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Rough Running On 851 Sp3 - Fuel Starvation?

Discussion in 'Technical Help' started by AndrewS, Aug 21, 2019.

  1. Hi All

    I took the SP3 out for a run this morning, first time it's been started for several weeks. It started first time, as it usually does, and ran fine for 20 miles or so. Then I pulled out onto a main road and accelerated (only up to about 7500 rpm) then backed off. The engine started running what I can only describe as "rough", with the occasional backfire. I managed to get it home, slowly, and when parked in the drive, it idled fine and responded normally to a blip of the throttle. After I turned it off, I opened the fuel cap (don't know why!) and there was a very loud rush of air - no great smell of petrol so I assume the air was going into the tank, i.e. there was quite a vacuum in there.

    Any thoughts welcome. I wondered about a blocked/kinked breather, but I'm not sure where it runs!

    Thanks

    Andrew
     
  2. It certainly sounds like a blocked breather.
    This is the best picture I could come up with for showing the location of the tank breather.
    Screen Shot 2019-08-21 at 18.48.36.png
    a. Is the fuel feed from the pump
    b. Is the fuel return line
    c. The breather hose
    d. The drain hose
    The manual for the '89 model doesn't show a drain hose. I hope that helps.
     
  3. Thanks for that Derek - I think that's from the Factory Workshop Manual? Do the breather hose and the drain hose come out of the bottom of the tank and through the airbox to somewhere underneath the engine? I think I've seen the drain hose (runs near the battery vent?), not sure about the vent hose. Is there a one-way valve or anything on it? Sounds like it might be a "tank off" job... On the SP4 I think it exits from the top front of the tank?
     
  4. Yeah sounds like blocked breather
    is there a top vent at the front of the tank like an SS?
     
  5. Thanks @bradders - the top vent was on the SP4.
     
  6. as said, if it's like the SS then steel tube can be totally blocked, usually with corrosion, or the one way valve either blocked or fitted the wrong way or even the hose collapsing but fix shouldn't be too bad.

    upload_2019-8-21_22-33-53.jpeg
     
  7. Yup, has to be the breather.
    Good luck with pinpointing the problem.
     
  8. You never forget your first "blocked breather" initiation, mine was around 1973 on a 1967 YDS3.
     
  9. Mine was around 2000 on a BMW R80/7
     
    • Like Like x 1
  10. Had a guy at our sailing club who became known as Captain Calamity.
    He set off up the estuary with his not very fit and well wife on board, in their Wayfarer dinghy with an outboard on the back. About a hundred metres upstream, the outboard puttered to a halt. As the water was quite shallow at that spot, he chucked his wife overboard to hold the boat. However, she couldn’t hold the boat against the tide and had to let go. The tide was coming in, so as he was drifting away, the water for her was getting deeper...
    At that point he called the RNLI.

    Moral of the story?
    Always remember to open the breather valve on your outboard’s petrol tank.
     
  11. what a story, quickly revealed the Captain's controversial priorities - can you jump out love?, nothing to worry about..
     
    • Like Like x 1
  12. He once asked me what direction the wind was blowing from!
    He just had no idea.
    Luckily he didn’t stay long...
     
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