anti-siphon valve clarification.....

CliffA

Well-Known Member
Dec 29, 2009
4,712
Lake Norman, NC
Boat Info
2001 Sea Ray 340DA
Name: 'Happy Place'
4.5kW West. Generator
Purchased Nov. 2014
Fresh Water Use
Engines
Twin Merc. 6.2L (MPI)
640 hp (Total)
Raw Water Cooled
V-Drive Transmissions
long post, but hopefully helpful:

i thought i would share something i recently learned so that this archived post may someday help someone new to boating...i have only been boating for around 5 years so i still have a lot to learn.....for those that are familiar with marine fuel delivery systems you can stop reading this post as this will likely be something you already know....

all of this boating season i have been experiencing an issue with my boat engine that took me a while to diagnose....the symptoms were that the engine would start, idle and overall run great...however, when i cruised for around 30 mins at 3,000 rpm's (or above) the engine would suddenly start stumbling, backfiring and sometimes stall....usually i could get the engine to restart and idle OK and even run the rpm's up when not in gear.....but when i would put the drive in gear the engine would stumble again....after i let the boat sit for a few mins the engine would start and perform normal again for a little while and then the symptoms would return....i went through all of the troubleshooting steps i know to try and determine if this was a fuel related problem or an ignition related problem....i have a fair amount of knowledge on how gas engines work and troubleshooting skills which were developed over many years of tinkering with cars and trucks....i won't list all of the things i tested and checked but if anyone wants to know send me a PM and i will list them for you....

i finaly determined that the symptoms were most likely coming from a fuel related problem that was causing the carb to starve for fuel once the vacuum built up in the fuel tank enough to prevent or restrict the flow of gas....i researched the subject and symptoms and found several forum posts that mentioned checking the anti-siphon valve...i had already checked all the things that i could think of that would cause this problem and found nothing...as i was reading the post that mentioned checking the anti-siphon valve i was thinking to myself 'why are people concerned about a valve in the gas filler hose that prevents someone from stealing gas from the fuel tank'.....that was my incorrect idea of what an anti-siphon valve is....i finally came across a post where the anti-siphon valve and its function was explained....the function of the anti-siphon valve in a marine fuel delivery system is to prevent gas from siphoning out of the fuel tank if the fuel line ruptures or otherwise becomes disconnected from the fuel filter and/or fuel pump...it is usually located in the fitting that is on the fuel tank that the fuel line connects to.....if/when this valve becomes blocked or partially blocked with 'crud' over time the flow of gas going to the carb or fuel injectors will be restricted to the point the engine will stumble and possibly stall, just like i was experiencing...

last night i removed the anti-siphon valve from my fuel tank and found it to be an estimated 80-90% blocked...i replaced it with a new valve...i have not tried the boat on water yet since replacing the anti-siphon valve but i have a very good feeling that this was casuing my problems...

so if anyone reading this post experiences similar symptoms with their engines perhaps you can learn from my experience and consider checking and/or replacing the anti-siphon valve....here is a link to a good article that describes the anti-siphon valve and its function...

http://www.sterndrives.com/anti-syphon-valve.html

cliff
 
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i forgot to include i read that a partially clogged anti-siphon valve can prevent the engine from reaching its max WOT rpm's due to insufficient fuel flow...if anyone is having problem reaching their WOT rpm range the anti-siphon valve should be included in things to check....

cliff
 
Last edited:
I replaced the anti-siphon valve in my '92 290. It was right at the top of the fuel tank. Here is a few pics of the valve and the pick-up tube and screen removed from the fuel tank:

vid00391.jpg


vid00383.jpg


vid00384.jpg
 
I've got the almost identical issue as CliffA. Only after running for a while, and above 3000 rpm will I get an occasional backfire and stumble. I've gone through and changed, one by one, plugs, plug wires, distributor cap, ignition sensor, rotor and coil. Each item had not been changed since I got the boat and all were visibly in need of change. The coil was oozing a clear mineral type oil. The problems still persists. Researching online continues to point back to the anti-siphon valve as a high probability player so that will be next. I don't mind that I've changed out the other items as now I have a baseline for future maintenance.

One question I do have is brass or aluminum? I see one place that states "Use 15 (Brass) calibrated for HP 200 & above and 30 (Aluminum) calibrated for HP below 200." I don't really see this anywhere else but want to know if anyone knows if this is true or not. For < $20.00 I won't mind changing a 13 year old part out but would like to get the right one.

http://www.amazon.com/Anti-Siphon-V...7314&sr=8-2&keywords=marine+anti+siphon+valve
 
Some anti-siphon valves aren't the whole assembly as pictured either. They resemble a barb fitting that screws right into the top fitting on the tank. Inside the barb is a spring and check ball
Clean that with some Carb cleaner and it will be good to go.

Doug
 
My second pic.
 
I posted that 2 days ago ha...ha! You have an old Commodore 64 with the CRT monitor?
 
I've got the almost identical issue as CliffA. Only after running for a while, and above 3000 rpm will I get an occasional backfire and stumble. I've gone through and changed, one by one, plugs, plug wires, distributor cap, ignition sensor, rotor and coil. Each item had not been changed since I got the boat and all were visibly in need of change. The coil was oozing a clear mineral type oil. The problems still persists. Researching online continues to point back to the anti-siphon valve as a high probability player so that will be next. I don't mind that I've changed out the other items as now I have a baseline for future maintenance.

