kz750 sounds like an air compressor

Background:
Got a KZ750 off of a friend, who rode it for several years before gifting it to me. He rebuilt it in a shed, using parts salvaged from a junkyard. He rode it for a few years, gave it to me, I rode it for two before mothballing it for three.... Replaced battery, changed oil, cleaned plugs, cleaned carbs & tried to start -- nothing. After rooting around some cagey wiring, I downloaded a schematic & re-wired what I had (there's a lot of stuff on the schematic that's not on my bike...). Anyway, bike starts and runs.

BTW, bike starts right up with kickstart, occasionally sounds like I'm ginding something when I use electric starter. I can grab KS lever & turn engine a little, bike starts up.

Here's the question:

750 twin, Mikuni carbs; one side (cylinder)gets hot, the other warm. Plugs are not spec, but they are what has been on the bike, when it DID run for several years (oil on plugs, sometimes black soot, but usually oil). Executed a relay mod on the coil & spark went from white/ orange/ yellow to PURPLE (looking for blue...). This is an old, ACCEL coil, running both plugs. Switching plug wires does NOT change the hot/ warm arrangement on the cylinders. What's worse, its sounds "weird", a little like an air compressor, not the way I remember it sounding -- suggestions? Am I missing somthing or do old KZ750 twins sound like air compressors and I never noticed before? Do I need a new ignition coil?

Can email audio; can fax/ email schematic (how I wired bike)

Thanks.

Sounds like you have an air leak on one cylinder. Have you checked the diaphragms on top of the carbs?

Also check the intake manifolds for leaks.

Thanks, I will check diaphrams & manifold intake, but for clarity, it's not a hissing sound, it's the quality of "rumble". It doesn't sound right and I was worried that something wasn't right, mechanically. I was also worried about the uneven header temperature.

The bike is a 2 cyl, with 2:1 exhaust. I assume that both pipes should heat up similarly. Again, one gets hot (burn skin off), the other warm (hot but won't burn).

thanks

smellydog wrote:

Thanks, I will check diaphrams & manifold intake, but for clarity, it's not a hissing sound, it's the quality of "rumble". It doesn't sound right and I was worried that something wasn't right, mechanically. I was also worried about the uneven header temperature.

The bike is a 2 cyl, with 2:1 exhaust. I assume that both pipes should heat up similarly. Again, one gets hot (burn skin off), the other warm (hot but won't burn).

thanks

If not already done, would perform compression test. Cold engine not running, throttle held wide open during test.

To determine which cylinders are combusting, water-spray test is far superior to burnt skin test :pinch:

Good Luck!

Sounds like one cyl is not firing or not firing all the time. This can cause unusual noises. First confirm you have spark to both plugs. Do you have points or electronic ignition? If points one side could be stuck open. If you confirm spark to both plugs then you have either a carb problem or valve/timing problem. Does your plug electrode look wet? Smell gas? Do you have a compression tester?

Compression: used the kick-start/ thumb over the sparkplug hole method. On both cylinders, compression blows my thumb off of the spark plug hole.

I do have spark on both plugs (spark purple -- was a whitish/ yellow color, then I did the wg relay mod & now the spark is purple...).

I have points, although I can't get the cover off... (single wire, leading to the coil).

Plug electrode looks dry, but area around the electrode is oily/ wet. I do not smell gas on the plug.

I do smell gas when I run the bike with a small amount of backfire in the pipe -- I thought it needed to warm up. No knocking or pinging. Although it does sound like an air compressor....

Should I assume that I did a poor job cleaning the carbs?

When I re-wired the bike, I was left with a black, three wired part about the size of my thumb, but thinner. I called a couple of local mechanics, but no one seemed to know what it was or what it did. I traced the wires and they dead-ended in the loom (didn't connect to anything). It's sitting in my toolbox right now -- could that be important, and the source of my problem? If so, where does it go?

