Been having some fuelling issues with my custom bike for a while now. Have narrowed it down to being very clearly an issue with fuel getting to the carb float bowls. I am running a high flow fuel tap with a 6mm barb, widening it to 8mm line through an inline filter (glass, so I can see what the hell is going on) and then widening again into the 10mm pipes under the carb and feeding in from the left hand side. When disconnected from the carb, the fuel runs freely from the fuel tank and at high volume through the line and filter with no problem at all. When hooking the carb up to a slave tank and running the bike, the fuel runs freely through the 6mm, 8mm and filter and the bikes runs perfectly.

BUT... The minute I hook the fuel tank up to the carb, I get what appears to be an airlock of sorts and the fuel will not flow through the line and the bike will eventually die as no fuel is getting to the float bowls.
Properly stumped now.

As a possible solution, am thinking of wiring in a fuel pump (original CBR1000F one that I have). Does anyone have a bike still using a fuel pump? If so, can you please take some pictures of the loom where the relay and the pump plug in, Specifically, the order and colouring of the wiring and where in the loom it is found. Yes, I do have a manual and a wiring diagram, but it is not very clear. My bike is fully customed and the loom has been cut about and so will need to try and splice in these points.

I should note that originally, the fuel line went over the top of the carbs and entered from the right (as can be seen in the full photo), but was having the same problem after I added the inline fuel filter. I changed it to the right hand side thinking it would be both a shorter route and also remove some of the high points in the line where air might collect rather than bleed out backwards to the tank. Any other helpful thoughts and solutions would be appreciated as well.


Bike as it looks now.

Current fuel line setup.

 

Hi Kleinbaai,
As a very quick and easy check, have you tried with your current lines / filter set up removed and a use single piece of clear pipe from the tank to the carbs?
This would say if it is the fuel filter that is causing the problem with the suction on the crabs being insufficient to pull the fuel through the filter.

 

Hi Al.

Yes, am gonna try that as my next move. Am just waiting on some elbows to put in the line as the clear pipe tends to kink more than standard hose when trying to bend around corners.

 

What year motor is it? How long does it take for the engine to die when connected to the tank? If you leave the gas cap open, does it still die?

 

I guess I should have added that the tank is well ventilated and have no issues with vacuum stopping the fuel flow. The motor is likely an 89, at least that is what the logbook says. Will never really know as the previous owner bought and fitted a motor from a breakers with the numbers removed... This was way back. AS for the engine dying, it takes a while, time differing as sometimes a wiggle of the line will let more fuel through, sometimes not.

 

@Kleinbaai OK, so you were asking about fuel pump. The 89 1000 didn't use one. Just saying. As for fuel flow using gravity feed, the amount of pressure that it is supplying the carburetors is directly related to the elevation difference between the level of the pickup and the entrance into the carburetor via the float valve. Naturally that is minus any restrictions in the line. An inch or two can make a significant difference when you're only talking a couple inches to begin with.

For Honda's that do use a fuel pump with carburetors, like on the CBR600F3, the control for the fuel pump was not just a simple relay that was turned on or off with the key similar to the way a Fuel Injected engine is. I have noticed that there are many fuel pumps available from ebay for the CBR600F3's that probably could be fashioned to work if you have to go that route.

 

Yep, had been thinking about whether the tank was high enough to get the necessary gravitational pressure to feed the carbs properly. It used to have no issue with this, and the problem only really started once I introduced an inline fuel filter. I did that because there was no filter at all when I got it, but now there is one with the new fuel tap. Also, when I took the carbs apart for cleaning, noticed there are some small individual filters in the float bowls themselves. The carbs are now spot on, so no issues there. Am going to try it without the inline filter again and see what happens, If I have to, will also re route the lines back to how they were when I got the bike so the fuel comes in from the right again.

 

Have now removed the filter and loosely fitted a clear pipe with 2 90 degree elbows in it. Initially, fuel ran and filled the pipe a bit then stopped. As I moved the pipe , pulling it down, it filled some more. First thought was that the level in the line was in line with the float bowls and when I pulled it down, it seemed to fill to that level again... Aha, I thought, is that what it does al the time, is the fuel line never full, but just to the level of the float bowl when the bowls are full and the inlets shut off? I would have thought that the line would still fil up as the air trapped in it should bleed back into the tank?

So then fitted it fully and opened the tap again... A trickle is all I got. Didnt fill the pipe at all. None the less, started the bike and warmed her up. On choke, did notice a small trickle as the bike was running, this increasing in flow as I blipped the throttle. I have a video, but not sure how to upload that here, so this is a link to it on my Google Drive. https://drive.google.com/file/d/1kxb...ew?usp=sharing

 

Another question... The fuel pump. Is it absolutely neccessary to have the relay? I know it wires in to the harness and means the pump will only run when the bike is making spark as a form of safety feature, but as it is a low pressure pump, will it stop pumping of it's own accord once the pressure has been reached. If so, this will make wiring it in so much easier as I can wire it in so that it comes on with the ignition very easily.

 

Can you disconnect the fuel line on the outlet side of the 2nd 90 degree elbow? Does the fuel flow freely at that point out of the hose at that point? Can you work to get all the air out of the line? The 2 different levels of fuel that are important is the top of the fuel in the tank relative to the top of the fuel in the bowls. That's the pressure difference. Are the carburetors properly vented?

As for the pump, I'm sure you could wire it that way. I think they also do it to reduce the amount of current running through it as well. The contacts in the pump are a weak point, hence the link in my signature.