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[RESOLVED] After a lot of reply posts here and in other locations with great ideas (and some who didn't read it and suggested I do what I already did - you know who you are 
) I got it fixed. I went back to the basics. At times it felt like fuel and at other times it felt like fire. But the change of coil and condenser didn't change anything so I went back to the basics.
I reset this newly rebuilt carb back to factory preliminary spec. I fired the bike up and started dialing it back in. This time it worked. Everything is smooth and it is running strong again at all speeds. Don't know why it didn't happen that way the first two times but it didn't. Some times you just have to be humbled.....
As frustrating as this has been it has also been a great experience in several ways. First, I learned I can reach out and get some great ideas from folks on this forum. Thank you. Second, I relearned that 'suck, squeeze, bang, and blow' has to be balanced with quantity of fuel and timing. Third, I again embraced the concept that critical thinking and understanding the simple systems on these old beasts will get you past most difficulties. When in doubt, go back to the basics. And lastly, looking at, disassembling and assembling adjusting and replacing the parts and systems I was dealing with is the best education you can get (even if you've done it before). Reading about it in a service manual is nothing like doing it. I'll be riding this old girl on The Cross Country Chase later this year and have to be able to repair on the road. These experiences are invaluable.
Thank you all.
[ORIGINAL POST]I’m officially perplexed. 1942 WLA 45 Flathead. Stock setup, 6 volt with an M88 carburetor. Was running great. One day after a 200 mile ride, suddenly it wasn’t running great anymore. Starts and idles just fine. After first starting it, with it sitting in the driveway warming up, it will accelerate smoothly. But, after it’s warm or if you get on it and go for a ride - as you increase throttle it starts to stumble and buck. It won’t pull itself through first gear without kicking and bucking. Back the throttle down again and it will smooth out once more.
Over the couple of months I’ve been chasing this, the behavior has changed somewhat. When this first started happening you could smooth out the higher RPM’s by choking it. That’s not the case now. Choking it now when it starts to stumble will kill it.
Initial thought was intake manifold leak after it warmed up. Intake manifold holds pressure (built a tester) and there is no reaction to stater fluid when misted around the manifold while it’s running.
Checked and adjusted the valves. They were tight but they were adjusted to the correct measurements. No change.
Flushed the tank. Made sure gas line was clear and fuel flowing properly. No change.
Had the carb rebuilt by an expert (it needed it). No change.
Replaced the coil with a new one I had here on the shelf along with plug wires and plugs. No change
Replaced the condenser. No change.
Like I said, it will start and idle just fine. When it is cold with no load on it (sitting in the driveway idling) you can accelerate it smoothly. But let it warm up and put a load on it like accelerating up through first and into second gear and it will start to buck and stumble. Adding choke does not help. When it starts to stumble I also notice just a bit of black smoke from the exhaust pipe that isn’t normal. Back the throttle down and it will smooth out. I’m up for any suggestions. Appreciate your input.
) I got it fixed. I went back to the basics. At times it felt like fuel and at other times it felt like fire. But the change of coil and condenser didn't change anything so I went back to the basics. I reset this newly rebuilt carb back to factory preliminary spec. I fired the bike up and started dialing it back in. This time it worked. Everything is smooth and it is running strong again at all speeds. Don't know why it didn't happen that way the first two times but it didn't. Some times you just have to be humbled.....
As frustrating as this has been it has also been a great experience in several ways. First, I learned I can reach out and get some great ideas from folks on this forum. Thank you. Second, I relearned that 'suck, squeeze, bang, and blow' has to be balanced with quantity of fuel and timing. Third, I again embraced the concept that critical thinking and understanding the simple systems on these old beasts will get you past most difficulties. When in doubt, go back to the basics. And lastly, looking at, disassembling and assembling adjusting and replacing the parts and systems I was dealing with is the best education you can get (even if you've done it before). Reading about it in a service manual is nothing like doing it. I'll be riding this old girl on The Cross Country Chase later this year and have to be able to repair on the road. These experiences are invaluable.
Thank you all.
[ORIGINAL POST]I’m officially perplexed. 1942 WLA 45 Flathead. Stock setup, 6 volt with an M88 carburetor. Was running great. One day after a 200 mile ride, suddenly it wasn’t running great anymore. Starts and idles just fine. After first starting it, with it sitting in the driveway warming up, it will accelerate smoothly. But, after it’s warm or if you get on it and go for a ride - as you increase throttle it starts to stumble and buck. It won’t pull itself through first gear without kicking and bucking. Back the throttle down again and it will smooth out once more.
Over the couple of months I’ve been chasing this, the behavior has changed somewhat. When this first started happening you could smooth out the higher RPM’s by choking it. That’s not the case now. Choking it now when it starts to stumble will kill it.
Initial thought was intake manifold leak after it warmed up. Intake manifold holds pressure (built a tester) and there is no reaction to stater fluid when misted around the manifold while it’s running.
Checked and adjusted the valves. They were tight but they were adjusted to the correct measurements. No change.
Flushed the tank. Made sure gas line was clear and fuel flowing properly. No change.
Had the carb rebuilt by an expert (it needed it). No change.
Replaced the coil with a new one I had here on the shelf along with plug wires and plugs. No change
Replaced the condenser. No change.
Like I said, it will start and idle just fine. When it is cold with no load on it (sitting in the driveway idling) you can accelerate it smoothly. But let it warm up and put a load on it like accelerating up through first and into second gear and it will start to buck and stumble. Adding choke does not help. When it starts to stumble I also notice just a bit of black smoke from the exhaust pipe that isn’t normal. Back the throttle down and it will smooth out. I’m up for any suggestions. Appreciate your input.
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