70 yamaha2 stroke won't start

Discussion in 'Outboard Maintenance' started by formerWAflyfisher, Aug 19, 2011.

  1. formerWAflyfisher

    formerWAflyfisher Well-Known Member

    So I had a previous thread about gummed up carbs.  Well I cleaned them up, soked them and put them all back together.  I reinstalled them on the motor. Drained the fuel tank and lines, changed the plugs but the motor won't start. 

    I made sure every passage way in the carbs were clean by blowing compressed air through each one. then installed the rebuild kits.  I think the motor has good spark.  If I spray some carb cleaner down the throat of the engine it'll turn over once but then dies.(I only did this once) I checked the drain plugs in the carb bowls and they are filled with good gas. Does the motor need to be primed in some way?

    Any help would be greatly appreciated. 
  2. AfterHours2

    AfterHours2 Stripper in my own Mind!

    Sounds like you still have a fuel issue based on what you said about the carb spray. If the spark is good, you may need to purchase a cheap tester and test in the evening, then you might need to look into a possible fuel pump kit. Sounds like it is the only thing that is fuel related that you have not replaced. Those yami carbs can be tricky and it is not uncommon for something to be missed, so you could still have an issue in that area. Im in no way a marine tech but have had my run ins with those Yamaha motors and half the time it is something very simple. Good luck..

  3. twitch

    twitch Well-Known Member

    Did ya remove the O ring and diaphragms on the center carb(prime start system) before you soaked the carbs. If not that is where I would start looking....

    Mix up a small batch of premix fuel and put it in a spray bottle(alcohol resistant), instead of starting fluid.
  4. Frank_Sebastian

    Frank_Sebastian Well-Known Member

    When you pump the fuel line ball does it get firm after a little pumping? It is possible that the float needle slipped out while installing carbs cutting off gas? Are you using a portable tank or a built in one? Most built in tanks have a check valve at the pick up tube and they stick a lot with the ethanol fuel. Make sure you have gas into the carburetors and take care if you use the spray bottle gas method. It sounds dangerous to me.

    Best regards,
  5. formerWAflyfisher

    formerWAflyfisher Well-Known Member

    Ok, So I mixed up some gas and went a little heavy with the oil 16:1 then sprayed some in the opening for the plugs and down the throat of the carb.  Then cranked it and the motor started right up ran for a few seconds then died.  It was no problem to get it started this way and it would run until the gas ran out.  (I made sure I sprayed evenly in each cylinder) SO it is the fuel system, right?

    I disconnected the fuel line just below the motor and pumped gas straight from a one gallon tank of fresh gas into the carbs.  The bulb gets hard and there is gas in each bowl of each carb. (I can hear the gas squirting in and when I check the drain on the bottom of the bowls they are full)  So wouldn't the problem have to be in the carbs?? What actually supplies the pressure for the gas to be sprayed from the jet into the cylinder? Thanks for all the help guys, it is greatly appreciated?
  6. Brett

    Brett > PRO STAFF <

    Not really pressure, more like vacuum.
    The basis of carb function is the venturi effect.


    The pressure drop isn't a lot, so it doesn't take much to foul it up.
  7. mmjamp

    mmjamp I Love microskiff.com!

    If the engine isn't starting under normal conditions, but does when you spray gas into the throat of the carb, then it's in the carbs. Are you sure you have the idle air mixture screw adjusted correctly?

    If so take them back off and go through them again, not looking for thrash but something you did wrong.

    The engine runs off vacuum, which is created with crankcase cylinder pressure and the reed valves.