CB750C Carb 2 leaking fuel, entire bike runs only on choke

[deeveeance]

Hello all, picked up a project for only the cost of the uHaul and gas by trading an unused laptop.

1982 CB750C

Previous owner had new baby, had the bike he wanted to work on, no time, sky is blue. He mentioned a gas leak, and he thought it was the bowl gaskets. Had the part, no time.

Swapped the laptop, took the bike home.

As the title says, after taking it apart and replacing just the gaskets (in hindsight, that was stupid and I should have just checked everything else on the carbs), carb 2 is leaking excessive amounts of fuel via the ejection ports, routed to the bottom via thin tubing. Bike only runs on choke, then dies down.

I was doing a bit of searching, found a couple videos, and a thread on here with the advice that the "idle circuit in the carbs are plugged" on someone else's bike with the same issue.

Anything else I should check for? I have the Clymer's book coming in the mail next week, but thought I'd ask here to see if anyone else ran into this issue.

Thanks in advance, sorry for the novel!

 

[DetailGuy]

Having sat, depends on if the gas was left in or carbs dried out...

A full carb clean is the only way, check the seating of all the rubber sections..

Check out
#motorcyclemd Cody is a master Honda Bike mechanic, he has plenty of video on similar things.

I took my other bike to a shop, for another repair it too leaked all over the shop. He too cleaned all the carbs put them back on,leaked again..mmm took them off to find another problem,so yet it does happen always fixable..could be old bad gas that has clogged the system.

#mustie1 is a big bike, carb guy on utube, good luck.

 

[scoleshi]

Hello all, picked up a project for only the cost of the uHaul and gas by trading an unused laptop.

1982 CB750C

Previous owner had new baby, had the bike he wanted to work on, no time, sky is blue. He mentioned a gas leak, and he thought it was the bowl gaskets. Had the part, no time.

Swapped the laptop, took the bike home.

As the title says, after taking it apart and replacing just the gaskets (in hindsight, that was stupid and I should have just checked everything else on the carbs), carb 2 is leaking excessive amounts of fuel via the ejection ports, routed to the bottom via thin tubing. Bike only runs on choke, then dies down.

I was doing a bit of searching, found a couple videos, and a thread on here with the advice that the "idle circuit in the carbs are plugged" on someone else's bike with the same issue.

Anything else I should check for? I have the Clymer's book coming in the mail next week, but thought I'd ask here to see if anyone else ran into this issue.

Thanks in advance, sorry for the novel!

When filling the float bowls after disassembly, make sure the bike is on the main stand not the side stand. Yes I did have this issue a few times in a row. After putting on the main stand, the issue stopped happening. I figure the float somehow gets hung somewhere during filling and will not float if it is on an angle after being empty, or the overflow starts before the float can close the float needle valve. Fuel will just keep coming out of the overflow if on enough of an angle.

 

[Tomos]

You may have several problems with that carb.
- draining screw tip is damaged,
-draining screw gasket blocks screw (to tick or damaged)
-bowl seat mating surface with screw tip is damaged,
-over flow bowl tube(brass tube)is cracked or loose in bowl.
I hope this helps.
Regards Zoran

 

[DetailGuy]

You can do a leak test, make a frame over a plastic box, fill with some fuel to see if it is leaking prior to fitting to the bike.

Any good new have you managed to repair them ?

 

[ricerocket71]

i wold recommend looking at each carb (one at a time).
take of the float cover and turn gas on. you can then use your finger to lift float. (if gas stops that one is good).
continue to do this to all four. (most times it's only one float dirty or stuck )

Good Luck

 

[kedun]

i wold recommend looking at each carb (one at a time).
take of the float cover and turn gas on. you can then use your finger to lift float. (if gas stops that one is good).
continue to do this to all four. (most times it's only one float dirty or stuck )

Good Luck

Can you do this with the carbs still on the bike?

 

[Stan C]

Can you do this with the carbs still on the bike?

