Air bubbles

I've got a 78 kz1000 with pod air filters. I have clear fuel line on and as the bike sits and idles every 30 seconds or so a large air bubble will come out of the carbs and into the gas line. The fuel line always has air bubbles in it. It's not the tank inlet, it will do the same thing with the gas cap wide open. Also, I'm going to replace the intake manifold because of issues with the idle not dropping back down when revved up. Could these things be related?

don't panic about the air bubbles if it dosen't affect the way the bike runs and the problem with it not returning to idle properly is usally a symptom of improper fuel or air screw settings if it has fuel screws then you should try to richen them up try going half a turn out at a time on each screw if it has air screws go in a half a turn at a time on each a symptom of bad intakes is usally that your rpm goes up quite a bit when warmed up also you will get popingand luittle back fires and the engine will run quite hot. the fuel screws are on the front of the carb air screws are on the air box side of the carb. what carbs does your bike have on it.

Thanks Timebomb. I didn't mention them but the bike has all three of the symptoms you described for bad intakes. I had the carbs redone by Wiredgeorge when I started this project a year ago. I've only put 300 miles on the bike since then so I'm hesitant to turn any screws until I've eliminated any other possibilities.

I'm not sure of the model of carbs. They say Mikuni on one side and MIC on the other.

Those came with 26mm The vent lines are on the carbs . If WG rebuilt them you can pretty much eliminate the carb settings He test drives all rebuilds on his test bikes. Like TB said the air bubbles are nothing to worry with.Is your throttle cable hanging maybe pulling the carb open a bit?Why are you changing the intakes ? whats there problem.?

These are the 26mm he rebuilt for me.

Well. The bike has all the symptoms described earlier. The idol increases as it gets warm, the engine seems to get hotter than it used to and it has small popping backfires frequently. I did check the throttle cable first, no problem there.

I'd look at the mechanical ignition advancer to see if it's sticking. It's under a CD sized cover on the right side of the crank.This advances and retards the ignition when the rpm changes.

Some light oil like 3-in-one or sewing machine oil will work. Apply the oil to the moving parts like the springs, weights and other pivot areas.

It should return to where it's at when you twist it slightly.

ssalyer wrote:

I'm not sure of the model of carbs. They say Mikuni on one side and MIC on the other.

Look by the choke lever and on the flange there should be some ID numbers. Find them and post it..Did you check for air leaks??

ssalyer wrote:

I've got a 78 kz1000 with pod air filters. I have clear fuel line on and as the bike sits and idles every 30 seconds or so a large air bubble will come out of the carbs and into the gas line. The fuel line always has air bubbles in it. It's not the tank inlet, it will do the same thing with the gas cap wide open. Also, I'm going to replace the intake manifold because of issues with the idle not dropping back down when revved up. Could these things be related?

On a 2-stroke bike like an H1-500 Kawasaki for example, a not-returning-to-idle is a potential piston killer as it might indicate an air leak. An air leak causes a LOT more heat in the cylinder and can burn a hole in the piston. I'm not 100% sure if an air leak, ie. a 'lean' condition, is what you have -- but a bike running lean runs HOT. And you said yours is running 'hot.' And an air leak normally produces the 'won't return to idle' symptom too.

Pull your spark plugs -- if the plugs are hottish-looking, such as: the ceramic bits are white, everything looks too-super-clean -- sounds like you're running lean, possible due to an air leak. Just pull the plugs and if they're not the desired light brownish color on the ceramic bits -- yep your air leak suspicion, which is why I'm guessing you want to change out your carb intake manifolds, may be correct.

Here's how to test your intake manifolds for an air leak before you $pend money on new ones: one at a time, while the engine is running, spray a flammable liquid such as WD-40 in small squirts on one manifold at a time. When you here the engine revs spike after a short squirt of wd-40 you've probably found an air leak in the manifold. DON'T STOP THERE. A common mistake when troubleshooting is 'Ok I found the problem', then fix that, then you still have other problems -- don't assume there's only the one leak. Spray onto one manifold at a time, wait for the telltale engine rev spike, and whether you hear it or not move onto the next manifold; check all possible sources of an air leak.

Now, being a 2-stroke biker myself, I'm not even sure if an air leak, which creates a 'Lean' condition in the fuel-air mixture, causes this 'bike doesn't return to idle when revved' issue.

Could be lots of other things. Throttle cable sticking a bit? Check this by manually forcing the carb slides closed by pushing on the 'butterfly' metal throttle cable linkage on your '78 kz1000's carbs --- I have the same model/year bike, and this 'butterfly linkage' is easy to see on your carbs -- it's where the two ends of the throttle cable are hooked on to.

As for the air bubbles -- everyone who deviates from the stock black-colored fuel lines notices that. It's meaningless if the bike's running OK. If you see gas and some air in your clear fuel line, the important bit -- the gas -- seems to be dropping down from the tank to the carb, doesn't it. That's the important bit!
Now, if the fuel line is 100% air and *no gas* in it -- that's when you can worry.

From all the responses it looks like the air in the fuel line is no big deal, Cool!

I'll be working on the bike sunday afternoon. I'll do the tests mentioned and report back here. thank you everybody for your insight. Scott