'81 KZ650 Surging / cutting off when hot

So I posted a while back about some difficulties w/ my kz650, and I ended up replacing the engine, and having to replace the piston rings in the new engine. That's basically all I did, put it back together, and I'm now making decent compression again.

My issue now is this: when I get it running, it surges (seems kinda sluggish when I roll on the throttle, then takes off like a crazy machine). I've read about this issue on this site, but I've never seen anyone say what causes this.

Now, about the bike (sorry, still learning about terms, parts, etc.). My kz has a 4-1 exhaust straight through (no muffler). It also has pods, which I've read aren't helping me. I purchased an airbox to replace them, which hopefully will be here within a few days.

When I ride the bike, it will go for a few minute, but if I shut it off or just stop, it will die and won't restart. I took apart the carbs and cleaned the pilot/jets out and everything seemed clean. When I bought the bike, I was told the jets were tuned for the current setup, but I don't know how to verify this.

I've checked my sparks, and I'm getting good spark all around. I have new plugs, and my coils are functioning properly.

Any suggestions on the surging and rough hot start are much appreciated. Thanks!

Jonnie

TriniTT wrote:

....I purchased an airbox to replace them, which hopefully will be here within a few days....

When I bought the bike, I was told the jets were tuned for the current setup, but I don't know how to verify this.

If you bought a KZ650-CSR airbox, I hope some good airbox ducts are included (the ones for the 81 650-CSR are no longer available).

Does the bike have the stock VM24 carbs w/ accelerator pump?
You can verify the main jets by removing them & looking for a number stamped on them - stock is 92.5, your setup will need several sizes bigger. There's probably a way to see what size the pilot jets are - you could pull those out & take a look at them. Maybe go to a carb parts site or Z1Enterprises.com for info. I'm fairly sure the CSR VM carbs don't have slots on the jet needles for clips, but I guess you can check to see if there's a shim somewhere to see if they have been raised.

The ad for the box was labeled as having a good duct included, so that's what I'm hoping for. I'm not sure that the carbs are stock, but I'm assuming so. I'm a car guy, and new to the bike thing, so I'm still learning.

I didn't notice any numbers on the carbs, but I also wasn't looking. Also, I'm not sure what the reference to "jet needle clips" is to, because I don't recall there being any clips on the jet needles. However, I only removed the jets themselves, and not the needles, so I didn't get that deep into the carb.

I'm considering taking the carbs to a pro, but there aren't any local shops so I'll have to scout one out.

Sorry about the jet needle wording. I meant to say that I'm fairly sure the CSR jet needles don't have the slots (or grooves) for a clip (a clip is used to raise the jet needles). Without the grooves, a shim (washer?) is used I believe.

Carb ID. If the carb tops have 3 screws, then they're probably the stock VM24 carbs. WGCarbs.com should have some photos to help you identify your carbs. Or you could use the diagrams at Kawasaki.com.

Have you checked the valve clearances?

When the engine warms up, the valve stems grow longer and the valves stop sealing if there is insufficient gap where the cams press down on them.

After just a few minutes, or even seconds, of cooling, the valves will seal again.

I believe the carbs are the stock ones, and they look like these:



When I looked at the valves, they looked like they could use new seals, but I was just replacing the piston rings at the time and I wanted to get everything back together. It's not hard to take this motor apart, so I may go ahead and order new valve seals and install them just to be certain.

Did you happen to measure the clearances? Your bike uses shims under buckets so you use a feeler to measure the gap between the cam and the top of the bucket (while the lobe is pointing away from the bucket).

Hmm, I didn't, but good to know. I just ordered new valve stem seals, so I'll measure them when I take the valve cover off to install the seals.

Double check what carbs you have. The various Mikuni slide carbs look kinda similar. The VM24 carbs have raised ridges around the carb body. Also, the #2 CSR carb has an accelerator pump on the bottom of the float bowl. Measure your carbs at the carb bore near the throttle slide. If you don't have calipers, use a thin piece of cardboard cut to the correct width (24mm). The photo is a set of 81 650-CSR VM24s:

I'm sorry, what am I measuring to 24mm? The carbs you have pictured look identical to mine, with the raised ridges. I'm sorry if I come off as such a newb, but the reality is, I am

I looked in the service manual, and I can't find any place where it says the distance I need to measure b/t the cam lobe when it's facing away from the valve and the valve bowl cover. I see a lot of other valve measurements, but I can't seem to find that one. Any suggestions?

If your carbs are identical to those in the pic, then you don't need to measure them. They're 24mm at the inside diameter of the carb bore near the throttle slides. The measurement is usually taken on the engine side of the slides (but it's the same on either side of the slides). The slides are the cylindrical things that move up & down when throttle is operated.

Lookup valve lash, or valve adjustment, or valve clearance. It's called a bucket, not bowl-cover .

I don't know what level of mechanic you are, but that's not entirely a job for a first time mechanic. Checking the valve clearance is fine, but actually adjusting the clearance is a lot more involved. You have to remove the cams, sprockets, buckets, chain-tensioner etc. You have to be able to time the valve train on the engine. It should all be in the manual, and as long as you are very very patient and follow every step, it's not a big deal. Any rushing and it's very easy to make engine-life-ending mistakes. Plan on taking a day or two if they need adjustment, and make sure you have a very clean, organized work environment. Actually if it needs adjustment, you'll probably have to order some shims.

The reason they make it so difficult is to reduce the valve train to the minimum number of moving parts. It's also one of the most reliable valve trains ever designed from that era. But service is definitely a big job for someone new to it all.