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Carburetor number three fuel screw question?

Cmax72

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I got my bike running and up to bench sync. My bike shows signs of being a mixed bike; it seems to have some CB750F parts though most of the parts and the serial number are CB750c. I know that the person who sold it to me built it from spare bikes he owned. I have also found some but not major damage work (stripping and cracking) from someone working on it or making marginal parts fit. My carb is a VB42A.

My question has to do with the bike being a "Frankenstein" build. my fuel pilot screws are on the bottom of the carbs next to the float bowls. The fuel pilot screw my third carb is what looks like a brass cap screw. The other three screws on the other three carbs are a unique screw style screw head. They have a flat head driver fitting and a "wing" which sticks out and stops on a flange on the float bowl. (confusing to just describe). I am wondering if the odd cap screw on the third carb is a set screw, not to be changed, for syncing the fuel adjustment on the other three carbs to its fixed setting, and if so why isn't it on the number two carb like all the other set adjustments? Or is this just a screw that the guy who built the bike had around and used to replace the unique style head?

Why is this site rejecting my JPEG files!? well no pics I guess

Thank you for your time
Cullen
 
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Wait. Ok as I sit here thinking about it I feel stupid. Its not just a screw its a needle and a better explanation would be its a plug head not a cap screw because the screw head doesn't hit anything, it just threads down and inside the screw hole.. what im now theorizing is he used a new fuel needle from a rebuild kit to replace just the one needle on the number three carb. It still could be a factory set needle too but its not just a screw. its a needle. duh.
I'm going to look up pics of my carbs and see if all four fuel screws are normally the same or not.
 
There are at least two basic types of screw used there, ALL being brass. Whatever is used there should be all the same type. '80 model used the longer EPA type with loctited on aluminum/zinc heads with the tabs (your wings) on them to prevent the idle mixture being moved more than just a little bit, they are known as 'idle limiter caps'. The 3 screws on OP's bike that are the same are that type. They have longer heads and the aluminum tab part can be removed by heating the screw up to melt the loctite or grinding the tab off or even grinding a notch in the fuel bowl tab that the idle screw hits. The screws are supposed to be tamper proof and the head is undercut right under it to break off easily to make adjusting it useless and why the odd one may be used there. There is a second type brass needle used on non-pollution models that has a shorter head that is not easy to break off. They can be mixed if needed to get a running bike. I've seen the idle limiter type with the head broken off to have somebody file a screwdriver slot to adjust what is left.

You sync to carb 2, not any other. All 4 carbs adjust to the same setting when you set idle mixture, preferably around 2 turns out from a light hit, the needle seats can damage easy as spit there.
 
Thank you. this helps a lot. I just had the tab head breakoff of my number two carb. I have a rebuild kit here but am always hesitant to swap parts. Im going to pull the number two and see if I have a replacement here. I'll check on the needle length. I am communicating with the admin to try and figure out why the site is rejecting my pics. Hopefully I get them all adjustable and then adjusted to the same setting today.
 
I have another question if you have a moment. The new needle screws in my kit are the right size needle but the threads are fine. As I search all the replacement needles available online I can't tell if the threads are the same and I am remembering that this kit claimed already to fit my bike. How would you ensure you have bought a set of new needles with the correct threads online? Moto shops near me are sparse and not helpful.
 
Dunno what that thread is but I would match to what is already in it. I have never seen but one only mixture needle thread only for DOHC 750. The early short on the top needles and the later EPA ones with cap on them are the same at the thread and the needle tip extending outward from that. Only the breakaway top is different. There are several other Hondas that used the VB carb type though, it may be that some of those used a different thread. The 650 SOHC, CBX, 400 & 450 Hawk and CX500 & 650 v-twin all used them. Gold Wings used something very similar too.
 
I cut a flathead notch into the bad needle. Cleaned them all up, threw them back in and I’m now rollin. Bike sounds great. Thanks for the help. I learned some things.
 
You sync to the number two carburetor... That is your baseline, we use an old motion pro Mercury gauge been doing it for years never had a problem.. as far as your idle mixture screws we have soldered small t's in order to make adjustment easier because they are a pain.
 
You sync to the number two carburetor... That is your baseline, we use an old motion pro Mercury gauge been doing it for years never had a problem.. as far as your idle mixture screws we have soldered small t's in order to make adjustment easier because they are a pain.
And I would also suggest getting a sink tool holds the nut why you turn absolute must
 
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