Thanks.

smellydog wrote:

...can't get the cover off... (single wire, leading to the coil)...Plug electrode looks dry, but area around the electrode is oily/ wet. I do not smell gas on the plug...do smell gas when I run the bike with a small amount of backfire in the pipe -- I thought it needed to warm up. No knocking or pinging. Although it does sound like an air compressor....
Should I assume that I did a poor job cleaning the carbs?
When I re-wired the bike, I was left with a black, three wired part about the size of my thumb, but thinner. I called a couple of local mechanics, but no one seemed to know what it was or what it did. I traced the wires and they dead-ended in the loom (didn't connect to anything). It's sitting in my toolbox right now -- could that be important, and the source of my problem? If so, where does it go? Thanks.

What's preventing removal of the points cover?

If not already done, would perform clear tube test to assure correct floatbowl fuel level.
An otherwise perfect carb typically runs excessively rich where the fuel level is too high.

Would also recommend assuring correct valve clearances. Acceptable cold compression figures do NOT assure continuing good compression upon attaining normal operating temperature.

Could be wrong, but guessing the mystery black, three wired part is not an issue here.

"Cleaning" plugs rarely works for me. I would replace them with brand new factory spec plugs.

Good Luck!

The cover is held on by stripped out phillips head screws. Tried to remove but have enough experience with stripped hardware to know when to stop before making problem worse. Put off removing the cover, hoping I didn't have to....

I like the advice but don't know how to level/ sinc carbs or measure valve clearances.

How does a clear tube assure floatbowl level?

How do you measure valve clearances -- take the engine off of the bike? Then take the valve covers off of the engine? Then what?

New plugs is a no-brainer; I shoulda done that first. ...I'm stupid....

smellydog wrote:

The cover is held on by stripped out phillips head screws. Tried to remove but have enough experience with stripped hardware to know when to stop before making problem worse. Put off removing the cover, hoping I didn't have to....

I like the advice but don't know how to level/ sinc carbs or measure valve clearances.

How does a clear tube assure floatbowl level?

How do you measure valve clearances -- take the engine off of the bike? Then take the valve covers off of the engine? Then what?

New plugs is a no-brainer; I shoulda done that first....

With points, will sooner or later need to remove cover for access to points as matter of routine maintenance, ignition timing, cleaning and lubing advancer unit, and expedite hand-turning crank incident to checking valve clearances.

Very slowly and carefully using a reverse drill bit will often removes the damaged screw without drama. Be sure the target punch hole is dead center into the screw, and don't push hard on the drill. Just go real easy to avoid breaking the drill bit. That's REAL easy.

Don't need to sync carbs right away, as that comes later (one of the final tuning concerns).

The clear tube test is described in many earlier threads. Would type "clear tube test" in the forum search box and press enter. It's easy to check with bike on centerstand and no disassembly required other than perhaps removing a float bowl drain screw.

Most KZ's require removing the valve cover to check the valve clearances. Some models have access plugs for access to screw-type valve adjusters. But I'm guessing your bike has shims and requires removing the valve cover to check clearances.

Really should acquire a good shop repair manual covering the particular model. An official FSM (Factory Service Manual) is imo the best, but an aftermarket shop manual would also be a good investment.

Without a shop manual to help with required routine maintenance, and without performing required routine maintenance, am afraid the bike's road-worthy life won't be long for this world.

Good Luck!

Sears sells a kit with small sized reverse drill bits/removers.

You can also take a sharp chisel and start it into the side of the screw and after you've started a cut, angle the chisel at about 45 degree and it will remove screw if it's not corrroded in. You might have to chisel off head, remove cover and work from there, PB Blaster works great. But sounds like you may have point problem. Adjust or replace points and slightly lube point cam. Let us know...

Since you say you got spark on both plugs I'd have to look at gas. Although with points I'd really advise you to get that cover off. If I've stripped the head to the point of uselessness I'd grind it off, remove the cover and juice up the shank and try and get a vice grip on it. But as for the noise I'd now be looking at the carbs. Ya might have missed something. You do need a good manual!