To rich at full throttle

[Fyrfota]

I have an 4hp Yamaha, it always starts without problem and works fine up to 40-50% throttle but when I put into full throttle it gets to much fuel, it even blows out of carb in front. When I hold a flat screwdriver at the tube where the fuel comes out it immediatelly rev up to maximum. When I remove the screwdriver it goes down again blurring.<br />I have changes all the membrams in the fuelpump and the "expensive thing" between the carb and the crankshafthousing (aluminiumpart with three "flaps". No difference!<br />Can it be the floatthing in the carb?<br /> Anders

 

[jim dozier]

Re: To rich at full throttle

Have you or anyone else ever had that carb apart. It sounds to me like there is a problem with the main jetting. If it was the float it would probably be worse at idle when the engine couldn't keep up with the excess fuel. You have removed the fuel pump as a potential problem. You have removed the reed valves (the expensive thing between the carb and the enging) as a potential problem.<br /><br />I would take the carb apart and see if:<br />1. Someone has damaged the main jet orifice<br />2. The main jet has come loose or fell out<br />3. Something associated with the jet has broken or come loose.<br /><br />PS Don't stick anymore screwdrivers in the carb throat, delicate pieces of equipment there, maybe where the problem started.

 

[Fyrfota]

Re: To rich at full throttle

Thank's Jimd<br />Yes I have had the carb apart and it looks brandnew in every detail!<br />Can someone before me have change the jet to a bigger one for some reason?<br />It's really a lot of fuel coming out on full speed (neutral, in a barrel).<br />It even builds up a small fog infront of the intake, and if I blow it away with compressed air it speeds up a bit, so it for sure gets too much fuel.<br />Anders

 

[Fyrfota]

Re: To rich at full throttle

Me again!<br />The gasket between the membran and the aluminiumhousing has a small damage in the area that is closest between the enginepressure and the fuelintake to the carb. I guess when the rev goes up the pressure slips between the two chambers and forces the fuel through the jet and chokes the engine...<br />Anyone?

 

[jim dozier]

Re: To rich at full throttle

Does this engine have a fuel pump or is it gravity fed? If it has a fuel pump check the internal diaphragms in the pump. You may have a hole that is injecting fuel directly into the crankcase and some of it is coming out the carb.

 

[Fyrfota]

Re: To rich at full throttle

It has a fuelpump but I have changed everything inside this one too. Today I experimented with the throttlelevel - no diff.<br />When under load it works on about 50% of top rev for about 30 secs and then slowly goes down and starts smoking - too much fuel again...<br />The Yamaha authorized repairshop said it's like old women - get a new one. They couldn't solve my problem so I think it works like an anchor in the future, while it still loooks brandnew...<br />It's a shame!<br />Now spent about 120$ on it for no good...

 

[bigshrimpin]

Re: To rich at full throttle

Fyrfota,<br /> Any luck with this? I have a similar problem with my 4hp.

 

[jim dozier]

Re: To rich at full throttle

Do these engines have adjustable main jets or fixed main jets like most larger outboard engines? This would be in addtion to an adjustable idle mixture screw. Generally the things that make an engine run rich in order of likely hood are:<br />1. Stuck open carb floats<br />2. Leaking fuel pump diaphragms<br />3. Inaccurate main jet setting.<br /><br />If you have adjustable main jets I would move #3 to #1. If they are fixed I would verify with the dealer that they are correct and then verify that they are screwed in snug. There aren't that many ways for extra gas to get in there. Usually, not enough is the more common problem.<br /><br />Also the fuel for the idle circuit is initially controlled, I believe, by the main jet. So if the main jetting is screwed up, the idle setting will be screwed up. Sometimes people will over adjust the idle mixture to make up for incorrect main jet settings. Then the engine may idle but not work off idle. Get a manual and make sure your baseline settings are correct. If there is an adjustment for the main jet, get that right first before you start fiddling with the idle mixture.

 

[Yepblaze]

Re: To rich at full throttle

Perhaps you have lost or installed improperly the "orafice tube" located above the main jet.<br /><br />Not sure if your model is so equipped but take a look.