How to tell if running lean, or rich?

EricJRW · Jun 5, 2012

I was reading a thread about the VRO, and it was said that running rich can burn up a cylinder and or motor.

So, is there a way to tell if you are running lean or rich? I'm running pre-mix, and since there is no high-speed adjustment on the carbs, I'm wondering if that's as good as it gets? Of course there is the needle-valve, but it's my understanding that for idle, so I'm not sure if that's a factor (low RPMs and all).

Thanks,

Eric

oldcatamount · Jun 5, 2012

Re: How to tell if running lean, or rich?

If your motor is running lean, it will "sneeze" at low rpm, if it's running rich it will smoke and foul the plugs. The needle valve should get you a happy medium at about a quarter turn counterclockwise from where the motor will sneeze. Turn your needle valve clockwise until it seats (lightly) and then back it out in 1/8th increments until the motor stops the lean sneeze and then go another 1/8th turn. It should smooth out there.

EricJRW · Jun 5, 2012

Re: How to tell if running lean, or rich?

Thank you. My manual was not clear on the number of turns out from lightly seated. The goofy table made me think it could be two turns, or 2.5 turns. I did two, and I don't think my motor was sneezing, but I'm not really sure. I think I will try opening the needle-valves up a bit and see what happens.

By the way, how loud is the sneeze? I've never heard this term, but it makes sense. I wonder if there's a You Tube video?

Also, with two carbs and two cylinders, can just one pair be sneezing?

Thank you,

Eric

oldcatamount · Jun 5, 2012

Re: How to tell if running lean, or rich?

The needle valves should be open at least 1 and 1/2 turns to start with and be sure both carbs are adjusted as close to identical as you can get them. A "lean sneeze" will sound like a mild, muffled, backfire.

EricJRW · Jun 5, 2012

Re: How to tell if running lean, or rich?

Excellent. Thank you so much for your help.

Chris1956 · Jun 5, 2012

Re: How to tell if running lean, or rich?

Eric, Running lean will cause heat and can cause damage. Fixed carb jets can get clogged and cause this condition. Running rich is usually not an issue. BTW - the idle mixture setting will never cause a motor damage. The motor may not idle correctly, however.

EricJRW · Jun 5, 2012

Re: How to tell if running lean, or rich?

I just rebuilt the carbs and am confident that orifices and nozzles are clean. Replaced all fuel lines too. Motor idles better than it ever did (you should see the gunk I cleaned out of the bowls). After reading that VRO thread though, I got nervous. I'm not getting any sneezes, but it does shudder a bit (but even my '04 BMW oilhead boxer shudders a bit, so I thought this is just a 2-cylinder thing). Just backing out 2 turns did not seem scientific enough to me, so I'm very grateful to understand how the needle valve affects a running motor.

Continued thanks,

Eric

nwcove · Jun 5, 2012

Re: How to tell if running lean, or rich?

you can also keep an eye on the condition of the plugs.....dark brown is the way i like to see them (jmo), and i dont even mind seeing them a bit on the black side. but grey/white is a sure sign of running on the lean side. ( a lean condition isnt always caused by an internal issue with a carb tho, any air leak around gaskets on the vacuum side can lean a motor out)

EricJRW · Jun 5, 2012

Re: How to tell if running lean, or rich?

nwcove said:
you can also keep an eye on the condition of the plugs.....dark brown is the way i like to see them (jmo), and i dont even mind seeing them a bit on the black side. but grey/white is a sure sign of running on the lean side. ( a lean condition isnt always caused by an internal issue with a carb tho, any air leak around gaskets on the vacuum side can lean a motor out)

Great tip. Thank you. The boat should be out for it's first extended run on the rebuilt carbs this weekend. I will bring my spark plug socket and pull the plugs at some point to take a look.

PS. ALL gaskets were replaced... So I feel good there too.

Big Bubba · Jun 5, 2012

Re: How to tell if running lean, or rich?

Yeap, I agree nwcove, on the color of the spark plug being dark brown then it is running like it should.

Chris1956 · Jun 6, 2012

Re: How to tell if running lean, or rich?

Medium Brown is also fine. Even lite brown is OK. lighter than that can indicate a lean condition.

How to tell if running lean, or rich?

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