85 250CR cutting out at full throttle

Right Well I'll keep the level where it's at for now, seems to work so far. I'll have to ride it a bit more before I can judge anything properly.
It's not flooding any more (always used to spit a little bit every now and then) and at least on my short test ride didn't run out of fuel in top gear. But have to keep it pinned for a bit longer to tell for sure I just threw on a larger front sprocket so I can maybe keep it at full throttle in top gear for a bit without overrevving.

Hmmm, mechanic is logic !
But we must test to know how it works.
we learn everyday from our errors.

I remember that I tried to adjust my carb on my 400 wr in every ways,
until I realize that my red sem ignition coil was for 4stroke.
I thought an ignition was the same on 2 str or 4 str.
I install a 2 stroke ( blue or black) and it works perfectly.

what oil mix are u using? that can be used to fine tune the jetting as less or more oil in the fuel can effectively richen or lean each "charge" out. if you want to go higher than 40: 1 make sure you use a top quality synthetic 2/ oil like motorex. im running motorex at 75 and 100: 1 and I have the needle clip probably 1 notch too lean but the bike runs super smooth down low and onto the band and I ride it appropriately. desert ride coming up so I will richen the needle for that! enjoy your next ride.

I'm dragging up this old thread because 3 years later I finally found the actual reason for the misfires: the spark plug gap!
I've never had to bother measuring the gap on any of the dozens of other 2-strokes I have so it never crossed my mind to check it. As it turns out, the B10ES plug was the only one that worked out of the 10+ plugs I tested, because it was the only plug that had a 0.5 mm gap out of the box whereas all other had 0.7-0.8mm. I finally got fed up with fouling plugs out in the woods and decided to look into it again and this is the result. I'm now running a B8ES gapped to just over 0.4 mm and it seems to work just as good or better than the B10ES and with no fouling so far

There's still the occasional misfiring period though, which was there with the B10ES too. I'm not sure what's going on there but it might be a jetting issue as I've just replaced the carb. Same behavior as in the video I posted earlier in this thread except it seems to clean up after 10-15 seconds. It happened 3 times during an hour ride, and only after riding very slowly for a bit. But other than that, I can now finally ride with normal plugs.
I figured I'd update this thread because maybe it helps someone else who finds this thread when searching for symptoms.

go for an ev or racing plug. i had a plain jane in my kato and noticed some farting occasionaly and ol mate said to go racing plug and it cleaned up entirely.

I'm waiting for some B8EGVs to arrive, can't get them anywhere nearby so had to order online. I'm pretty curious to see if that will make a difference.

One of the first things that should have been checked is the air filter element. A bad element will give you an over rich fuel air ratio and has you looking at everything else

I did most of the testing without the filter because I first thought it was a jetting problem and the carb was coming on and off all the time. Didn't want to stretch the intake boot more times than needed. So if anything it should've been running lean.

I just replaced the intake and carb last week and jetting seems to be dialed in pretty close to where it should be but probably need to raise the clip one position. But I'll wait for the new plug before I do any more. Given how picky this ignition is about its plugs any small changes I do now might be a wasted effort when I change the plug.

I had an interesting experience with a 1972 Yamaha AT2 that a friend brought to me because of a carb problem. I cleaned the carb,checked the ignition to make sure that the timing was correct. I checked everything I could think of to no avail. I called my friend and told him I could not find the problem. He told me to keep it and give him 5 dollars for it when I saw him at school. I got all the loose parts together and found the airbox. I decided to mount the airbox and try running it again. The upshot was that it needed the airbox for enough resistance to open the reedvalve. Without the airbox it would idle smoothly but would bog badly. With the airbox it ran perfectly. I offered it back to my friend, but he said he didn't want it anymore So my father let me relace the rear wheel then made me sell it to him for what I had into it so he could give it to my younger brother.

I did test both with and without but it did not seem to make any difference in my case so I just continued without it while trying to get it to work.

