Yamaha 90 2 stroke rough at low rpm

[charles90yam]

Looking for some direction on this problem. 90 Yamaha 2 stroke runs rough at low rpm (1000 - 2000 approx.) with the fuel enrichment switch in "normal" position. The switch has 3 positions - normal, open, closed. Idle is OK, higher rpm is OK, but low rpm is shaky and rough. When I switch to the "closed" position on the fuel enrichment switch it runs excellent at low and higher rpm, but will not idle. As soon as I slow down to 1000 rpm or go into neutral, it backfires and shuts off. It was actually running worse in the "normal" mode before I started troubleshooting different problems. In the last 2 weeks I have replaced the plugs (they were fouling), then the fuel pump (helped), water separator, and fuel filter. I also started running non - ethanol gas for the last 2 tanks. I am thinking the problem may be the fuel enrichment switch - does that sound like the right direction?

[charles90yam]

Add on

Forgot to mention that I also run it on a remote tank with non-ethanol, just to eliminate a fuel issue.

[99yam40]

the enrichment system should turn itself off after motor is warmed up while in normal position.
While in the off position there is no fuel flowing threw the enrichment system at all cold or warm.

Sounds like you need to pull and clean carbs and make sure enrichment system is doing its thing.

Yamaha service manual for your motor will have all the steps and specs for testing and adjusting the carbs after reinstalling and all other systems too

[throrope]

I suspect your carbs got gummed up.

Changing to non-ethanol fuel and additives won't clear them once gunked. Also, if the fuel went bad, adding good to it, won't help. I've also heard others found ethanol in premium priced non-ethanol advertised fuel. There's a good reason why all new cars and outboards are fuel injected.

I had similar issues. Thought I cleared the problem, but because I didn't trash the old fuel and supply, I sent more gloop in and found myself in the same place.

I since replaced the tank with smaller top side tank and all hoses. By rigging a cheap fuel pump with hose, fuel fitting and 12 volt plug, after trips I now pump all fuel out and into the car, dump the water separating filter and drain the carb bowls. I even flop the open tank so the heavier than air vapors drop out letting all fuel evaporate. No gas to go bad.

good luck and keep us posted.

[charles90yam]

99yam40 and Thorope - thank you for the responses. It looks like I should be looking at the carb and not the fuel enrichment valve. It does make sense considering the gas problem that I had. I just received my Yamaha service manual yesterday so I will prepare to clean the carbs.

Thorope - did you clean out your inboard gas tank? What did you use to do it? I would like to do that to mine to make sure all the goop comes out. My local dealer wants $1,000 to empty the tank and "scrub" it. I would like to be able to do it myself.

Has anyone done this, and what do you use? Thanks again for the help.

[throrope]

99yam40 and Thorope - thank you for the responses. It looks like I should be looking at the carb and not the fuel enrichment valve. It does make sense considering the gas problem that I had. I just received my Yamaha service manual yesterday so I will prepare to clean the carbs.

Thorope - did you clean out your inboard gas tank? What did you use to do it? I would like to do that to mine to make sure all the goop comes out. My local dealer wants $1,000 to empty the tank and "scrub" it. I would like to be able to do it myself.

Has anyone done this, and what do you use? Thanks again for the help.

I tossed my tank and lines. The old gas went into our fuel injected truck diluted with fresh gas over a few tanks with no issue. New tank is a smaller Moeller tank purchased online delivered free to the front door. My dealer wanted $300 plus $3.75 per gallon disposal fee. The complete replacement cost less not including disposal.