Fixing Leaky Carbs | Honda CB750 Carburettor Repair | Motorcycle Project | Part 12

Fixing Leaky Carbs | Honda CB750 Carburettor Repair | Motorcycle Project | Part 12
In this bitsa CB750 Restoration episode, I finally fix the leaking float bowls of my Honda CB750 K series project bike. Fixing the leaky carbs turned out to be more of a mission than I’d originally anticipated, but I think they’re now successfully repaired.
Here's what I used in this video:
Blow torch: ebay.us/Wstfop
Soldering Flux: ebay.us/QdHNtI
Fire extinguisher: ebay.us/Mks7nC
JIS Screwdrivers: ebay.us/R2brPs
Auxiliary fuel tank: ebay.us/otHb8C
Brake and Clutch Cleaner: ebay.us/ibW7Zu
After becoming very quick at removing and replacing the carbs, and setting the float levels I came to the realisation that it wasn’t the fuel valves causing the leak. I suspected either the drain plugs or the brass tubes used for the breather/vent pipe.
I took off the offending float bowls and the pipes didn’t look too clever. There weren't any obvious splits or holes, but they did have a lot of pitting and black marks that could have been hiding anything. I filled the float bowls with fuel to see if they leaked, and they did straight away.
I part emptied the float bowls so that only the drain plugs were covered by fuel, to check that it wasn’t them. It wasn’t!
Researching leaking breather/vent tubes it seemed the 3 options are, new float bowls, solder the splits, or remove the tubes and put in new. There aren’t any new ones that I could find, and second-hand ones might also have problems.
So I decided to try the solder route first. I have a little experience of putting water pipes together, but I expected trying to fix invisible holes in a tight space might be beyond me! :)
I had read that you needed to get the tube as clean as possible, then use flux to get the solder to run into the splits or holes. As I couldn’t see the splits or holes I tried to cover as much of the surface as I could with solder.
After my first attempt, one of the bowls still leaked. I realised that I had concentrated on the bottom end of the tube and that the issue might have been at the top of it. So I had another go.
Although my soldering wasn’t pretty, in fact it was horrible and lumpy, it did the job so I was quite pleased. I tidied it up with some abrasive paper so it didn’t look too bad.
This seems to be a fairly common problem, so I am pleased that I now have a technique in my armoury for when I come across it again.
For more helpful how-to guides and restoration project logs, visit our blog: www.spannerras...
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[@chrisrynston8749]

Great perseverance. I love your videos because they are real world. 👍

[@SPANNERRASH]

I appreciate that! Thanks so much.

[@davidmuir6818]

I have the exact same issue with one of my carbs. Now I finally know what to do. Thanks for this video 👍

[@SPANNERRASH]

You're welcome David, really hope the guide helps you fix your leaking carb!

[@andreokazaki14]

Hi. It's me again. Over the weekend, I was going over my CB750 K0 to get it ready for spring. I found the carb for cylinder #2 was leaking. I discovered that the overflow tube has a small hole or crack. I came back to this video to see how you fixed yours. I will have a go at soldering it as well. I checked all of them to ensure the others were okay. I have a quick and simple way to test them, which shows exactly where the leak is. I put the thin tube on my carb cleaner and sprayed into the overflow exit where the hose attaches. If you block the small hole on the inlet end of the brass tube with your finger while spraying carb cleaner, the fracture in the tube will leak and show exactly where a repair is needed. I just thought this technique could be a time saver on your future projects.

[@SPANNERRASH]

Great tip. I'm glad my video is helping. Hopefully your soldering will be a lot neater than mine. It is a strange fault, that isn't obvious at first. Good luck, and let's get out riding. Love K0's by the way, really miss mine.

[@andreokazaki14]

Update. I used a soldering iron so that I could keep the heat concentrated in the area of the fracture, in case the brass tube was soldered in place. No more leak. Thank you for another helpful video. It's a good feeling, fixing something that most people these days would simply replace. @@SPANNERRASH

[@SPANNERRASH]

@@andreokazaki14 Brilliant, really pleased the fix worked out for you!

[@andreokazaki14]

I have been riding and restoring vintage bikes for almost 25 years. I'm not really a Honda guy and your CB750 videos have helped me through some unfamiliar territory while getting my CB750 K0 ready to ride this spring. Watching you work through some of the challenges on your bikes has saved me some headaches on mine. The repair manuals only cover so much, and you fill in some of the gaps.

[@SPANNERRASH]

It's always good to see how somebody else has done it. The big problem is you spend a lot of time working something out, becoming an expert in it, then you dont come across it again for decades :) Isn't it amazing how the pictures in repair manuals never seem to show what you want to see! I loved my K0, but struggle not to be new (to me) bikes so it had to go, I still miss it, loved the colour.

[@mulliedog1]

Patience is a virtue!! Great to see you working through the problems.

[@kimsmith4409]

Excellent thanks 😊

[@ronnyfacio]

Nice! Cheers from Mexico!

[@aluminumshapingobsession2814]

Use clear vinyl hose on the overflows so u know which carb is leaking.

[@SPANNERRASH]

Great idea. Took me a while to spot the the leaky pipe inside.

[@aluminumshapingobsession2814]

I’ve been there man…!

[@oilinmyblood3906]

Hey spanner rash! I wanted to ask you if you realize that the carburetors on that motor are not the actual carburetors that would have come with it those carburetors you have go to a f model not the K model, if I were you brother I would switch then out and see what happens

[@SPANNERRASH]

Hi Jose, thanks for pointing that out, I thought they were F series carbs. For some strange reason they came with the bike. I'm hoping to get these set up and use them, although I bet my life would be easier if I swapped out for K carbs like you suggest. I may give up on these and find some K carbs if I continue to have endless issues. :)

[@oilinmyblood3906]

@@SPANNERRASH yeah seriously I would give them a go too, I have a gut feeling that is the issue because the F model engine has higher compression, a slightly more aggressive cam and is making more horsepower, so I'm almost certain those carburetors are not adequate for that motor, I definitely would give them a shot and get a set of K model carbs...

[@SPANNERRASH]

@@oilinmyblood3906 I didn't know the cam and compression ratio was different on the F model, i'll keep my eyes peeled on ebay in that case. :)

[@oilinmyblood3906]

@@SPANNERRASH yeah it's the truth brother! Anyone can look it up and see for themselves, I'll be waiting for the video, by the way I go by Joe, not Jose, it is my name in Spanish but I don't really speak Spanish, I can't wait to see your luck with a set of K carbs, I really think it should do the trick

[@jorgeeltoritoperez7238]

How much a carburetor job cost ?

[@SPANNERRASH]

I usually buy the rebuild kits off eBay, usually around £10 per carb. But if you want somebody to rebuild them for you there are usually people advertising through owners clubs that take on the work quite reasonably.

[@dannievaughn8588]

Common problem... I had a 750 4k that leaked gas into the air box. Brought it to honda and they didn't fix it right. Never did get it right.

[@SPANNERRASH]

Yeah its a pain, you always end up needing to replace the seals and the fuel valves on these.

[@aluminumshapingobsession2814]

I had the same problem. Ended up one of the brass riser tubes had leaks along the seam…just for insurance, clean and run a bead of solder on all 4. If one’s leaking chances are they all will soon. By The Way use a solder gun NOT A TORCH!!!