[Irving]

This 30 second video shows how the the bike acts in neutral when slowly given increased throttle and how it acts when suddenly given wide open throttle. This is of course an issue when riding as well. Just demonstrating in neutral.

Notice that when given full throttle at about 16 seconds the RPMs are all over the place and hesitating/bogging down. And then when given wide open throttle from idle at 24 seconds the motor instantly quits. It will instantly die every time when given wide open throttle from idle.

The engine has been rebuilt with new rings, new cam chain, all new gaskets, valves lapped and new valve seals. Carburetors are cleaned and fully rebuilt with new floats and stock #88 and #38 jets. Float height is properly set at 21mm. The main jet needles are set on the leanest (highest clip) setting - this is how it seems to run the best. The mixture screws are set 1 1/4 turns out as called for in the service manual. The air filters are stock air boxes and appear pretty nicely clean.

Other new parts include: ignition coil, condenser, throttle cable, fuel petcock + filtering screen.

The bike is rideable as long as you don't open up wide open throttle at low speeds. Once you're in 4th or 5th gear and going 40mph+ you can then hold open full throttle without apparent issue. I've had the bike up to 70 mph in this state. It is of course a problem though and I'm dying to fix it but I'm completely stumped as to what is causing this. Any thoughts?

 

[Irving]

I've just ordered some CL200 jets to try out. They are #95 main and #35 idle instead of the CB200 #88 and #38. I have the stock air boxes but the exhaust is smaller. So I figure it's worth a shot to try larger jets.

 

[Irving]

Figured I'd give an update. This bike's battery had been near death so I replaced it with a brand new one and I also threw in some brand new spark plugs. The hesitation problem shown in the video above was still fully there. The fresh plugs and battery did make a very slight overall improvement though, which is good. The last battery wouldn't even start the bike. I had been kick starting only.

However, I then replaced the #88 stock sized main jets with the CL200 #95 main jets and it made for a vast improvement! The bike will still die if given full throttle from idle in neutral. But the high end rpm hesitation/bogging down/cutting out that was happening is now gone. I can race right up through all gears now at full throttle without issue and it just overall runs quite a lot better. If you give it full throttle from low enough of a speed it will still die out, but nowhere near like it was before. Maybe I still need a few sizes larger for the mains?

I tried many things including multiple carburetor cleanings before finally experimenting with jet size. I guess just changing the mufflers really can greatly impact the required jet sizes. I had definitely heard that if you change to higher flowing air filters you will need to re-jet. But even with stock filter boxes this bike apparently has needed larger jets due to the mufflers.

 

[mike in idaho]

Conventional slide carburetors don't have accelerator pumps, if you whack the throttle suddenly WFO at low idle the engine will stumble and/or die. That's just how they work, open the throttle at a more normal rate and the carb can meter enough fuel to keep up with the increase in air flow. The normal way to fine tune off-idle throttle response is to change the size of the cutaway(angle) on the slide. The cutaway works as an accelerator pump on this style carb, bigger numbers are richer, smaller numbers leaner.

 

[Irving]

Conventional slide carburetors don't have accelerator pumps, if you whack the throttle suddenly WFO at low idle the engine will stumble and/or die. That's just how they work, open the throttle at a more normal rate and the carb can meter enough fuel to keep up with the increase in air flow. The normal way to fine tune off-idle throttle response is to change the size of the cutaway(angle) on the slide. The cutaway works as an accelerator pump on this style carb, bigger numbers are richer, smaller numbers leaner.

So you're saying that I should not expect to be able to give wide open throttle to this bike at low speeds without it bogging down and dying like it has been? I'm fairly new to bikes but that just doesn't sound right.

 

[mike in idaho]

That's the reason Honda started putting CV(constant velocity) carbs on bikes, covers up the "hole" in the fuel delivery at low rpms when the throttle gets yanked open.

 

[Irving]

That's the reason Honda started putting CV(constant velocity) carbs on bikes, covers up the "hole" in the fuel delivery at low rpms when the throttle gets yanked open.

To give less fuel on full throttle?