Suggestions for handling a Carb Sync

I bought my first bike very recently, its a 1982 KZ650-H2/CSR650. When I bought it it wouldn't start due to the gummed up carbs. After a neat trick my mechanic friend showed me to clear them a bit (got it started and was able to drive it home) Him and I tore it all down and cleaned it all up and it starts right up now.

The issue is its definitely in need of a carb sync, the engine wont hold an idle properly, it doesn't rev down when you rev it up (it will hold RPM for like 10 seconds) and it is running really rich (I can smell it) and loud. It also dies if you give it no choke at all.

So here is my question, being that I don't have a carb sync tool, and have never done a carb sync, is it better to take it to a shop for 80 bucks or to buy a carb sync tool and learn to do it myself. My mechanic friend was visiting so it will be me on my own.

If I should decide to do it myself what other settings besides the air mixture screw will I need to set? The mechanic mentioned something about a "curb idle" that may need to be adjusted, and so he doesn't suggest I try it myself this year.

Sounds like you might also have an air leak between the carbs and the engine.

I don't think so, when we took it apart I checked all the boots really well and when we put it all together we used a spray bottle to check for leaks, it all seemed nice and tight.

Syncing is going to be one of the last steps in a tune up. It sounds like you have other bugs to start troubleshooting first. If it only runs on choke, there will be more cleaning you need to do first. Make sure you have gone through all of the jets. Sometimes some are hidden under plugs or under each other. Obviously, the first thing is to get a factory manual.

derekthemajestic wrote: I don't think so, when we took it apart I checked all the boots really well and when we put it all together we used a spray bottle to check for leaks, it all seemed nice and tight.

Cool - the not coming back down from idle sounded slightly air leak-ish. Does it have a push and pull throttle cable and both have been lubricated properly? Typically a sync wouldn't affect that to that degree, I don't think. jmo.

Some guys have built manometers for fairly cheap, others buy them. You'll be hard pressed to get one for less than $80. If you want to learn, buy one, if you just want it sync'd, pay somebody and you will probably never need to do it again.

You definitely still need to do some additional carb work. As suggested above, get a manual and adjust them accordingly. You sure your cables are not binding?

With a manual and a little time, you can get them REALLY close and running pretty good. As a final touch once your comfortable with how its running then I would suggest just taking it in and have them synced. Unless you think your gonna wanna do it again later.. bur for a one time deal, just have it done.

Here is a link that may not be 100% relative, but may give you some basic reference.

Carb Tuning

As to the throttle cable, I am not sure what "push pull" means, but my friend said he lubed it, and you can watch it from the side of the bike, the motion is smooth and even, and when the bike is off returns really well.


So when we took the carbs apart for cleaning we counted the turns to bottom out the air mix screws prior to their removal (6.5 turns) and when we put it all back together we put it to the same number of turns.

I had my mechanic friend helping me but he's more cars (he has been riding for 25 years but never worked on anything from my KZ650's family of bikes) so he had no idea how far off that was, so when I spoke to the mechanic I am considering taking it too (telling him all we had done so he knew what he was up against) he started laughing and said to always start at 3.5 turns and go from there. My Clymer manual doesn't share that at all.

So we are gonna start there and see if we can get it closer to right this weekend, that being said I'm wondering if he was right about the "curb idle" and such.

The part that really worries me is that the previous owner claims the carbs were professionally rebuilt a year ago, and while all the parts look brand new, the tuning seems to be pretty far off... I'm just worried that the air/fuel mix might be set that way to compensate for some poor tuning in another part of the carb, is there anything that opening the air/fuel up that much might be trying to compensate for?? (if this sounds ridiculous it might be, I really don't know anything more about carbs than what my lawn mower has taught me.)

Ohh and thanks for all the help and advice guys!

DoubleDub wrote: Sounds like you might also have an air leak between the carbs and the engine.

Derek, I have a tendency to agree with what DoubleDub says here. It is possible that you could have multiple issues that are confusing the issue. However, an idle that "hangs" as you describe is a classic sign of a bike that is lean at idle. The most common cause is hard boots that have hairline cracks. On the other hand, it could be a jetting issue. Are you still using the factory air box and factory exhaust? Pods and a 4 into 1 are common mods and they require re-jetting to compensate.

My suggestion would be to purchase a factory service manual and do all of the maintenance as if it had never been done. First and foremost, do the ignition tests. It doesn't make a wit of difference what your carburetor issues are if your ignition isn't functioning correctly.

What I like to do on a newly purchased, but old bike is to start by changing the oil and filter. Then preform the valve clearance checks/adjustments. Next, run a compression test and possibly a leakdown test. Then run through all the charging and ignition tests. Set the ignition timing, and then address fuel issues. Make sure the petcock is functioning, and verify that you have good fuel lines and vacuum lines in the case of vacuum operated petcocks. After all of that stuff, then you can worry about cleaning, jetting, adjusting, and syncing carbs. The sync is the last thing you do after you are sure that everything else is right.

If you want to do your own work, then I would suggest you buy a Morgan Carbtune. I have used the newer Motion Pro sync tool and I'm not terribly impressed. However, you will spend more on the tool than the $80 you were quoted to have a mechanic sync the carbs.

BTW welcome to the club. It may seem like a lot of work that I have suggested, but it will pay off in a trustworthy bike that you will be able to ride and enjoy instead of sitting on the side of the road waiting for a rescue.

Bingo....

derekthemajestic wrote: As to the throttle cable, I am not sure what "push pull" means, but my friend said he lubed it, and you can watch it from the side of the bike, the motion is smooth and even, and when the bike is off returns really well.

Your bike should have 2 throttle cables One opens the throttle when you twist the grip to accelerate; the other closes the throttle when you twist the grip to the closed position. The closing of the throttle will happen without the second cable because there's a spring down at the top of the carbs that also pulls the throttle closed. However, if for some reason the spring breaks or something hangs up the throttle that second cable will allow you to close the thottle. Without it, if the spring doesn't do its job, you're off to the races.

As a test, you may want to unhook the throttle cable at the carbs and see if the idle is still slow to return to low rpms. If the problem is still there you need to double check for air leaks at the carb holders (maybe you didn't do that procedure correctly the first time) or check the carbs to ensure the slides are moving freely. Ed

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