Yes you can, I own a 1979 CB750K, at present and did own a 1980 F for some years. If you use a 1/4 ratchet with short extention and philips bit, you can undo the (3) screws to each of the float bowls. More room is obtained by removong the starter cover. Pay particular attention to the second carb from the left as it has an accelerator pump feature. As long as you remove the bowl straight downward without tilting it, you should have no problem. Be careful of any small bits that may come loose such as the pump plunger itself. Also, if you drained the carbs at one point and you tried to refill them do it ON THE MAIN STAND NOT ON THE SIDE STAND. I had the very same issue and corrected it by refilling the floats with the bike on the main stand. Hope that helps. The comment that kedun made is excellent advice as well. This will confirm if your float needle is working properly by pushing it upwards gently, then turning on the gas. If no gas comes out, chances are the needle valve is OK. While you have the bowls off, unscrew the jets and make sure that they are perfectly clean inside and out. The secondary jet (one with multiple holes up the side) is very critical. It CAN NOT be dirty, ANYWHERE ! Also, inside the gas tank there can be NO DIRT ! !

 

[Chrisraven]

I would agree with all the above advice, a couple of points maybe worth mentioning, have you got an in line fuel filter in case there is dirt/rust in the tank. And when you are cleaning out the carb jets be extra careful if you try to clean out the jets with a wire it is very easy to damage them. An unltrasonic cleaner is the best, I use one for everything I do, does a great job of cleaning all the little crevices that gunk likes to hide.

 

[Stan C]

I would agree with all the above advice, a couple of points maybe worth mentioning, have you got an in line fuel filter in case there is dirt/rust in the tank. And when you are cleaning out the carb jets be extra careful if you try to clean out the jets with a wire it is very easy to damage them. An unltrasonic cleaner is the best, I use one for everything I do, does a great job of cleaning all the little crevices that gunk likes to hide.

I put an inline filter in the line but the bike died after riding around the corner. After I removed the filter, it ran fine. I think because the filter was made in the far east where filters look like filters and feel like them and cost the same and probably actually do filter but . . . . . too slowly. My solution was to siphon all the dirt from the tank after swishing all the fuel to the gas line side and using a flashlight to find the fine dirt at the bottom and throw the "filter" away as the stock bike has no filter. (Intentional ?) Most likely as these bikes don't run well with dirty fuel just like anything else.

 

[Chrisraven]

I use the bronze or brass metal filters, clear housing so you can see when and if they get dirty. To run the bike without a filter is really just asking for trouble, the jets inside are really fine and when they plug up the only way to clean them is with an ultrasonic cleaner. The dirt just wedges itself into those crevice's and will not come out any other way. If you are having trouble adapt the fuel line out of the tank from 1/4" to 5/16 then get a open flow filter at least then you will get enough gas through to do the job.

 

[Stan C]

Having sat, depends on if the gas was left in or carbs dried out...

A full carb clean is the only way, check the seating of all the rubber sections..

Check out
#motorcyclemd Cody is a master Honda Bike mechanic, he has plenty of video on similar things.

I took my other bike to a shop, for another repair it too leaked all over the shop. He too cleaned all the carbs put them back on,leaked again..mmm took them off to find another problem,so yet it does happen always fixable..could be old bad gas that has clogged the system.

#mustie1 is a big bike, carb guy on utube, good luck.

For what it is worth, I had installed the float bowls after cleaning the carbs and after reassembly, and refilling, some leaked. I noticed that the bike was on the side stand, so I tried to remove and replace the bowls with the bike on the main stand. After doing that, I had no leaks and no problems ever since. I believe that there is a design issue inside the carburetors that allows the float mechanism to catch or bind somewhere if the bike is placed on the side stand after the bowls have been emptied of fuel. Once they are filled, and remain mostly full, this would most likely not occur.

 

[brettp]

There might be some truth to that. The floats may have some room on the sides of the pin to slide left or right slightly. And when the fuel is drained from the bowl, could shift.