TOPIC:

When I fired up my '81 kz650 today my #1 cylinder was dead (header pipe on #1 only mildly warm, all other sizzling hot).

It's getting a spark and I dropped the bowl on the carb and there was gas and everything looked fine from under there. I'll check compression next but from there I'm not sure what else to check. I've got old car experience but not so much old bike experience. Any advice would help - especially advice on common problems with these bikes.

The last time I rode it was last week and it was stop/go traffic for an hour and the bike got really hot. I didn't notice any problems then but now I'm wondering if that caused something to overheat.....???

I should add that this problem is at all speeds - from idle up to the higher rpms. I drove it around for fifteen minutes and the problem didn't go away.

Replace the plug on #1

mine was doing it too and went away after replacing plugs.

Try taking #4 out and switching them around.

del

if it's a spark problem...

the wasted spark setup fires through both sides at once. it will fire through the plug, or through a path of less resistance if one is available. so, it can be firing one plug just fine and not the other.

maybe spark gap too big on that plug or too small on it's mate? maybe a crack in wire insulation, carboned plug, carbon track on outer ceramic? an open circuit boot or break in the wire itself will exacerbat the problem by increasing the resistance of the path through that plugs gap.

Is this a new problem? Was the bike OK and this just started up?

step 1. 1&4 fire together. Swap 1&4 plug wires. Does the problem move to #4?

yes? Bad plug, plug wire, plug cap or connection of wire to coil

no? bad spark plug, compression issue (probably valve clearance related) or fuel problem

If no, go to step 2:

step 2: replace plug: Does #1 fire? If the answer is yes, the plug was bad. If no go to step 3:

step 3: Check compression. To check, you will need a compression tester. I suggest one that has a rubber tip that can be press fit into the plug hole. Check compression with throttle wide open (might as well check all cylinders). If compression on #1 is very low or zero, check valve clearances and adjust as necessary. If the valve clearances are OK, you will need to figure out if you have bad rings or bad valves on that cylinder - post a separate thread. If clearance is nil or less than it should be, adjust valve clearance and repeat compression check; it should be OK and the problem with non-firing should be fixed. If compression tests OK from the get-go, go to step 4:

step 4. fuel delivery issue. Since ONE cylinder is affected, it is likely the #1 carb having a plugged fuel pipe or jet; mostly likely the pilot jet. Take off carbs and clean them.