runs with choke only

Hey, im new here. I've had my bike about a year and a half now, and been too stubborn to ask for help, but i need it. I have tried everything. My bike is a 76 kz750 twin. It's stock except a K&N air filter, and I opened the exhaust up. it runs rich, and I'm getting about 20 miles to the gallon. I have to keep the choke turned on at all times, but can turn it down when it's warm. I runs a little rough at idle, and if I roll the throttle i can turn the choke off, but as soon as I let go, it sputters and dies. I've tried working with the pilot screw, but it doesn't change anything. Could it need a new pilot jet due to not seating right?

clean the pilot jets and pilot circuits. also make sure the pilot screws are the correct number of turns off their seats.

definitly will need the pilot circuits cleaned properly.

Carb Cleaning 101

By M. Shively

The elements of internal combustion engines are: correct fuel/air ratio, spark at right time, adequate cylinder compression.

There are many passageways and openings to check and clean. All are important in function and when obstructed or not working properly, have subtle to radical effects on engine performance. Vacuum leaks and carburetor synchronization also effect performance and should be inspected and adjusted following the below procedures.


Warning: Remove all rubber parts before you begin. These parts usually include vacuum diaphragms, needle valves, o'rings, hoses, and other parts. Spray cleaners will damage these parts. Do not disassemble individual carbs from the carb bracket.

Air & Fuel Passageways: Trace and learn individual fuel and air circuits from beginning to end. Machines can only drill straight through the cast passageways. To change direction, another angled passageway must be drilled. The union is plugged with a brass or bronze bead. Inspect and clean each passageway with spray cleaner, brushes/pipe cleaners/etc, and compressed air. Remove any discoloration and debris. Look for spray cleaner to exit from one or more passageways.

Jet Cleaning: Inspect jets by holding to light and look through them. You should see an unobstructed round hole. Clean the jets with one or more of the following: jet cleaning wires, soak solutions, carb spray cleaners and compressed air. Re-inspect jets after cleaning and install when clear of obstructions. Some main jets have paper-like gaskets. Most have metal spacers between the jet and the emulsion tube. Some screw directly into a brass emulsion tube which is machined for a 7mm wrench at its float chamber exposed base.

Inlet Fuel Valve: Inspect the needle valve & spring. Press down the tiny metal rod that protrudes from the butt or float end of the needle valve. The spring should move freely and return the rod to its location. Check the needle valve's seat area for a groove or other wear. It should appear highly polished. Some needle valve seats are rubber and wear may not be visible. Inspect the needle valve jet seat. You can clean the jet seat with Q-tips and semi-chrome polish if necessary.

Carb Body Castings: Blow air through the atmospheric vent holes located on the dome of each float bowl chamber. Air should exit via hoses or brass nipples. Inspect the emulsion tubes and passageways (cast towers that jets thread into) for discoloration and debris. Clean interior emulsion towers with a soft bristle gun cleaning brush. Clean each Venturi (main carb bore).

Needle Jets & Jet Needles: Clean the needle jets, jet needles, and passageway or tower that needle jet screws into. Clean the emulsion tube (pipe between needle jet and main jet) (Main Jet may screw into emulsion tube). Jet needles are part of the throttle slides. See below…

Throttle Slides: There are several types of throttle slides: Mechanical linkage, vacuum, diaphragm, and cable. Disassembling the jet needle from the slide is not always required for cleaning. If you have vacuum piston type throttle slides (large diameter solid metal slide), avoid cleaning the lubrication from sides and caps. If piston type check cap vents and passageways with air. Clean if necessary and re-lube. If you have rubber vacuum throttle diaphragms, inspect for dry-rot, defects, and tears by gently stretching rubber away from center. Do this until all areas around diaphragm have been inspected. Replace any defective part as described above. Clean carb body areas around diaphragm including air passageways and air jets. Diaphragms have a locator loop or tab fabricated into their sealing edge. Observe this locator upon reassembly. Avoid pinching the diaphragm when reinstalling caps.

