Clutch Slipping, or Oil to blame? (79 KZ650SR)

Hi all!

1979 KZ650SR.

My mechanic is unable to do my yearly check-up (oil/filter change, cable/chain adjustments), so I decided to tackle it myself - a first for me!

Last Friday, I changed the oil and filter, and used Rotella T6 15w40 oil, with an OEM filter cartridge.

On Sunday, it was about 65 degrees F outside, so I took the KZ for a 40+ mile ride, no issues whatsoever. Last night it was 55 degrees and I had to run down the street (~5 miles round trip), so I hopped on the KZ. At 4.5-5k RPMs, with decent throttle (half?), the clutch would start slipping. I got home and realized there was no slack in the clutch lever, so I figured a quick adjustment would fix that. While I was at it, I thought I'd figure out how to lubricate the cable too, in case that was an issue.

This morning, after much trial & tribulation, I got the clutch cable off the lever and lubricated it. When I put it back on, I left a couple of millimeters of slack in it, and ran it on the center stand to ensure the clutch was fully disengaging - it was.

Took the bike on a 25-mile ride to test it out, and as I rode and the bike warmed up, the slipping occurred less, but was still intermittent, most notable in 4th and 5th gears, but it was only doing it probably 25% of the time, and I did several runs to higher speeds (there are some 50mph roads near me).

I spoke to a buddy of mine who wrenches on his own bikes all the time, and he says my clutch is probably starting to wear out and will need replacement soon.

More mental notes I took: It only happened when I quickly cracked open the throttle, not on smooth roll-on of throttle. Only noticeable in 4th and 5th gears, and almost always at 6K+ RPMs.

I tried adjusting the clutch cable at the midpoint connector, but have no idea what that's supposed to look like when tight or loose. I just know that after fiddling with it for a while, there was a little free play in the clutch lever.

Questions:

1. Could the oil be causing a problem? I've always had my independent mechanic do my oil changes, and I don't know specifically what brand motorcycle oil he used in the past (I've had this bike since 2017 and he's done at least 2 oil changes since I got it).

2. What causes an intermittent slip in the clutch? Every time it slipped, I'd quickly get off the throttle and then I could go back on and no issue.

3. How do you properly adjust the midpoint connector? I get the top one (at the lever), and while I haven't touched it, I think I understand the one behind the side cover.

4. If I do need a clutch, how hard is it to do? I am not that mechanically inclined, so I don't want to bugger anything up.

5. Is it temperature related? Like a 10-degree ambient air difference causes slippage?

Thanks!

-John

Change the oil again
Try T4 non synthetic Rotella
Clutches don't like the Synthetic

Well, crud. I have follow-up questions then, if you don't mind:

1. Do I have to change the filter again too?
2. Is 15w40 a good weight to use still? The T6 I used was 15w40, recommended by some other KZ650SR owners.

Thanks,

-John

You need a JASO MA certificated oil: Delivers the desired friction performance motorcycles need to prevent slippage in the wet clutch.

Most likely a combination of tired clutch springs and a misadjusted clutch. People make way too much of different oils and clutches in my experience. You can try the non synthetic oil if you want, no need to change the filter, but that won't change the fact that your clutch springs are 40+ years old (unless they have been changed previously.)
Oh, and the fiber plates almost never wear out unless someone is drag racing the bike.

Greybeard wrote: Clutches don't like the Synthetic

Have used synthetic oil in various bikes for well more than 20 years and have yet to have any clutch issues - maybe it takes a while for the issues to show up?

Rotella T6 15w40 is JASO MA certificated

Greybeard wrote: Change the oil again
Try T4 non synthetic Rotella
Clutches don't like the Synthetic

That has not been my experience, and I have put more than 62,000 miles on my KZ650. Synthetic works perfectly fine, but non-JASO-MA compliant oil can cause clutch problems. Ed

I stand corrected.
It is not the synthetic oil that is bad for your wet clutch, but rather an additive found in many (not all) synthetic oils called MOLYBDENUM that makes wet clutches slip

650ed wrote:

Greybeard wrote: Change the oil again
Try T4 non synthetic Rotella
Clutches don't like the Synthetic

That has not been my experience, and I have put more than 62,000 miles on my KZ650. Synthetic works perfectly fine, but non-JASO-MA compliant oil can cause clutch problems. Ed

Rotella T6 is syntetic

Nessism wrote: ...............

Rotella T6 is syntetic

Exactly. I've been using Rotella T6 for the past 4 years with no problems. Before that, I used Mobil Racing 4T synthetic oil for about 10 years. I found that over time my starter clutch began to slip with Mobil Racing 4T, but after changing to Rotella T6 that problem went away. Ed

You need to adjust the clutch first at the actuator on the engine, then take up most of the slack mid cable, then finish off at the lever, motorcycle specific oil is the ideal oil to use, a lot of car oils have friction modifiers in them to aid lubrication which can cause wet clutch slip, I assume you have a manual.

Hey all, thanks for the helpful information!

When the rains die down, I will adjust the clutch cable at the engine attachment. That was the only one I didn't touch (because I didn't have a manual this morning). Getting the cable out of the lever was a royal PITA - I ended up taking the lever off to get the cable out of the adjustment knobs, as there wasn't enough slack in the cable to be able to thread the adjuster and lock nut out of the lever housing.

I downloaded a PDF of a KZ650 Service Manual (Kawasaki) this afternoon, so I have the steps to do it. It's for a 77 KZ650, but the pictures match my 79. I haven't found a 79 D2-specific manual (yet).

I see the steps in the manual for adjusting the mid cable, but on my bike there appears to be the adjusting nut (tall), a locking nut (short), and a 3rd one that's in the middle height-wise... what's that for? The adjusting nut and locking nut are both 12mm, but the 3rd one is 10mm. I played with all 3 of them for quite a while, with no discernible difference in clutch lever play.

I'm hoping a simple adjustment will do the trick.

As for the Rotella T6, I took pictures of the front and back of the jug, and it's got a "blank bottom" donut cert on it - top says "API Service CK-4", with "SAE 15W-40" in the middle, and nothing on the bottom half. As others have said, it lists JASO MA/MA2 in its certifications.

If it ends up being the clutch, is there a kit I can buy that includes all the necessary hardware?

Oh, if anyone is in the Washington, DC Metro area, I'd be happy to socially distance with you while working on the bike if you can walk me through it. I'm new to this mechanic stuff, and it doesn't come easily to me. (computer programmer by trade)

On a side note and to add another level of experimentation, I also own a 2005 Kawi Vulcan 800, which I did an identical oil change (same oil, OEM filter, same day). I've only put about 15-20 miles on that bike since (I've put 75 on the KZ in the same time period), and have yet to notice any slippage. I'll put it through the same riding and see what happens with that bike. That bike doesn't have a tach on it, but I run it up the RPMs quite frequently

Thanks!

-John