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KZ400 Single Carb Woes

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17 Apr 2022 13:08 - 17 Apr 2022 13:24 #865622 by YamaKawa
KZ400 Single Carb Woes was created by YamaKawa
Have a 79 KZ400 that was never satisfied with the stock carbs, no matter how many times it was cleaned, balanced and tuned so I got tired of messing with them, yes even by a shop. Got an 11th Hour manifold and a VM34. 

Bike runs AWESOME when WOT, lots of power, and dare I say feels quite a bit more peppy over stock carbs; however, at anything less than 4000 RPM the bike stutters and sputters underload ONLY AFTER first exiting said load (i.e. shifting, coming to a stop, etc).

The idle is all cattywampus: sometimes wanting to stall at 800-900RPM, sometimes idling PERFECT at 1100 RPM, sometimes idling high at 1600-2000RPM, sometimes it will randomly rev to 3500-5000 and hang there for several seconds. Majority of time the idle is constantly surging 1200-1500. Reminiscent of a vacuum leak, but I cannot find any (sprayed head-to-manifold, manifold-to-boot, boot-to-carb, carb-to-filter as well as choke areas. NO LEAKS. Throttle cable is not binding, slide is fully closing.

The bike was running perfectly fine last year before winter. YES I have cleaned the carb and did so to the extent of spraying every tube and orifice, including removing the choke assembly cleaning that pocket out as a just in case. The tank has ALL brand new gas and ZERO sediment/crud. At this point I'm at the end of my wits. Bike was fine last year with the same jetting but now is only  at 5k+ RPM. Only making me contemplating getting a modern bike to ride more and more (eyes set on a Z400 with fuel injection). My roommates told me to throw some StarTron in there as it apparently works miracles on small engine machines (he used to work for a repair company), but other than that, I'm out of ideas short of getting a color tune.

Pilot jet: 27.5
Needle Jet: 159-Q0
Needle: 6F9
Main jet: I wanna say 140, but don't remember (it's somewhere between a 130 and 190)
All else: stock (slide, air jet/screw, etc)

Gonna throw some new plugs in when I get home as a just in case.

EDIT: to mention that bike also felt like it was running EXTREMELY hot for some reason. It was only 50 degrees outside, ran for 15 minutes.

1985 Yamaha XJ700N Maxim (airhead) - SOLD
1979 Kawasaki KZ400H (LTD), bought with 8416 original miles. - 'Tis my baby
1978 Honda GL1000 - SOLD
1995 Suzuki Katana - SOLD
Last edit: 17 Apr 2022 13:24 by YamaKawa.

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18 Apr 2022 00:26 #865646 by YamaKawa
Replied by YamaKawa on topic KZ400 Single Carb Woes
So just an update, I pulled a black and it was entirely black. So I guess while it ran fine last year, the plugs are probably fouling this year as a result. I'm gonna be dropping the pilot and just start over with tuning LOL
If I can find the jets I bought last year, I'll have access to a 15, 17.5, 20, 22.5 and a 25 pilot. Debating whether I want to start over with the 25 or just step down to the 22.5...

1985 Yamaha XJ700N Maxim (airhead) - SOLD
1979 Kawasaki KZ400H (LTD), bought with 8416 original miles. - 'Tis my baby
1978 Honda GL1000 - SOLD
1995 Suzuki Katana - SOLD

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  • Scirocco
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  • Never change a running system
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18 Apr 2022 01:35 #865650 by Scirocco
Replied by Scirocco on topic KZ400 Single Carb Woes
Did you check the ignition timing and is your timing advancer moving and not stuck by rust?

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18 Apr 2022 08:23 #865664 by YamaKawa
Replied by YamaKawa on topic KZ400 Single Carb Woes
They were fine last year when I checked the gap on my points. Never hurts to double check though. Will do tomorrow. I don't get off until 730 and have maybe 20 minutes of light at best by the time I get home.

1985 Yamaha XJ700N Maxim (airhead) - SOLD
1979 Kawasaki KZ400H (LTD), bought with 8416 original miles. - 'Tis my baby
1978 Honda GL1000 - SOLD
1995 Suzuki Katana - SOLD

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