1983 Kawasaki KZ750 - Horrific gas mileage?

Hey all,

This is my first time posting here after poking around for a little bit. I'm 19 and I recently bought my very first motorcycle (1983 Kawasaki 750 Spectre). It was in great shape when I bought it, still running and it had all the original parts still on it. Unfortunately, the bike had been sitting for about 15 years and obviously developed some problems. I'm not super engine savvy, but my dad knows a fair bit so we've been trying to get everything running right.

Anyways, onto the problem: I'm getting about 55 miles to 3 gallons of gas....which is terrible. I have a 4 gallon tank and I need to set it to reserves at roughly 55 miles, so I'm assuming my reserves hold another gallon. That places me at around 70 or so miles per tank when I know I should be getting upwards of 120 at least. The carbs aren't leaking any gas and the bike is driveable (I've been riding it everyday), but one of the cylinders definitely isn't firing properly nor does it idle for any longer than a couple seconds. I've replaced the spark plugs and wires, took the carbs off once and scrubbed the varnish out of the jets (they were pretty clogged), but I never actually soaked them in a carb cleaner. I should also note that I cleaned the carbs out before I cleaned all the rust out of the gas tank (there was A LOT in there). I'd imagine some of that rust could have gotten into the carbs again, but I'm not entirely sure. In addition to those things I've changed the oil, checked the petcock and replaced my handlebars,

I'm sure this question has been asked a bunch for various models here, so I apologize in advance if this topic annoys anyone. I really am trying to learn as much as I can about getting this bike to run properly so I figured someone here might be able to point in the right direction as to what I should do to fix these problems. I'd love to be able to take this motorcycle to college with me this weekend, but I can't quite do it on that gas milage. Thank you for any help!


Also, I attached a picture of the bike. Note that, that isn't a real gun. It is a military blue gun used for training purposes that is spray painted black. For whatever reason, this is the only picture I can find showing the new, lowered handlebars and my dad appears to have placed that prop in the picture he took :side:

If you haven't done so, you probably should add an inline fuel filter. That should prevent more rust particles from getting into the carbs. After adding a filter, I would clean the carbs again as that's a primary suspect, and make sure the diaphragms are not damaged. Ed

I was going to help because I saw the gun. Since it is fake, never mind.
Just kidding. After you install the filter (the correct type) you need to do a real carb clean, which includes the dip. The varnish you cleaned out of the bowl is probably in the small passages and blocking things that should not be blocked.

650ed wrote: If you haven't done so, you probably should add an inline fuel filter. That should prevent more rust particles from getting into the carbs. After adding a filter, I would clean the carbs again as that's a primary suspect, and make sure the diaphragms are not damaged. Ed

Good idea with the filter; I'll go pick one up tomorrow. How complicated are the diaphragms? I've never looked into those so I'm not sure what to expect.

koolaid_kid wrote: I was going to help because I saw the gun. Since it is fake, never mind.
Just kidding. After you install the filter (the correct type) you need to do a real carb clean, which includes the dip. The varnish you cleaned out of the bowl is probably in the small passages and blocking things that should not be blocked.

Is there a specific inline filter I should buy? Yeah the carbs definitely need to be soaked overnight. Not quite sure why I didn't do that the first time...

Yes, you need the type for gravity feed fuel systems. Such as lawn mowers, lawn tractors, etc.

koolaid_kid wrote: Yes, you need the type for gravity feed fuel systems. Such as lawn mowers, lawn tractors, etc.

If you can get a metal bodied filter so much the better.
I put plastic on mine at first, the engine heat softened it up so it kind of folded up and shut off most of the fuel flow.
Not good! :ohmy:

Nice looking bike. I really like the spectres but you don't see many over here.

Here's what I bought after trying a couple different ones. It's not cheap, but it's really small, filters down to 10 microns, flows up to 6 gallons per minute, and will last forever. Ed

kzrider.com/forum/3-carburetor/582476-ru...-one-cylinder#583562

If one cylinder is misfiring, you should take care of that ASAP. Check the engine oil for fuel contamination.
What type of spark plug wires & caps did you install?
Double check the spark plug wire connections to make sure they are good.

Check the coils w a multimeter (note that they may be within spec, but a coil can be bad as there can be internal shorts that the resistance tests may not reveal... the coils are old).
Alternate test: ride the bike at highway speeds, then stop & feel the coils to see if they are getting hot to the touch - press hard w your fingers. They should be warm, not hot. Don't get burned by the valve cover - practice feeling them first when the bike is cold.

Another thing you can do is swap the coils around to see if the problem follows the coil or stays on the bad cylinder(s).

Clean & lubricate the timing advancer.
Separate the rotor from the body to clean it & fill the groove w grease (if yours has the groove).
Get a new engine cover gasket beforehand.

If you need a manual, search ebbaayy for KZ750 Shaft Manual.

With that kind of milage it must be running like crap.

Something like this should do. Not as fancy as Ed's, but not as pricey either. Plus it is translucent, so you can tell if it is flowing fuel or not.
You will want one for a 5/16" fuel line, of course.

martin_csr wrote: If one cylinder is misfiring, you should take care of that ASAP. Check the engine oil for fuel contamination.
What type of spark plug wires & caps did you install?
Double check the spark plug wire connections to make sure they are good.

Check the coils w a multimeter (note that they may be within spec, but a coil can be bad as there can be internal shorts that the resistance tests may not reveal... the coils are old).
Alternate test: ride the bike at highway speeds, then stop & feel the coils to see if they are getting hot to the touch - press hard w your fingers. They should be warm, not hot. Don't get burned by the valve cover - practice feeling them first when the bike is cold.

Another thing you can do is swap the coils around to see if the problem follows the coil or stays on the bad cylinder(s).

Clean & lubricate the timing advancer.
Separate the rotor from the body to clean it & fill the groove w grease (if yours has the groove).
Get a new engine cover gasket beforehand.

If you need a manual, search ebbaayy for KZ750 Shaft Manual.

Wow, thanks for the input! I took the bike to be inspected today by someone who spent years working for Kawasaki on older bikes (works for BMW now). He took the bike out and actually thought it was running pretty well all things considered. With that in mind, he thinks the problem is all in the carbs and recommends that I take it apart completely, clean the piss out of it and soak it. He had an ultrasound carb cleaner, but only for a set of 2 and not 4

I'll go about disassembling the carbs and cleaning everything out soon hopefully. If that doesn't fix/make anything better I'll definitely move onto what you've recommended, though I'll take a look at the oil in the morning for sure. The spark plugs I bought were the NGK B7ES's and the wires were from Z1 Enterprises, which someone had recommended on another Kawasaki forum for my bike.

baldy110 wrote: With that kind of milage it must be running like crap.

I took it out on the highway today and ran it up to 90. I've actually gotten 61 miles out of it this time and still haven't needed to put it on reserves. I'm guessing it's because I wasn't shifting as much, but I hope it could have cleared up a bit since I haven't actually run the bike at those speeds before. I can still see gas in the tank without shaking it, so I'll probably be able to go about 10 or so more miles before reserves, putting me at around 23 mpg. Not great at all nor what it's capable of, but it's still better than my car. I'm just not going to have quite the same amount of time to work on it now that I have to go back to school

If it's only getting 23 mpg you really should make the time to fix it. Something is wrong, and riding it like that may lead to larger, more expensive problems. Spend the better part of one day removing, cleaning, and re-installing the carbs and be done. They can be cleaned without ultrasonic, overnight soaking, etc. Ed