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TOPIC: KZ750H Restoration

KZ750H Restoration 16 Feb 2020 11:50 #819447

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I had a look but couldn't see anything. The stock jets and needles are listed and my needles are all stock. I'll re-balance the carbs on Wednesday and set the idle speed and idle fuel screws. If it's running smooth after that I'll probably just ignore it :) Will have a look at the spare set though - then I'll know what I'm in for if I can't get it running well.
Current Project: 1980 KZ750H
Project Thread: kzrider.com/forum/11-projects/612054-kz750h-restoration

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KZ750H Restoration 16 Feb 2020 16:38 #819465

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Are the stock needles adjustable? I tend to doubt it.

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KZ750H Restoration 16 Feb 2020 21:25 #819474

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I have no idea. I'll have a look at my spare set later today. I figure there'd be something in the manual if they were...
Current Project: 1980 KZ750H
Project Thread: kzrider.com/forum/11-projects/612054-kz750h-restoration

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KZ750H Restoration 17 Feb 2020 13:17 #819503

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Pulled the needle out of a spare slider. The stock needles are non-adjustable. So I have all stock carbs and should be all good on Wednesday once I get the idle adjusted (according to Delkevic the exhaust should be fine on stock jets).
Current Project: 1980 KZ750H
Project Thread: kzrider.com/forum/11-projects/612054-kz750h-restoration

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KZ750H Restoration 27 Feb 2020 06:20 #819942

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Slowly making progress. Got the rest of the wiring done. Just need to wrap the main part of the loom later today. Ended up ditching the idea of a connector under the tank to separate the main from the front harness. Problem was, I'd already cut the wires and put the connector on before I realised that there just wasn't enough room!. 15 uninsulated butt connectors and a load of heat shrink later and all is good (sort of, it really stung having to repair a brand new harness!).

Started her up again and am having a few problems when she's warmed up. Engine idles OK (when it's cold it rattles like an old diesel, but that calms down when things warm up). When the engine's warm and I rev it to about 5-6000 rpm it will bottom out and stall when I let the throttle off. I'm new to carbs so would just be guessing if I tried diagnosing this. However, my thoughts are as follows:
- try adjusting the idle again first (screwing the idle fuel screws in to lean it out?)
- recheck float height (was about 3mm across the board, spec is 4+/-1 mm so may be a bit rich)
- check for vacuum leaks (again), although I assume the problem comes from running rich, which would mean any vacuum leaks would just help the situation.

Any help/advice/suggestions would be hugely appreciated.



Current Project: 1980 KZ750H
Project Thread: kzrider.com/forum/11-projects/612054-kz750h-restoration
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Last edit: by calum.

KZ750H Restoration 27 Feb 2020 06:26 #819943

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I'd leave the float where they are at. 3mm is in spec and you're consistent across the board. Have you performed a running sync yet?
1982 KZ750-M1 CSR
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KZ750H Restoration 27 Feb 2020 08:29 #819950

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OK, thanks. I've synced the carbs (running, warmed up). All the jets are new and I've done my best with the idle screws. Carb boots are new and the throttle shaft seals have been replaced.
Another thing I noticed was that the idle speed dropped just before it was properly warmed up. When the oil temp gauge (on the filler cap) got to around 40 C the idle dropped from about 1200 to around 900. I adjusted the idle back up to around 1200 and it ran OK (although idle speed did seem to fluctuate a bit).
Current Project: 1980 KZ750H
Project Thread: kzrider.com/forum/11-projects/612054-kz750h-restoration

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KZ750H Restoration 27 Feb 2020 08:41 #819951

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How are you adjusting the idle mixture?

I see you are running of an aux tank, I assume you've plugged the vacuum connection for the petcock? Vac leaks will make for an unstable idle and cause other problems. If you've syncd and properly adjusted idle mixture then I would focus on vacuum leaks.

Is the timing advancer moving freely? Considering how clean your build is I would think so but those sticking can cause issues.
1982 KZ750-M1 CSR
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KZ750H Restoration 27 Feb 2020 11:26 #819957

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Today when then idle dropped once it got warm I just used the thumb screw under the fuel line (idle speed adjust) to take it back up a notch. Last time I had it started I used the idle fuel mixture screws (on the top of each carb) to try and get it running smoothly. I checked the carbs were still in sync (I'd had them out since the first sync) and then tried adjusting the screws. Basically anything below 1.5 turns out or above 2.5 turns was awful. Anywhere in between and I couldn't really hear a difference.

I have the second carb plug (where the vacuum pipe to the petcock goes). Although I did forget this the first time I started it.

Timing advancer is good. I have a spare though, as I may have yarded on the nut when doing the valve clearances and I may have sheared of the protrusions that go into the nut and I may have had to do the valve clearances by turning the alternator rotor... :whistle:
If all else fails I'll swap out the timing advancer as it's on the list anyway.

One other thing I noticed: throttle return seemed a little sluggish, although it may have just seemed that way as the tacho has a really slow return. Listening to the engine I think it may have been OK. Will post a video later.
Current Project: 1980 KZ750H
Project Thread: kzrider.com/forum/11-projects/612054-kz750h-restoration

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KZ750H Restoration 27 Feb 2020 12:50 #819968

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Here's a video of the idle drop. While trying to sort out the orientation I accidently mirrored the video, so don't worry if everything seems backwards :).
The bike is running pretty rough at the start of the video. I think I had just revved it and it had dropped down again. I then blipped the throttle to get it steady again. Not sure why I adjust the idle speed at the end of this, it may have been the last of a few attempts to get it smooth and I just decided to try with a higher idle (although the idle only goes up to 1200-1300 rpm).

One thing I forgot to mention before is that it was pretty cold today - you can see a bit of snow on the driveway in the picture above and you can see my breath in the video when I adjust the idle speed. However, the bike hardly needed any choke and I had the choke off completely within a minute or so. Would also point to running rich, question is why...

From the video description:
Idling after warming up. Survives a little bit of throttle OK but almost dies when I give it more revs. Toward the end of the video I increased the idle speed to even it back out. The problem didn't go away though.

Current Project: 1980 KZ750H
Project Thread: kzrider.com/forum/11-projects/612054-kz750h-restoration

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Last edit: by calum. Reason: Fixed video, added details

KZ750H Restoration 27 Feb 2020 14:15 #819974

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Your engine sounds like it´s running on 3 and a half cylinder. Have you try to check the temperature of the header pipes are all equal?

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KZ750H Restoration 27 Feb 2020 14:23 #819975

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I checked them by feel, so no, not properly. I have been meaning to check the spark and I did notice that when pulling the vacuum plug on carb no. 2 (to put the sync tester hose on) that a bit of smoke/vapor came out. Nothing came out of the others. Also, when doing the compression test I noticed the plug on cylinder two was a bit black but the others were grey.
What's the easiest way to check things? If I can find out if a cylinder is not firing properly I could eliminate the new plug leads/caps as the source by switching the caps on two cylinders.

Edit: Compression was 128, 130, 128, 128 psi

Edit2: A friend is bringing me an infrared thermometer tomorrow so I can check the header temps.
Current Project: 1980 KZ750H
Project Thread: kzrider.com/forum/11-projects/612054-kz750h-restoration

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Last edit: by calum.
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