KZ750 acts like it's running out of gas - Kawasaki Motorcycle Forums
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post #1 of 32 (permalink) Old 09-22-2016, 07:15 PM Thread Starter
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KZ750 acts like it's running out of gas

I've been trying to chase down this problem for a little while now, and it's driving me a bit crazy.

It started a little while back, after riding the bike for almost the whole summer and it running great, intermittently it would act like it had no power, felt and sounded like one of the cylinders wasn't firing, basically the same symptoms as when you're running out of gas. It didn't happen very often, and after a few seconds it would go back to normal and things would be fine for a while.

Things escalated a bit, and it happened on my way to the shop. Parked it outside, grabbed a couple things in the shop, came back and one of my carbs was pouring fuel into the parking lot. Got it home, set all the float levels and cleaned the carbs (which I had done recently, mysteriously one was way out already) and figured that my problem was solved, until it happened again a few days later, started pouring fuel out of a different carb this time. Again, took it home, re-checked all the float levels and set them a millimetre lower than spec (spec is 4mm) and crossed my fingers. At this point I also checked my valve clearances, ignition system, and timing. Clearances were good, got fat blue spark from all 4 plugs, no resistance issues in the leads, and timing was good. I also synced the carbs with a homemade manometer, so not a great job, but got them pretty close. Now I'm still having this issue. At pretty much anything other than W.O.T. the bike sometimes decides to act like its fuel starved. It doesn't happen often, but when it does it's hard to keep the bike going. If I have my hand off the throttle at a light or something it will die instantly, but I can always start it right back up again. When the engine is under load it loses almost all power for a bit until it works through whatever is wrong with it and then it's back to normal. Other than this it's running great, if a bit lean, and idling better than it ever has, so I don't know what else to look for here.

Anybody have any thoughts as to what I might be missing here? Ideas on what to check next? It's not dumping fuel anymore, which makes it rideable, but it's not a good problem to have. Seems to happen more at low rpm than high rpm.

It's a 1982 KZ750 with Keihin CV34s and pod filters (I know, I know, but it was running fine for a long time before this stuff started happening) and a 4-into-1 exhaust. It's got Dyna S ignition and coils.

1982 KZ750 LTD
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post #2 of 32 (permalink) Old 09-22-2016, 07:53 PM
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I had the same issue with my GPz750, I don't run pods but I have a ZR7 header. I ended up having to rejet. Which had me running rich so I cut the stem off the intake cap for a bit more air and it's running fine now.

As for the overflowing carbs it sounds like something is still gumming up. Can you get access to ethanol free gas? Ethanol likes to dissolve things like paint and seals and fuel lines and deposit the crud in all the wrong places. What are you using for fuel lines? Are you using a inline fuel filter and which one? Most filters are meant for fuel pumped systems and won't allow enough fuel once they become only slightly clogged.

82 GPz750/KZ750R1
82 KZ1100A
79 KZ1000E1 Shaft <<Parting out!!
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post #3 of 32 (permalink) Old 09-23-2016, 08:44 AM
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Carbs need to be properly rejetted. You won't pull at the top of the rpm range with stock jetting. The cylinder cut-out issue is likely not related. Try opening your fuel cap when the bike feels like it is starving for gas. May be a venting issue. If you have vent hoses on the carb assy, check to see they are not kinked. Make sure any fuel filter is rated for fuel flow that will work on a bike. A lawn mower fuel filter looks the same as any other but won't flow enough gas. Make sure the fuel filter is installed in the correct direction. If you don't have a fuel filter on the bike, you should have one. Last thing, have a vacuum petcock? Put the petcock in running position and remove the fuel line from the fuel inlet and set it in a catch can overnight and see if any fuel comes through. If you have a manual petcock, put the petcock in shut position and remove the fuel line and put it in a catch can overnight and see if there is a leak.

If the bike runs on three cylinders for awhile, take a spray bottle of water and squirt the head pipes; a non-firing cylinder will produce headpipe heat where the water splatters and pops off. The cylinders that are running correctly and combusting will vaporize the water. If the same cylinder is the one where there are issues, these can be addressed more intelligently.

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post #4 of 32 (permalink) Old 09-23-2016, 02:16 PM Thread Starter
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I have a new inline filter bought from a suzuki dealer in town, and it's installed in the right orientation. Should I have a tank vent? I'm pretty sure I don't. I replaced the stock vacuum petcock with a manual petcock a while back because it stopped working, as far as I know it doesn't leak but I'll try your test. No vent hoses on the carbs.

When the does have this issue I've checked the fuel filter, and I've always been able to see it full of fuel, which leads me to think it's either a constriction farther down in one of the lines or the carbs, or maybe it's this tank venting issue. Unsure as to why that would have started now, after months of riding without issue.

1982 KZ750 LTD
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post #5 of 32 (permalink) Old 09-23-2016, 03:01 PM
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Whatever was working got clogged.

82 GPz750/KZ750R1
82 KZ1100A
79 KZ1000E1 Shaft <<Parting out!!
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post #6 of 32 (permalink) Old 09-23-2016, 11:50 PM
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All fuel tanks are vented or fuel would not flow. Mine is vented thru the cap.
With a manual petcock be sure to shut it off when the bike is parked. Some carbs will not shut off properly so you have to do it yourself. My gpz's carbs would shut off any fuel flow when parked but my 454 will not so i had to rebuild my vacuum petcock as it was dumping fuel into my air box.

1985 LTD 454
sold the gpz
avy change coming one of these days
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post #7 of 32 (permalink) Old 09-25-2016, 04:21 PM
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All vintage bikes vent the tank through the gas cap as far as I know. I am sure your bike's gas cap is vented.

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post #8 of 32 (permalink) Old 09-26-2016, 12:20 PM Thread Starter
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Ok good to know. I guess I'll run some carb cleaner through the gas cap and see if that helps. Sounds like it could be my issue. I was noticing in the summer that sometimes I would get back to my bike and there would be streaks of dried gasoline on the top and down the side of my tank. I assumed it was caused by fuel evaporating through the gas cap, so that might explain how it got clogged.

1982 KZ750 LTD
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post #9 of 32 (permalink) Old 09-26-2016, 02:01 PM Thread Starter
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Just tested this out and it looks like it's not the problem, unfortunately. On my ride this morning I left the tank cap open for a while and still got the same bogging down and low power issues. It seems to happen most at low-rpm and idle, and I often get exhaust popping and backfiring shortly before it sorts its issues out and returns to normal. I'm going to pull the tank petcock and filter to see if there's any issues, and replace my two fuel lines (petcock to inline filter and inline filter to carb). After that I guess it's got to be a carb problem right? Any thoughts on what acceptable minimum inside diameter for inline filter and fuel lines is for these bikes?

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post #10 of 32 (permalink) Old 09-27-2016, 09:48 AM
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The gas coming out the tank vent was a sign the vents are not clogged. The vents are basically holes in the rubber part under the lid that allow venting and if gas comes out then they are not clogged. The gas comes out because if the tank is full and you park on side stand and it is warm out and the gas heats and expands, it finds some place to go.

Carbs still need to be rejetted properly but that doesn't account for the poor running issues. Since you claim all the normal culprits for poor running are fine, there isn't much else that I can think of.

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