'80 KZ750 RPM Issues!!

Hello I'm new to the KZ world and just purchased a 1980 KZ750H. The bike has 23k miles and is in pretty great condition. I had some issues with the carburetor but it was fixed professionally and has been working great since. I've been riding the last two days with no problems really besides my bike would shut off when downshifting at stops and it wouldn't idle without me holding the throttle open a little bit. I looked in to some stuff to adjusting my idle and when i went to adjust the idle screw between carbs 2 and 3 I got my bike to idle at around 1500 RPM's and thought i had fixed my problem. As I took off down the road i got around 3500 RPM's and they just took off towards 6000 RPM's and I couldn't stop it without pressing the kill switch. I had tried to re adjust the idle screw back and turned on my bike letting it idle in neutral. Then i go to rev it to test what would happen and it took off again!! Just kept revving. I'm almost positive that it's just a problem that i created with the idle screw because it was running perfectly before then but I just want to get some outside thoughts. Anything helps you guys, I really appreciate it!! Thanks.
Creighton

sounds like it could be a few things related to tuning, but def check out your push/pull cables first. and throttle snap spring. Your throttle could be snapping back, but it still engaging.

M

Thank you I will check that out and let you know how it goes.

Creighton

Could be a vacuum or carb boot leak.

Any video links on how to check your guys suggestions would be great! I'm new to motorcycles so videos are very helpful. I checked the throttle cables and they have a lot of slack in them nothing seems right or stuck anywhere. I am still messing with that idle screw since that's originally what caused this problem for me. I got my bike down to around 1000 RPMs while idling but when I rev the engine up to 3500 it sticks there revising continuously or it increases rapidly and I have to kill the engine. Anything you guys could do to help me get back on my bike as soon as possible would be greatly appreciated. Thanks. Creighton.

i have the same bike, and just got her back on the road a couple weeks ago. im experiencing the same issue, i found a sweet spot in the throttle screw, where it doesnt rev up, butmit feels like the idle is low.... just a bit low (i have yet to install my tach so im really not sure).... it took quiete a bit of fiddling with it to get it to stay running and find the sweet spot...

are you running stock air box, exhaust, jetting?

i have yet to get to diagnose my problem, but plan to try this weekend... ill keep you updated....

dont adjust the throttle screw until your fully warm and fully off choke... (mine has a choke sweet spot too, about 5/8 on, where it likes to warm up at) not sure if its related....

also, check for kinks in the fuel line for your dieing issue, mines fooled me a couple times due to kinks....

For certain it has a vacuum leak somewhere. Most often it's the carb mount rubbers, either cracked or not sealing on the head, or the carb hose clamps are loose or slipped.

Some times a really lean mixture will cause a like yours.

I have the same exact bike with the same exact issue. and i'm new to motorcycles. this is kind of killing my want to ride at all.

First - I agree with DGA and Dom.

I don't have that model of KZ (dunno why either? LOL) but I did have the exact same issues with my 550a this past spring just after I got it. It would start fine, run fine for a while and then suddenly the RPM would race outta control, to the point where I was using the clutch and engine braking to bring it under control at stop lights etc...
The issues I dealt with to resolve it were related to 2 things:
1) vacuum leaks. (usually carb mounts but also check all vacuum ports for boots or proper hoses)
2) carb syncrhonization. (for uneven and problematic idles and noise)

The runaway RPM I believe was more than likely just vacuum leaks but I did both because I wanted to know.
There are many posts here on how to easily check for vacuum leaks around the carb mounts; the reader's digest being: run engine, spray something like WD40 or even starter fluid (less messy) around the mounts and see if the RPM changes or fluctuates. You may even see the fluid being sucked into teeny gaps...

Does this issue occur only after the engine has been running a while (ie: warmed up)? This is usually a sign that heat is causing expansion of the materials involved and a tiny gap is opening up (typically this is between the carb mount and the cylinder head surfaces) and thus, if you spray something at these joints, the engine is sucking it inside and fowling the carb's efforts.

It doesn't have to be all the carbs either. Just one with a leaky carb mount is enough to mess-up a perfectly running engine otherwise.

Carb synchronization was my second step to reduce idle noise which I still don't fully understand but it did work.

Most of the time, the manuals state a good idle (as measured by the tachometer) is between 900 - 1100 ish (this is true to my 550, 650 and 1000 at least). Higher idles just reduce fuel efficiency but some engines are more finicky and there has been discussion here on just how accurate our tachometers are in the first place, so you go with your gut and what the bike ultimately demands for consistent idle.

Finally. If your bike starts and runs well initially and then it changes to this behavior, then it's very unlikely to be an adjustment such as your idle screw. Thus, my belief in the vacuum leak somewhere. If you have POD air filters, ensure your bank of carbs are still seated fully into the carb mounts. We've seen here on this forum where guys' carbs bounced their way out and introduced problems. It was even noticeable in one guy's pictures he posted. So check that too if it applies.