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carb rejetting on 78 kz1000
- sweeted
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- timebomb33
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1973 z1 2-1974z1-a,2-1975z1-b dragbikes1015cc+1393cc, 1977kz1000,1978kz1000,1981kz1000j, 1997 zx-11, 2000 z12r,1428turbo nitrous pro-mod and a shit load of parts thats all for now leader sask.,CANADA
I THINK MY POWERBAND BROKE
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- sweeted
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- Patton
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...adjusted pilot screws but no changes occurred....
A typical symptom when carb pilot circuit is less than pristine.
Good Fortune!
1973 Z1
KZ900 LTD
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- keith1
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- TeK9iNe
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start at 20 pilot/120 main and 4th clip needle......should get you 7/8ths there......dont pay the shop......
Sounds pretty good for a start... and YEAH. DONT PAY A SHOP!
Be patient friend, just a few changes and it will all come together for ya.
Get some seafoam from NAPA, and pour it in then fresh fill with gas.
Start with your mixture screws 2 turns out.
Once the bike has settled idling off choke (hot), turn them all in 1/2 turn and listen to see if the idle rises or falls. Continue to turn them all the same amount either way hunting for the highest idle point. Its kinda a rough way to do it, but it will get you riding ok.
Good luck.
Motorcycle Shop Owner/Operator
79 Kawie Z1000 LTD
81 Kawie Z1000 CSR
83 Honda VT750C A
85 Kawie GPZ900 A2
86 Zukie GS1150 EG
93 Yamie XV1100 E
Lucky to have rolled many old bikes through my doors
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- nads.com
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There we go again. You messed with your needles. And you have a 122 jet. You could have left the needle height as it was and installed bigger mains. Like it should be done. What are the bigger jets made for then? Your aware that needle adjustments richen or lean the mixture. Then why take the carbs completly apart to change the needle height for no good reason? I'm not impressed with the fact everyone is being told to change the needle height. I'm not singling you out here, your not the only one that said it. It's just damn hard for some to do, and could cause other issues.a pro pipe is a pretty wide open pipe, what type of carbs you are running is very important with my 29 smoothbores i use 122 mains and 20 pilots and needle all the way up "bottom clip position". that pipe works the best with big cams and lots of compression and a very good cylinder head its too much pipe for a stock engine
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- timebomb33
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1973 z1 2-1974z1-a,2-1975z1-b dragbikes1015cc+1393cc, 1977kz1000,1978kz1000,1981kz1000j, 1997 zx-11, 2000 z12r,1428turbo nitrous pro-mod and a shit load of parts thats all for now leader sask.,CANADA
I THINK MY POWERBAND BROKE
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- Patton
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The carb pilot circuit governs fuel mixture from idle to 1/4 throttle, influenced by pilot jet size and pilot screw adjustment. The pilot circuit loses effect as the jet needle and needle jet assume command from 1/4~3/4 throttle position.
As the tapered needle rises from 1/4 throttle to 3/4 throttle, the open space around the needle gradually enlarges, which allows progressively more fuel through the needle jet. A lower clip position raises the needle, which enlarges space around the needle at any given throttle slide position (richer). A higher clip position lowers the needle, which reduces space around the needle at any given throttle slide position (leaner).
From 1/4 throttle to 3/4 throttle, main jet size doesn't matter, because the tapered needle position inside the needle jet is governing the amount of open space around the needle (through which fuel may flow), regardless of how much fuel is available through the main jet. Whether a small or huge main jet size, only so much fuel from the main jet will be allowed around the tapered needle through the needle jet opening. For illustration, given the same size outlet hole, a large drain pipe and a small drain pipe will flow equal volumes through the hole.
At about 3/4 throttle, the needle has gotten so small that it no longer restricts fuel flow through the needle jet. From 3/4 throttle to wide open throttle, size of the main jet orifice limits the amount of fuel available to pass through the needle jet. A too small main jet will be too lean at wide open throttle. A too large main jet will be too rich at wide open throttle.
The transitions from 1/4~3/4~wfo throttle positions are gradual and usually unnoticed. For example, from idle~1/4, the pilot circuit gradually loses effect as the jet needle and needle jet assume command from 1/4~3/4 throttle. And then from 3/4~wfo, the main jet assumes command. And all of these work in coordination with the throttle slide position and its cut-a-way design inside the carb bore.
None of this is an excuse to start arbitrarily messing with the needle clip position. If acceptable past performance should begin to suffer, changing the needle position is likely not the best solution.
Good Fortune!
1973 Z1
KZ900 LTD
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- Old Man Rock
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kzrider.com/index.php?option=com_docman&...d&gid=461&Itemid=108
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1976 KZ900-A4
MTC 1075cc.
Camshafts: Kawi GPZ-1100 .375 lift
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ZX636 suspension
MIKUNI, RS-34'S...
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Phoenix, Az
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- Patton
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Could do the throttle chop exercise with existing main jets. If plugs are black and sooty, try smaller main jets. If plugs are too light, try larger main jets.
As to needle clip position, I use the middle (or whatever the FSM recommends), unless there's suspected poor carb performance at say 1/2 throttle position (faster highway speeds). Plug readings from throttle chop exercise at 1/2 throttle may also give good evidence of fuel mixture being delivered at mid-throttle cruising. If plugs are black and sooty, try higher clip position which lowers the needle for leaner mixture. If plugs are too light, try lower clip position which raises the needle for richer mixture. The main jet size doesn't matter much until reaching 3/4 throttle.
Regarding pilot jet size, I look for a smooth steady idle and strong pull-away from idle. This presupposes a pristine pilot circuit where the pilot screw has noticable effect on idle speed rpm. And properly synced carbs. Sometimes, the next larger size performs better, because the oem size was small to begin with for easier compliance with epa requirements.
A prerequisite to addressing carbs is that all other tuning considerations are up to snuff, such as compression, valve clearances, good properly timed spark, adequate fresh fuel supply, float bowl fuel level, etc. Because even perfect carbs can't compensate for deficiencies in these other areas.
Good Fortune!
1973 Z1
KZ900 LTD
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- nads.com
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