Fuel Screws: Fuel screws have sharp tapered ends. Carefully turn one fuel screw in while counting the turns until it seats lightly. Warning: These screws are very easily damaged if over tightened into their seats. Record amount of "turns-in" and remove the fuel screw, spring, washer, and o'ring. The fuel screw is part of the enrichment (choke) circuit...clean passageways as described above. When carbs are assembled, spray low PSI compressed air into diaphragm air vents located at intake side of carbs. Throttle slides should rise, then fall when air is removed. Lightly lube external moving linkages. Reinstall carbs and follow through with carburetor synchronization.

Throttle Cables: Lubricate cables periodically. If cables are disconnected from carbs or removed for replacement, etc . . . remember cable routing and ensure proper reinstallation routing. Avoid bread-tying, sharp bends, and pinching cables. Adjust cables so throttle grip has about 5mm of play or throttle slides or butterfly valves may not open completely (full throttle)(wide full open).

Float Bowls: Inspect float bowls for sediment, gum or varnish, crystallization, and defects. Clean all pipes, tubes, passageways, and embedded jets with cleaners and compressed air. Remove and clean the drain screw and area. Inspect bowl gasket and replace if necessary. Clean and inspect overflow pipes and tubes, look for vertical cracks.

Floats: There are several types of float materials: plastic, brass, black composite, tin, and others. Handle floats carefully. Avoid bending, twisting, denting, or other means of mishandling. Most floats are adjustable by bending a small metal tab near the float axle end. Do not change the float adjuster tab unless tuning fuel service levels. Clean metal floats by soaking or by spraying cleaner and wiping clean. Other material type floats may require replacement if cleaning is necessary. Inspect the needle valve (float valve) and seat. Check needle valve's spring loaded pin. It should depress and return smoothly and without resistance. Check the needle valve's tip for a worn groove. Replace needle valve and seat if either symptom exists. These parts wear together and must be replaced as a set.

Synchronization: This is a fine adjustment performed usually and preferably with the carbs installed and the engine running. The unusual part is performed with gauged wire with the carbs on the work bench. Carburetor synchronizing balances Venturi vacuum at the exhaust side of each carburetor, resulting with smooth idling and optimized performance at all throttle openings. Synchronization is checked using a set of gauges which are either air vacuum type or liquid mercury type. The gauges are connected to vacuum ports on the intake manifolds via nipple tubes or if sealed with screws, sync gauge adapters will be needed. With the engine running at temperature, and with a fan or means of forced convection aimed onto the engine, the carbs fuel screws and idle are adjusted, then the synchronization is adjusted via adjustment screws on the carbs. A reserve fuel tank is recommended for convenience of accessing carbs during this procedure. See gauge instructions and repair manuals for detailed use of synchronization gauges.

Notes: While carbs are apart, record the jet sizes. Look for a very small number imprinted on the body of the jets. Verify that numbers are the same for all jets on models with in-line cylinders. A few transverse-4 models and V-engines, the inner and outer carbs use some different size jets and it's important to not mix them up. If you have dial or veneer calipers, measure and record float heights. Perform measurements with floats just touching needle valves, though not depressing the needle valve rods. Replace fuel and vacuum hoses. Be sure to use fuel rated hose for fuel. Install or replace in-line fuel filters. It's a good time to remove and clean interior petcock fuel filters. Inspect carb manifolds for dry-rotting, inspect all clamps and air ducts. Inspect, clean, lube, and/or replace air filter(s).
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Lemon Juice Carb Cleaning(Found on the internet)

If it was me, I'd pull the top covers off, pull the slides out, drop off the bottom bowls, remove pilots/mains. Then with the rest of the carbs still assembled boil them in some lemon juice. You'll need a big roaster pan, a little over a gallon of lemon juice (cooking supply stores), and an understanding wife/girlfriend cause the sh*ts gonna stink a little.

