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Discussion Starter #1 (Edited)
So I pulled my 1982 CB450SC Nighthawk out from storage and noticed something strange while riding it for the first time this year. It started great, choke worked fine, but while riding it when I went to down-shift the revs stayed high (around 4000 RPM), with the clutch pulled and my hand completely off the throttle it stayed revving high for at least 15 seconds every time I went to downshift. So I got it back home and thought maybe the carbs might need to by synced so I pulled out the vacuum gauges. After hooking them up and starting the bike I started adjusting the mixture screws and noticed no change in RPM's no matter what I did to the mixture screws, I turned them all the way in (seated) and no change to the RPM's, so tried something crazy and unscrewed them all the way out (yes both mixture screws completely removed) and the bike continued to run with no change to the RPM's. Even sprayed carb-cleaner around all the boots looking for a vacuum leak but found none. So can someone please tell me how the heck a bike can run with no mixture screws installed, I can't for the life of me figure out what's wrong and I'm about to take a sledge-hammer to this thing…..PLEASE HELP???
 

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My guess would be a broken or stuck throttle return cable.
 

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My guess would be a broken or stuck throttle return cable.
I was going to guess gremlins or just possesed!!

I would think even with a bad return cable it would still technically flood with the fuel screws out of the carbs.....personally I would pull, clean and inspect the carbs, looking for lacquer or that white crap ethanol likes to leave behind after evaporating especially in the jets.
 

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I'll guess hanging throttle cable, too. At 4000 rpm, the mixture screw circuit is "out of the mix".
 

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Discussion Starter #8
Do you notice an idle change during start up with choke on and off?

Did anyone say sticky float yet?
Choke works normally and bike starts fine, idle goes up when choke is pulled and down when released. If the float was sticking then there should be gas overflowing but there is not. My thinking is maybe the choke is staying on a little bit which would bypass the mixture screw operation?
 

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If you can feel the throttle physically returning to closed position, it is likely a sticking slide in the carb.
 

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Discussion Starter #10 (Edited)
If you can feel the throttle physically returning to closed position, it is likely a sticking slide in the carb.
That might explain the high revs remaining when the throttle is released when riding, but how does the bike keep running/idling with no mixture screws installed?
 

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The idle screw (idle set/idle stop screw)has almost NOTHING to do with the engine running or not....The idle air/fuel MIXTURE screw does...BUT only within a limited rev range...
A fully closed, or complete lack of the mixture screw is essentially meaningless above idle speeds....Or if another circuit is active..........
IF you are above that range, the engine is getting its mixture from another fuel/air circuit ....Or perhaps it is managing to draw air and getting the fuel from a flooding condition....
An engine CAN run without a carb at all IF it manages to get a burnable air/fuel mixture somehow....A carb just makes the mixture close to optimal, and controllable.....
 

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Discussion Starter #12
The idle screw (idle set/idle stop screw)has almost NOTHING to do with the engine running or not....The idle air/fuel MIXTURE screw does...BUT only within a limited rev range...
A fully closed, or complete lack of the mixture screw is essentially meaningless above idle speeds....Or if another circuit is active..........
IF you are above that range, the engine is getting its mixture from another fuel/air circuit ....Or perhaps it is managing to draw air and getting the fuel from a flooding condition....
An engine CAN run without a carb at all IF it manages to get a burnable air/fuel mixture somehow....A carb just makes the mixture close to optimal, and controllable.....
Sorry, I meant to say "no mixture screws installed" not idle screws........I edited it.

So if I'm understanding you correct, there is something bypassing the mixture screws.....like a slightly opened chock? Or my idle screw was engaged to much? Would this cause the idle to "hang" when riding?
 

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YES..Yes.. and it could......There is some other method, means, or circuit providing fuel/air to the engine....Maybe to just one cylinder......

A removed mixture screw IS an air leak...to run it has to be getting/adding fuel from somewhere.....

Your accelerator pump diaphragm could be allowing enough fuel into the carb....Lots of possibilities......
 

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Discussion Starter #14 (Edited)
Ok, that makes sense. The chock is easy to eliminate, just remove the choke cable so then there is no chance it can be open a little bit bypassing the mixture screw circuit..........but how do you test the idle screw, it has to be engaged somewhat to keep the bike running but not to much to bypass the mixture screw circuit?

All diaphragms are in perfect condition.......
 

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Doesn't engaging (pulling the knob) the choke on a warm engine cause it to stumble and almost die instead of revving up madly?

If not, mine has issues....

Regards,
 

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ok, as mentioned previously the mixture screws, idle circuit, are pretty much bypassed at 4K rpm.
First thing to do is get the rpm back down to 1200. At just over 1300 the second idle circuit port is exposed rendering the mixture screw ineffective by in large.
So verify that the choke is indeed open AND the high idle cam portion of it is free and returning to the off position. There's a small spring that is wound around the linkage that could be bound up or sticking causing the high rpm w/choke off
201_4644-001.JPG
 

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Discussion Starter #17
ok, as mentioned previously the mixture screws, idle circuit, are pretty much bypassed at 4K rpm.
First thing to do is get the rpm back down to 1200. At just over 1300 the second idle circuit port is exposed rendering the mixture screw ineffective by in large.
So verify that the choke is indeed open AND the high idle cam portion of it is free and returning to the off position. There's a small spring that is wound around the linkage that could be bound up or sticking causing the high rpm w/choke off
View attachment 57421

Choke is working great..........

I pull the choke and start the bike, bike revs up to 3500 and sits there as it should.
When bike is warm I push in the choke and revs drop down to 1200 (or so).
I can blip the throttle while sitting there on the stand and bike revs up then drops down to normal idle.
It's only when riding does the revs "hang" at about 3000rpm for about 10 seconds after letting go of the throttle, and all this happens with (or without) mixture screws installed ?
 

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The spring between the throttle plate linkages is in place?
The needles in the pistons are not fixed but floating in the holder?
Pistons move freely? Best test is with the carbs inverted and pistons pushed down so you can see the actual piston movement by spring action only.
Float level is correct at 15.5mm?
 

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Discussion Starter #19
The spring between the throttle plate linkages is in place?
The needles in the pistons are not fixed but floating in the holder?
Pistons move freely? Best test is with the carbs inverted and pistons pushed down so you can see the actual piston movement by spring action only.
Float level is correct at 15.5mm?
Yes spring is there........needles are loose (no washer mod added), I would have to remove the carbs to do your inverted pistons push-down test, can you take the tops off and add some WD-40 to the slides in case they are sticking? Floats are not adjustable on this bike.
 

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Yes spring is there........needles are loose (no washer mod added), I would have to remove the carbs to do your inverted pistons push-down test. Floats are not adjustable on this bike.
True they are not adjustable BUT if there are aftermarket float needles in there I can almost guarantee the float level is too high/low. Too high could mask the mixture screw issue being removed.
Yeah, it's a PITA to remove the carbs but we're dealing with something obscure here so it may need to be done a couple more times.
 
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