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Question on the round-top carbs and how they operate

panshovelmike

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Hello everyone, new member here. I've built several bikes of my own and worked on plenty for friends, but my experience has been limited to Harleys and YamahaXS650's. I recently picked up a 78 CB750 SuperSport for cheap and I'm trying to get it running. Previous owner said it ran before being parked in the back yard, where it sat for years uncovered with the airbox un-attached. I removed the gastank, seat, and airbox. Pulled the original keyhole carbs so I could go through them, and found LOTS of problems created by previous people. Aside from being REALLY corroded and gunked up, most of the bowl screw holes had been stripped out and the original screws replaced with pointy coarse thread wood screws. Accel pump was nothing but black goo. The underside of the carbs (inside float bowl) are corroded/eaten away to the point that the main jets threads are exposed. Anyway... these carbs are going to need alot of work to be rebuilt IF I can even use them. I had a buddy who had a set of round-top carbs laying around and offered them to me, so I'm going to try to use them for the time being, and put the keyhole carbs on the back burner. I have them partially disassembled for cleaning before I install them. They are over-all in pretty decent shape, I just have a few questions.

First question is regarding the choke. The carbs use a "door" which slides up and down for the choke, I've heard this referred to as guillotine type choke. The choke on the center 2 carbs seems to work right, but the outer 2 carbs are a little diff. As the door comes down to close for full choke, the doors on the outer 2 carbs seem to stop just a bit before bottoming out as the center 2 do. They close almost all the way, but I can see just a crack of daylight under them. If I push them down with my finger they will seat all the way closed, but will spring back if I let go. Is this how they should be, or do I need to adjust the lever-operated choke linkage to make them seat properly like the center two do? I can see that there are long threaded coupler-type nuts on the metal choke linkage which can be used to adjust them.

Second question is regarding the throttle operation. I disconnected the large throttle-return spring on the center of the 4 carbs. Operating the throttle manually, it seems to me that the carbs aren't opening up as far as they should for full throttle. Measuring from the top of the round cap on top of the carbs, the throttle rods move 5/8" until it feels like it gets stuck. I feel like they should open more, but I don't have a known good set laying around to compare them to. I'm including a photo to show how far they open. I have not removed the slides from the carbs yet, as I'm not sure how to properly remove them. I know they are synced and don't have experience with that, I don't want to throw anything off with my ignorance. I need to remove the 2 large arms which operate the throttle (one arm for each 2 carbs on a side), so that I can figure out which carb(s) are binding and limiting the travel and so I can get in there to clean... but I'm not sure how to proceed. Any info greatly appreciated, thanks in advance.

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The choke is still closed enough for it to work but it cant hurt it to adjust them so they close even.

By the picture you have, the throttle is not opening all the way. If you loosen the arms like you stated you will loose the sync anyway. You need to invest in some gauges to sync them because carb sync is one of the biggest factors in getting a smooth and quiet running 750. Unscrew the tops of the carbs and take the return spring off, you should be able to open the throttle all the way and then pull the slides out of the the carb bodies.

You will also need different carb boots and different air filter seltup as the round tops are all around smaller carb then the ones you are replacing. You may need new throttle cables also, I dont know if you can take all of the slack out of the cables that are for the carbs you are replacing because they have alot more cable free length then the round top carb cables.
 
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