Plug problems

[Eric Wettschreck]

2000 Yamaha Big Bear 400.

It keeps fouling the plug. I'm talking like every hour of run time or so, the plug fouls. This is a new problem as of about a month ago.

The choke is open, so it's not running rich.

I'm no certified ATV tech, but I'm not a dummy to engines either. I can not for the life of me figure out why all the sudden my plug is fouling.

Any suggestions on where to start looking?

[Dave S]

What else is the motor doing while it's still running. Some of the better gear heads might be able to tell you more, but my first guess is you might be running rich even with the choke open?

Check your air box and the boot between the air box and carb for obstruction.

Smoke from the exhaust (more than usual)?

[Eric Wettschreck]

This is the weird one for me. The thing runs great. Exhaust is normal. Air filter is new, boot isn't plugged, etc.

I'm used to fouling plugs if I'm running rich, but I really don't think it's running rich. However, I could be wrong.

[Dave S]

What does the tip of the plug look like (color)? Gapped right?

[Eric Wettschreck]

I have not gapped a plug in years, maybe that's the problem. In the past with my Yamaha sleds that came jetted factory rich I used to burn a B8 instead of a B9 plug. That solved the foulding problem but I think it's an apples to oranges comparison. This is a problem that has not been there since day one, it just recently started.

The plug is dark, not the nice tan it should be. This is telling me it's running rich.............but...........dayng.........it's a simple one lunger 4 stroke. Choke is open, air filter is clean, getting plenty of air.

Yeah, it's a toughy.

[sparcebag]

Have ya compression tested? Sticky rings?

[Dave S]

I did some searching around earlier and the only other thing that I could find was others experiencing the same thing were saying it could be the rings/piston.

I have my doubts on this. The 400 Yamaha motors that I know of have been a great motor as long as they're maintained well. I don't think you're the kind of guy that doesn't maintain anything.

You said it runs great, but for some reason it's running rich. Maybe the colder weather? But that doesn't make sense either as colder weather, if I'm not mistaken would be more likely to cause a lean condition (cold air is more dense).

With that being said, a couple other things that comes to mind are maybe your valves need to be adjusted or jetting is set too rich for the weather conditions?

[Roofer]

Did you leave it sit long. I have read about piston and rings problems if it sits too long. Is the oil level OK? Valves could be one thing, but that wouldn't be "all of a sudden", would it?

It is air cooled, right? Did it ever overheat? Would you know?(dummy lights work?)

[Macgyver55]

Boilerguy, there are several things to consider for this problem. First, do a compression check to eliminate rings and valves as a possible cause. You should have at least 120 or more lbs of compression. If it is prone to use oil in between changes, valve seals could be a possible cause. Also, check the crankcase breather to verify that it is not plugged or frozen with moisture.

It could also be a float or needle seat problem causing a rich condition. Another thing to check for is a plugged or restricted exhaust system, although I would think it would be running a bit sluggish if any of these are the cause.

Let us know what you find....

[Eric Wettschreck]

Did a quick down and dirty compression test. 135 lbs. So I'll assume rings and seats are good. I understand assumptions are bad, but the numbers are what they are.

I understand the "It's been sitting" sccnario. Trust me, this machine is not a sitter. It's my work horse. Also, it's not an oil user. I check it often and it never changes.

I thought about the restricted exhaust thing, but I'm going to not really consider that as exhaust flow seems fine. The only indidcator I have of this is what I feel is normal flow from the pipe.

Macgyver, I was somewhat hoping it wasn't a carb problem, but I'm inclined to think that it is. I'll take it off tomorrow after work (yeah, I'm working sunday) and let you know what I find. Admittedly I'm not a carb expert but these carbs are fairly simple.

Thanks for the input fellas.

[Tyler23]

yea the carb would be the next thing i would check, its possible that some grit or something got in there and caused it to run and little rich. make sure ALL the jets and holes and completely clean then if it still fouls plugs atleast you can completely rule out that its not a carb problem. may not be a bad idea just to rebuild the whole carb while its out, the rebuild kits aren't expensive