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Discussion Starter #1
I've just got my CB450 back together after a rebore and top-end overhaul. It starts and idles just fine but as it warms up it start to get rough - this is with no filters fitted for the time being. The mixture screws are turned right in as far as they'll go and both sides are still too rich. The bike was like this before I dismantled it. I have new float valves and the float height has been set on the bench with a credit-card gauge and fuel flowing. When I go the bike the carbs were exceptionally clean but I stripped and checked them anyhow and replaced the gaskets. One of the main jets was originally 145 but I've borrowed a 130 so both carbs have 130 mains. I checked the rest of the components and they're all the right parts.

Off-idle the engine runs cleanly with a crisp throttle response. I've been very careful to set the ignition timing as well as valve clearances.

I wonder if anyone has any specific pointers to assist in getting this sorted out? I've spent a good deal of time researching this and reading through the factory manual, but nothing I've tried has made any difference. The air and fuel passageways look good but someone with more experience of these bikes may suggest something I've overlooked.
 

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What makes you think it's too rich at idle?

There should be a starting setting for those idle mixture screws in the manual - put them there and see how it runs. They certainly should not be closed.
 

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Discussion Starter #4
What makes you think it's too rich at idle?
It soots up the plugs very quickly at the factory default setting and a sign of a too-rich idle is the bike starting and running well when cold and then progressively worsening as it warms up. The running begins to improve when the screws are fully home (they're NOS screws). Exhaust smells of unburnt fuel. If I open it up to 3000 rpm it clears. It also bogs down when warm when the throttle is opened slightly. I could set it up with my gas analyzer but it would soot the filters up in seconds the way it is. Maybe it's pulling fuel from somewhere else other than the pilot circuit, or there isn't enough air being fed in via the emulsion tube.

I have new air filters and they make no difference, so I left them off for the time-being.
 

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I think the stock jets are 145 or 150's, why are you using 130's? No idea if that could be the source of your problems though.

Edit, I'm all wrong. FSM shows jet is a 130!

BTW manual says air mix screw should be 1-1 1/4 turn out.
 

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Weird question.... During the rebuild did you remove the throttle butterfly plates?

I ask because they aren't simply flat discs, and the beveled edge MUST be correctly oriented
 

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I think the stock jets are 145 or 150's, why are you using 130's? No idea if that could be the source of your problems though.

Edit, I'm all wrong. FSM shows jet is a 130!

BTW manual says air mix screw should be 1-1 1/4 turn out.
Stock main jets on 14H carbs are 130.
 

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Hmmm... Over-filled carbs, wrong type idle mixture screws, loose pilot jets, blocked pilot air jets spring to mind.
carb_operation.jpg
 

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Stock main jets on 14H carbs are 130.
I swear my 450 came with a 145 in one carb and a 150 in another, ended up getting another 150 so they matched and bike seems to run well. Mine is a 74', I remember reading somewhere that they came with 145 or 150 but perhaps I was hallucinating.
 

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I swear my 450 came with a 145 in one carb and a 150 in another, ended up getting another 150 so they matched and bike seems to run well. Mine is a 74', I remember reading somewhere that they came with 145 or 150 but perhaps I was hallucinating.
That info is correct.......
 

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I don't know why we're discussing main jet sizes, since the problem is at idle, and the main has almost no control over the idle mixture.
 

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Ah, but it does.... The idle and slow circuits are fed fuel from the main through a small hole in between the two jet "towers".....
While the slow jet again restricts fuel flow, it is capable of flowing enough for the idle to slow transition, and ALL that fuel is more than required at idle....
The primary determinant of how much fuel goes in at idle is the mixture screw... (assuming the butterfly is at its most closed position and blocking the additional holes into the venturi)
There is NOT a hole in the bottom of the jet where it is exposed to fuel in the bowl......ALL the fuel must go/come through the main.....

All your suggestions in post #8 are valid....They are simple carbs, but finicky......
 

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Discussion Starter #13
I checked the butterflies and it looks like they've never been disturbed and are oriented correctly. Just to be sure I removed the idle screw springs and checked the adjustment screws are seating correctly. With the engine running I can't see any fuel spray from the needle jet at idle that may be contributing to the richness. I think I'm going to have to take the carbs off again and start from scratch. The only variable that I can see that would make a significant difference is float height. Neither float is leaking and they're both the correct weight.
 

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Ah, but it does.... The idle and slow circuits are fed fuel from the main through a small hole in between the two jet "towers".....
While the slow jet again restricts fuel flow, it is capable of flowing enough for the idle to slow transition, and ALL that fuel is more than required at idle....
The primary determinant of how much fuel goes in at idle is the mixture screw... (assuming the butterfly is at its most closed position and blocking the additional holes into the venturi)
There is NOT a hole in the bottom of the jet where it is exposed to fuel in the bowl......ALL the fuel must go/come through the main.....

All your suggestions in post #8 are valid....They are simple carbs, but finicky......
Steve is correct - on a 450 carb, all fuel for both high speed and idle circuits passes through the main jet first.
 

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While the main jet has some effect on the amount of fuel flowing at idle, it is a very small effect, because of the comparative sizes of the jets. Changing between a 130 and a 145 jet would hardly be measurable. A plug in the pilot air jet would have a huge effect, as would a loose pilot jet. The pilot jet is easy to check, the pilot air jet is harder, unless the blockage is at the air inlet in the throat of the carbs.
 

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While the main jet has some effect on the amount of fuel flowing at idle, it is a very small effect, because of the comparative sizes of the jets. (Not so...) Changing between a 130 and a 145 jet would hardly be measurable. (If you do the math, the AREA of the opening INCREASES by nearly 25%, so theoretically the 145's could flow about 25% more fuel than the 130's as well) A plug in the pilot air jet would have a huge effect, as would a loose pilot jet. (Agreed) The pilot jet is easy to check, the pilot air jet is harder, unless the blockage is at the air inlet in the throat of the carbs.
​In addition, float level setting make a big difference as well....
 

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Discussion Starter #17
I dismantled both carbs again but I still can't find anything wrong. The body drillings are clear and the fuel shuts off cleanly with no dribbling. I also checked to make sure the slides were correct (with the two holes engine-side). I re-checked the float levels and was happy with how I'd got them set originally with the carbs set vertically, but I did re-check them horizontally to ensure the fuel shuts off in the running position.

All of the jets are firmly in place. I've also checked that the floats aren't binding.

As an experiment I tried increasing the float height by a small amount (0.7mm) and this does make a difference in that the bike now idles with the mixture screws about 1/3 turn out, but the adjustment sweet-spot is too knife-edge for my liking. It looks like fuel height could be used to eliminate the problem, though this could then shift it somewhere else - probably lean throughout the rest of the range given that the whole carb is calibrated off the fuel level.

Has anyone run into a situation where the fuel height or pilot jets have needed to be changed from stock on an unmodified bike?
 

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I almost asked if he was referring to my mental state. This island isn't quite as isolated as it was when I moved here 30 years ago which is both good and bad.
 

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As an experiment I tried increasing the float height by a small amount (0.7mm) and this does make a difference in that the bike now idles with the mixture screws about 1/3 turn out, but the adjustment sweet-spot is too knife-edge for my liking.
I can move mine about 1/8 turn in either direction before there is more than a subtle difference. Are you sure you have the correct screws? And the O-ring, washer, and spring in the proper order? Maybe post a clear closeup of one; some kits have poorly made ones.
 
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