Can't get running...

Just purchased an old 80 kz750 4 cylinder. Guy unfit it from swore up and down it ran last year tho apparently doesn't know anything about bikes. Been trying to get the thing up and running to no avail. I don't know anything about bikes so everything has been a learning experience but I'm at a loss as to what I'm doing wrong or why I can get it going. Any ideas would be greatly appreciated.....

Welcome aboard!
That is a common problem with bikes that have not been ridden in a while. Among the more common culprits are
Dirty and gummed up carburetors plus a weak battery.
There could also be problems with seriously out of adjustment valve lash, stuck rings or even badly worn cylinders. There could also be ignition problems.
On the home page of tis website are some really useful links. Scroll down and find the link for the model identification guide for 1980. Once you have identified the exact model you have, then look for a link to the factory service manual for your year and model. The fsm has pretty good troubleshooting sections as well as an extensive section on routine maintenance. It is best to assume that no maintenance has been done. Start at the beginning of the list and work your way through them. This will solve many problems and prevent many more.
There are also some good articles on wrenching available and I hope that 650Ed will share his very useful introductory post.

Fuel tank. I would drain & clean the fuel tank. at the minimum rinse it with fresh gasoline & check it for rust particles. burn the leftover fuel in your lawnmower or car. clean/check the petcock screen. the petcock may need a rebuild kit or at least the diaphragm. while the tank is off, do the ignition, carburetor & airbox stuff below. ... warning: fire or explosion danger. no smoking or sparks.

Airbox. clean the airbox & install a new air filter. the old end cap foam can crumble & get sucked into the carburetors & engine. check that the airbox ducts are soft. If hard, replace them. or expect to. removing the carburetors will be difficult or impossible if they're hard.

Clean the carburetors. remove the bowls, then use your eyeballs & common sense. eebbaayy pics are handy. if the carbs don't look too bad, then they may only need a cursory cleaning. spray carb cleaner thru the orifices & verify that the orifices are clear. The pilot jets have polyester plugs w o-rings that need to be removed to clean the orifices --- very important. If the carbs don't look so great, you may need to soak the carb bodies. the vacuum slide diaphragm rubbers may need replacing. You'll probably need to replace the bowl gaskets & o-rings. expect or try to replace rubber parts as the old rubber gets hard & deformed.

While the carbs are off the bike, that would be a handy time to check the compression, as you won't have to fiddle with the throttle.

Ignition system & spark stuff. make sure the spark plugs are correct & properly gapped. I would install new.
Redo the spark plug wire connections. unscrew the ribbed compression boots at the ignition coils & untwist the wires from the spark plug caps. there's probably oxidation on the plug wires, so nip off the ends a tad. or go ahead & get new 7mm copper core. you may be able to reuse the outer sheathing from the stock plug wires so that new wires fit right. silicone spray would help in removing the sheathing.
Ignition coils. check the coils for cracks or damage. they may need replacing, even if they appear fine. grey or green Dyna coils.
Timing advancer. clean, check & lubricate the timing advancer. I use high temp wheel-bearing/disc-brake grease. the engine cover screws probably will be stuck, so use an impact wrench & properly fitting bit. Liquid wrench penetrating spray lube should help. I use a hand impact tool --- with the best fitting bit, put the tool in place, then whack the end of it with a hammer. do a test on a deck fastener or something for practice using the tool & to make sure the tool is set correctly to unscrew the bolt.

Engine oil. change the oil & oil filter. Shell Rotella-T 15W-40 in the white bottle. or Chevron Delo.
Z1Enterprises.com has an oil filter & air filter for your bike. A local place may have an oil filter. fram CH6012. wix 24941.

Manual. 1980-84 KZ750-4. 1983-85 ZX750. 750turbo.com --- Index . occasionally unavailable.
yellow/gold cover w color wiring diagrams.->> GPz750 Base Manual.pdf. good scans.
The one that says full english is a cruddy, photo-copied version.

Wiring diagrams. mbsween. bike-night. KZ750L3 . Scroll down & click "The Bible" link.
Note: some of the wiring diagrams are mislabeled in the links: the 1981 Ltd is an H2, the 82 is an H3, & the 83 is an H4.
....

The first time carb cleaning success rate is very low unless the person doing the work takes their time and does a complete job. That means fully breaking down the carbs and soaking/cleaning all the various parts in either carb dip or an ultrasonic cleaner. There is no need for "carb kits" other than simply rubber O-rings, and in fact most carb kits are full of inferior aftermarket jets you don't need anyway.

The KZ750's employ 34mm CV type Keihin carbs which are a little more fiddly to clean than the similar Mikuni counterparts. More specifically, to ungang the carbs you need to fuss with the choke linkages and that's a pain. Once you get over the mental hurdle it's not hard though. Best to bite the bullet and do the carbs right the first time. It saves time in the end.

Once you get over the mental hurdle it's not hard though. Best to bite the bullet and do the carbs right the first time. It saves time in the end.

Excellent, especially the mental part. I put off doing them, but then decided to take the plunge on my VM24 carburetors & did the complete disassembly. I thought it was easy & I do mean easy. The same thing is done 4 times on a lot of stuff & most things only go back together one way, so it's hard to mess up. CV carbs might be easier as there are no mechanical throttle slides to mess with. Use good fitting tools & replace as many rubber parts as possible. Do the job on an uncluttered work table. I used 4 plastic divided lunch containers to keep the parts from each carb separated from the others. Probably the stupidest thing a guy could do is knock the bank of carbs off the table onto the concrete garage floor.

If lack of fuel through the carbs is the problem after sitting for a period of time, most likely the pilot jets are plugged solid with corrosion due to ethanol.

I've seen it before where folks go through the whole carb, believe they have cleaned everything spotless, but the pilots remain plugged up. Be sure and hold them up to a light and confirm the jets are clear. Or play it safe and replace them.

A guitar string or strand from a wire brush is handy for cleaning pilot jets.
Steve
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