Topic: Bought a 72 CB750 K2, running but carb leaks, front-brake, iffy clutch

[edwardmorris]

Hello All, this is my first post on these forums. I bought a fairly nice 72 cb750 off of craig's list. L:ooks like the engine is in good shape, both kick and electric starter work, but every time I turn the fuel on at the petcock, the carb starts dripping gas pretty badly. Haven't been able to ride it yet because the front break doesn't work (can't tell if its stuck coz the wheel spins), and the seller told me that the clutch wouldn't engage until the lever was fully out. When running, it sounds pretty quiet for a 4 pipe megaphone exhaust. My goal is to be able to ride it at least minimally before the end of this season and then brave into the world of rebuilding it over the winter. This is where I need advice, I'm guessing I need to start at the carb leak first, then look at the front brake, then the clutch. Cosmetically it looks like crap, missing tin on the left side, brush painted horrid black and replacement front fender. I will upload some pix as soon as I can. Any and all advice is much appreciated.

[harisuluv]

Where is the carb leak coming from?  Fuel line connections, fuel Ts, float bowl to body gasket surface, overflow tubes? 

Pinpoint it and then we can go from there.

[edwardmorris]

Where is the carb leak coming from?  Fuel line connections, fuel Ts, float bowl to body gasket surface, overflow tubes? 

Pinpoint it and then we can go from there.

Its very hard to tell but I think it has to be the overflow tubes. The fuel lines look bone dry and this is severe when bike is on side stand and fuel line is on. I literally just bought it, so haven't had time to empty the gas tank, take it off and take a better look. But its bad, and only on the left side so far. I will try and add some pix when there is daylight. My garage lighting is horrid.

[harisuluv]

Ah I see. Sounds line overflow rubber tubes aren't installed.

[edwardmorris]

Ah I see. Sounds line overflow rubber tubes aren't installed.

This just gave me hope! If I can I'd rather avoid tearing open the carbs until I know for fact I'm starting the rebuild. Would you be able to confirm this and provide a solution if I posted some pix? Or would you need more?

[harisuluv]

Well if there are 4 long rubber tubes hanging off the bowls then they are installed.  If they are coming out the bottom that just eliminates some of the other places it could leak.

Unfortunately freshly harvested CBs will need to have those carbs come off. 

Post some pics of the carbs and where the leaks are.

[ekpent]

Where do you live Ed ? and do you have a pic of the bike. Best way to introduce yourself and get good help. . Welcome to the forum   

[Tews19]

Welcome to the site and yes please post pics!

[vrm]

I can't help thinking that you remind me of me two months ago. I bought two distressed 750s. One with a bad motor but otherwise not bad. The other a parts bike with a good motor. I have had to fix innumerable problems that I won't go into here except to say that tonight I fixed the last problem that was keeping me from riding and coincidentally  it was the number three carb leaking. I dropped the float bowl and saw that corrosion on the float pivot shaft was keeping the float from moving. I got the shaft out, cleaned and back in without losing the needle. It's no longer leaking and I should be able to ride to work tomorrow. Don't give up. You'll get there.

[ekpent]

I can't help thinking that you remind me of me two months ago. I bought two distressed 750s. One with a bad motor but otherwise not bad. The other a parts bike with a good motor. I have had to fix innumerable problems that I won't go into here except to say that tonight I fixed the last problem that was keeping me from riding and coincidentally  it was the number three carb leaking. I dropped the float bowl and saw that corrosion on the float pivot shaft was keeping the float from moving. I got the shaft out, cleaned and back in without losing the needle. It's no longer leaking and I should be able to ride to work tomorrow. Don't give up. You'll get there.

I like it another guy starting to tell a tale and talk.Don't be bashful and don't pick a fight with those Australians though-------         

[70CB750]

Welcome, Edward!

It can all be as simple as cleaning carburetors, cleaning hydraulics of the front brake and adjusting the clutch.

Download manual from here:

http://forums.sohc4.net/index.php?topic=17788.0

and on top of the three things do the 3000 miles maintenance.

Also dropping the oil pan to see what is in there and clean the oil pump strainer would be good.

