carb rejetting on 78 kz1000

just purchased 78 kz 1000 and need some info on rejetting carbs.i have a vance and hines pro pipe and k&n filters but not quite sure where to begin on main jet sizes or what all should be changed as far pilot jet setting.any info will be helpfull

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

solved my pipe problem by getting a sidewinder with baffle. still running the 26mm carbs with the k&n filters with stock 107.5 jets and bike starts but smokes and wont get a smooth idle.carbs feel like they are sucking for air.adjusted piolt screws but no changes occured.all cylinders are fireing.need help before i go into shop

sweeted wrote:

...adjusted pilot screws but no changes occurred....

A typical symptom when carb pilot circuit is less than pristine.

Good Fortune!

start at 20 pilot/120 main and 4th clip needle......should get you 7/8ths there......dont pay the shop......

keith1 wrote:

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.

timebomb33 wrote:

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

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.

the needle postion is the mid range postion i always adjust that if i'm ever having mid range drivability problems which he is going to have if he uses a huge pipe like a v&h pro pipe on the street also alot of the smoke your seeing is from running to lean it's lean mis firing not getting a complete burn so the extra unburnt mixture is going straight out the pipe and your bike will look like the exxon valdez. richen up everything a bunch.20 pilots 120 or better mains and raise up your needles it shouldn't have any flat spots or at least not as bad as your going to have with stock jetting. by the way nads i've been runnig a very simalar setup both at the terack and on the street for many of years and my bikes flat out rip with no flat spots or other problems. i wouldn't have it any other way it does take a bit of time and care to chanmge needle heights but it is worth it.

My 2 cents, admittedly over-simplified, but here goes anyway ---

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!

Download the zipped file... Covers everything you need to learn about carbs and jetting... Includes VM manuals...

kzrider.com/index.php?option=com_docman&...d&gid=461&Itemid=108

OMR

Freer breathing exhaust and air filter often need larger main jet for best mixture at wot. There are some rules of thumb ranging from 2~4 sizes larger being most preferable. But the true test imo is plug color at wot (preferably tan), determined by so-called "throttle chop" exercise. The main jet size that works best for someone else's similarly equipped bike may be just fine for your bike, or could be one or two sizes different from what will work best in your bike.

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!

The higher the needle, the larger the main jet opening becomes. Would a larger main not leave a larger opening as well?