85 250CR cutting out at full throttle

Just watched the video and it sure sounds electrical.
Go back to your standard jetting , if it's ported radical then it should be using MORE fuel not less so the main should be bigger not smaller.
If it does run clean you might POP it with that small jet.
Looking at your stator there seems to be a lot of corrosion on the laminates front and back and this is where the system earths.
Clean it up and try again.
The flywheel looks ok for the moment , the minor damage is on the generator part [ the ignition is the smaller ring of magnets down inside]
That flywheel and stator are WR you know , but handy if you want to do night mx.

i would say that the squish is way tight dont have the specs but would say min 1mm perhaps the cylinder was mated to a modified head originally, you could try a thicker base gasket to bringit inline.

Any idea where I might get one of those?

It should need more fuel, yes. But if more air flows over it, the same jet will give more fuel So it's not at all certain that it would need a larger jet.
The other stator looks better and resistance between the black wire from the stator and the frame was close to 0 Ohm, so I think that means there should be good earth everywhere on the bike? (from the black wire to the stator -> to the plate -> to the engine -> to the frame).
I figured they might be WR parts. But the bike and engine are in fact CR. Altho all parts considered it's pretty much 50/50 CR/WR by now I think

Here's another video from 3 or 4 years ago when I had a very similar problem on my racing sled in progress. Everybody including me was convinced it was an ignition problem. But after replacing everything there, jetting up and down, and still not getting anywhere, replacing the carb is what made it go. It reved 11k rpm without any hesitations or misfires before I even started on the jetting.


Same everything, just a different carb thrown on:



I may have to try just putting a VM38 on it? I should have a couple of them laying around.

I'm not ruling out an electrical problem but having just replaced everything, I think it's getting pretty unlikely that it is ignition related.

Weird, I love these problems .. not for your sake though Nico, just puts the grey matter into full gear trying to come up with that answer.

a mate had a cr250 years ago that had a similar drama and it was the coil to frame earth giving problems. the bolts had come loose

I moved the needle clip to the bottom position (leaning it out as it's definitely still getting too much fuel), didn't help.
Next I replaced the needle valve and seat with brand new ones (again), this actually seemed to have a little bit of an effect, the behavior became less eratttic, more constant. Right now it's more like a 4-stroke bog and less of a random cutoff. That was good, at least something is happening. Now I'll take the carb off again and re-adjust the float level, I think it might be a bit too high still.
If that's not it, I have one more idea, which is that the exhaust may be filled with fuel..

Re: earth, all connections check out ok but what I don't really get is why the coil doesn't seem to be earthed by anything more than the black wire? The housing is all plastic and it's not earthing at the coil itself from what I can see.

The float level was a bit high still, I've now set it to 19 mm. I am not sure where to measure though, the owners manual is a bit unclear. I measured at the point where the pins on the floats push against the fork. It was about 17 before measured at that spot.
Anyway, not entirely unexpected, no change....
Next I removed the silencer, again no change. Then removed the entire exhaust system and except for it reving more in general, it was still cutting out. The exhaust was oily/wet but not filled with fuel.

I did find some new cracks in the intake rubber, so that's what I'm trying to fix now. Altho it can't have leaked all that much, it idles very nice and normally air leaks should be most noticable on low rpm, not high. But it could be that the leaking gets worse when the engine starts vibrating more, and no matter what a cracked intake isn't helping, so fixing it.

Took out the reeds too and they still looked good. Very wet though, which I guess might be a hint?


I just realized that by lowering the clip I actually made it run richer....
That means I have to move it again.. NOT my favorite job!

bloody needle clips, lucky its not a moto villa, you have to remove the head stay to tilt the carb

earthing of the engine mounts is a hidden one as well. the frame isn't freshly powder coated is it?

you've got to be running this this issue down by know, there cant be much left!

years ago a mate had a sss Datsun with twin carbs and it started playing up.

at 5000 revs it would suddenly ping madly and blow clouds of smoke. wouldn't hold an idle either.

(he was a mechanic) after 6 months of carby kits, carb experts, replaced electricals, and everyone's "expert" help, a drunk was watching the action one Saturday afternoon and noticed the pcv hose "move"..... $3.50 and all was well. It had a split and was causing a vacuum leak at idle. At full revs it would "main jet" the sump oil into the carbs.

just a helpful story keep your enthusiasm up. its probably a $3.50 part or a $350 one!

