Petcock (pet cock...say what?) Question

I just tried rebuilding the vacuum petcock on a kz550 and the prime setting worked at first, but after changing lines around a second time to work the kinks out of the fuel filter, the prime setting no longer works at all. Will the prime setting work even if the float bowls are full of gas?

It was rebuilt because the crank case was flooded with gasoline and the petcock was the most likely culprit.

There is a small trickle of fuel while the engine is running in the ON position but the bike is un-rideable. It stutters and refuses to go beyond 4000rpm and I can't tell if it's running out of fuel or if it's a spark problem. The number one plug was fouled from gas/oil but I've never seen the bike run so poorly. All new fuel and vacuum lines to the carb but it runs like crap.

Grebnaws Inc. wrote:

I just tried rebuilding the vacuum petcock on a kz550 and the prime setting worked at first, but after changing lines around a second time to work the kinks out of the fuel filter, the prime setting no longer works at all. Will the prime setting work even if the float bowls are full of gas?

It was rebuilt because the crank case was flooded with gasoline and the petcock was the most likely culprit.

There is a small trickle of fuel while the engine is running in the ON position but the bike is un-rideable. It stutters and refuses to go beyond 4000rpm and I can't tell if it's running out of fuel or if it's a spark problem. The number one plug was fouled from gas/oil but I've never seen the bike run so poorly. All new fuel and vacuum lines to the carb but it runs like crap.

Sounds like the petcock is still mal-functioning AND float needles are leaking AND the overflow circuits are clogged. Also, maybe a float is sticking or leaking, and perhaps the float levels are set too high. Such problems could exist in one or more of the carbs.

PRI is same as having a direct line from tank without any petcock.

Here's a simplified version of what happens as the fuel flows from the tank into the carb.

Fuel flows into carb float bowl through an orifice in the float needle seat.

Float rises pressing pointed tip of float needle against seat orifice (supposed to completely seal the seat orifice upon float attaining correct height via buoyancy in the fuel). A leak at the needle/seat orifice -- a "leaking float needle" -- due to crud, grit, worn or damaged needle tip or seat orifice, or whatever reason, allows fuel to keep on entering the float bowl.

When excess fuel enters the float bowl past a leaking float needle, the excess fuel goes somewhere -- usually through an overflow system designed into the float bowl, which excess fuel drips to the outside. If the overflow system is clogged, the excess fuel may rise upward into the carb and flow both ways -- forward toward the engine, and backward toward the air filter.

Excess fuel which has not escaped through the overflow and has consequently risen up inside the carb -- flowing forward toward the engine -- gets past the intake valve into the combustion chamber and flows down around the pistons into the crankcase.

Excess fuel which has not escaped through the overflow and has consequently risen up inside the carb -- flowing backward toward the air filter -- either drips from a pod type air filter, or collects inside an air box.

Excess fuel which has not escaped through the overflow and has consequently risen up inside the carb and flowed backward into an airbox, escapes from the airbox either to the outside through the air box drain hole or by draining into the crankcase through the crankcase breather hose.

When the fuel delivery system is working correctly as designed (needle not leaking), there should be no excess fuel being allowed into the float bowl regardless of the petcock position. And any excess fuel which might be allowed through a leaking needle is supposed to escape through the overflow.

When the engine isn't running, turning a properly functioning manual petcock to the OFF position, or leaving a properly functioning vacuum petcock in the ON or RES position, stops fuel flow into the carbs so there is no "excess" fuel to escape.

Caveat - a serious float leak coupled with a faulty floatbowl circuit, may allow excess fuel to enter the crankcase while the engine is running, and of course while riding the motorcycle.

A frequent oil sniff-test is recommended, along with assuring: absence of float needle leakage from whatever reason; correct float height setting; properly operating floatbowl overflow circuit (including clear drip tubes); properly functioning petcock; and always using an in-line fuel filter.

Am thinking probably time for a good carb overhaul.

Good Luck!

Thank you for the description Patton.

I cleaned/rebuilt the carbs at the beginning of summer. Once it was bench sync'd and installed the bike ran like a top! A few months later and now it's running badly. We had some fuel delivery problems outside of town and did a makeshift fuel routing job. Now that we're working on the bike again all kinds of mysterious problems are happening.

The petcock knob was extremely tight after gasket installation. I wonder if it was so tight that we actually moved the circular gasket to a new position while turning it a second time. I'll buy a shorter, disc style, filter and easier to use hose clamps and get back to work on the bike this weekend. The longer lawnmower style filter doesn't fit well without kinking the line, and even if that isn't the problem it sure looks like amateur work.

Grebnaws Inc. wrote:

...cleaned/rebuilt the carbs at the beginning of summer. Once it was bench sync'd and installed the bike ran like a top! A few months later and now it's running badly. We had some fuel delivery problems outside of town and did a makeshift fuel routing job. Now that we're working on the bike again all kinds of mysterious problems are happening.

The petcock knob was extremely tight after gasket installation. I wonder if it was so tight that we actually moved the circular gasket to a new position while turning it a second time. I'll buy a shorter, disc style, filter and easier to use hose clamps and get back to work on the bike this weekend. The longer lawnmower style filter doesn't fit well without kinking the line, and even if that isn't the problem it sure looks like amateur work.

Some quick thoughts ---

Fuel delivery issue could be from:

Blockage in fuel tank venting (could test run with cap ajar to see if it makes any difference);

Petcock clogged (blocked drain screen or filters up inside tank) or improperly assembled or faulty parts;

Inline filter -- have heard (but have no experience to verify) that some lawnmower filters don't flow enough fuel for the KZ engines. If marginal flow when new, perhaps it worsened quickly as it became increasingly obstructed with the crud it was filtering from the fuel;

Possibly got some bad gasoline at a re-fill.

Take your time when routing new fuel lines so they loop smoothly with a straight run into and from the filter, into the carbs, and from the petcock. Clamps are okay, but should not be necessary. Sometimes its a tight fit, but usually there's a way with enough patience and ingenuity.

Good Luck!