Boil them for about 15 minutes, then rinse them really well and blow them dry with compressed air (make sure all passages are clean). You'll be amazed at how clean they come in that short of a time.

I just did a set yesterday after hearing about it from a buddy who's used lemon juice on a couple dozen racks of carbs. After a couple of minutes at slow boil the lemon juice starts to foam up a little (careful not to boil over) and you can see all the sh*t just dissolving.

After boiling them for 15 minutes (might have to roll them around once to make sure you get the entire carbs) rinse them really well and blow them dry with compressed air. The carb bodies will be kinda chalky looking at this point. I used a toothbrush and PB blaster to put a little luster back into them and then put my internals back in.

I had pulled the bowls off, removed the floats, float needles and seats, jets, and air/pilot screws as well as the top covers and slides just to make sure no crap got itself wedged inside those passages.
After putting the carbs back together and bolting them up not only do they look great but the bikes running MUCH better with all the jet passages finally cleaned out.

Here is a link to a site for KZ twins.

www.armbell.com/kz400/viewforum.php?f=12...56ee1c2&mforum=kz400

Thanks a lot guys. I'm definitely going to try the lemon juice trick. Im not sure about the needles, will they have the number printed on them? And just another question but, my main jets are 125, does that sound right? Everyone else on here seem to have smaller ones.

so I completely took them apart and cleaned them with carb cleaner and nothing. Then I took them apart again, and rebuilt them, still nothing. Do you think it could still be clogged? Should I just do the lemon juice thing? I was a little hesitant just thinking about it because it just sounds like it would be bad for it.

almost gaurantee that you are not getting the in body passages (cast in) blown through...are you 100% sure that you were able to remove everything that can be removed prior to cleaning the bowls sides out....

trianglelaguna wrote:

almost gaurantee that you are not getting the in body passages (cast in) blown through...are you 100% sure that you were able to remove everything that can be removed prior to cleaning the bowls sides out....

i m with him....running on choke only is a classic sign there is still smeg up in carb bodies not allowing the pilot jet to function as it should.....jets clear also?

Could it possibly be that the timing of the ignition was off. I have to keep the choke on because it runs rough if i don't. I also may have really screwed things up. in trying to start it and adjust the timing, I tried to start it and it sounded normal for a bit, then it sounded like the starter chain broke. I took the side panel off and the starter teeth are fine and so is the chain. It sounded like a spinning starter, then catch, then spin again.

so now i'm into the guessing mode...which does not mean i "know" your model...twins are known to burn out starters...you can get a starter cheap sometimes on ebay.....there is a way to test the starter by popping it out and making shop bench connections...i however never have done that so ...also...i think you may wanna....pull the carbs....and take everything apart on the bolws ...ONE MORE TIME......but relax it is important to have fun this time ...you'll be a regular carb guru by now..lol....sooo just relax...do it on a really really clean bench....on a sunday morning ...after coffee and some food...put on some mellow jams and get everything claen and all set to do it...ONE more time!!!...get a big metal bowl bout the size of a dog water dish...and get about a half a can of carb cleaner in the bowl...at least an inch or two,,,,the cheap auto store spray ones......and some fine pices of copper wire...really fine ones unwrapped from an old lamp crds braided type centers....with a flash light peek in each hole as you remove a jet.....look really good in everyplace somthing could hide in...you will find some tiny tiny tiny jets that need a jewlers/reading glasses screwdriver to extract....then put on some safety glasses and a compressor spray nozzle...go where you wont kill the mrs flowers and waste about two cans of carb cleaner spray
.....i once had a circuit i was missing and my very best cleaning efforts were actualy cramming more gunk into it...on my third try i found it....that is the way i did it...on sunday morning with a relxed...i'm gonna get it today ....approach...and there it was...i removed it...cleaned em super good....was like i had a new bike......good luck

your bike has a one way clutch or what is known as a sprag it sounds like it is worn out the sprag will only engage one way when it gets flat spotted it will do what your describing. if your ignition timing off it wont do what your describing it is in your carbs.