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I am coming at this problem with some experience working on my own bikes to include carb rebuilds. But this is something new and I really need help with troubleshooting.

My project bike is a '82 KZ440 parallel twin. When I got it for really cheap it had no air box. It ran badly so I rebuilt the top end. It has an old Kerker 2-into-1 exhaust with a cap on the back but no baffles. I installed pod-type air filters.

I have made a whole new wiring harness for the bike and I have good fat blue spark at both plugs. On choke, the left side will not run (the pipe does not get hot) but the right side runs fine. If I swap plug wires the problem remains, and also if I swap plugs (problem also remains with new plugs). So I think it is in the carbs. The left side is definately getting plenty of fuel. The float levels are correct and the float valve shuts like it should. The overflow tubes are clear. The carbs are clean as best as I can tell, but having cleaned a few racks of carbs I do understand the fact that I may be missing some funk in there somewhere. But I do think they are cleaned properly and thoroughly. I have brand new carb holders (boots) that are not leaking. All fuel is running from a brand-new auxilary tank (the kind made for this kind of work) and through a fuel filter. The vacuum line to the carbs has simply been plugged up.

When the bike is on choke, the right side fires up just fine, revs fine, idles fine. But when I begin to move the choke lever, the left side will fire and both sides will rev very high for about 2 seconds, then immediately die.

Why is the left side not burning the fuel that is delivered through the carb?

Could the jetting be SO far off with the air filters and exhaust that it would cause this problem?

Oh, compression is good and both sides have very near the same compression.

I'd appreciate any ideas you may have on this bike, and thanks.

Not that I've had a lot of experience with KZ carbs, but I have worked with dual carbs on other bikes. Did you synchronise the carbs after the rebuild? Is the choke mechanism adjusted so that it moves both butterfly valves the same amount?

Carbs are synched. Y'know I have not bench-synched the choke but that wouldn't be hard....

Have you swapped the coils? Let see, you change plugs, no change. You change plug wires, no change. Try swapping coils.

Johnj wrote:Have you swapped the coils? Let see, you change plugs, no change. You change plug wires, no change. Try swapping coils.

I dunno, John, from her description it sounds more like a carb issue. If it was a coil problem, it would be either no spark at all or no spark once the coil gets hot. The way she's described it, the left cylinder doesn't fire until the choke is moved, which to me sounds more like a fuel/air problem.

Just out of curiosity, Ms Coffee, what process did you use to synch the carbs? Are they flat slide or CV carbs?

If the carbs and choke are synched properly and the problem remains, a vacuum leak is usually the culprit. One trick to check for leaks is the WD-40 test. With the motor running (even if it's only on the right side cylinder), squirt some WD-40 around the intake boot area at the cylinder head on the left carb. Then adjust the choke until the left side fires up and see if there's any white smoke coming from the exhaust. The white smoke is caused by the kerosene content of the WD-40 burning in the cylinder and is a positive confirmation of a vacuum leak.

There's only other thing I can think of and that depends on if the carbs are the CV design. Sometimes the rubber diaphragm on the top of the slide can get pinched or otherwise damaged during reassembly, causing the slide to malfunction. The spring could also be broken, giving the same result. Might be worth a look to pop the cover off the top of the carb and check the slide, diaphragm and spring, but only if it is a CV carb.

Thanks folks.

These are CV carbs. There's no intake leak, I did check that....

I synched them with a mechanic's mercury set (after a bench synch).

There is only one coil on this bike, it's a new coil designed for an old Harley (5-ohm as reccommended for this bike) and it's doing fine.

Off to pull the carbs again and check the diaphragm....

Try swapping carbs then.

What do the left and right plugs look like after it dies?

fireguzzi wrote:What do the left and right plugs look like after it dies?

Clean and soaking wet with fresh fuel.

Johnj wrote:Try swapping carbs then.

I may swap diaphragms and floats and float needles, but this is a rack of carbs. Taking the rack apart is going to be a major hassle.

I found one very tiny pinhole in the "bad" side diaphragm, but not nearly enough to cause this trouble. The bike ran and idled with much worse diaphragms when I got it.


I just swapped idle screws, going to try it again.

At least it's raining, I'm not tempted to jump on the Vmax and go riding....