Oil/Gas backup into airbox, out exhaust

Hi guys, I finally got my '77 running (FINALLY! it's been 12 years since it ran) only to have a nasty liquid mixture (i'm assuming and totally guessing it's a gas/oil mix) backup into the airbox. a decent amount too, it then spits out the exhaust pipe as it exits the airbox.

i took out the air filter, ran it again and saw the liquid coming up from a small hole just in front of the hole that the air exits from. i also noticed there is a small hose/tube thing that goes back towards the engine.

i have no idea what's going on, maybe someone has experienced this before, maybe i'm explaining something totally obvious, i don't know. Please help!!

thanks!

Could be non-functioning carb float bowl overflow circuit allowing fuel to rise up into carb and flow toward air filter and toward engine.

Fuel getting inside air box may drain through crank case vent hose directly into crankcase (this is bad).

Fuel from carb throat may also get into combustion chamber and flush past pistons into crankcase (just as bad).

Would sniff-test crankcase oil. Then change oil and filter. And not run the engine again until the problem is corrected (so excess fuel inside floatbowl may drizzle out through the exit nipples onto the garage floor as designed).

Excess fuel being admitted into the floatbowl is often due to a "leaking float needle" which usually means an imperfect seal at the needle/seat interface. A good reason to keep the petcock turned off while the engine isn't running.

Good Luck!

i'm not the most mechanical person in the world, so forgive my sounding like a layman, but 2 questions:

1) shouldn't it just be gas coming out of the carb ports? the fluid in the airbox was a really dark, much thicker gas/oil sludge. where is the oil coming from? could it be coming out of the crankcase vent hose into the airbox if there's too much oil in it?

2)is this float bowl overflow circuit (or needle, or seal) something i could replace? ie is it something relatively easy to access or would i need to disassemble a lot first? (i'm not even sure what it is)

Thanks!!

-and i went out to my bike at 1am to turn the gas valve to the off position, thanks for the advice!

Zen Pre KZ wrote:

...
1) shouldn't it just be gas coming out of the carb ports? the fluid in the airbox was a really dark, much thicker gas/oil sludge. where is the oil coming from? could it be coming out of the crankcase vent hose into the airbox if there's too much oil in it?

2)is this float bowl overflow circuit (or needle, or seal) something i could replace? ie is it something relatively easy to access or would i need to disassemble a lot first? (i'm not even sure what it is)....

(1) Thinking gas only from carb throat is mixing with oil sediment inside airbox from crankcase vent to create the sludge. Breather is supposed to separate gas/oil fumes in crankcase and send gas to airbox and oil back to crankcase. Overfilled crankcase can indeed cause excess oil mist getting into airbox.

(2) On the stock '77 KZ650 carbs, am thinking the overflow is a non-removeable hollow tube from inside bottom of float bowl. Gas level reaching the top of the tube (meaning the fuel level is already too high) is supposed to run out the tube through the nipple on the outside bottom of the floatbowl. The nipple should have a drain hose attached and routed over the swingarm to underneath the bike. Beginning with the entrance hole at top of tube, be sure the passage is clear all the way to end of drain hose. Sometimes a clog is blocking fuel flow through the entrance hole, or the hollow tube, or the drain hose. If excess fuel is being allowed into the floatbowl and can't escape through the overflow, it rises up into the carb throat and flows both ways (forward toward the engine and backward toward the airbox.) Cleaning inside the floatbowl requires removing the carbs and then removing the floatbowls. Might be lucky enough to find the blockage limited to a kinked or clogged drain hose, so would first check the drain hoses from the floatbowl nipples to underneath the bike.

Where carbs are removed, would consider disassembly and thorough cleaning.

Good Luck!

awesome. i actually just had the carbs cleaned/tuned/etc. by a mechanic at the shop my dad takes his bike to. so they should be top notch. i also replaced the overflow tubes, so they all flow clean.

anything else i could do before trying to fire it up again? would firing it up and letting it run a while (in hopes of running the kinks out) be a bad idea?? my plan was to let it run a while, maybe take it around the block, then change the oil & filter. good/bad idea?

thanks for the help!!

Zen Pre KZ wrote:

awesome. i actually just had the carbs cleaned/tuned/etc. by a mechanic at the shop my dad takes his bike to. so they should be top notch. i also replaced the overflow tubes, so they all flow clean.

anything else i could do before trying to fire it up again? would firing it up and letting it run a while (in hopes of running the kinks out) be a bad idea?? my plan was to let it run a while, maybe take it around the block, then change the oil & filter. good/bad idea?

thanks for the help!!

Don't forget to sniff-test the crankcase oil for any gasoline odor. If there's any doubt about proper lubrication with existing oil for whatever reason (fuel contamination, yuckiness, etc.) would change both oil and filter before cranking up the engine.

For routine oil/filter changes (where the oil isn't fuel-contaminated or really nasty), it's better imo to allow a little warm-up before draining, which allows more of the solids to remain in suspension for more thorough draining.

In this case, if positive there's no fuel contamination, and the oil doesn't look seriously nasty, it would probably be okay to take it around the block before draining the oil.

Oil and filter are relatively inexpensive. Can't hurt to play it safe by draining it cold, replace both oil and filter, and then run it for awhile. Keep an eye on the level, assure no fuel contamination, and do a warm drain the next time.

Good Luck!

Sat for 12 years huh? I'm assuming you did an oil change before cranking her over. Or just added oil to her. I bet you didn't get all the old oil out of her and then over filled it. Then it re-liquified and is coming out of the vent. you need to warm it add some marvel mystery oil and do a quick oil change. Note the amount of liquid coming out of the bike and it's condition. Bet it's black and stinky.

yeah 12 years. guy had an accident (albeit a small one), got freaked out and garaged it. after a lot of convincing, he finally sold it to me. mechanically it's in UBER awesome condition. with a fresh battery even the electric starter still worked.

i did a few things right away, bleeding the brake lines and doing an oil change being among the first. the oil was NASTY, so was the brake fluid. i think i've decided to run it around the block once or twice, then do an oil/filter change and hope for the best.

thanks for the help!!