Gas in airbox

1980 KZ550A

Hi all,

So after a top end rebuild, running right, then having gas pour out from the overflow I pulled the carbs and replaced the jets and float valves, I used clear tube to set the float height. After a few short rides I noticed a loss of power. This morning I went out to go over a few things before a ride, and the was a ton of gas in the airbox. I mean like half full. I drained the gas and checked the float height again. All 4 carbs float height are normal. I cant smell the oil for gas in the crankcase as everything smell like gas at the moment, but the oil level is higher than normal and a lighter color so I suspect that the is gas in the oil. I know I will need to change the oil, and pull the carbs yet again. But before I do does anyone have any ideas of what is going on to cause this?

If you have gas flowing into the airbox while the engine is off you have at least 2 problems.

First - your petcock is not operating properly. With the engine off the petcock should prevent fuel from flowing from the tank. If by chance you have a manual petcock rather than a vacuum operated one, do you shut it off when you shut off the engine as you should or if you have the stock vacuum operated petcock are you leaving it in the "RUN" position as you should? If so, you need to rebuild or replace the petcock because it is not working properly.

Second - assuming you set the fuel level correctly, at least one of your float valves is not closing completely or may be sticking open sometimes. Even if you replaced the float valves they can stick open if any crud from the tank gets in them. Do you have an inline fuel filter? If not, you should install one and then carefully clean the float valves to ensure they close all the way every time the fuel has reached the proper level.

One other thing to check is the overflow tubes in the carb bowls. These sometimes develop cracks. When they do fuel will flow from them even if the fuel level is set correctly and the float valves are operating properly. This seem unlikely in your case since there is sufficient fuel flow to have fuel enter the airbox, but it's worth checking while you have the carb blows off. With the carb bowl in hand, just fill it with liquid and see if the liquid leaks out the overflow. Ed

Sometimes the petcock's vacuum diphragm fails and fuel flows backwards down the vacuum line and into the carbs/airbox while the bike is parked. Something to check out anyway.

If not already done, fuel level should be checked by performing the clear tube test on each carb.

Good Fortune!

Nessism wrote: Sometimes the petcock's vacuum diphragm fails and fuel flows backwards down the vacuum line and into the carbs/airbox while the bike is parked. Something to check out anyway.

That would be my bet.
Steve

Thanks for the advice. It does have a vacuum operated petcock. I have performed the clear tube test on all 4 carbs and they showed normal float level. I know the overflow is working on the carbs as gas coming from the overflow is what prompted the carb rebuild. Does anyone have any idea as to how I would test the petcock?

Since you have a vacuum operated petcock it should be easy to see if it leaks when the ENGINE IS OFF. With the petcock in the RUN position just put a catch pan under it and disconnect the vacuum line. If fuel comes out the petcock has a problem. If no fuel come out disconnect the fuel line from the petcock. If fuel comes out the petcock has a problem. Don't disconnect the fuel line from the petcock with the petcock in the Prime or Reserve position. Also - NO SMOKING or any open flames while doing the test. It's best to do this outside - just in case. Ed

So I tried testing the petcock, and could not get gas out of the vacuum line, ordered a petcock rebuild kit from Z1E anyways. Figure it wont hurt nothing if there is no problem. In the mean time, would it be a dumb idea to plug the vacuum line and ride with the petcock on prime? Also all of the inline fuel filters I have seen have been overly large. Any ideas on where to find a small fuel filter? Thanks

If no fuel came through the vacuum line that is a good thing. That line never should have fuel passing through it. If you plug the petcock vacuum port be sure to also plug the vacuum port on the carb or the bike will run like crap. Then you should be able to run the bike on prime without a problem, BUT be sure to turn the petcock handle back to RUN whenever you park the bike with the engine off. The problem with leaving it on Prime when parked is that if a float in a carb doesn't fully close fuel will flow from the fuel tank into the carbs and possibly into the cylinders and corrupt the engine oil. Ed

Djckay wrote: ... Any ideas on where to find a small fuel filter? Thanks

Readily available at most motorcycle dealers, motorcycle repair shops, Z1 Enterprises, etc.





Good Fortune!

Napa 3027

www.napaonline.com/Catalog/CatalogItemDe...R-FIL3027_0289039844

Here's a link that shows what I use; very small filter and super-fine filtering. Not cheap but will never need to be replaced since the stainless steel filter element can easily be cleaned and re-used. Ed

www.kzrider.com/forum/3-carburetor/59406...ne-and-filter#633243