Can't Keep Steady Idle, 81 Kz750 Spectre

Bought an 81 or 82 Spectre (dataplate says made in 81; emissions sticker says meets specs for model year 82 vehicles) several weeks ago for $350. The guy said it was his daily driver a few years back but it sat for a couple years. He said he took the carbs off and sprayed them but couldn't get one carb to synch with the others.

I started giving it a once over recently and took off the fuel & air systems. Removed, disassembled and cleaned the carbs well but didn't uncap & remove the mixture screws -- the carbs weren't very dirty and I didn't want the hassle of having to try to fine-tune each carb mixture screw individually.

The foam air filter disintegrated when I touched it; most of the vacuum system lines were stiff and hard. My impression was that the breathing and vacuum systems hadn't been tended to for a long time.

I got the bike to run "sort of OK" but the idle wouldn't stay put.

I saw a post on this forum showing how to remove the whole vacuum system, and I did; removed and capped every line except the one to the fuel tank petcock's diaphram.

Bike started up ok, took a long time to run ok without the choke on, but then I couldn't find the right spot for the knurled idle set-screw (the "all carbs" knob that you turn, underneath the carbs, that sets the idle bar for all 4 carbs at once).

The engine would race at 4,000 rpm and I'd turn down/unscrew the carb knob -- it's continue racing, then I'd have to twist the throttle grip quickly a couple times before the rpms would drop.

Then it'd idle nice for a few seconds and then either drop fast to a stall-out; or rise back up pretty quick to maybe 3,000 rpms.

Any ideas on what might be wrong?

Oh, one other thing, might be related or might have nothing to do with it: the bike does not have a battery. I'm using a 12v car battery hooked with standard jumper cables. One time, as the bike was running, I unhooked the positive alligator clip from the battery and the bike RACED up to 4500 or so RPMs. I reconnected the positive cable and the rpms went back down again (rectifier issue??).

The whole thing has me scratching my head.

Step 1 - Buy a battery.
Another Step 1 - buy an air filter.

It is a waste of time trying to tune a motorcycle that is missing vital pieces.

Thanks ed.

How is the battery vital to the bike's idle? Doesn't the stator put out all the juice needed for the bike to run & drive?

No, the ignition system needs juice from the battery to run. These will not run with a dead battery. And when you cleaned your carbs did you make sure all the small passages and jetting inside the carburator are clean and clear. Part of cleaning it properly is removing hte idle mixture screws and making sure the passages are clean. Just count the number of turns till they seat and replicate that when you put it back together. Also do a complete tuneup before trying to solve problems on an unknown machine. Relace plugs, chain oil, check valve clearances ect.

On my bike when it's time to replace the battery the bike runs very poorly and has a bunch of odd symptoms. Ed

Now that you point it out, the bike ran differently with three different batteries. First I used a 12v car battery with jumper cables. Then I used a lawn tractor battery with my heavy-gage but shorter home made jumper cables. Then I used a questionable old motorcycle battery -- wouldn't fit into the holder so i also had to jumper it.

the bike ran differently with all these configurations. I never thought it would actually be the BATTERY causing the bike to run different!

Does anyone know specifically what ignition parts require the battery, causing this weird syndrome?

Just never would have thought that the battery itself would effect the idle of the bike. I've run cars, trucks, minivans, ATVs, other motorcycles, lawn tractors, etc with all kinds of whacky battery setups and have never had such a weird thing happen before.

Any electical system SME's online to explain why this bike needs exactly one type of battery and that battery has to be in good shape not just to START the bike, but in order for the bike to idle smoothly?

Thanks again guys. Always nice to have a place like this to go so i can ask these weird questions. what did we do before the internet? 9well I remember what we did: bought paper manuals, went to the library, drove around asking mechanics, etc)...

At idle, the "dynamo" on the smaller displacement bikes is piss-poor (there are upgrade kits to go to the 3 phase alternator setup used on the bigger bikes). Couple that with ancient electrics and the best electronic ignition that could be found, in 1981 or 82, and voila, you got an enigma wrapped in a riddle. I'd say by and large the gremlin most chased on these old timers is electrical in nature.
Welcome to the insanity.

DiamondSkyBlue1000 wrote: At idle, the "dynamo" on the smaller displacement bikes is piss-poor (there are upgrade kits to go to the 3 phase alternator setup used on the bigger bikes). Couple that with ancient electrics and the best electronic ignition that could be found, in 1981 or 82, and voila, you got an enigma wrapped in a riddle. I'd say by and large the gremlin most chased on these old timers is electrical in nature.
Welcome to the insanity.

