'77 KZ650 Troubles and Questions

Hey all,

Sorry for the wall of text!! But I would really appreciate any insight you all have to offer.

I've got a 1977 KZ650 B, that is a looker but not running as well as it could. I'm new to mechanical work in general, so I'm not sure what my next steps should be.

1) First things first, previous owner removed airbox and put in pods. Still needs to be jetted for pods. I do hear popping coming form pods at times. Carbs and gas tank were cleaned by previous owner.

2) Excessive blow by-- smoke billows out of crankcrase breather filter. There's a 10 inch long tube with a pod filter at the end, and it looks like oil is bubbling on the filter when it gets up to temp. Should I replace the filter? Just changed the oil and the oil level is fine. I'm assuming this is caused by 3)

3) Low compression in one cylinder, wet test resulted in higher compression so it's probably the piston rings? I need to run the test again just to be sure, because I forgot to hold the throttle wide open during the last test :pinch: The rest of the cylinders are at about 150psi. Will retest when I get home tonight.

4) Needs choke or idle turned way up (at 3000rpm) in order to not sputter and die. Can get it to idle somewhat well at 2000rpm, but it gets dicey and it usually needs the choke.

5) Eats gasoline-- probably due to high idle and maybe fuel/air mixture being off? Doesn't seem like it's leaking anywhere, though there is a little residue on the outside of the carb bowls. No puddles or anything.

6) Random question: if you notice in the pic below, I've only got one throttle cable? Did a previous owner remove the second pull cable? It snaps back just fine, at any angle.
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My question is-- do I bother getting the bike rejetted and/or tuned by a mechanic while the piston rings are bad? I'm willing to learn how to rejet carbs and fix the piston rings in the winter time, but for now I just want to ride the rest of the summer out.

Also, are there any carb adjustments (mixture or idle screws) i can make to make it idle a little better for now? Just to last the rest of the season until I fix everything. This guy points it out I think: Carb tuning

Any other tests I can do? Was planning on checking for vacuum leaks, but there don't seem to be any cracks in the boots visually.

Thanks for any help, it's much appreciated! So happy to be a part of this community.

Here's who you're helping!


-Stephen

I recommend you seek out and buy an airbox.

The previous owner didn't do that engine any favors. It looks like when the airbox was removed he just let the bike run way lean and way too hot. I say this becasue the header pipes appear to have turned dark. That should never happen to those stock K650 pipes because they are double walled.

Along with a compression test run a leak-down test to determine where compression is leaking. Ed

How's the sparkplug center insulator look? If it's white, the engine's running far too lean, and could melt a piston/burn valves.

So, ran a few more compression tests (with the engine cold), and now have more questions than answers.

(Cylinders L->R = 1->4)
Open throttle, dry test
1) 150 psi
2) 142
3) 155
4) 146

Open throttle, wet test
1) 240 psi (?!)
2) 162
3) 185
4) 172

So does this mean all of my piston rings are bad? How did Cylinder 1 go up to 240psi?! I used too much oil?

Here are images of my spark plugs, if someone could help me decipher them! They are in order (1-4) imgur.com/a/9nBdn#0

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Just out of curiosity, I tried retesting with throttle closed (same as I did weeks ago when I got one cylinder considerably low). I've been using Marvel Mystery Oil, so perhaps that's helped a bit?

Closed throttle, dry test
1) 140 psi
2) 120
3) 150
4) 135

Here's a picture of my (dirty) carbs haha i.imgur.com/UDY58xr.jpg

650ed wrote: I recommend you seek out and buy an airbox.

The previous owner didn't do that engine any favors. It looks like when the airbox was removed he just let the bike run way lean and way too hot. I say this becasue the header pipes appear to have turned dark. That should never happen to those stock K650 pipes because they are double walled.

Along with a compression test run a leak-down test to determine where compression is leaking. Ed

Thanks for the reply Ed!

I've been looking for stock airboxes but I can't really find anything, any tips?

I think you're right about it running hotter, cause on the few occasions I've ridden it for an hour, it stalls out eventually and needs a cool down for a few minutes before I can get going again.

I'll have to go rent a leak-down tester this weekend to try that out as well.

First - compression tests should be done wit the engine at normal operating temperature. The pistons are actually tapered in toward the top, so when they are hot they expand to the proper fit. A cold test will normally show lower reading than a hot test. Having said all that, judging by the readings, I don't think compression is a big problem. I would look for other issues such as the carb jetting being to lean for pods.

By the way - I see limited benefit in a test where oil is added. If the compression does not rise it shows the leak may be a valve or head gasket, so that may have some value. Other than that, the compression will always rise becasue the oil is displacing air volume in the combustion chamber, and oil cannot be compressed. Using a KZ650 as an example, when the piston is at top dead center there is precious little open space above it because of the 9.5 to 1 compression ratio. The volume of each cylinder is 163cc with the piston at bottom dead center but only roughly 17cc when the piston it is at top dead center. So if you add let's say 5cc (which is 1 teaspoon) of oil to one of those cylinders you have reduced the compressed cylinder volume by nearly a third which will drive the PSI on the gauge very high. So any cylinder with oil added should naturally show higher compression regardless of the condition of the rings (unless they're missing or have pieces missing) . What makes matters worse, if you don't add EXACTLY the same amount of oil to each cylinder the pressure readings between cylinders can vary wildly. Ed

Would do the following:

Sniff-test and examine the crankcase oil for possible gasoline contamination;

Check the valve clearances to assure being within specs;

Eliminate the filter at end of tube from the crankcase breather, so that the tube exits to open air (the filter is a useless troublemaker);

Clean the crankcase breather.

