CliffA
Well-Known Member
- Dec 29, 2009
- 4,712
- 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
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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