Not really. Actually, many of the "smaller" bikes (my KZ650-C1 for example) came with the 3 phase setup as original equipment. There were in fact 3 different flavors of the 3 phase systems used on KZ650's. One was an excited field version that used a field coil, and one used a permanent magnet, there were also some models that did use a single phase system. Below are the KZ650 types. I don't know which flavor (if any) of these the '81 KZ750 used.

First gen. uses 3 wires
1977 B1 standard...3phase excited field
1977 C1 custom.....3phase excited field

Second gen. uses 2 wires
1978 B2 standard...1phase permanent mag
1978 C2 custom.....1phase permanent mag
1978 D1 sr.........1phase permanent mag
1979 B3 standard...1phase permanent mag
1979 C3 custom.....1phase permanent mag
1979 D2 sr.........1phase permanent mag
1980 E1 LTD........1phase permanent mag
1980 F1 custom.....1phase permanent mag
1980 B4 standard...1phase permanent mag
1980 C4 custom.....1phase permanent mag
1980 D3 sr.........1phase permanent mag

Third gen. uses 3 wires
1981 D4 sr..............3phase permanent mag
1981 F2 ................3phase permanent mag
1982 F3 ................3phase permanent mag
1983 F4 ................3phase permanent mag
1981 H1 csr.............3phase permanent mag
1982 H2 csr.............3phase permanent mag
1983 H3 csr.............3phase permanent mag up to KZ650DE052650

Fourth gen.
1983 H3 csr.............New type 3phase permanent mag starting at KZ650DE052651

Which one of those sets of numbers explains why three different batteries and one different jumper cable make my bike idle four different ways?

rustbuckett wrote: ...81 or 82 Spectre ...Removed, disassembled and cleaned the carbs well but didn't uncap & remove the mixture screws...foam air filter disintegrated when I touched it... most of the vacuum system lines were stiff and hard...got the bike to run "sort of OK" but the idle wouldn't stay put...couldn't find the right spot for the knurled idle set-screw...engine would race at 4,000 rpm and I'd turn down/unscrew the carb knob -- it's continue racing, then I'd have to twist the throttle grip quickly a couple times before the rpms would drop.

Then it'd idle nice for a few seconds and then either drop fast to a stall-out; or rise back up pretty quick to maybe 3,000 rpms.

Oh, one other thing, might be related or might have nothing to do with it: the bike does not have a battery. I'm using a 12v car battery hooked with standard jumper cables. One time, as the bike was running, I unhooked the positive alligator clip from the battery and the bike RACED up to 4500 or so RPMs. I reconnected the positive cable and the rpms went back down again (rectifier issue??)....

Would suspect air leaks in the carb holders as possible cause of the racing idle.

Would verify integrity of the ground connection where the negative battery cable attaches to rear of engine.

Also wondering whether particles from the disintegrated foam air filter might be clogging carb passages.

Am uncertain why unhooking the positive alligator clip from the battery caused the engine to race up to 4500 or so RPMs, or why reconnecting the positive cable caused a return to the lower rpm.

Typically, an inoperative regulator will cause over-charging, and an inoperative rectifier doesn't allow charging. Am thinking the bike at hand has a combo rectifier/regulator, unlike earlier models factory equipped with a separate rectifier and a separate regulator.
Would assure the black/yellow wire from the rec/reg is providing a good ground.

The ignition system draws from the battery (as it will do without any charging system components, and with the engine running perfectly well until the battery runs down). I believe, perhaps erroneously, that the ignition would perform just fine drawing from a good hot automobile battery strapped to the passenger seat -- the bigger the better -- provided the jumpers are connected properly (including negative to engine ground).
The chassis electrical components also draw from the battery.
The charging system serves to keep the battery charged.

Good Fortune!

Thanks for the detailed reply. I went to AutoZone and bought a new battery, it's charging now.

I cleaned the barbs well; i don't think the air filter particles clogges the holes, but I think I'll take all four off again anyway and do the fuel level 3mm measure check. I set the float valve height but didn't check the fuel level.

I'll also remove the carb holders and clean them and check them real well for any signs of potential leaking. I wondered if those things might be letting some air squeak into one or more cylinder...

Will check the ground wires too. I also noticed the starter pushbutton switch needs a good disassembly & cleaning, so maybe there might be several little wiring glitches.

Gotta build an air filter element. i have the plastic cap, cage and bottom. But I need to get some foam. Maybe will see if the foam filter from my Honda Foreman 400 parts ATV might fit....

Tomorrow I've gotta do some more divorce paperwork though so I might not get to it for a day or more. Been working on bikes instead of filling out lawyer forms! Jeez, i wonder why... those divorce forms are a lot more fun than motorcycles.

Thanks again.

You don't need to build an air filter element. The 750 Spectre element is still an "active" Kawasaki part, so any dealer can provide it. Cost is about $15. Ed