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How much "surgical" skill does it take to restore carbs?

zzaacchh

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I have inherited a '78 CB750K, and have just completed a bunch of work to get the bike to the point where it *starts*, which is encouraging. But, several of the carbs are leaking through the overflow tubes. I've taken the carbs apart a four or five times, cleaned them as best as I could, but the darn things are still leaking. I've replaced the needle valves with some replacements that i picked up on ebay. When I put them back on the bike though, they still leak. Maddeningly, though, it's different carbs that are leaking now :)

I love the idea of being able to keep this bike in tune myself, but I'm starting to wonder/worry that I just don't have the surgical skills to complete this job. I'm an electrical engineer, with lots of experience building and fixing things generally, but don't have much experience with engine maintenance. I can't decide whether I should (A) Sending the carbs to one of the shops on the internet that specializes in carb repair (which sounds like a $400 job) or (B) take the whole bike to a motorcycle shop 40 minutes away (which sounds like a $1000 job). Or (C), just keep trying and trying myself.

Looking for wisdom here. Anyone have guidance on whether I should pursue path A, B, or C? I greatly appreciate your help!
 
Are the needles the correct rubber tipped needles? The early carbs are solid metal tips and the later style had rubber tip. Did you set the float heights correct? Did you verify the floats actually float? Did you clean inside where the needle seats? If they where really dirty you may need to get in there with qtips and carb cleaner and spin the around in there to get them clean. Look in there and verify there isnt a knick in the seat which could be leaking.
 
Two of mine had TINY splits along the overflow tubes. Really hard to see, but if you fill the bowl with isopropyl alcohol, you will see it leak out the drain/overflow. Also, I had to change out one drain valve that was corroded on the tip. I soldered the tube splits up and seem to have it fixed. Otherwise, patient diligence and care should do it.
 
I would suggest purchasing an ultrasonic cleaner. You will be shocked by how much gunk comes out of a single carb in one of them. I got a 2.5 liter one for about $85 dollars at Harbor Freight and it works well. So far I have done 8 carbs with it and all have came out super clean after 4-5 cycles in the machine. +1 on using a Q-tip in the float valve seat. I use a small amount of Brasso on a Q-tip chucked in my cordless drill to "polish" the seat. Seems to work well. Good luck!
 
dirtdigger , pidjones , and saltracer: thanks for your super helpful replies and sorry for the very delayed responses.

I'm assuming the needles are the right ones as both the old and new have rubber tips and look identical. I have done a couple of rounds of cleanings with carb cleaner, but I didn't really understand that I could do good diagnostics with the carbs still apart by checking their willingness to "float" against alcohol. I'll take them off and apart again this week. I'll also try to look closer for small faults along the overflow tubes. Maybe I'll be able to get a photo to share.

One thing I did notice is that a couple of the carbs were hesitant to "spring back" after I pushed down on them. It looked like this spring force was being generated by something internal to the needle valve itself. I worried that maybe my new ebay needles were defective?

Finally: thanks for the tip on brasso. Someone told me the same thing recently, but suggested toothpaste instead of brasso. That would be easier to try, as I have toothpaste :) I searched the forum and only found one other mention of that idea. Does that sound concerning?

Thanks guys. You have a inspired me to give this a few more rounds of repair attempts!
 
Every rubber-tipped float needle should have a spring loaded tip that "springs back" when you press the float against it. If they are old, you might have to soak them in carb cleaner to get varnish out of them. Pressing the tip (with the valve out) should readily demonstrate the spring action. I've had old ones that were stuck and needed freed up, but never new ones.
 
Thanks, pidjones. All of the internal springs on the needle valve tips "bounced back" when they tips were disconnected from their individual carb float assemblies and I pushed the tip back with my thumbnail. But once they are all connected to the floats, only two of them were strong enough to be able to lift the float back up reliably -- at least when the carbs were upside down. I figured that they would behave once I flipped them over and installed them on the bike but am now realizing that I never actually verified that. Maybe that should be a bigger cause for concern...
 
Thanks, pidjones. All of the internal springs on the needle valve tips "bounced back" when they tips were disconnected from their individual carb float assemblies and I pushed the tip back with my thumbnail. But once they are all connected to the floats, only two of them were strong enough to be able to lift the float back up reliably -- at least when the carbs were upside down. I figured that they would behave once I flipped them over and installed them on the bike but am now realizing that I never actually verified that. Maybe that should be a bigger cause for concern...
Or a couple floats are too heavy - did you weigh or float-test them?
 
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