Anyway.. Here I thought my problems were over, but today was a disaster. I got stuck in some deep mud and I blame the ignition. I was trying to make my way through some deep mud, lots of throttle and plow my way through pretty much, when it started misfiring again, lost power and momentum, stalled and I got stuck.. With mud up to the axles. Took me an hour to get out of there mostly because once I did get it going it wouldn't rev high enough to get out of there by itself.
So today lesson was don't go in deep mud until I'm sure that the ignition woes are completely gone.

What's interesting is that it never recovered from this misfire, and it had been running perfectly fine until that point. Afterwards I tried cleaning it out by riding nice flat roads for a couple of km but it just wouldn't stop misfiring. And when I got home and shut if off, it wouldn't start again. The plug was light brown and dry, definitely didn't look like it was running rich.
So there seems to be more to this than just the plug.
It's happened a couple of times now that the misfiring started after some really wet patches. It misfires when everything is bone dry as well but water definitely doesn't seem to make things better.

I'm starting to get really sick of it, I just want to go out and ride but I don't dare to go more than a couple km now because I never know when the ignition is gonna leave me stranded.
Are there any other ignition systems that will bolt on?
I've been thinking of getting this but I would like a heavier flywheel http://hpi.be/item.php?item=068K177
Any suggestions?

I have checked for leaks and after replacing the intake I can't find any. It also runs fine on low rpm even when it's in misfire mode. Base gasket or seal leaks should have a much bigger impact on low rpms than on higher. The misfires don't start until it starts to get on the pipe.
I think the spark is just too weak. It's probably a weak coil or something but SEM parts are expensive and tbh I've lost all confidence in it already so I'm not going to spend $100+ on a used coil in the hope that it will fix it.
I've just ordered the HPI ignition with lighting (external flywheel) and an extra flywheel weight. We'll see how that works but it can't be much worse than how its going now.
If the EGV plug magically fixes the ignition I'll put the HPI on my 400WR instead which is completely without spark (also SEM..), so no loss either way.

the sem units are junk. the motoplats are much more reliable but still old..mine have all seemed to behave well.

have you checked the coil, sounds like a field failure
take the coil off test the ohms low and high voltage
also replace the plug cap with an NGK one
clean all contacts and use dielectric silicone grease to prevent moisture

What kind of Ohm values should I be getting if the coil is okay?
I have tried 2 different coils and neither of them seemed to make any improvement over the other but I will go and measure some Ohm values.

Re: plug cap, just making sure here so I'm not missing something obvious, it's supposed a non-resistor cap for best result, right?

if you go online and look, pretty much all older type coils have the same valuee
on the cap yes I have always used non-resistor
back to the coil, funny but you could have two marginal coils
ignitions hate heat and vibration, dirt bikes provide both

I have read through this complete thread. Do yourself a favor and replace the complete SEM ignition with a good motoplat or new aftermarket unit so you can go out riding and having fun with your Husky. What you are experiencing is typical for a SEM unit that is bad. The symptoms come and go, but usually show up when good and hot. I did have a SEM coil that would sputter when cold and would start working after riding just 10 minutes. Rarely do they completely die, which would be easy to diagnose. Someday maybe I will tell the story about how one failing SEM coil caused me to brake my wrist up in the mountains on a single track trail miles and miles away from everywhere.

Marty

Hi Bill,
Do you use your chainsaw under water?
We expect the dirt bike magnetos to keep working for 30 to 40 years, even when we do river crossings and the covers are poorly sealed and allow moisture in.

I sat down and invented a way around low output SEM's and crook CDI's and listed it here ages ago. Total cost ????? about $30 Australian.

I remember that, could you post a link to your page?

I remember when I bought my used 95, WR360, after I got it home, I decided to take the magneto

cover off, and at least half a cup of water came out.

How was it even running?

some magneto run full of oil, by design. so liquid wont really impair function...water would certainly be bad tho..