Hello from NC! Considering complete teardown/rebuild on my '81 KZ650 CSR H1

Hi everyone,

I've had my '81 KZ650 CSR H1 for a little over three years now. It had 10k miles on it when purchased through Craigslist, and it's now up to 12k miles after riding around town and a few longer trips. It's pretty much bone stock, except for a chrome crash bar (came with it), 12 volt cigarette lighter, smartphone mount, and battery tender hookups. See attached for some pics!

When I initially brought it home, I took it to my local vintage bike mechanic who did a thorough safety inspection as well as some basic services including cleaning/tuning the carbs and new brake fluid. It's run pretty well since then with hiccups here and there, but I think with help from this community I can get it running 100%. Everything "works" though.

Here are the problems I'd like to address:
1. The engine takes quite a long time (10-15 minutes) to warm up, which I know is quite common for old carbureted bikes. The main symptom here is that if throttle is applied when not fully warmed up and under load the engine loses almost all power but this can be alleviated if I ride for 5-10 minutes with 25% choke applied. If I twist the throttle further, the power kicks back in.

2. When riding along and applying throttle I can actually hear a distinct sucking noise coming from under the seat, which I know is where the air intake/filter is located. Not sure if this is actually normal or not.

3. At exactly 4200 RPM and only there, I hear a rattling noise from what I suspect is the engine. I can hear it when either revving up or down. I'm not exactly sure where on the engine the noise originates from, but my suspicion is that it could be the cam chain being too loose? I've also read that it may be one of the exhaust headers with a loose inner sleeve.

4. Drive chain is tighter in certain places than others. I've cleaned and re-lubricated following service guidelines since having the bike but just tried to tighten the drive chain for the first time since it was making noises when letting off the throttle in gear. I found the tightest section along the length and simply used that to measure the recommended 3/4" slack when adjusting the bolts on the back of the rear hub. After a test ride the noise was gone and I didn't notice any other issues. Is this normal, or should I purchase and install a new chain and possibly new sprockets?

I'm pretty comfortable with mechanical stuff. I've done my own car and motorcycle service stuff when feasible. I have both the Kawasaki service manual and Clymer book for reference. New tools are not an obstacle and would be a welcome addition to my garage.

At some point, I'd really like to take on the arduous long-term project of completely tearing down the bike, refinishing all necessary parts, installing many mechanical and electrical modifications that I've had bookmarked, and rebuilding from the ground up. I can reply with a complete list for anyone interested. I love the bike, and I plan to keep it forever. So learning everything about it in and out and making it my own is very important to me.

Thanks!

Welcome Blinker.

Yes, depending on the condition of the drive chain, you may need a new one.
The one on my ZRX was very noisy when I bought it, and a replacement sure helped there.
Have a look at the air filter set up.
Don't normally hear much sucking coming from there, unless you have four open pod air filters.
Long warm ups are normal, but yours seem s a little too long.
Some carb cleaning may be in order.
Check the function of the can chain tensioner.
The newer Automatic ones tend to fail over time. They can be converted to a manual lockable type.

Good idea to go over all the service maintenance schedule from a service manual.