the Helix operates the choke plate linkage and begins to open up the choke chimney.
Hence what technically is an UN-Choke !

Thank you for the explanations of fuel pump operation. Now I feel like I have a much better handle on the complete system.

Here's an example of what I should have been doing:
https://youtu.be/lTKFC6mWKqU

When this man is "pumping" the throttle, he is actually operating the carburetor's accelerator pump, of which in turn enriches the fuel/air mixture for a cold start.
Fuel injection systems do not use an accelerator pump, and is why we don't need to touch the throttle lever.

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Engine --->Motor ...I catch myself using them interchangeably as well. Wrong, but I do.

Correct on the accelerator pump. Again, my lack of reading the manual resulted in ignorance of the way this setup operates. Fuel injection systems rely on rail pressure to deliver fuel to the injectors and the ECM varies delivery during cold engine operation to richen the mixture.

I made the assumption that the low pressure electric fuel pump on my system would deliver fuel to the carb and aid in cold starting. Now I understand 'pumping' the throttle handle drives the accelerator pump for that initial shot of fuel to make it rich.

Thanks for the explanation of 'UN-Choke'. That helps me to further understand the system. I've always owned outboards and this is my first I/O. I've long used in-line primer bulbs on the outboards and push-to-choke on the ignition. My older outboards used choke plates and the newer one uses a primer solenoid to squirt fuel into the throttle plates. Both are similar concepts to the correct method of cold-starting my Mercruiser......now that I know the method.

Again, I feel like a dufus because I never took the time to read the manual. But, now I know the proper starting procedure and I can have a much happier boating experience.

My engine setup is such that the electric fuel pump does not run until there is some oil pressure. When left alone in warm weather especially for a few days the gas in the bowl evaporates out and I have to crank it several times to get the bowl full again. I was going to install a momentary switch to run the fuel pump, and then I realized that cranking the engine until there is oil pressure before it starts, eliminates what some consider, one of the biggest causes of engine wear, i.e. having an engine start without lubing with oil.

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kevinj said:

Engine --->Motor ...I catch myself using them interchangeably as well. Wrong, but I do.
Yeah..... it's an old Pet Peeve of mine!
I was fortunate enough to have a father and grandfather who were savvy to engines, motors, etc,....... so I learned the differences very early on!



I made the assumption that the low pressure electric fuel pump on my system would deliver fuel to the carb and aid in cold starting. Now I understand 'pumping' the throttle handle drives the accelerator pump for that initial shot of fuel to make it rich.
Yes, and it may require several initial shots of fuel, and occasionally a few shots even after start-up.

Thanks for the explanation of 'UN-Choke'. That helps me to further understand the system.
Perhaps again my Pet Peeve.

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Got a chance to go out this weekend and used the proper cold starting sequence: 2 full sweeps of the handle and stop back at first neutral detent. Vroom....Vroom, she fired up within two seconds of cranking.

Kind of amazing when you do things properly.

I have done this with every boat I've owned: Put a squeeze ball pump in the fuel line between the water separator and engine fuel pump. when ever my boat sits for any length of time, I give the squeeze ball a pump or two and then start the engine, always fires within 1 second or two. Also, the squeeze ball pumps act as a check valve by design.