Fouled spark plug weak spark

Hey folks,
I bought a used ATV with a 250 cc Yamaha motor (4 stroke). It has seen it's share of use and neglect and developed a rough to non existant idle (runs good otherwise). The previous owner decided the carb was the problem and, knowing which end of a screwdriver to hold onto, started "adjusting". When adjusting didn't fix anything,he took things apart and put them back together and adjusted some more.
I bought it with the thought that nothing could be that complicated on a single cyl motor.
I replaced the dryrotted rubber manifold that connects the card to the cylinder, set the mix screw to where it should be (shadetree knowledge)and got it running with an OK idle that gets worse after a few minutes. Runs rich (black smk from exhaust)still and I have the air mix screw as lean as I can get it.So, my routine is:
Remove the spark plug, shoot some carb cleaner on it, take a couple of swipes with a wire brush, reinstall and start the motor, use the quad for the day, and repeat this process any other day I need it.

The spark plug is always fouled. Just realized that the plug that was in it (and that I bought a new one the same)is one heat range colder that what is called for.

Could this be my problem, or enough of it that I can put the hotter plug in and live with the idle thing? Or is it possible that the hotter plug will make my idle problem better also?

If anyone can steer me a little, I'd sure appreciate it.

Thanks, Dave

Possibly the carb float is too low or sinking. Is there any place for fuel to seep past old used gaskets?

In general.. a hotter plug won't foul as bad.

have you looked for a stuck choke.. or an air restriction.. all those will enrich the mix and cause sooty black smoke..

soundguy

Dave, I dont know beans about that particualr engine, but still can tell ya if the fuel air mixture is running so rich it quickly soots up the plug with that flat black colored carbon (excess fuel) ITS GONNA TAKE MORE THEN A HOTTER PLUG TO FIX IT. Sure, Id run a hotter plug n that may help a little, but it sounds more like you have a carb or fuel problem... Still, a weak spark can contribute to premature plug fouling so you may wanna check out the ignition n try that new hotter plug n get the fuel mixture right n see what happens??

John T

(quoted from post at 06:37:51 09/30/08)
have you looked for a stuck choke.. or an air restriction.. all those will enrich the mix and cause sooty black smoke..

soundguy

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I can't figure out how the choke works. I try moving the lever (on the handlebar) while looking for something to move in the carb, but nothing happens. Since you mentioned it, it came over me (I get these brainstorms at the damndest times) that a part I thought was in a normal position is actually restricting airflow and is probably the choke and is stuck closed. I'll check that first thing in the morning and see if I can wedge that thing open somehow. If that's the problem, I can be creative on making it work.

Thanks, Dave

dave, as far as the choke on your yamaha, assuming its a mikuni carb(most popular) when you look into the air intake side of the carb, there should be a hole at either the 10 oclock or 2 oclock position, maybe 1/4 inch or so in diameter. that is the choke hole. there is a plunger inside there that when the choke is put on, the plunger opens the hole to allow air to enter there and pick up the fuel charge. the throttle must be closed to choke the engine. if you open the throttle, the air bypasses and goes thru the main venturi. be sure all passages in the carb are clean, and not plugged up. a welding torch tip cleaner is a good tool to open em with. be sure to put an in line fuel filter on that machine too. seems like all the atv"s hate this new formula gasoline. it doesnt hurt to keep sta-bil in the fuel too.

Sounds like you found it.. there will be a throttle plate/butterfly , and the choke plate int he front where the air intake is.. likely the cable broke and the choke may be closed.. I'll bet once you get it fixed your fuel economy goes way up!

soundguy

Thanks! I was wondering about a filter and will put one in tomorrow. I'll check out the choke thing also. Gotta figure what the stabil equal is over here though.

Dave

(quoted from post at 07:51:25 09/30/08) I'll bet once you get it fixed your fuel economy goes way up!

soundguy

Click to expand...

I read on forums here that this thing should use 2-2.5 liters per 100km (+/- a little IAW use). It takes 6 or 7 liters now. Could be onto something here . See what I can figure out tomorrow.

Thanks, Dave

any type of stuff that with preserve the fuel for long term storage will help. whatever brands you have available. in a pinch, you can add some of the "coleman lantern" white gasoline fuel. it is loaded down heavy with fuel stabilizers.

Sounds like choke to me..

soundguy

Dave... If this Guy was a "Screwdriver" Finatic then it's quite possible that he has ruined the Fuel Adjusting Seats in the carb., Probably tightened them too Tight, just a Thought. Larry KF4LKU

Like others said air cleaner full of grass, debris, weeds, seeds.

Don"t overlook the inlet needle and seat as they wear badly over time in the mikuni carbs, the enrichener cable if equipped should have just a little free travel to ensure that the plunger is fully seated with the lever or knob fully shut off.