yz 250 eats sparkplugs

[Chris888]

my yz 250 will use a sparkplug for about 2 or 3 days fine and then on about the 3rd or 4th day ill start the bike it will be fine for a couple of miniutes then just die and wont start again until i put a new plug in. then that plug will do the same.. a couple of rides and then dies. ive tried irridum plugs to b8es plugs all the same. could it be to much resistance in spark plug lead to the coil? or carbon on piston? ??

 

[kaboom1371]

what does your plug look like when you take it out? black? brown? white?

 

[Chris888]

it is usually a brown to black colour but mostly tan brown

 

[helio lucas]

is your yz 250 having some digest problems? after eating all the plugs... ;)

if every thing is in good condition: stator, reeds, carb and vent tubes clean, good compression and good seals
then i would bet on jetting
maybe you have just one circuit very rich...
http://justkdx.dirtrider.net/carbtuning.html

to check if every thing is in good condition you will need a service manual...

 

[kaboom1371]

sounds like your jetting is close to where it should be. what are you mixing your fuel at?

 

[SpDyKen]

Fuel mix does not foul plugs. You can run anything from 16:1 to 100:1, and have your bike run great, when properly jetted. (You simply get better, or worse, lubrication, as a result; you decide.)

Only two possibilities, IMHO. Either a mechanical problem or a jetting problem. You need to eliminate any question about mechanical problems before spending time jetting.

 

[SpDyKen]

sounds like your jetting is close to where it should be.

I respectfully disagree.

 

[whenfoxforks-ruled]

You need to perform a leak down test. You need to rule out the crank seal condition. The right seal can leak trans fluid in at the worst time. How does your top end measure, check piston skirt clearance and new rings. Check the thrust washer clearance on the big end of the rod and main bearings. How are the reeds, silencer packing, dents in the pipe, intake sealed up good, air filter and boot to carb? Then jet it. That is why everyone wants to blame it on the oil ratio, its easier?

 

[bwood]

Fuel mix does not foul plugs. You can run anything from 16:1 to 100:1, and have your bike run great, when properly jetted. (You simply get better, or worse, lubrication, as a result; you decide.)

Only two possibilities, IMHO. Either a mechanical problem or a jetting problem. You need to eliminate any question about mechanical problems before spending time jetting.

+1

Do a search for this over the last month or two, it seems like this topic has been getting beat to death.

Here are some of the one that stick out:
http://www.dirtrider.net/forums3/showthread.php?t=168787&page=1&pp=10

http://www.dirtrider.net/forums3/showthread.php?t=168459

 

[2strokerfun]

Easiest method to fix this: rebuild the top and lower ends, of course including seals, and either buy a new replacement carb, or buy new jet needle and needle jet, float needle and seat, and then jet your carb perfectly.
Your bike is long on the tooth. They do this. Doesn't mean you have an insurmountable problem, just a "where do we start" problem. Do the above and the problem should be fixed for another six years or so when you have to do it again.
Good Luck!!

 

[Chris888]

hey thanks for all the replies.

i think the jetting is all ok as the spark plug is a tan brown colour how it should and it runs perfectly fine like it always has, the plug just all of a sudden dies after a few rides. the bike still has good compression but ill probs rebuild topend soon. last rebuild was about 7monts ago. could it be because of reeds?? they didnt look the best wen i checked them. or just sumthing to do with resistance to the spark plug from all the stator and coil and stuff making the spark plug work harder and die.

 

[2strokerfun]

I'd sure start with the reeds, if they are worn. And it doesn't cost anything to check your resistance--unless you have the electrical ability of a caveman, like me.

 

[whenfoxforks-ruled]

A bad float valve can do it also, it is not as dramatic though. The seat and valve should be changed yearly? The reeds are good or bad? It will make a difference in throttle responce. Overall, I suppose it could load up the engine with excess fuel? Is the plug tan everywhere? No black on the threads? 7 months on a 250 rebuild? How many hours is that?

 

[Chris888]

yeah ive checked the resistance of just the plug for the spark plug and it was bad so im gunna put a new plug onto the coil lead.
also try get sum boyesen reeds or sumthin.

also i only bought the bike about 1 month ago and the guy i bought it off said it was rebuilt 6 months ago. ive done probs 10-20 hrs
but still good compression and every thing and piston looks fine.

 

[Chris888]

also the plug is tan over all of the tip and a little bit of wet black at end of thread

 

[oldguy]

I would be willing to bet your carb float is to blame. Either set wrong or just worn out. Often the float will get a pinhole leak or just supersaturated with fuel and fails to float to the proper level thus not shutting off the fuel flow. The excess fuel builds up in the crank and then fouls out the plug. Your statement that the plug is wet shows excess fuel in the chamber.

You also could have a worn float needle or seat which would also do the same but my first look would be at the float itself.

I had the same problem with a 92 yz250 a couple years back. The bike was jetted perfectly but ate plugs. I would run it all day for a couple days then go to the track start it up, get half a lap in and the plug would be fouled. Change the plug and be good for a couple more days

 

[GarbatyA4]

I would be willing to bet your carb float is to blame. Either set wrong or just worn out. Often the float will get a pinhole leak or just supersaturated with fuel and fails to float to the proper level thus not shutting off the fuel flow. The excess fuel builds up in the crank and then fouls out the plug. Your statement that the plug is wet shows excess fuel in the chamber.

You also could have a worn float needle or seat which would also do the same but my first look would be at the float itself.

I had the same problem with a 92 yz250 a couple years back. The bike was jetted perfectly but ate plugs. I would run it all day for a couple days then go to the track start it up, get half a lap in and the plug would be fouled. Change the plug and be good for a couple more days

So.. What you're saying is that the float is actually sinking providing that there is a hole in it?

 

[mgouras]

So.. What you're saying is that the float is actually sinking providing that there is a hole in it?

If the float fails to work because it is saturated, it "sinks" in the gas. These means the float valve remains open even though the float chamber is already full of gas. The excess richens the mixure while you ride. when the engine is not running the fuel can just overflow into your cylinder.
A bad float valve can have a similar effect.

 

[Chris888]

ok thanks heaps ill have to go through the carby then and check the float.at least its not the electricals.

 

[Chris888]

bike seems to be all good now. i repacked the muffler and has been fine since..

 

[whenfoxforks-ruled]

Good to hear! How important are mufflers on 2 strokes? It is just like tuning the carb.