Bike runs insanely rich when below 1/4 tank gas

[donmega]

Anyone have the same problem?

1996 kat 750

anytime I get below 1/4 tank its damn near impossible to even keep the bike running and runs like total shit.

I can smell tons of gas, bike doesnt want to rev, and idles rough as hell and I have to hold the gas to get it to stay running.

When it happens I have to let it sit for a while, like over night with a full tank then all is good.

happens everytime I get below 1/4 tank and it took a while to put my finger on it but this is the 5th time and I knew it was comming and it did so Im sure its directly related now.

[The B00t]

Your tank is probably filled with water and other crap. I would drain it, flush it out and look at what comes out. It is probably starting to rust. Someone else here can tell you how to take care of that.

[katana bob]

strange. It could be trash/water in the tank. where do you have the petcock set? it should be in "on". would a leaking gas cap gasket cause the vacuum from the motor to pull too much fuel in when the tank gets low?

[donmega]

Petcock is on ON position

Tank is very clean inside, no water cause I have went through too many tanks of gas to counts this summer and if there was any watter Im sure its gone by now.

I did drain the tank dry twice when working on it and it was clean comming out also.

[The CyberPoet]

Are you sure it's running insanely rich? It might be going insanely lean and the fuel you smell would be fuel that made it past the cylinders without detonating (usually accompanied by after-fires -- which you did mention in another thread, come to think of it).

Three primary possibilities, IMHO, two easy to test, one requires eyeballs to look at closely:

1. Petcock diaphram & spring borderline bad. As long as there sufficient fuel in the tank, there's enough pressure to help keep the diaphram open. As the fuel level drops, the pressure on the inside of the diaphram from the fuel itself drops and the fuel supply starts to dwindle.
TEST: When it starts acting up, switch petcock to PRIME while the bike is running. If the symptoms disappear in about 30 seconds (or less), this is the issue. Replace the petcock diaphram (get a rebuild kit from SpecialK) and all vacuum lines.

2. Vent tube and seal around gas cap not breathing right. As the gas drops, vacuum forms in the tank. Filling the tank eliminates the problem because you are opening the cap and equalizing the pressure.
TEST: Place a spare key into the gas cap and ride with it there. When it starts acting up, pop open the gas cap. If the symptoms disappear in about 30 seconds (or less), this is the issue. Replace the seals and drain/breather tube.

3. Fuel line and/or fuel filter installed between the petcock and the carbs in such a way that they are running uphill or flat at one point (or the fuel filter is not pointing straight down, such as installed on it's side). As long as there is sufficient fuel pressure from the gas in the tank, it's a non-issue, but as the pressure on the liquid from above drops, the fuel no longer has the push to make it through the flat/uphill section.
TEST: Remove tank and eyeball fuel line routing & fuel filter placements carefully. Relocate as necessary. IMHO, place the filter as close to the actual carb hose attachment as physically possible, to help ensure it's downhill all the way (or find smaller fuel filters).

Cheers
=-= The CyberPoet

[mystahagy]

I actually have similar symptoms sometimes when my tank gets around 1/4. I recently put in new fuel lines, and I suspect that they don't necessarily take a path that is the easiest to flow, remembering that it's gravity that fills our carbs, not a pump. I think one of the lines actually curves back up before going down into the carbs. I suspect that less pressure from the tank leads to lack of fuel at the carbs. I've been just trying to keep my tank filled, but it's something I'm gonna reroute soon, and also double check my fuel filters.

[mike10562004]

+1

[katitoy]

3 days ago, my fuel gauge needle hit a little below the red line. Then suddenly while turning the corner, I pulled the clutch and engine died. I restarted and noticed that their was no power and sound of my muffler is different. I was able to run about 2 miles then power was back and muffler sound is just right. Then problem occurs again.

What do you think folks?

Gas tank or gas filter problem?