Trouble shooting, warmed up runs rough.

I have a 76 kz70, and have been tinkering for a long time. I have it so that it will start no problem and run great untill it really warms up. Then one cylinder will pop sometimes and if I try and rev it it will cut out and die sometimes. Thats when the trouble starts. If I try and use the starter, the bike acts like it doesnt have the juice, and sometimes it wont kick-start either. It's like the bike just wont fire if it's warm. I'm thinking the coil just doesnt want to fire when it's warm, and the charging sytem might not be putting out enough juice to keep the battery fully charged even though I'm riding at 4K. Any imput before i buy a coil and piss my wife off for buying something that doesn't fix my problem?

Checked the valve clearances? When engines get up to operating temperatures, metal expands, possibly causing valve(s) to partially open, causing your problems. On these types of engine, you should hear a litttle valve noise, when you don't, WORRY! as it's possible to burn valves if they can't close.

Get a shop manual, the tools needed,feeler gauges,the tool to depress the shim follower & start checking the clearances.

Checked and adjusted them end of last season and I can definitely hear the valves haha. I just wasn't sure if a weak spark would occur after te coil heated up? It's always strong when I check it when it's cool. You have any imput on the charging systems on these bikes? Are they decent on output?

The head actualy exspands more than the valves.Aluminium - Thermal expansion (25 °C) 23.1 µm·m−1·K−1.
Steel - Thermal expansion (25 °C)11.0 ~ 13.0 µm·m−1·K−,1depending on composition.

yes have seen coils go dead after they worm up get it wormed up then check spark

Thanks for the advice. I took it for a short ride. Started to sputter t a stop light so I held my rpms high till I got home. I killed it then tried to restart it, sure enough it wouldnt start. Checked the spark and the right cylider is a weak yellow spark where as when its cool its a thick blue spark. I have an excuse for the misses now as to why I need a new coil . Tanks for the help again guys.

If not already done, would check the charging system by measuring voltage between battery posts at idle (look for 13 volts) and at 4000 rpm (look for 14+ volts).

Good Fortune!

Ignition Coils And Sparkplug Wire Choices

What came with the bike were ignition coils that even when new were marginal, add years of cooking under the tank in stop and go traffic, and the coils will fail when hot.

Replacement coils can be bought at www.z1enterprises.com . I believe "Emgo" is the brand. They also carry the popular Dyna 2.2 and 3 ohm coils along with replacement sparkplug wires.

Now, you have a choice on sparkplug wires:
1. Copper plug wires with no supression caps(the best choice, will produce greater spark, but will produce audio noise on Radio's & T.V.'s)

2. Copper plug wires with supression caps(a built in 5000 ohm resistor, that can some times fail, like what came with the bike).

3. Copper plug wires with no supression caps, but using resistor plugs(the "R" in the plug number, like B8RES).

4. Supression plug wires with no supression caps.

NEVER use resistor plugs and supression caps or supression plug wires, as now there will be two (2) sources of resistance in the secondary windings of the ignition coils, greatly reducing the spark energy.

Another choice is the Accel 3 ohm coils, with a similar primary(small wire) electrical connection using ring terminals as the Dyna ignition coils. Whatever brand of coil you go with, make sure the primary wires will clear the mounting hardware, as the spacers will come really close, possibly causing an electrical short, blowing fuses. Some riders have replaced the metal spacers with Nylon or Plastic non conductive ones, reducing the chance of an electrical problem.

7mm sparkplug wires will fit the stock ignition coils(if they have replaceable wires, indicated by screw off caps on the coils with ridges). The sparkplug wire fit into a port with a brass “Stinger” that goes into the sparkplug wire or lead(The Brits, Canadians and other countries call the sparkplug wires “High Tension Leads”).