Topic: Air in the fuel system CB750K4

[tato81]

Air in the fuel system
- Patient: Honda CB750 Four.
- Description of the situation: As a motorcycle will stand a little longer (a week) after unscrewing the tap of fuel does not pass fuel from one cable. Only detaching the cable from the carburetor or filter or tap causes the fuel begins to flow. I noticed that in the drive, there is no problem because, as it already sucks up fuel flows.
- Excluded:
- Tap: the first thing I thought is that the fault of the tap. I have a spare so I replaced and continue the same.
- The air filter and airbox. I gave the new filter and nothing. I also have another airbox and both filters is the same.
- The fuel filter and lines. In fact, it coincided with the replacement. I set up again the old filter and was also a problem. I gave the other wires and still nothing.
- Venting of the fuel tank. Before there is my problem, I sealed cap fuel tank giving a leather pad. I dumped her and I still have a problem

Does anyone have any ideas what could be wrong? I wonder if, after all once again not to return to the fuel filters and buy some decent and not Chinese. Maybe it was a coincidence that the old can not overflowing. I wonder whether if it is defective fuel tank vent is whether also has an effect on air in the system.

[pjlogue]

Check your float bowl fuel levels using clear tubing method.  Your bowl vents may be clogged or the fuel valves inside the float bowl may be sticking shut.

-P.

[tato81]

thanks man, I'm going to check it out

[Bodi]

This question is a bit unclear.
"in the drive, there is no problem"
So what's the worry?

You won't necessarily see fuel flow when you open the petcock. If you aren't having problems when riding, I don't know how to fix this non-problem. If you get good fuel flow from the petcock outlets with no hose attached and the bike isn't starving for fuel then nothing is wrong.

You do need some tank venting - not much. The original "donut" cap sealing gasket should not be replaced with a solid disk... but I can't see any way that would be possible. The centre portion of the cap has the air vent.

Air bubbles in the fuel line or in an aftermarket filter don't cause much trouble, the fuel flows around them as the bubble floats up against the flow. You can have flow a problem if there's a big bubble in an upward loop of fuel line, and the fuel line routing should never go much at all above the bottom of the tank. Short, direct, and sloping down is ideal.

Inline fuel filters should be OK, although not usually necessary if the petcock filter is intact. Maybe where you are the fuel is not so clean as we get, and the filter is needed?

[tato81]

The problem is in the fact that after removing the fuel tap, float chamber are dry in 2 carburetors, fuel shortage. The air in the fuel line between the fuel tank and carburetor. How to disconnect the fuel line from the carburetor, fuel begins to flow, and after reconnecting the fuel fills the chamber float.


So look at my clear tubing method; Is not too low? The fuel level is 7-8mm











On 25.05.2012 the user "Spaner 1" posted in the topic "CB750 Carb bowl Sight Glass Mod. Amazing Discovery!"

"There are venting passages in the upper carb body too which are connected by that black rubber hose between carb 1 and 2 and 3 and 4 to 'equalise' the air pressure above the fuel in each carb and allow for fast fuel intake..... any blockage in that system = low/slow fuel intake even if your carb drain test is good and fast .... a lot of s**t to consider/ check isn't there ! "

maybe there is a problem. I changed recently, these rubber hoses

[Duanob]

It's a gravity fed system. The first problem I see is you have a low spot in your fuel system. Try and route the fuel line so there is no low spots, just a drop. Also you might try it without the filters and see if that helps. I'm not familiar with the tap and fuel line split on a K4 750, but if you have two nipples on the tap and two separate fuel lines check the tap by pulling the fuel lines off, put a bowl underneath and open the tap and check the flow from both nipples. Also check the level of fuel in your tank. It's a no brainer but I have done it before, couldn't figure out why i wasn't getting fuel after putting a gallon in. After filling the tank, no problems.

[tato81]

It may be easier than I think. It really can be "low spot fuel system". Thanks Duanob. And what do you think: the fuel level is correct? I have floats set as SHOP MANUAL. Corrected it to 4 mm?

[DH]

It's a gravity fed system. The first problem I see is you have a low spot in your fuel system. Try and route the fuel line so there is no low spots, just a drop. Also you might try it without the filters and see if that helps. I'm not familiar with the tap and fuel line split on a K4 750, but if you have two nipples on the tap and two separate fuel lines check the tap by pulling the fuel lines off, put a bowl underneath and open the tap and check the flow from both nipples. Also check the level of fuel in your tank. It's a no brainer but I have done it before, couldn't figure out why i wasn't getting fuel after putting a gallon in. After filling the tank, no problems.

+1, and keep in mind that both of the outlets don't necessarily flow fuel at the same time. Pull the hoses like mentioned, pinch one hose,
and the other hose should flow fuel. Reverse the process and restrict the second hose, and leave the first hose open. Fuel should flow.
Each half of the valve is designed to flow fuel as needed by by each pair of carbs. If it passees this test, The valve is working properly.

[tato81]

I do not know if I understand this test. My English sucks!

[DH]

I do not know if I understand this test. My English sucks!

If fuel flows from one nipple only, block it off and see if fuel begins to flow from the other one. and if so, the valve is working as it should.
check for proper operation in the reserve position as well. This only tests the valve, you may have other issues as has been suggested earlier.

[tato81]

Ok, now I understand. I'll check it

[pjlogue]

The fuel filters might be the problem.  I have had them cause problems with fuel flow when there was an air pocket in them. try removing them and see if your problem goes away.  If it does and you want to have a filter install it vertically where the fuel feeds into the filter from the bottom and exits from the top.

-P.

[Duanob]

It may be easier than I think. It really can be "low spot fuel system".

Thanks Duanob. 

Your welcome.

And what do you think: the fuel level is correct? I have floats set as SHOP MANUAL. Corrected it to 4 mm?

I prefer 2-3mm from the lip of the bowl but yours seems quite a bit lower. Maybe recheck the float heights again?