Yz250 fowling plugs/no idle. No changes to cause the problem.

I am double posting this so I can get feedback from the general public and the Yamaha crowd. Sorry if you see it in the Jetting/carb forum too.

Here is the deal I am out of ideas, so I came here to see if if there is anything I am missing before I start mass tear down.

Problem: out of the blue, nothing has changed my YZ has started fowling plugs at an insane rate. I fowled one yesterday playing in the pasture and 2 today in the parking area headed to the trail. I have not changed anything on the bike and it usually runs perfect. It will not idle at all, if you let go of the throttle it goes straight to dead. I also have an increased amount of smoke from the exhaust at idle and under power.

Machine Specs:

1996 YZ250

recent top end (20 trails hours maybe) has great compression

PC pipe and silencer

Carbon fiber reeds

175 main

42 pilot

NGK BR8ES plug

32:1 ratio

Things I have tried:

new plug ( stock and one heat range warmer)

Complete tear down and cleaning of carb (btw carb cleaner burns like a :prof: when you get it in your eye.)

cleaned the air filter

reeds look good

adjusted mixture screw

attempted to adjust idle speed, but has no effect.

I am open to suggestions, do I need to look toward case seals? Could a tank of bad gas cause all of this? Anything helps and thanks in advance. :smirk::worthy::smirk::worthy:

Are you sure you mixed the oil / gas at the same ratio you always do? Changed type of pre-mix you are using?

Have you checked compression?

Sorry re read your post. Now I feel stupid.

Same brand of premix and same ratio as always. I am going to try a fresh tank of premix tomorrow, I drained the tank while I had it off checking the reeds.

Sorry re read your post. Now I feel stupid.

It happens to even the best of us:thumbsup:.

Right side crank seal.....

Cracked reeds....

How is the spark and compression? Check elec connections, reeds,and carb. Dump the fuel and you should have revealed it by then.

just a thought, but my 99 did that (but not as BaD) and it turned out that some of the jet needle groves had broke and the bronze bushing the jet needle slides in and out was pretty much oval, might want to check that if you havent

Does it do it going down a steep hill??

Iv'e found that the older (two part) kehien carbs will leak in the body after the internal seal is dried and gone. Remove those funny looking torx screws inside and inspect the seal between the carb halves.

812david has a good point too.....

Dose your idle adjust by turning the choke knob like it did on my '95 250?

I bought a 1995 yz250 new in 95, rode it till 99 and it starting the same thing. After cleaning the carb, and rejetting leaner it would detonate out of 1 corner and the foul a plug out of the next. Before i could get it fixed my motor blew. After resleave and a total rebuild it still fouled plugs like crazy. I talked to the people at Carb Parts Warehouse and they said the carb on it was a pj (adjust idle with the choke tower) and they would wear out. I thought that was B.S. but bought a new pwk from them and installed it. All I did was open the air screw about 1/2 turn from where they had it and it ran perfect from then on. If you have a friend with a good carb on thier bike see if they will let you try it to see if it is the carb. That maybe the best way to find out if it is a carb problem.

Well after another complete tear down of the carb, a fresh tank of 91 octane 32:1 premix, my bike is back to normal. It took me forever to get it to start and it smoked like a train for about 5 mins. It finally cleared up, idles great and is back to a normal amount of smoke. I am assuming that I got a bad tank of fuel or there was a piece of trash in the carb I knocked loose in the second cleaning.

Thanks everyone for the help:worthy::smirk:, I was losing my mind trying to figure it out. I never expected a tank of gas to suddenly go bad, but I guess it happens.:smirk:

I also thoroughly cleaned the reeds and cage while I had the carb off, I am wonder if there was maybe a small piece of trash hung in one of the reeds keeping them open.

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