85 250CR cutting out at full throttle

My 85 is driving me nuts! It cuts out every time I hit full throttle. At part throttle it gets a lot better but obviously I need it to work at full throttle..

The carb is stock and ran fine with a stock cylinder and stock jetting. The trouble came when I had a little accident with the cylinder (sleeve cracked) and had to replace it. The cylinder I bought has had quite a bit of porting done to it, and stock jetting won't work any more. I've got the low speed pretty much under control now but can't figure out what I'm doing wrong with the rest. It idles fine, is not hard started, so I think it's just a matter of jetting.. So far I've changed the idle jet, air screw setting, and main jet, the rest is stock.
A stock main jet is 390, which didn't work, cutting out. I figured it was getting too little fuel so I put in a 440 for starters. That made things worse, still cutting out and when it finally did stop cutting out it 4-stroked like there was no tomorrow. So, less fuel then. I've now tested everything from 440 down to 300 with steps of 20 and it's still behaving pretty much the same as it did with 390. The plug has been anywhere between black and wet but I'm not sure if that's anything to go by as the bike is pretty much unridable like this so I can't do a full throttle run.

So, the symptoms:
Cutting out when accelerating at full throttle.
Cuts out when giving it full throttle in neutral as well, sound a bit like a rev limiter/launch control.
Behavior seems to get a bit better in higher gears.
If I'm careful enough on the throttle, it'll cruise fine.
Idle is fine.
Opening choke doesn't change the behavior, only makes it want to die at idle.
In neutral, if I close the fuel valve and give it full throttle, it'll eventually rev up.

To me this suggests it's still getting too much fuel. But having to go down from 390 to lower than 300 seems a bit much?

I don't have any smaller jets but I'll try and get hold of some so I can try and see what happens, but I just wanted some input. All ideas are welcome!

all indications point to being rich on top, double check reeds, float level ,exhaust packing and compression then try smaller mains.whats your squish set at.

Tough description.
Cutting out .
Does it stop ? or if you shut the throttle a bit will it run again.
A rev limiter sounds like 4 stroking [ on off on off on off ------>]
Put the 390 jet back in and look elsewhere.
If it revs up with the fuel off then it's getting too much fuel .
Check your float height and needle/seat [ leaking ? ]
Run it with the air filter off.
Are you using unleaded fuel and if so ethanol?
I have had trouble reading plugs since the demise of leaded fuel.
Has the 85 cr got a reed valve setup because if it hasn't and the porting is too radical or the piston to bore clearance is too big then you can get fuel blowing back through the carb into the filter [ leading to ever increasing richness ],if it has then are the reeds seating because the result will be the same.
We've been spoilt with reed valves and forget what a pain pre reed 2/st were.
Good luck
Ps. I have a 88/Wr400 here that sounds like it's hitting the rev limiter but it hasn't got one and replacing everything hasn't helped [ different carb , different ignition ] .
Previous owner gave up and sold it.

I've changed the needle valve and seat today which made a very small improvement. Also tried lowering the float level a bit, and I could actually do a short full throttle run. It still cut out all the time but at least eventually it got going.
I had a friend take a test ride and his opinion was that the cutting out was happening way too fast for it to be jetting related, he thinks it's an ignition problem.

The bike has been vibrating pretty bad since I put on this new cylinder and head and I wonder if it might be cutting out because of the vibrations, it gets pretty bad when the revs go up. Either something happened to the crank when the sleeve cracked (bits were in the crankcase) or I didn't tighten the motor hard enough when I put it back in the frame.

I replaced the packing at the end of last season, will have to check compression and squish.

It's too late over here to start it but I'll shoot a video tomorrow. It cuts out for a fraction of a second, about 2-3 times per second. It eventually will rev up even if I keep it at full throttle.
I inspected the reeds when I replaced the cylinder, they looked good. Our premium fuel has no ethanol mixed in and I think lead has been banned since the 80s here

ok, bits went into the crank...last time something like this happened to me it toasted all the ends of the reed valve. I would pull it out and have a good look for cracks, broken ends etc.

its symptoms are of an engine clearly getting way too much fuel on the main jet.. and as the jetting is doing nothing and the needle and seat is good. check the floats for a hole in one, lower the float some more. ...

WAIT these issues would cause probs at all revs.... hmmm..

maybe your ignition system is failing when the load of full throttle comes on. new plug?? try another...check the plug lead and cap.. a common bug. check all wiring and connectors. remember.... you've pulled it apart, whats been changed or not reinstalled properly....

Here's a vid of how it behaves. First half of the vid is with a 380 main jet, second half with a 340. It's a bit snappier with the 340 and the problem becomes less of a 4-stroking bog and more of a cutout.
It behaves exactly like this when riding too, too much throttle and it cuts out.

Judge for yourselves


(PS It's a CR with WR plastics)

The replacement cylinder came with reeds, so I used those The old blades did have some damage at the ends.
I hadn't thought of the floats yet, but as you say that should affect things even at idle and low rpm.
I've tried a couple of different plugs, different temps and both new and used and the behavior didn't change between them.
But I think the ignition system is the main suspect at the moment. Something is causing it to miss at higher rpms, might be a bad coil or a bad connection somewhere that gets worse when the bike starts vibrating more. I haven't touched the ignition system though, but maybe the parts were bad already but still good enough to work with stock but maybe porting and possible head mods were too much for it.

