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Discussion Starter · #1 ·
My "Stock" 74' CB 450 K7 is extremely difficult to start ? You can't give it too much choke or throttle for it will flood easily ? I just gave it a tune-up check Points adjustment and valve adjustment all spec out to the FSM. Performed a compression check on the bike after warming it up for a few minutes, pulled "equal" compression on both cylinder at 150 lbs.

Which comes down to the carbs., is there a " base setting " for the air mixture or fuel mixture screw ? The only screw that I see on these stock carbs., is the screw on the side of the carb. near the front of the carb./manifold side ?

I like to see if an adjustment if needed , would help with my hard starting problems ?

Of course, I haven't rebuild or cleaned the carbs. yet but will. The bike runs fine down the road, idles well, and exhibits no "quirks" besides hard starting. Any Help or Direction would be Greatly Appreciated !
 

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Discussion Starter · #3 ·
There is an idle screw on the side linkage of both carbs. but the screw I'm making reference to is the one on the side of the carb. which is located at where the front of the carb. which slides into the intake manifold ? The Link you made reference to does not indicate what carb. this is on my 74' 450 ?
 

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Yes, Brian's reference was to the later 450, and yours is the older DOHC twin. While you do want to get the idle mixture to its best setting when fully warmed up and ridden for a while, it won't play much of a role in hard starting - the choke is a big part of it and the use and need for choke varies according to your area, altitude, etc. I've found over the years that consistently, carbureted Hondas with no accelerator pump (all DOHC 450s and the SOHC 350s) usually start the easiest by turning on the choke and leaving the throttle at idle when cranking with the electric start, then as it begins to fire, turning off the choke a little bit and elevating the throttle slightly as needed to keep it running while slowly turning off the choke as you learn what your engine's needs are based on all the outside factors like weather (temperature), dampness and elevation. I've found that most are harder to start when you open the throttle significantly while cranking, in part because you help lower the vacuum draw created by the choke and closed throttle butterflies which helps draw fuel into the engine. Doesn't work the same as a car - especially old school carbureted cars, where you pumped the pedal a couple times to give a couple squirts of raw fuel into the manifold and allow the choke to set - or even a newer car which will still start when the throttle is opened because the injection systems take throttle position and cold start temp into account when metering fuel into the engine.
 

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Discussion Starter · #6 ·
Yes the Choke Lever is loose where it pivots but operates as it should. Haven't checked float levels or cleaned the carbs. yet.

So again, the adjustment in the front/side is probably a Fuel Mixture screw. Do you know what the base setting would be, how many turns out as a starting point ?
 

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I believe it does richen the mixture - but interestingly, the FSM has a misprint (my PDF copy anyway) and describes the movements both directions as making the mixture richer

carbadj.jpg
 

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Thank You and I'm assuming turning the screw out ( counterclockwise ), richen's the mixture ?
The tapered part of the screw controls the volume of the rich pilot mixture that is passed through to the port(s) downstream of the throttle plate, increasing it as the screw is turned CCW, but only up to the position where the tapered point is too far out of the orifice to have any effect. You are matching the amount of premixed fuel/air to the air passing the throttle plate, which has little or no fuel from the needle jet.
carb_schem.jpg
 

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Most Honda FSM's contain errors, some are egregious.
Make sure you have the correct mixture screws - the 14H and earlier carbs have a real pointy one, like in Tom's graphic above. Later carbs have a stepped, more blunt tip, and also have a little washer involved. They are not interchangeable at all.
Get the bike idling properly - then adjust the mixture screw in and out a little until you get max rpm. You may have to adjust the idle down a bit after this. The suggested start point is 3/4 turn for earlier bikes, 1-1/4 turn (or so) for the later carbs, but that's just a suggestion.
Just as info, all the fuel in this carb passes through the main jet first - the pilot jet gets fed after the main jet.
Here's an annotated drawing that shows how the carb works.
 

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Discussion Starter · #12 ·
Mine are set at 1 turn out, so increasing that even more would probably make the hard starting problems even greater. Maybe "decreasing" the mixture screw ( turning it in ) would help ?

Or maybe I need to look at the float height adjustment ? Eventually I'm going to have to pull the carbs. and thoroughly clean them.
 

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Mine are set at 1 turn out, so increasing that even more would probably make the hard starting problems even greater. Maybe "decreasing" the mixture screw ( turning it in ) would help ?

Or maybe I need to look at the float height adjustment ? Eventually I'm going to have to pull the carbs. and thoroughly clean them.
Please read my post again - turn the screw until you get max rpm, it's that simple - whether it's one turn out or 1-1/4, doesn't matter that much.
If the bike is still hard to start the problem lies elsewhere. That mixture screw has almost nothing to do with starting the bike.
You've already mentioned 150 psi when warm - I'd consider that marginal for a CB450 DOHC.
 
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