Number 5 cylinder - no spark on spark plug

[3rdInfantry]

Question: I have a 1944 cckw 352, with an engine that has a cold plug on number five cylinder. Tester shows no spark. All other cylinders register a spark. New plugs, work on distributor, coil and spark plug wires and testing of wiring have not resolved this issue. Next will be compression testing for each cylinder, and in particular number five cylinder; followed by valve adjustments.

I have had spark at higher rpms, and whilst driving it, number five cylinder kicks in on occassion.

Any ideas?

Thanks!
3rd Infantry

[mudflap]

Hello 3rd Infantry,

Is your spark tester the type that replaces the spark plug, and connects between the end of the plug wire and ground?

If so, when you check each cylinder, do the other 5 cylinders show a good hot spark, but #5 shows nothing?

Assuming this is the case, I would check the following:

1) Check resistance of #5 plug wire. Ensure the wire ends are securely attached to the conductor in the wire, and that they snap positively onto the cap and plug. Check for any signs of pin holes or abrasion on the wire, allowing the spark to jump to ground. Swap wire with #6, and see if condition follows the wire.

2) Check distributor cap. A crack or carbon track could be shorting the spark to ground or an adjacent cylinder. If it looks old, replace it. The NAPA Echlin line is a good source for high quality ignition parts.

3) The only other thing that I can think of that could cause weak/no spark in only one cylinder would be a worn lobe on the distributor cam - although this seems like an extremely unlikely failure mode. Be that as it may, watch the distributor while the engine is cranking, making sure the points fully open and close for each cylinder.

You mentioned that #5 occasionally kicks in while driving. Do you mean that normally the engine is misfiring, but occasionally it smooths out again - like someone threw a switch? If so, that sounds like a classic ignition problem.

Best Regards...

[Bill_Wolf]

In addition to the input from mudflap I would also add the following checks.

Compression check all cylinders.

Use a vacuum gauge to check readings.

http://www.cckw.org/pdf/vacuumgages1.pdf

[3rdInfantry]

2017 02 02

To all who replied to our Inquiry,

Thank you all for your replies assisting us to repair an ailing cckw 352 GMC 270 engine.

The cause was a failing distributor. We rebuilt it, and also changed out the points, condenser, rotor and cap.

Wires and plugs were already previously replaced.

The engine runs well now.

All the best,
3rd Infantry