YLTO-Superhawk

Hmmm, sure enough I had placed the tube for the back on the bottom. Having placed the tubes in all their proper places though I find I have a little bit of gas in the float on the front carb, but absolutely nothing draining out the float drain valve on the back carb. So.... plugged tube perhaps? Or something else before I tear into this thing.

Whats the SIMPLEST way to check the tube, and (roughly) how much disassembly is going to be required? Is there a part of the service manual that might be helpful?

YLTO-Superhawk

Is cleaning the carbs as daunting as it LOOKS? I ask because I just finished pulling and overhauling a 4 pack on my 79 CB750, and while after three times of doing THAT I feeling a bit like an expert, this bike here looks like a major project to dig into.

smokinjoe73

No its not, its easy except be really careful NOT to break the 90* bends that the choke cables make as they enter. They are wimpy plastic and you can break them even if you are trying not to (like me).

Is the bike running? You may just have a clogged drain so opening it doesnt flow gas. Its really obvious where the 2 fuel lines leave the tank, check them for flow with vacuum on the petcock.

BTW, we kinda just finished a huge thread on this very topic, so search around. May have been called starting issues

7moore7

Make sure gas is coming out of your petcock first, then work down into your carbs. The dangerous/easy way to do this is to disconnect the fuel lines from the carbs, hold them over a bowl, and turn the starter. You should get a pretty good flow of fuel from both of them.

And like joe said, they're not that difficult. Just watch out for those plastic choke cable pieces and don't mix front and rear carb jets and needles and such (as parts of them are different). If it's been sitting awhile may not be a bad idea to clean them anyway.

RWhisen

You won't have gas flowing until you create a vacuum on the petcock like when cranking the motor.

YLTO-Superhawk

I understand there wont be gas flowing until the vacuum is created. but ive cranked the engine repeatedly. Theres SOME gas in the front bowl, so Id expect some gas in the back bowl as well. The lack of it suggests that line is blocked, or something.

At least, I think thats what it suggests.

7moore7

You could make sure that the bike runs by pouring gas directly into the fuel lines going into the carbs. It'll run for a good bit before it dies...

If it backs up and you're still not getting fuel to the bowls, your problem is probably with a float or jet. If it runs, you're probably looking at a petcock rebuild.

YLTO-Superhawk

Ooooh, excellent idea.

Whats the likelihood that some kind of vacuum leak, causing a lack of suction on the petcock valve, is the issue here?

7moore7

Well, it's pretty easy to diagnose, as it's just one line (you should be able to visibly inspect it) or do what smokinjoe said and just suck on the vacuum line- there's enough strength in your lungs to activate the petcock. If that doesn't work, it's also pointing to rebuilding the petcock.

Your petcock is set to "on" right?

YLTO-Superhawk

Yes, the petcock is on, though they could have done a better job of marking that IMO. Had to look at the pictures on here to be ABSOLUTELY sure. That and the fact Im getting a LITTLE gas into one of the bowls.

I followed both the gas and the vacuum lines "into" the bike tonight. None of the 3 lines looks particularly easy to get to / see the end of. How much disassembly is necessary for example to pull the vacuum hose so that I can suck on it? How much to pull the gas lines out of the carb so I can check for blockages? I flipped through the manual but didnt see any obvious discussions on how to do these things. Just some stuff on removing carb / etc. Was hoping not to do that (at this point)

smokinjoe73

To really do anything other than tilt you head and squint, you are gonna have to do some dismantling. Seat off, tank unbolted & propped which is as difficult as just taking it off, but you can pivot it on the back bolt an use a tie down or coat hanger to hold it up.

Then remove the airbox which is more screwturning than anything else, at that point you can test the petcock. For that its easier to pull the tank & sit it on a trash can or milk crate. Use some other tube and suck on it to create vacuum.

Gas should gush from the 2 large outlets on the tank. If not you have answered all questions.

If you are dilligent and careful there is nothing tricky until you remove the carbs which you dont have to yet.

Ug. I just wrote this same whole story for the last thread with the same issues. Leave the 3 lines attached to the carbs and do the above proceedure. There is no way around it.

YLTO-Superhawk

I do appreciate you taking the time to write it again. The extra how to insight is a great piece missing from the manuals.

Speaking of that, I have two tubes that theoretically FIT on the vacuum line. Switching didnt seem to help the issue, and the manual stick drawings are NOT helpful. Is there a good way to know which one is the vacuum line? It SEEMS from the drawings that I can discern to be the one that runs parallel up the left side of the frame beside the fuel line.

aja

You are right, vacuum hose is on left, the vent hose is on the right and slips over the small nipple on the bottom of the tank itself.

I seriously doubt that the lines running to the carb are blocked. They are huge, and there is an in tank filter so how something big enough to block it got by the filter would be incredible. If you have vacuum but still aren't getting gas, its most likely your petcock diaphragm is worn or ripped and needs replacing.

smokinjoe73

One is a breather that goes to somewhere else on the tank & then down in front of the rear tire. The vacuum line goes to the base of the carb.

Have you done the tank removal yet or are you still squining and tilting your head?

YLTO-Superhawk

the tank is off, though I did want to confirm the hoses before I spent a lot of time trying to solve a problem I didnt really have.

I have 5 hoses in total. The two fatter hoses clearly are the gas lines and clearly go to the sides of the petcock. I have one other fat hose that only really fits on the underside of the tank.

Whats left were the two smaller diameter tubes. And now youve answered that for me definitively, so im ready to proceed with all these things you guys have suggested.

smokinjoe73

Its kind of funny to hear after how many times I have ripped the hoses and tank off over the years. Not to mention my recent motor swap. Its kind of like taking off your shoes & retiing them.

You can always just video the whole disassembly or take alot of pics.

there is also a repair manual on here somewhere and lots of good diagrams of where stuff goes, but alot of the hoses are superfluous. Like 80% of them can be trashed & you will be happy to be rid of them (the whole PAIR system).

You really only need the vacuum and 2 fuel lines.

Go to section 1 page 25 (1-25) in the manual