It's a ridiculously bad construction anyway, why couldn't they just do it like on the VM carbs.. On a VM it takes a minute at most, it took me the better part of an hour on this TM! The plastic cover thing that keeps the throttle wire in place is almost impossible to get out, and when putting it back together you need about 5 hands to hold everything in place while trying to get the screws at the bottom of the slide in. Also why couldn't they have an idle screw like every other carb? Having to adjust idle speed using the throttle sucks and in my opinion is dangerous. Can't have any play in the wire making it too sensitive.
I was pretty annoyed at all of this, and I blame me moving the clip in the wrong direction on that

riiiiggghht...its a job for your friendly trained octopus,( if you can get him from the pools people!)

wasnt this bike supposed to have a vm on it from the factory?

Man I just listened to that clip again and I gotta say that's a dud spark plug if ever I heard one. Are you using racing plugs? my kato 250 would do that if I opened it up when dead cold. I couldn't get on the main jet until it was warmed up a tad and a hard twist would kill the plug and that's what I would have. it would short out down the insulator but still show a spark on the lead when kicked. but load it in the engine with some throttle and bubow... no go. sounds exactly like that.

also, using a std ngk plug would see it badly missing in the higher rev range after some use, put in a platinum egv and all good.

I see you have an SEM ....they are renown for issues, particularly if they get dirty inside the cover. they get hot and fritz... usually totally though. ive never heard of one causing those issues.

its a bugg*er allright....

I've tried all kinds of plugs, from B6 to B9 with and without resistor, new and used. The bike had a B8ES in it when I got it and ran fine with that, so that's what I have in it now.
I do have some BR10 racing plugs but haven't tested yet, I assume they will foul instantly with it getting this much fuel.
What plug do you recommend?

you could try a timing light connected to a battery and a drill attached to the fly wheel spin it up and see if the timing signal is consistant at higher rpm also can check spark strength at higher rpms this way, might help rule out ignition problems.

It works!!

This morning I first moved the clip to the top position. Using a welding clamp as an extra hand made the clip moving process a lot quicker. Started it, and still the same behavior, if anything it seemed to have gotten worse because it seemed to be cutting out at lower rpm than before.
Deep sigh...
But I removed the plug for inspection. For the first time it was neither dark black nor wet, so I figured at least the jetting was going in the right direction. I put in a new B10ES plug (which is supposed to be the equivalent of the stock Bosch W2C plug) just for the heck of it, and what do you know... It reved right up! I was shocked and actually couldn't believe it. I let it warm up for a bit and took a quick test ride. It runs great through all gears without any hesitation, and goes like a stabbed rat when it gets in the powerband

I think the main problem has been the jetting all along, and because of that it has probably been running so rich that whatever plug I put in was instantly killed, making it an ignition problem as well. I know these plugs can be very sensitive in combination with some ignition systems, and apparently SEM is one of them. I will try different plugs again now, just to get to the bottom of this. I didn't measure, but I noticed that the electrode gap on the B10 plug was quite a bit less than on the B8.

To make sure I don't kill the ignition by using the wrong stuff now, is the plug cap supposed to be of resistor or non-resistor type? The current one has a 1 kOhm resistor.

thank Christ for that, now richen it 1 clip at a time until it starts to play up and go back 1 clip. rich on the needle will make the transition to main jet hit a bit harder. Porting may be doing that as well. note unleaded fuels wont color the plug properly. if you can get some av gas you could do some plug chops to check the plug color at 1/3, 1/2 and full throttle. the needle jet is usually the culprit for seizures.

The clip was in the middle position from the start, so I think the most I can go is one down. But will try for sure, the last thing I need now is a seizure.. The spec sheet says the needle should be in the 4th position, but doesn't specify which side to count from. I assume it's from the top. But then it also it says needle position, not clip position, confusing me even more...

Another question to avoid seizures then.. Where on the fork arms am I supposed to measure the float level? The arm ends up far from horizontal so the 17-19 mm value kind of depends on where on the arm it's measured. If it's measured at the tip of the arm that gives a higher float level than measured at the center where the floats push against it.

Excellent - glad you have been able to sort it.
T

looks straight forward to me. I adjust them till they don't flood fuel out the overflow and don't use all the fuel in the bowl before it can be filled by the needle and seat while flat out in top gear... sorry.....