Ahh right, didnt see that. Good investigating there & something to remember. Look forward to the update when crank key arrives. Great satisfaction in descovering that sort of fault.

My new flywheel key came in and I was surprised to see that it came right out of the Yamaha bag with a center punch dimple on one side exactly how MarkT had commented as a cure for loose fitting keys.

I lapped the flywheel to the tapered shaft to ensure a clean and proper fit. I borrowed my sons TUSK flywheel tool to hold it in position while I torqued it to 70 ft/lbs even though spec said 48-52. I then put my pneumatic impact on it and gave it another 1/8 of a turn. So I guarantee it’s not slipping again…..

Besides the new key, I also re-soldered a loose blue wire that I had found on my condenser. I also swapped out one of the floats because it seemed to hang up on its guide pin every time I inverted the bowl. One float would fall right out but the other had to be touched in order to fall off its guide pin. The spare float seemed to pass the inverted drop test every time. I also installed a new Yamaha petcock.

These first two videos had me thinking problem solved! It seemed to be idling properly again.
youtube.com/shorts/1AgZvltCAhY?feature=shared


Then I took it for a ride………
Please note that everything is ass-backwards because I simply put my phone in selfy-mode and clip it to my shirt. You’re best just to listen and not concern yourself with the view. Lol
youtube.com/shorts/XMG0wWvcu4Q?feature=shared

This long final video shows an F’d up idling condition after the test ride. About halfway thru the video I backed down the idle speed screw assuming it would stall, but it just continued to stutter and stammer.
youtube.com/shorts/u_sIPBwEnHc?feature=shared

I did go back out there and put a brand new out of the box 4054 plug gapped to 20 thou. No improvement. So at this point I will begin chipping away at all the other suggestions that you guys gave me. I will update this thread with my progress, or lack of….

I've come to accept the fact that when you own over a dozen of these old enduros it is quite likely you will always have one on the lift as you scratch your head and work thru this sort of thing.

EDIT
No need to comment on top end noise. My cell phone brings that to the ear with every video I take. Not saying they don’t have some top end noise. But my cell phone leaves you thinking it’s about to implode….

It's still electrical when it does the misfire which is why I wasn't convinced the flywheel key partly damaged was the total answer & why i mentioned about loose windings on the source coil or lighting coils seeing the source earth's through the lighting coils. That said, if it was yours with the unsoldered wire on condenser I was sure you'd found it. So back to the source coil. All pretty unlikely though as they're pretty robust. Try this, plug mag black direct to the plug coil to eliminate bad connections through the system. Be sure the black itself is ok from condenser to the top plus the crimping of the wire plug is good. Look I'm the dark for anything erratic in the mag while running, like one flash per instant miss or reverse of that an instant "non small spark" of the points at the moment of misfire. Points will give a constant minor spark to see in the dark but I'm talking about extremes either way but not easy to see as just gives one misfire then ok until it does it again. Motor mount bolts tight 'as', or run an earth from motor to the frame or even up to the coil mount. I listened with headphones on & watched the tachometer needle & can see it falter instantly per misfire. Plug cap eliminated or changed. I've lost my way now I've confused you all. One more--if yours has the short black on the mag loom for checking timing or whatever it's for-- does it Y into the loom or just soldered on side on. If a Y it could be a bad 3 way joint there--I'm clutching at straws.

Tomorrow’s test will be swapping the ignition coil, wire, and cap with a complete spare that I have on hand. As I had mentioned in one of my earlier posts, the plug cap came off the coil wire as I removed it from the spark plug. So I trimmed a bit off the cable and reinserted the cap on the wire. So it may still be the culprit.  Stay Tuned!

EDIT
The coil windings all looked good with none being loose. I thought I took a photo, but evidently not……

Just throwing out ideas: (though it does sound electrical)

* make sure atmospheric vent lines to carb are not blocked. Also, center hole at bottom of carb airbox side open,
though as stated appears electronic.
* Coil resistance check
* Cap resistance check
* Hook up a timing light just to watch it blink on and off looking for constant working circuit.
* if condenser was loose and ran like this, then hooked up made no difference.
possibly grounded condenser, no dwell time. Test for open and grounded circuit.

* The unthinkable ->> Is main or throttle on/off switch momentarily GROUNDED water, corrosion, etc??

One thing I’ll add to the mix. It pulls as hard as normal for wide open throttle. It’s getting to wide open throttle that stutters & stammers.

It appears as though it was the ignition coil or maybe just the cap and plug wire, but it runs good again. When I recently traded my SCR950 for an RT3, part of the deal was a $400 line of credit on used parts. I already spent the entire $400 and in that batch for parts was an ignition coil for a DT250 which I bought for $7.50 total. No shipping because I pick up my parts orders from Flashback Enterprises.

Here is another idle video where I again backed out the idle speed screw half way thru the video to see if it would stall.
youtube.com/shorts/m05Qf1mGXno?feature=shared

Here is another ride around the yard video with the phone on selfy-mode.


This weekend I will take it out on the back roads and give a good run of 50 to 60 miles just to be sure it is all sorted out.

Thank you all for your patience and your many suggestions. I had established a pecking order as to how I was going to individually try all the suggestions and got lucky with an early winner. (I hope)
 

Glad you got your bike running well and I keep trying to get a glimpse of that Chevelle.

Thanks!

Here’s a good glimpse of my Malibu. I’ve owned it for 23 years. Typical story. I had one when I graduated high school. You could say I’m stuck in the 70s. lol 

imgur.com/a/zFbbHQc

Good looking car. Thanks for the viewing. 70s living? Any day!