One question I do have is brass or aluminum? I see one place that states "Use 15 (Brass) calibrated for HP 200 & above and 30 (Aluminum) calibrated for HP below 200." I don't really see this anywhere else but want to know if anyone knows if this is true or not. For < $20.00 I won't mind changing a 13 year old part out but would like to get the right one.

http://www.amazon.com/Anti-Siphon-V...7314&sr=8-2&keywords=marine+anti+siphon+valve


Trucky, i have done some fairly extensive research about the symptoms our engines are showing.....if the anti-siphon valve does not help, the chances are very good you have an ignition related issue vs. a fuel related issue....i spoke to a certified marine mechanic about this at length....the fact that the probelms only arise after a fairly hard cruise indicates the problem is likely electrical and ocurrs only after heat builds up in the engine.....according to the mechanic what is happening is when the engine is running around 3000 rpm's the water pump is circulating around 20 gallons of cooling water through the engine per minute which is adequate to keep the temp of the engine internals at acceptable levels....when you slow the boat down to at or near idle after running hard for a while the output from the water pump drops to around 2 gallons per minute....the internals of the engine continue to build heat beacuse there is not enough cooling water being circulated at idle to effectively cool the engine down fast enough....eventually the engine will cool and return to normal running temps....when the temp of the engine rises when you come off of a hard run the heat is transferred to the electronics and everything else on or around the engine....if an electronic component is marginal the heat build up can cause it to fail either completely or intermittenly....then after the suspect component cools down for a while it sometimes can return to normal operation.....the mechanic told me the 3 primary electrical components of the Thunderbolt ignition system that are subject to heat failure are the ignition coil, the ingnition sensor in the distributor, and the ignition control module....

since you have already replaced the coil and ignition sensor you may be having problems with the ignition control module...the mechanic told me the main purpose of the module is to maintain a steady ignition timing under varying conditions...he suggested to use a timing light to check the steadiness of the timing both before and after the problem happens....if you have a Thunderbolt V ignition system you do not want to put the ignition into 'Base Mode' by grounding the purple/white wire....just check the timing with the light and verify that the timimg light flashes on the same timing mark each time...if the light flashes on different marks the ignition contol module is likely bad and needs to be replaced...i have read where some people have used a bag of ice to place on the control module when the problem ocurrs to cool down the module....if the problem goes away when the module is cooled down that would be an indication the module is bad also...

and by the way, the original ASV in my boat had an aluminum housing and an aluminum (or stainless steel) checkball...the new valve i put in has an aluminum housing with a brass checkball....sort of a 'hybrid'....

good luck...

cliff
 
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Hi CliffA,
Not sure if you'll find this post........did the anti siphon valve fix your issue? Searched everywhere for "engine stalling at higher rpms" and cannot find anything. Having this issue on the port side. I do have a fuel cross over but not able to test it at this time to see if something with the port side tank is the issue. Thanks for posting this.

long post, but hopefully helpful:

i thought i would share something i recently learned so that this archived post may someday help someone new to boating...i have only been boating for around 5 years so i still have a lot to learn.....for those that are familiar with marine fuel delivery systems you can stop reading this post as this will likely be something you already know....

all of this boating season i have been experiencing an issue with my boat engine that took me a while to diagnose....the symptoms were that the engine would start, idle and overall run great...however, when i cruised for around 30 mins at 3,000 rpm's (or above) the engine would suddenly start stumbling, backfiring and sometimes stall....usually i could get the engine to restart and idle OK and even run the rpm's up when not in gear.....but when i would put the drive in gear the engine would stumble again....after i let the boat sit for a few mins the engine would start and perform normal again for a little while and then the symptoms would return....i went through all of the troubleshooting steps i know to try and determine if this was a fuel related problem or an ignition related problem....i have a fair amount of knowledge on how gas engines work and troubleshooting skills which were developed over many years of tinkering with cars and trucks....i won't list all of the things i tested and checked but if anyone wants to know send me a PM and i will list them for you....

i finaly determined that the symptoms were most likely coming from a fuel related problem that was causing the carb to starve for fuel once the vacuum built up in the fuel tank enough to prevent or restrict the flow of gas....i researched the subject and symptoms and found several forum posts that mentioned checking the anti-siphon valve...i had already checked all the things that i could think of that would cause this problem and found nothing...as i was reading the post that mentioned checking the anti-siphon valve i was thinking to myself 'why are people concerned about a valve in the gas filler hose that prevents someone from stealing gas from the fuel tank'.....that was my incorrect idea of what an anti-siphon valve is....i finally came across a post where the anti-siphon valve and its function was explained....the function of the anti-siphon valve in a marine fuel delivery system is to prevent gas from siphoning out of the fuel tank if the fuel line ruptures or otherwise becomes disconnected from the fuel filter and/or fuel pump...it is usually located in the fitting that is on the fuel tank that the fuel line connects to.....if/when this valve becomes blocked or partially blocked with 'crud' over time the flow of gas going to the carb or fuel injectors will be restricted to the point the engine will stumble and possibly stall, just like i was experiencing...

last night i removed the anti-siphon valve from my fuel tank and found it to be an estimated 80-90% blocked...i replaced it with a new valve...i have not tried the boat on water yet since replacing the anti-siphon valve but i have a very good feeling that this was casuing my problems...

so if anyone reading this post experiences similar symptoms with their engines perhaps you can learn from my experience and consider checking and/or replacing the anti-siphon valve....here is a link to a good article that describes the anti-siphon valve and its function...

http://www.sterndrives.com/anti-syphon-valve.html

cliff
 

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