Good Fortune!

the fastest way to get the bike running better is to find a stock air box and get new rubber boots for it.

the bike it's definitely running lean and that is hard on any engine, especially an air cooled one. Piston rings, valves, and valve seals don't tolerate excessive heat very well.

the best way to evaluate the engine at this point is to get the fuel mixture right again, either by installing an airbox or re-jetting the carbs, then make sure the carbs are running in sync. put a new set of plugs in and run it. see how bad the smoke and blow by is and if it is just one side or both.

there should be a return throttle cable, but they do work without one.

650ed wrote: First - compression tests should be done wit the engine at normal operating temperature. The pistons are actually tapered in toward the top, so when they are hot they expand to the proper fit. A cold test will normally show lower reading than a hot test. Having said all that, judging by the readings, I don't think compression is a big problem. I would look for other issues such as the carb jetting being to lean for pods.

By the way - I see limited benefit in a test where oil is added. If the compression does not rise it shows the leak may be a valve or head gasket, so that may have some value. Other than that, the compression will always rise becasue the oil is displacing air volume in the combustion chamber, and oil cannot be compressed. Using a KZ650 as an example, when the piston is at top dead center there is precious little open space above it because of the 9.5 to 1 compression ratio. The volume of each cylinder is 163cc with the piston at bottom dead center but only roughly 17cc when the piston it is at top dead center. So if you add let's say 5cc (which is 1 teaspoon) of oil to one of those cylinders you have reduced the compressed cylinder volume by nearly a third which will drive the PSI on the gauge very high. So any cylinder with oil added should naturally show higher compression regardless of the condition of the rings (unless they're missing or have pieces missing) . What makes matters worse, if you don't add EXACTLY the same amount of oil to each cylinder the pressure readings between cylinders can vary wildly. Ed

Wow, that was very informative, thanks Ed!

I warmed up my engine for a few minutes, so it wasn't cold per se, but it also wasn't hot enough to burn me!

The oil displacing air volume makes a lot of sense actually. Especially because my numbers are so off from each other, I wasn't exactly being precise with how much oil I was adding.

So, there's still a chance my piston rings are bad, but only a leak-down test will show me that at this point. I'll have to run that this weekend!

Thanks!!!!!!!
-Stephen

Patton wrote: Would do the following:

Sniff-test and examine the crankcase oil for possible gasoline contamination;

Check the valve clearances to assure being within specs;

Eliminate the filter at end of tube from the crankcase breather, so that the tube exits to open air (the filter is a useless troublemaker);

Clean the crankcase breather.

Good Fortune!

Thanks for the help!

Will have to make time this weekend to check valve clearances, never done it before so I'll have to do my research.

I'll try your other tips out as well!

The compression test can be done with the motor cold, The numbers you will be looking for will just be less than numbers given for a hot motor.. It's all relative... :dry:
Also a compression test is not a volumetric test, makes no difference what amount of the compression chamber the oil may take up. You are just looking for a series of numbers that shouldn't vary by a whole lot..unless there is problems...
Oil put into a cylinder has been around for over 70 years...not sure any one here can scientifically disprove such a standard... It's propose is to isolate the compression leak between either the rings or the valve seats... Yes all cylinders may rise but the bad cylinder will not rise in the same realm as the good cylinders. :woohoo: I've used it hundreds of times on various types of motors and never had it fail me... :silly:
All cylinders will not have the exact same reading due to various mechanical factors,including machining tolerances, as long as they are within 10% of each other you should be fine. :ohmy:
Ditch the crankcase breather for sure
Richen the carbs...If you can't find a airbox (returning a bike to the stock airbox set-up is not as simple as some people make it seem,especially if you don't have every single piece needed)If that can't be accomplished with the adjusting screws.... Time to get some bigger jets and learn a little carburetoring :laugh:

Wish I would have saw that you posted pics of your plugs,, I retract that your carbs need richened up..none of those plugs look to be running too lean...look at how black they are.
I change my recommendation to a good carb cleaning and rebuid, I'd also be concerned about the valve stem seals on cyl #2
There are several outstanding tutorials on carb cleaning on this site....

ezrider714 wrote:
"Also a compression test is not a volumetric test, makes no difference what amount of the compression chamber the oil may take up."

What??? No offense intended -you may have done this on hundreds of motors, but it defies the laws of physics.

Please try this - dump 1 teaspoon of oil in one cylinder and dump 3 teaspoons of oil in another cylinder then run a compression test on the engine to see if the compression of the 2 cylinders varies. You will quickly prove that the amount of the compression chamber the oil may take up matters a great deal. Ed