spongy Posted July 20, 2009 Share Posted July 20, 2009 Hi guys, Hoping some one here can shed some light on a small incident I had yesterday out on the water. After cruising for about 10 minutes going from spot A to spot B, pulling up to spot B, I notice the outboard running very rough at very low revs. I put it into idle and its just barely idling and then stops. I can't get it back up and running. After about 5-10 minutes it starts up again, but again very rough idle, but when I bring up the revs it feels all ok. Drop back into idle and again stops. Repeat the whole process again and 5 minutes later its back up and running roughly. I decide to make my way home, and after 15 minutes at cruising speed. I slow down to check the idle speed. To my surprise its all good - back to purring nicely as it should. I have never (over 4 years) had ANY problems with my Yamaha 70 2stroke until this incident. Starting and idling has never been a problem. Plugs have been replaced about 2 months ago. Can anyone advise what may have caused this? Im guessing either water in the fuel or bad fuel, etc. Thanks, Jeff Link to comment Share on other sites More sharing options...
Geoff Posted July 20, 2009 Share Posted July 20, 2009 Spongy How many hours on the clock , if a few then it may be the fuel pump. The 70 has a mechnical vacume style fuel pump. It is not uncommon for the diaphram to get a hair line crack or even a small split. This allows fuel to go directly into the bottom cylinder & in essence flodding that cylinder . At idle or low revs the engin is only working on 2 cylinders efficently. At higher reves the excess fuel is used up & the motor runs smothly. The diaphram is not expensive & reasonably easy to replace Geoff Link to comment Share on other sites More sharing options...
spongy Posted July 20, 2009 Author Share Posted July 20, 2009 Spongy How many hours on the clock , if a few then it may be the fuel pump. The 70 has a mechnical vacume style fuel pump. It is not uncommon for the diaphram to get a hair line crack or even a small split. This allows fuel to go directly into the bottom cylinder & in essence flodding that cylinder . At idle or low revs the engin is only working on 2 cylinders efficently. At higher reves the excess fuel is used up & the motor runs smothly. The diaphram is not expensive & reasonably easy to replace Geoff Thx for your reply. Do you still think its a cracked diaphram, when after about 30min, its all working fine again? I would imagine if its something like a split or crack (or any physical damage), then it would continue to be a problem. Link to comment Share on other sites More sharing options...
Geoff Posted July 20, 2009 Share Posted July 20, 2009 Thx for your reply. Do you still think its a cracked diaphram, when after about 30min, its all working fine again? I would imagine if its something like a split or crack (or any physical damage), then it would continue to be a problem. Spongy. Yes , in theory your correct. and after 15 minutes at cruising speed. I slow down to check the idle speed. To my surprise its all good - back to purring nicely as it should. It depend how long you you ran at idle speed , depending on the size of the crack it can take a short while for the cylinder to flood. Giving it a bit more thought it's possible that one of the plugs became temporally fouled & the run cleared it out. On the fuel side , do you have a fuel filter / water seperator fitted to the fuel line. If so has this been checked. Geoff Link to comment Share on other sites More sharing options...
a boat Posted July 21, 2009 Share Posted July 21, 2009 Hi Jeff, I would put this down to one of those things that can happen to outboards and perhaps some debris went thru the system and blocked the idle circuit, but then cleared itself. IF the engine behaved itself for the rest of the day and is now running fine, I doubt it will be the fuel pump, but as Geoff says prevenetative maintance is a good thing and the diaprahm is easy and cheap to replace. Cheers, Huey. Link to comment Share on other sites More sharing options...
spongy Posted July 21, 2009 Author Share Posted July 21, 2009 Hi Jeff, I would put this down to one of those things that can happen to outboards and perhaps some debris went thru the system and blocked the idle circuit, but then cleared itself. IF the engine behaved itself for the rest of the day and is now running fine, I doubt it will be the fuel pump, but as Geoff says prevenetative maintance is a good thing and the diaprahm is easy and cheap to replace. Cheers, Huey. Thanks Huey. I will remember to make sure the diaphram is replaced on next service. Geoff, I have a water separating fuel filter with drain plug. I did a visual inspection of the bowl and saw no water, but I will drain the bowl anyway to checck. Thanks again. Jeff Link to comment Share on other sites More sharing options...
Geoff Posted July 21, 2009 Share Posted July 21, 2009 (edited) Thanks Huey. I will remember to make sure the diaphram is replaced on next service. Geoff, I have a water separating fuel filter with drain plug. I did a visual inspection of the bowl and saw no water, but I will drain the bowl anyway to checck. Thanks again. Jeff Spongy If it is the diaphram (& we are not sure it is ) , I would not wait until the next service. Once they start to go it will just become worse rather quickly to the point the diaphram will fail & no fuel pumped to the carby's There is no way to fix this problem on the water & means a tow home Geoff Edited July 21, 2009 by Geoff Link to comment Share on other sites More sharing options...
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