[evanphi]

Brake lines would probably appreciate being replaced, and the caliper probably needs a once-over cleaning. I just replaced my original rubber hoses from 1975 with stainless last night. What an improvement! With the old lines, I could feel them bulging when I held the lever firmly. No wonder the front brake sucked!

[edwardmorris]

Thank you all for the warm welcome. I'm from NW Indiana. I haven't taken pix of my own, but here's one I grabbed from the listing. I will definitely take some after work either today or tomorrow and post them.

[Tews19]

Looks like a good foundation... Keep the updates coming.. Notice the airbox is not on...

[edwardmorris]

Looks like a good foundation... Keep the updates coming.. Notice the airbox is not on...

Is the airbox right after the oil tank and right before the carbs? I think its on there, isnt' it?

[70CB750]

air box looks tilted on the picture.

[edwardmorris]

Here are a couple pix of the carbs up close. Sorry for the crappy cell phone quality. I still can't pinpoint where the leak is but its almost instantaneous as soon as I turn the petcock. The best I narrowed it down to was right below the third carb.

[harisuluv]

I don't see any clamps on that fuel line and it is not 5.5mm. It is probably 1/4" and should have some kind of clamp on each connection.

Between three and four is the fuel t for number 3 and 4 carbs. You could be leaking from the hose connection to the T or from the orings on the fuel T or both.  Or perhaps neither.

To be honest the PO did a  poor job of installing the carbs and routing everything. As pointed out the airbox isn't really on all the way.  Sometime during the time you fix your leak you might want to consider taking the rack off and putting everything back on properly.

[edwardmorris]

Yea, I probably should try and take it apart and then put it together properly. For the immediate fix, are the 5.5mm hoses available easily or will I have to hunt for them? I'd like to replace them if they are not the right size even if they aren't the root of the leak. Labor day weekend isn't that far now, I'll try and empty the tank, take it off and see if I can get a better look at them and take the off for further inspection.

[70CB750]

Save yourself some frustration and get Tygon fuel lines.  They can be bought on eebay or you can just take a piece of the old line to John Deere dealer and they should have it in stock by foot.

I used vinyl lines when I first started fixing my bike up and they were a major nuissance.  Tygon will stand to the crappy gasoline we all use and it will seal on the nipple without any securing bands.  On top of that, Tygon line can be pulled off and put back on and it will not leak even after several repetition of that.

I believe I run this in my K0:

http://www.ebay.com/itm/NEW-TYGON-FUEL-LINE-3-16-ID-X-5-16-OD-BY-THE-FOOT-/310313612032?pt=LH_DefaultDomain_0&hash=item484021f300

But as I said, local John Deere should have it too, they use it in weedwhackers.

[edwardmorris]

I was reading around the forums, and someone here sold some ATV/Snowmobile fuel hoses that are 7/32" 5/56mm which sounds pretty close to what's needed. I looked it up and a local advance auto parts has it. Does the John Deere one come in the same size? If not, which is more recommended, proper size or longevity to gas exposure. AAParts has it for pretty cheap too, so replacing it a couple times may not be that bad....

[lwahples]

Looks like a good bike to start with.What are your plans? Just get it running or?

[edwardmorris]

Looks like a good bike to start with.What are your plans? Just get it running or?

Immediate goal is to get it running at least minimally for this season, then start restoration, possibly brave a rebuild. When the engine is running, its pretty smooth/even beats, so not too eager to tear it open just yet...

[Nic]

I see float bowls 2 and 3 are arse about. Swap them as well and rout the "new" lines properly. You need to figure out where the juice is leaking. If your float levels aren't too high and it's not the lines then it's more likely one or both of the Ts. I had my basket case all ready to start, turned on the fuel and it pissed out like it was raining fuel from the bowls. It was the O rings on both Ts, and easy fix if you don't live outta town like me.

[Tews19]

If it were me, send off the carbs to SOHC DIGGER and have him do his magic... Great price and work..... You need 50 post though.... He is near you as well... While the carbs are off, check the timing, points gaps, adjust the valves and the cam tensioner.... You should have a rider then.... I would get new tires before riding too.