TOPIC:

I am having a really odd problem after reassembling the engine on my 1976 Kz900-A4. I purchased a used head a couple of years ago and swapped my head out (finally) a couple of months ago. I also pulled up the cylinder block and replaced the gaskets from the crankcase up.

At this point the bike is back together and will start, but cylinders 1 and 3 will not fire.

I have verified spark on all plugs is consistent and good. We have readjusted the points to allow for this. I have rechecked the cam timing and the bike starts better now that it is correct, but even when we spray starting fluid into the carbeurators on those two cylinders, the pipes don't even get warm for those cylinders.

I find this really baffling because the two cylinders aren't on the same coil, same points, same exhaust (4-2 split), and they are on opposite ends of the firing rotation (or right next to each other I guess).

Other than compression testing those two cylinders is there anything else anyone can think of that might be causing these two cylinders to fail to fire?

A no-compression valve because of lack of clearance can easily be the problem. Check valve clearances BEFORE trying to start the bike. Next, check your plug wires. Note that one coil has a black wire connected to it and the other a green. Make sure the wires from the coil with the black wire are going to 1 & 4 spark plugs. Make sure the wires from coil with the green wire are going to 2 & 3. Those are by far the most likely problem areas. See the VM carb connection link on my web site. It is also possible the carbs have been hooked up incorrectly and the floats are not working as they should... not allowing gas to fill the bowls.

stuck closed float? non sealing valves?

Have you ever found a spark plug that would show blue fire out on the outside of the engine, but under pressure inside the engine nothing?
(champion plug) I'll send you one if you haven't
i was stumped by the plug thing once, now I have boxes of them.

Thanks - the coils and plug wires have been double checked (more than once). I have not checked the valve clearances. Can this be done with the head on the bike or does it need to be removed?

I'm pretty sure it's not the plugs as I have two new sets of NGK plugs I'm working with, but I haven't come accrossed that either. I'll try swapping them out again for good measure.

yea the cam cover has a close fit after taking off the tank and getting the area above it cleared out. a little wiggling after it's unbolted, slides out the side. easier to do than the airbox and carb removal but the compression test or a leak down test will keep you from wasting time. as a general indicator of whats up

DoubleDub wrote:

Thanks - the coils and plug wires have been double checked (more than once). I have not checked the valve clearances. Can this be done with the head on the bike or does it need to be removed?

Am I reading correctly that you changed the head but never checked the valve clearences ?
Very strange.

In my defense it's my first head swap and the head was pre-assembled and off of another 900 so I wasn't sure if it was necessary or not. You learn from mistakes I guess.

If it was running on another bike before swapping it to yours, valve clearance is not your problem. The valve clearance would have to be at least 5-10 shim sizes below zero to effect compression enough to make a cylinder not fire and this on 2 different cylinders? I don't think. Remove the plugs and turn your engine over and use your finger as a compression guage if you don't have one. Should be easy to tell if those 2 cylinders are no compression.

I will try that - this is partly why I am so confused about the cylinders not firing. There are no signs of actual ignition on either of these cylinders... I will check the compression by hand this evening. Thanks for the help!

If you have not checked valve clearances, don't assume the clearances were correct in some previous life of this head. Here is a "how to":
www.wgcarbs.com/269767.html

Lack of valve clearance is the most likely cause of compression loss.

Thanks - sounds like I'll have plenty to do this evening....

Thanks again!