[robertob]

OK CV carb gurus - I'm going crazy checking out a high-rpm misfire/something on my CB450.
It's got the stock CV carbs with velocity stacks & otherwise stock engine.

It runs & starts great most of the time, but acts up when I try to do a long, high-speed run.

I can accelerate at full throttle all I want with no problems, but after I get it wound out in fifth at about 6000-7000 rpm (somewhere around 80-85mph) it starts to bog down and fall on it's face. It doesn't seem to misfire, and it doesn't happen right away. Only after a minute or so of full-throttle top-gear acceleration.

I was thinking maybe clogged petcock but it's new and still clean.

I'm running 135 main jets, up from stock 130s that were too lean for the velocity stacks.

Maybe I should go up even more, but plug chops (not perfect ones, anyway) have shown pretty good coloring.

I hate jetting, BTW, I wish this thing had EFI so I could just tweak a parameter or two and be done with it.

 

[ROSKO]

"wound out at 6-7000"?????

Thats when the power should kick in, those 450's have no balls till 7 grand. Wound out is 9000-9500.

I had a weird flat problem on mine, ramp up, ramp down yadda yadda. Looked everywhere checking the jets, float height, petcock, fuel cap vent, air leaks... and nothing. Turned out that the coil bracket had cracke din two and was causing the spark to break up at high rpm (aka high vibration).

weird.

Replaced the coil bracket and all is good.




try maxing out each gear... don't even shift till 7500-8000


BORN TO LURK, FORCED TO WORK.
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www.VinMoto.org

 

[robertob]

Well, wound out in this case means wide open throttle in 5th.

Mine's down on power from where it should be - the exhaust cam is trashed and it has low compression, so maybe I shouldn't troubleshoot until I get the rebuilt motor in.

 

[peteN]

I'm just having the same problem with a '74 CB 450. This one is stock except for a set of uni filters on it and starts to surge at about 75-80 mph, Have a suspicion that it's running out of gas in the float bowls but don't know yet.

have you had any success with this one?

if at first you don't succeed, use a bigger hammer

 

[oldschoolorange]

running out of power? are you emptying the float bowls of fuel at wot?


Hey someone pas me the wrench........ Ahh screw it where is the torch

 

[robertob]

Honestly that's what it feels like but these are stock carbs and a brand new stock petcock. I wouldn't know how to check for such a thing and repair it if needed.

 

[robertob]

Oh, and my original post was off - it's around 7500-8000 rpm that it happens, not lower. I can cruise at 6000-6500 rpm (65-70 mph) all day long without a problem. It's when I bang the throttle open and try to hit top speed that it stumbles. I can hit about 85 mph before it happens.

 

[krapfever]

Is it possible you've got a problem with valves floating (i.e. worn valve springs)?

Shitty points?

Put a fresh set of plugs in it, run it WOT and immediately check the plugs. If you've got a fuel starvation problem, the plugs will look white.

Honda go sideways!