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KZ750N - Hanging revs off idle, hesitation and surging while riding 03 Jun 2020 10:34 #827216

  • ghostdive
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Hi, I finally got my bike running and riding. I've taken it out a couple times and it's not quite right:

First run, it seemed to have no power above 4,000. Plugs fine, pipes all hot, so I went out for another. Same deal, except when I rolled the throttle on as I hit a straight road, it suddenly gave me ALL of the power, which took me by surprise. The idle was at 6,000 all the way home. After I waited a minute to calm down, I started it again, and it idled at 1,050 as I'd set it, but when I twist the throttle it hesitates and shoots to 3,000 and stays there. When I back out the idle screw on the carbs, it quickly drops below 1,000 and shortly dies.

So, I have a couple ideas - I'll be readjusting my pilot screws, and spraying around the engine with carb cleaner to check for vacuum leaks. I'd appreciate any suggestions as to other tests as well.
1982 KZ750 Spectre - 6 speed swap, BS34s, 18" rear wheel

2001 ZX-6R

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KZ750N - Hanging revs off idle, hesitation and surging while riding 03 Jun 2020 12:23 #827222

  • zed1015
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Yes! Does sound like an air leak due to the fast idle and then dying.
First check for air leaks at the boots and then pilot jet blockages as that could also cause some of the symptoms.
Also check that the ign advancer moves smoothly on the crank if it's the spring /weight type and not fixed electronic.
Also what carbs are fitted .
If CV's do the slides drop the same smoothly or do any drop sharply indicating split diaphragms,
Are you running stock airbox ?
AIR CORRECTOR JETS FOR VM CARBS AND ETHANOL RESISTANT VITON CHOKE PLUNGER SEAL REPLACMENT FOR ALL CLASSIC AND MODERN MOTORCYCLE CARBURETTORS
kzrider.com/forum/23-for-sale/611992-air-corrector-jets-





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Last edit: by zed1015.

KZ750N - Hanging revs off idle, hesitation and surging while riding 03 Jun 2020 16:00 #827240

  • Nessism
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I'd replace the carb boots first off since they are the number one source of air leaks. They are critical to the running of the bike and if yours are not already gone they will be soon.

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KZ750N - Hanging revs off idle, hesitation and surging while riding 04 Jun 2020 08:27 #827291

  • ghostdive
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Thank you both! I quickly found a leak at 3 of the 4 boots, so I pulled them and used some secret sauce to patch them up - ordered a new set from Z1 last night, too. The hanging went away, and with some pilot screw fiddling I got them to drop nicely to idle - I'm not sure how fast they're supposed to drop, but it's usable.

I'm using a stock airbox with Mikuni BS34s, #37.5 pilots, #115 mains if memory serves.

The issue is now that there's stuttering and hesitation below 2700 RPM, so I'm going to hunt down some clear tubing and check the float levels. I should vacuum sync as well, once I get the equipment. Input is well-welcome, of course. Thanks again!
1982 KZ750 Spectre - 6 speed swap, BS34s, 18" rear wheel

2001 ZX-6R

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