May sound daft .... have you checked the electrical earths ? If your engine mounting bolts are loose, coil loose or the spare ground (if you have one), then the more it vibrates the more it will break that earth. If it is vibrating very badly I would be looking at the flywheel to check a magnet has not detached, and the then the crank.
I had a very slightly bent flywheel stub a few years ago. I didn't know better and just lived with it. Since we replaced it last time the motor was in pieces, it is now as smooth as ever.

Nothing sounds daft at the moment, I'll go over it I checked the engine mounts this morning, tightened the rear bolt a bit more, but at the front all seemed ok. Vibration wise I didn't notice any difference after tightening the rear.

Just checked compression and squish.. Compression was 185 psi, cold engine. Squish was only 0.6 mm, which seems a bit low.. I don't know what it's supposed to be but I was expecting it to be more, around 0.8-0.9 at least. Can anyone tell me what a good value would be? I could add a thin second base gasket to raise the cylinder and head a bit.

I've just remembered I've once had a very similar problem on one of my snowmobiles. No matter how much I changed the jetting, it would cut out at high rpm, very much like a rev limiter. I replaced the entire ignition system one part at a time to no avail. Finally put on a different carb, and it ran awesome. A friend had had the same problem with the same model carb (TM42 off a different snowmobile) and he never got it to work either. His theory was that it needed a very specific air box in order to work properly. All my testing now is without the air box (for easier access to the carb), so I'll have to try and put it back on just to rule that out.

I just tested with the airbox and filter in place, no difference.

Then started looking at the ignition, I did get a spare stator and flywheel with the bike so I have some parts to exchange. Don't have a spare coil though.

Here's what things looked like.

Some minor damage on the flywheel, not sure if this would affect anything?


Some paint has rubbed off the stator, on the sides.


But I didn't like what I found on the back side. There's silicone where the wires go into the stator. I don't know if the insulation might be bad there, but someone must've put it there for a reason.


Also things were pretty oily there, which can't be good. Doesn't seem to have come from the seal because it was nice and clean there. But the cup that the seal sits in wasn't tightened a whole lot, I could easily turn the screws in another half turn. So I'm guessing it's leaked there.

I've now cleaned everything and will see if I can find some values to measure the components and see if they are okay.

Ohm measurements..

Stator:
Red to green 1.68 kOhm.
Red to black 1.66 kOhm.
Green to black 22.5 Ohm.
According to some values I found, these values seem okay.

Coil:
Black to green 300 kOhm.
All other combinations no contact.
Can't find any reference values for the coil, so I have no idea wether this is good or bad.

Plug wire and cap about 1 kOhm.

I did the same measurements on my friends 86 400WR, Stator gave me pretty much the same values, only slightly lower. The coil was where it got interesting, his coil had about 800 kOhm between the red and green wires, whereas mine doesn't seem to have any contact.
I'm going to borrow his coil and see where that gets me

I have just replaced the entire ignition system one part at a time, from flywheel to plug. Still acting the same. So unless it's being extremely picky about the timing, I guess that pretty much rules out the ignition. I've tried different timings as well with no difference. Ground seems fine too.

Did you check the cranks? To test the seal on the transmission side. You can do a water bottle test by sticking the vent tube for the transmission in a bottle of water and see if it tries to pull the water up the tube. Make sure the tube is clean and sealed good to the tranny.

You said you got spare electrical parts with the bike. That tells you something that the previous owner may know about? One thing that I can say to check and it sounds dumm but it happened to me , is to make sure your spark plug wire is only touching plastic stuff. Mine was rubbing the radiator one time and it would not want to stay running , so I zip tied it away from all metal stuff and now it works just fine. good luck.

I think I read all of the above posts and saw no mention of moving the clip to lower the needle. Maybe you are never getting enough rpms to actually get onto the main jet.

The bike ran fine when I got it, it wasn't until I replaced the cylinder and head that it started acting up. I'm still curious about the stock squish height, does anyone know? The spare electrical parts look like they haven't been used for a very long time, and had a plate for a different bike so I think they were just spares that he picked up at some point. I don't know what bike it was for, the stator itself looked identical, but the plate it was on didn't fit the mounting holes in the bike. Anyway after moving it to the plate that came off the bike, it's on there now and doesn't seem to have made any difference

I will check the clutch side seal using the method you mentioned. It doesn't seem to be taking any oil, but if it does that might explain why it seems to be running rich.
Will also try moving the spark wire around a bit, but I'm using the same kind of wire on other 2-strokes where in some cases it is actually rubbing against the head with no effect.
Will also try moving the clip up and down. I think if anything it needs to go up (leaner) but I'll test both to see if it reacts to it at all. I didn't think it would make much of a difference, when the throttle is wide open I don't think the clip position makes any difference no matter what rpm? Worth a shot though.


I really appreciate all the input!

Put a screen around the main jet,
What it sounds like is the bike vibrates so much it's frothing the fuel around the main, so it's sucking lots of air bubbles along with enough fuel to keep it running.
This is an old trick had to do back in the day.
The old iron shook it's brains out but it causes problems when the resenent frequency gets just right.
Many of the old bikes had this problem some even came with a screen.
Later george

Do you mean one of those plastic "cups" with 4 small holes in it? It already has one of those But I do believe it's gotta have something to do with the vibrations and bubbling or maybe the needle valve acting up.

No it must have a screen like the shape of a small thimble that can be pressed(shaped around) main jet with a closed end on it.
Later george