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Reply With QuoteWhat brand of plug are you running. It’s starting to sound like the plug you have is to cold.
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Yamaha 115 2 stroke fouling plugs at idle
I had posted a week or so ago about an engine problem that I had. I cleaned the carbs, replaced the thermostats (per Cray, and many thanks for that) adjusted the floats to the recommended height, and installed new plugs. One thermostat was stuck partially open. I picked up 5 psi water pressure from changing the thermostats. I was running 15 at WOT and now I'm running 20, which means will allow me to raise my jack plate more to see if I can squeeze a little more performance out of it. Anyway....
I've owned the boat, nearly a year. It runs fantastic from about 1800 rpms to WOT. It has never ran good when running 4mph or slower, like I do while scanning with my SI. The motor loads up with unburned fuel and when I do give it throttle a huge plume of grey smoke comes out. When I putt around for more than 5 minutes, it really gets to running bad. I've put 4 sets of plugs in this boat in the 10 months that I've owned it. I put new plugs in it last week and after doing some slow cruising last night, they're fouled out.
The only adjustment screw on these carbs is the air/fuel mixture. I have it set more open than recommended so I'm leary of going any further. From my understanding, the more air, the leaner the motor runs.
I have checked the spark with a gap tester on each plug and they're jumping the gap that's recommended. I have not Ohm'd any plug wires, or coils, or anything like that.
One of my fishing buddies summed it up best last night. He said, "This motor sounds just like a street rod. It barely runs at idle and sounds like a beast when it's wide open."
Anyone have any recommendations?
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What brand of plug are you running. It’s starting to sound like the plug you have is to cold.
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Charlie Weaver USN/ENC 1965-1979
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NGK. I’m running the plug that’s called for in the manual but I did wonder if a hotter plug would help or solve the problem. These are B8HS-10. I know I can get a B9HS-10 and I think that 9 is hotter than 8 but I’m not sure of that
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The idle mix screws meter fuel, not air. If you have them opened a bit more then you have richened the idle mix. Which explains your symptoms.
I'd rather be fishing.
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Your running the right plug. Listen to Sliderman. He is a boat motor tech.
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Charlie Weaver USN/ENC 1965-1979
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My manual refers to this screw as the "Pilot Screw" It's at the top of the carb. Is that still the same thing you're talking about? There is only one screw on my carbs. One on each side of the carb.Originally Posted by sliderman
Crappie Wall Hanger II
You typically want to lean it out some to get a nice smooth low speed idle. However, too far can be bad news at wide open throttle as if too lean it can literally melt the tops of the pistons. Rule of thumb I follow is to lean it out at forward idle until you get your smoothest and lowest idle, then back the screw out 1/8 of a turn, then set your fwd idle speed to the recommended setting. I also compare my final screw setting to the initial recommended setting from the service manual and if I have screwed it in more than 3/4 of a turn than what it says I go back and check everything through again.
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Also are you using a good quality oil? Don't use the cheap stuff.
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Short answer= yes, pilot screw is the idle mix. From what I understand, you've richened the idle mix by turning it out and it's loading up at idle. Which will result in a rough idle and flooding if you allow it to run at idle for even a short amount of time.
Before you go any further, let me clarify a few things in order to ease your apprehension. It is true that lean fuel mixtures 'can' lead to internal damage. HOWEVER.... that is not of concern in your application. Being a water cooled, truely oil injected design, the engine will not idle on such a lean mixture. Where the hazard comes into play is when the main jet is swapped out with a smaller one that allows a lean combustion at high engine rpm and load.
If you haven't synch'd the carbs after re-installing them, that is your next step. Then adjust for best possible SUSTAINED idle in gear, in the water, not on hose.
I'd rather be fishing.
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Thanks for the replies. I use Yamalube and that's all I've used for quite a while.
sliderman, thanks. My engine doesn't have oil injection though. I mix it 50:1
I have no synched the carbs. I will try that next. Thanks again.
