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Recommended float valve/needle & a diagnosis story 09 Oct 2017 10:40 #772591

  • ayeckley
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Summary:
I've used a quadruple real-time clear tube float test shown in the attached picture to hopefully finally diagnose the engine problem that's been dogging me for two years now. I believe I need to replace (at a minimum) two float needles on a set of BS34's and I am wondering what everyone's recommendation is. Should I replace as a solid-tipped needle/seat set using OEM Mikuni parts at $37 per, or try just the aftermarket Viton-tipped needles that are out there? Or some other variation? The various toothpaste/polishing tricks didn't work well enough for me to eliminate the slow leaks (yes, I am already using a good quality filter upstream).

The full story:
I picked up this bike (the GPz 750 in my signature) a couple of years ago and as part of getting it back on the road I had cleaned and serviced the carburetors (replacing rubber components including diaphragms, Viton O-rings, etc.) while keeping the original Mikuni parts (or replacing with genuine Mikuni parts as necessary). At the end of that process I set the float levels using the time-honored clear tube method both on the bench and verified on the bike. The bike ran well but I mostly took short hops as I was repairing individual non-engine issues in the summer of 2016. Eventually (summer of 2017) I got ready for longer rides and discovered -- pretty consistently -- that after running at roughly 1/4-1/2 throttle for several (usually 5+) minutes, two cylinders seemed to cut out. After sitting it out for maybe 15-30 minutes I'd be able to re-start with all four cylinders but it usually wouldn't be until the next day that it would really run properly again. Since it seemed like a textbook thermal/electrical problem I worked my way through the usual suspects (maybe fouled plugs, maybe an intermittent coil, maybe an intermittent pickup, maybe a bad igniter unit, maybe bad plug wires, maybe bad ignition wiring harness, maybe a bad resistor in the plug caps, maybe low battery charging voltage, etc.) but to no avail. I also worked through all sorts of venting scenarios (clogged cap vent, vapor lock, too much pressure drop across the aftermarket fuel filter, faulty design of the aftermarket fuel tees, etc). Once I convinced myself that none of those things were at fault my thoughts eventually circled back to the carburetors. I had done a static clear tube test, but I didn't really know what was going on in the bowls when the problem occurred, hence the home-made getup shown in the picture. I could ride with the tops of the glass capillary tubing uncapped and once the problem occurred I could verify that the fuel levels were still correct on all four cylinders without first having to limp back home, get the tools out, etc.

Well, lo and behold on my first ride it acted up right on schedule and the fuel levels on #3 and #4 were way low (as shown in the picture). They had started out just fine at the beginning of the ride; right where I had set them via the clear tube method two years ago. Hmmm.

So my current thinking is that when I originally set up the float levels back in 2016 I didn't really pay attention to the speed at which I was performing it. I think I must have been waiting too long for the fuel levels to stabilize and adjusted the floats too low; by waiting too long for the levels to settle I had actually set them *based on the slow leaks*, rather than the "normal" filling rate. So - to fix this I need to re-set the float levels but I need to do it with needles & seats that don't have any leaks.

Oh, and In case anyone is concerned, I diligently use a Pingle tank petcock rather than the OEM vacuum petcock so no major worries about hydraulic locking the cylinders due to these leaks.
1976 KZ900A4 (1105 Project)
1976 KZ900A4 (Stock Project)
1978 KZ1000A2 (Completed Project)
1983 ZX750 A1 aka GPz 750 (Completed Project)
1983 ZX750 A1 (Almost Road-Ready)
1973 CL350K4 (Completed Project)
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Last edit: by ayeckley.

Recommended float valve/needle & a diagnosis story 09 Oct 2017 12:26 #772596

  • 650ed
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It almost sounds like fuel is not refilling the bowl on carbs 2&3 quickly enough. Since you have your 4 tube fuel level tool (very cool by the way) I suggest you try the following. With the engine and petcock shut off empty all 4 carb bowls. Hook up your fuel level tool. Then turn on the petcock and see the fuel level in all 4 carb bowls rises at about the same speed. If the bowls on carbs 3&4 fill much slower than those on carbs 1&2 there may be an obstruction of some sort on right side of the “T” fitting either in the fitting or #3 carb. Ed
1977 KZ650-C1 Original Owner - Stock (with additional invisible FIAMM horn)
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Recommended float valve/needle & a diagnosis story 09 Oct 2017 13:36 #772599

  • ayeckley
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You mean #3 and #4 I assume? I kind of did that already - once I had lost the cylinders I removed the sight glass for #3 and let it drain onto the ground as a bit of a flow test. Plenty of flow rate - also repeated the same for #4 with the same result. Both always refilled quickly to the low level then imperceptibly crept back towards the "correct" level over a period of many minutes.

I had considered the obstruction possibility, thinking that maybe the aluminum tee fitting and crossfeed tubes had blocked the flow somehow but it's flowing at a higher rate than any normal engine should consume.
1976 KZ900A4 (1105 Project)
1976 KZ900A4 (Stock Project)
1978 KZ1000A2 (Completed Project)
1983 ZX750 A1 aka GPz 750 (Completed Project)
1983 ZX750 A1 (Almost Road-Ready)
1973 CL350K4 (Completed Project)

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Last edit: by ayeckley.

Recommended float valve/needle & a diagnosis story 10 Oct 2017 04:58 #772626

  • Nessism
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Please do some research and double check but I think this float needle/seat is similar/same as what you need

www.amazon.com/Mikuni-Needle-Seat-Valves...eywords=N149.040-2.5

www.jetsrus.com/needle_valve_float_bowl_...l_gasket_mikuni.html

EDIT: I've messed around with the Suzuki version BS carbs a lot and have noticed that Kawasaki uses larger float needle/seats. For example, Suzuki GS bikes use a 2.0 and KZ bikes a 2.5. I'm pretty sure a 2.0 flows plenty for high speed work, so I'm not sure why KZ bikes use the greater flow parts. The high flow makes it more difficult to maintain the proper level in the carbs. Just thinking out loud here is all but I'd be inclined to use the 2.0 parts just because...
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Last edit: by Nessism.

Recommended float valve/needle & a diagnosis story 13 Oct 2017 12:26 #772895

  • kevski
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so I'm not sure why KZ bikes use the greater flow parts.

I guess when Kawasaki done the Daytona tests in 1973 the were set to run flat out and probably needed the higher flow rate so the probably had the insight to realize these bikes were going to be used for all sorts of speed trials as well as road use.

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