missing, backfiring, leaking,

Ok, My bike ran awesome, then I had to get into the throttle and go fast(just one of those itches). Well I let off and it's missing terribly. I get to where I'm going and it is leaking gas out of the first cylinder airbox inlet duct. The exhaust pipe on that same cylinder is evaporative not sizzling. Pretty much a dog. Hope you all can help. Tried to explain it best I could. Thanks...

rnuse wrote:

Ok, My bike ran awesome, then I had to get into the throttle and go fast(just one of those itches). Well I let off and it's missing terribly. I get to where I'm going and it is leaking gas out of the first cylinder airbox inlet duct. The exhaust pipe on that same cylinder is evaporative not sizzling. Pretty much a dog. Hope you all can help. Tried to explain it best I could. Thanks...

Guessing this is the 1978 KZ750 LTD?


Will parade some horribles for you ---

Perhaps #1 carb is flooding because float needle/seat interface isn't sealing (or maybe the float is leaking or stuck), and overflow function is for some reason insufficient. This would allow fuel up into the carb throat and then flow both ways, toward the air box and toward the engine. With gas leaking from the #1 airbox duct, gas may also be draining down through the crankcase vent hose into the crankcase. Gas in the carb throat flowing toward the engine may be getting on into #1 cylinder combustion chamber and washing down past the piston into the crankcase.

Excess gas into #1 carb could explain why #1 isn't firing, evidenced by cool exhaust pipe.

Would first prevent further gas from reaching carbs by whatever means is required, such as: turn petcock off, or clamp the fuel line, or remove the fuel line from the petcock, or remove the fuel tank.

Drain gas from #1 float bowl by loosening or removing the drain screw.

Drain the oil, and check it for gas contamination (would have a gas odor and might be obviously thiner than normal).

Replace the oil filter.

Add new oil.

Install a new in-line fuel filter.

Address and correct the gas leakage issue before allowing any more gas into the carbs.

Good Luck!

Wow close guess on the bike. Sorry I didn't mention that...
It's an 83 KZ750. I will check that.

Question though, even with the needle and seat stuck open wouldn't the engine begin to run correctly as the excess was sucked into the motor and eventually run like normal? Or is the flow THAT high with it stuck open?

rnuse wrote:

... even with the needle and seat stuck open wouldn't the engine begin to run correctly as the excess was sucked into the motor and eventually run like normal? Or is the flow THAT high with it stuck open?

It's the fuel level inside the float bowl. Being too high allows an over-rich mixture too wet to fire. For instance, a slightly too high float setting that's still below the overflow stage can nevertheless result in excessively rich mixture which causes the plug to soot and foul out and quit sparking.

It sure does smell like gas. So does the symptom of it suddenly running bad sound like I need to replace needle/seat or just adjust the float? Could I do it while still attached to bike being that it's on the outside? Basically I'm asking if this is going to take major surgery or not? the clymers manual makes it look more involved than what it seems. Thanks Patton, I'll be getting on this this weekend.

rnuse wrote:

It sure does smell like gas. So does the symptom of it suddenly running bad sound like I need to replace needle/seat or just adjust the float? Could I do it while still attached to bike being that it's on the outside? Basically I'm asking if this is going to take major surgery or not? the clymers manual makes it look more involved than what it seems. Thanks Patton, I'll be getting on this this weekend.

Being unfamilar with your particular carbs, am really just guessing. But it might possibly be (especially if running without an in-line fuel filter) that crud in the fuel has lodged between the float needle and float needle seat, and is also blocking the overflow tube. The float may be okay (not leaking, but might be sticking due to crud build-up) and not needing any height adjustment. The float bowl fuel level may be tested later after the leaking issue has been resolved, and I would not disturb the float level at this stage (don't mess with the tabs).

You could try smartly tapping side of float bowl with a small plastic mallet which sometimes allows the float needle to seat and perhaps release a sticky float. This is usually done while fuel is being allowed into the carb. Might also try some Seafoam or other brand cleaner mixed into the gas tank, which could possibly clear out some of the crud. But don't see much chance for any of this resolving the apparent lack of sufficient overflow function.

But a proper cleaning entails removing the carbs from the bike and disassembling them on the work-bench.

A Factory Service Manual covering the model and year of your particular bike would IMO be the best instructional guide. And you could check out this guide also: dansmc.com/MC_repaircourse.htm

Before proceeding, suggest visiting the Wiredgeorge Carbs website over there >>>>>>>>>> under THE ESSENTIALS and considering that as the best option.

Good Luck!

Agree with everything Patton recommended.

I had a similar problem earlier this season. It was crud/dirt under the needle preventing the float from shutting off fuel to the bowl. In my case I had not reinstalled the inline filter and the crud was picked up from the tank when I switched to reserve.

With the fuel switched on and the drain cracked, tapping on the bowl dislodged the crud...

Changed the oil and filter and installed a new fuel filter...no more issues.

This issue may not be carburetor related. It is very possibly an ignition issue. I am familiar with your carburetors and they seldom work then not work. It is possible that the float stuck on the carburetor where the cylinder isn't firing and flooded the cylinder but that is less likely than the cylinder quit firing because of an electrical issue and the gas just poured out. IF it was the #1 cylinder not firing, swap the #1 and #4 plug wires and see if the problem moves to the #4 cylinder. If it does, the problem is a bad plug wire or cap. If the problem stays on the #1 cylinder after swapping the plug wires, check the plug wire connection to the coil and replace the spark plug after setting the gap. If you do these things, you will eliminate the possible ignition related issues and can be fairly certain that the problem lies elsewhere. Other possible causes for combustion loss is poor compression on that cylinder.