Carb syncing question

[Dream]

I have an 81 cb750k recently rebuilt the carbs bench synced them also put the gauges on it and it seemed close at the time and finally coming back to it after 3 month. So first question is would adjust the valve shims change my synchronization? And also would having them no perfectly synched cause carb 4 to overflow with gas? Any help would be great thanks.

 

[SenseAmidMadness]

Yes, changing valve shims can change your carburetor synchronization if the carbs were previously synchronized with the valves not in perfect adjustment. When a valve gets adjusted to the correct clearance from an incorrect one, the amount of air it allows through at maximum valve lift also changes. In my opinion, carbs should always be re-synchronized after a valve adjustment.
Regardless of that, though, a set of carburetors that have been "bench synchronized" should always be properly synchronized on the running motorcycle because intake vacuum almost always varies between cylinders. On a good-condition engine it's usually only a slight variance, but it's still important.

And no, carburetor fuel level has nothing to do with synchronization. You have an issue in that carb bowl. Overflowing can mean a poorly-fitting or damaged float needle, a non-floating float, a damaged float, or something else. On the positive side, if you have a stubby JIS screwdriver (assuming you haven't changed the bowl screws) you can carefully take that bowl off without taking the entire carb rack off and figure out what's wrong.

 

[The Sea Raven]

Agree entirely with "SenseAmidMadness",. I've just had a similar problem with the carbs, you're lucky its Carb 4 as its the easiest to get to as its on the outside with the most clearance, so you should be able to get it off while the carbs are still on the bike, hopefully you have put cap head allen key bolts on the bowls
If its an 81' then it should have the plastic non-adjustable floats, so that helps. It will either be:

1/ A compromised float, which you can test by submerging in water to make sure its not taking in fuel and weighing it down
2/ Cracked overflow - again fill the bowl and just apply a few PSI of air back through the overflow covering the top to see if it bubbles, you can solder any crack.
3/ Sticky needle, I presume you changed the needles when you rebuilt, so that should be OK
4/ In my case, the press fit valve bush was just too low, it was catching the top of the float tang as the fuel raised the float, so it wasn't quite sealing the needle, , I just careful took a bit of the end of the float tang, so that when it cantered up with the fuel it was visibly clear of the bush and sealed well. The needles sometimes vary in length, and there may not be the clearance if they are just a little shorter. Have a good look to see it is definitely seating the needle valve before any contact with the bush

 

[Dream]

Thanks for the input I’ll let you guys know what I find next time I’m working on it

 

[Bo82CB]

When carb overhaul is in order on these bikes make sure to polish seats of float check valve, polish vacuum operated pistons and bores, jets… when I say polish I mean NOT using any power tool, only hand polishing.
You will be amazed how much this helps in smooth operation of these carbs! Syncing carbs after overhaul is minimal effort if everything done correctly. In one of the posts I posted procedure for carb overhaul I followed and result is amazingly running bike!

 

[Dream]

Agree entirely with "SenseAmidMadness",. I've just had a similar problem with the carbs, you're lucky its Carb 4 as its the easiest to get to as its on the outside with the most clearance, so you should be able to get it off while the carbs are still on the bike, hopefully you have put cap head allen key bolts on the bowls
If its an 81' then it should have the plastic non-adjustable floats, so that helps. It will either be:

1/ A compromised float, which you can test by submerging in water to make sure its not taking in fuel and weighing it down
2/ Cracked overflow - again fill the bowl and just apply a few PSI of air back through the overflow covering the top to see if it bubbles, you can solder any crack.
3/ Sticky needle, I presume you changed the needles when you rebuilt, so that should be OK
4/ In my case, the press fit valve bush was just too low, it was catching the top of the float tang as the fuel raised the float, so it wasn't quite sealing the needle, , I just careful took a bit of the end of the float tang, so that when it cantered up with the fuel it was visibly clear of the bush and sealed well. The needles sometimes vary in length, and there may not be the clearance if they are just a little shorter. Have a good look to see it is definitely seating the needle valve before any contact with the bush

Alright finally got to pulling the bowls off to find the issue, seems to be the overflow tub has a crack in it, I can’t see it with my eyes but I can see bubbles in the water every now and then. So what’s the best way to fix it.

 

[SenseAmidMadness]

The old-school way would be finding the crack and soldering it closed.