Update. Got some stuff done and have been chasing a gremlin.
Did the 5th oil change since rebuild.
3940 kms/2450 miles on rebuild
1609 kms/1000 miles since last oil change
Some small bits of plastic in sump and side filters picked out with a toothpick. Looks like some old fibrous bits still being washed out and some new small plastic bits, probably from the starter gear.
Some really fine filings on sump magnets. Oil was nice and clear though so I guess whatever was there has been flushed out.
New points installed, swapped in my old OEM condenser pack. Dwell set 22.5° - 22.9°, timing good.
While I had the top mount off to install my other condenser pack I decided to check head bolts torque again. This is the 8th retorque - bolts are still holding, acorns took maybe 5° Probably didn’t need to bother but it was interesting.
One intake valve had opened up a bit to about .006” from it's .003” setting.
Took it for a test ride last Sunday and it took a little longer to warm her up at 60°F versus the last 110°F with humidex day. Once warmed up still getting some intermittent stumble / break down at 3500 - 4000.
I know the first reaction is to suspect a carb issue but I know that's not it and it has to be an ignition gremlin. Also I would think if it was a carb problem it would be consistent and not sporadic, must be an ignition problem.
Gotta give a
HUGE……
to
@YamadudeXS650C
He said he had experienced the exact same stumble problem on his 75 and it was a weak connection in an ignition connection in his headlight bucket.
I was in there a week or so ago checking all connections so figured I'd start under the tank first.
Huh...........just about all the ignition bullet connectors were sloppy. I guess years of unplugging and plugging has opened up the female bullet connectors.
A couple in the R/W ignition wire came out of the female double bullet connector without any effort whatsoever.
Crimped all the connectors a little tighter with a small pair of needle nose pliers.
Only had time for about a 15 minute test ride today, no stumble! Cautiously optimistic, but gonna pull the headlight bucket apart again as well just to make sure.
I guess the vibes at 3500-4000 rpm we're just right to cause one of these to loosen up enough and cause an intermittent connection.
Dude, You sir are a genius! I owe ya one buddy, next beer's on me!