Might be that the carbs are sized such that air velocity at WOT is too low until you're at higher RPM? I've riddden two stroke dirt bikes before where it was a partial throttle game until you got into the power and only then could you use the rest of the throttle. Or maybe those were broken in the same way yours was. They were pretty janky. Just get fuel flow figured out and keep it on the pipe all the time and you'll be fine. 
In reply to mazdeuce - Seth :
That, and no reed valves, and probably whatever the cheapest porting that got the job done was at the time, and a fixed level of ignition advance... there are lots of valid reasons the power curve should look like this: _/
It will run right, once you get it set up. I had several of these things, my first had a front drum brake like yours, the rest were the later models with disc brakes. When properly adjusted, the front drum is effective. Premix effectively leans out the mixture. I used to run a couple of capfuls in the tank, no more, if the oil pump is working, just for luck. Make sure you have the carbs synced. You really can’t wing it. Get a flow meter thing and do it right. Make sure your throttle cables are working properly. Each carb slide should move the same amount when you twist the throttle. System worked good back in the day. There were adjusters in each cable section so you could get slack out.
When running right, it will idle and putt around at low rpm just fine. It will feel completely normal until around 4K and then it feels like you hit VTECH, YO!
I adjusted the front drum, and played with the front spokes a bit to get the wheel to spin closer to true. In order to keep the front wheel in the air on the center stand, I used some MR2 calipers as counterweight:
I rode it, and while the ride was smoother and the front brake more effective, the bogging was still present. So I adjusted the oil pump a bit leaner, and put some fresh gas in the tank- and the problem didn't change even a little. I really think the petcock isn't flowing enough, since the bog goes away entirely if you give the bike a few seconds of zero throttle to recover- it's an aftermarket piece and probably just doesn't move enough fuel for this thing. I'll have to find a NOS or rebuilt one since the one that came with the bike is beyond repair.
It sure looks good sitting here waiting for me to figure things out, though:
FWIW, a couple guys in our club have old Kawi stinkbikes. The 500 and 750 variety.
Apparently "powerband like a light switch", "poor brakes" and "rides like there's a hinge in the middle" are all features, not problems.
The latest issue of Cycle World magazine has a short article by Kevin Cameron on the H1, check your local newsstand.
Poking around, the aftermarket petcock I used does seem to be the most common replacement. I think maybe I need to devise a test to figure out if it flows sufficiently.
What else should I be looking for? Symptoms are as follows:
-Starts and runs decently
-Accelerates normally at first
-Cruising at constant throttle, will gradually begin to lose power until even WOT will not keep it moving
-At this point, if allowed to fall back to idle the engine will die. It can either be kept alive with some throttle, or shut off, and after 10 seconds or so will have enough power to take off again.
The engine will also "recover" if I let off the gas when it begins to lose power, but only briefly. Hence my thought that the bowls are being run dry.
Is it a vacuum petcock? I remember having some similar issues with a bike before I removed the vacuum diaphragm from the petcock and just used it on/off. That was an old part though that probably wasn't working, I wouldn't imagine a new one would have an issue.
In reply to EvanB :
Nope, fully manual on/off.
Gas cap vent partially clogged and not allowing full flow? Does it get any better if you crack the gas cap?
In reply to EvanB :
Nope, tried that too- I rode about half a mile with the gas cap completely loose.
FWIW the current issue of Cycle World (now issued quarterly) has a good article on these.
If you shut off the petcock just before it dies can you check the level in the bowls?
Is there anything (gasket, etc) that might be keeping the floats from dropping down fully?
914Driver said:FWIW the current issue of Cycle World (now issued quarterly) has a good article on these.
Hmm...wonder where I've heard that. 
stuart in mn said:The latest issue of Cycle World magazine has a short article by Kevin Cameron on the H1, check your local newsstand.
In reply to No Time :
Along that line of thinking, is something preventing the floats from dropping far enough to refill the bowls as fast as the carb is using fuel?
I did have them open for float adjustment recently, all seemed well but can check again. I think I'll pull the fuel lines off and make sure they all flow equally, then move into the carbs.
I am having 2 stroke carb. issues right now as well. My symptoms are quite different - full throttle feels like fuel starvation (clogged main jet?), but ran perfectly prior to this - so I don't think I have any advice. Do you have inline fuel filters on this bike? How do they look? Might try running without them.
Fuel flow results for all 3 petcock lines:
Left cylinder:
Center cylinder:
Right cylinder:
Each represents 15sec of petcock "on" time and probably flowed enough to run the whole engine just fine, so it's probably not that. I guess next it's time to go into the carbs, unless I can think of a way it could be an ignition problem. Hmm.
I rode the bike until it was at its' boggiest, then let it die and shut the fuel off as quickly as possible. Here are the spark plugs:
The left one has fuel on it but the other two are dry. Time to go into the carbs again when the bike cools down.
I'm getting pretty good at weaseling the carbs around without taking the rest of the bike apart:
They all had this powdery E36 M3 in them, which looks like what was in there the first time when it had sat for 40something years. The center carb's fuel inlet was completely plugged with it:
I cleaned (with spray and a guitar string) and reassembled all three- side note, the airbox has the coolest hose clamps I've ever seen:
Tried to kickstart it a few times before I noticed that the right carb's throttle cable was disconnected- so I unscrewed the top to remove the slide and the berkeleying threaded ring exploded as soon as it started moving:
The joys of working on "all original" stuff. I found a set of used carb parts for cheap on eBay, looks like at least one intact ring among them. They'll be here in a week or so, then I can maybe find out if cleaning the carbs fixed anything.
You're running ethanol free fuel in this, right? I haven't found an old motorcycle carb yet that didn't turn to white crust when exposed to e10 swill.
In reply to Ethnic Food-Wrap Aficionado :
Nope- you think that's the problem? My various 80s bikes have never had any issues unless left to sit for a long time, but this is my first experience with 70s bike carbs. Seems wild that they would develop issues within a couple months.
In reply to ¯\_(ツ)_/¯ :
Yep! I think that's exactly what you are looking at. I don't know if it's some weird reaction between the alcohol and aluminum, or if it's just the hygroscopic nature of the ethanol. But your float bowl above is exactly what it looks like in my experience.
I don't think oil and ethanol play nice together, either. You might not have the same problem there since you have an injection pump, but I found anytime I used e10 for premix it would separate out the oil and leave a sticky mess everywhere.
Anything of mine that I know might sit for more than a few weeks at a time gets ethanol free fuel. Fiats, dirt bikes, lawnmowers (heh, my lawnmower isn't a two stroke, but it runs on premix ). Everything.
Hopefully you can find some clear/pure gas without too much trouble, if not you can always order up some VP C12 and make it just that much better. 
Only place I have found ethanol free in our neighborhood is vanSant airport, and they really want you to have an airplane for it. Tell them you are flying an ultra light off your own field (Rt 212?) and they will sell. Bradens flat says no to a travel can, only pumps into plane. If you find somewhere more wheel friendly please post it, I want to know.
As Bill points out, we unfortunately live in an area where ethanol-free is hard to find unless you're flying. Van Sant would be somewhat convenient, and I know Doylestown airport has it as well. I think Lehigh Valley International actually has a gas station across the street which might have it, so maybe I need to come up with some reasons to visit Allentown.
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