'79 KZ650C misses/backfires

I am new to KZ bikes, please understand I have single cylinder mechanical experience and am trying to help my brother with his latest purchase. The bike cranks fine, will not idle well, even after it warms up. During test run, it spuddered like it was running on 2 cyl. and then it opened up to 4 cyl. As I slowed down, then hit the throttle, it started spuddering again and I let off , it backfires through the right tailpipe. I'm here to ask for some clue before going through the manual(Clymer's)and blind testing the fuel, intake, timing, carb. sync.,& etc., I don't mind trial and error,I just want someone with KZ experience to guide me in the right direction. Thanks for any help. Doug Miller, Vernal, MS

Hi, Welcome aboard! How long has it been since the bike was used on a regular basis? I ask be because if it's been much more than 6 months or so there may be gas crud built up in the carbs and the fuel may be "stale." The symptoms you describe could be any of several different things, but crud in the carbs will definitely make it act the way you describe, so if it's been out of service for a long time that's where I would start. Ed

carb's need to be cleaned properly that is my educated guess give that a try and let us know how it goes for you.

Thanks for the replies ! I suspected several things myself and could see me slinging parts with one hand and the manual in the other. My brother picked up the bike in Wisconsin at a yard sale. I have no idea how long it has been sitting up but he did mention filling it up wth premium gas. No, he didn't drain or clean old fuel out. Both replies indicate dirty carb. Any tricks I need to know before following manual step by step in removal and assembly ? I've no experience with a 4 carb. setup.

Once you get things cleaned up be sure to install a quality inline fuel filter, otherwise hidden crud in the tank will quickly corrupt the carbs again. Ed

Thanks, 650ed ! It has an inline on it that I'll replace when done. I'll let ya'll know how it turns out.

just follow the manual for disassembly and when you get to that point start asking questions if we tell you now it would take all the fun out of it.

I hear ya, timebomb33. I was just reading up on carb. sync. and I see a large can of worms in my future.

It's probably going to be dirty / gummed up carbs. However the first thing to understand is the definition of "cleaning". Most people when they clean their carbs take them off, pull off the float bowles spray some carb cleaner in and around the float bowl area until it appears clean. They then put it all back together and have the same problem. That does not consistute "cleaning" you must take all the jets out clean them up making sure the small holes are clear then MAKE SURE all the small passage ways are clean in teh carb bodies and you can spray carb cleaner though every passage. Make sure ever lever and slide operates smoothly. Check float level and if needed adjust. Put everything back together and install on bike.
Next make sure the gas in the tank is good or you will just be allowing bad gas to get into the carbs you spent time cleaning. Good luck

Thanks, Baldy. I need all the luck. I just finished removing the carbs from the bike. I'm familiar with carb cleaning, just not four at a time. I'll start cleaning tomorrow.

BTW, while removing the carbs, I noticed vacuum line to #1 carb was plugged off with a screw, no retainer springs on air box tubes, and no screws in the air box. Do I need to be concerned with any of this ?

Is it the vacumn line or the vent. The vent is the larger of the two. If it is the vacumn line, that one goes to the petckock to keep it open while the engine is running. Sometimes people will install a manual petcock valve on the tank if it has one then that line will need to be plugged.