What causes intermittent failure of Generac Guardian 8kw air cooled 724 cc

I am assuming you have checked goth plugs for spark

If it were only one cylinder not firing, I would suspect a bad magneto. Since neither cylinder is firing, I would assume a short in the kill wire or a bad board.

Directly above the oil cooler is a small terminal block with a wire #18 connected to it. This is the kill wire. Disconnect the kill wire, and do not let it touch ground. Recheck for spark on both cylinders.

If still no spark, we need to look at the magnetos.

If there is spark, disconnect the wire number 18 from the J1 terminal of the control board, and ohm from wire 18 to ground. It should read no continuity.

If it reads continuity, there is a short in this wire and it must be traced down and fixed.

If it does not read continuity and there was a spark with wire 18 removed, the control board is bad.

Let me know what you find and we can go from there.

The part number for this board is C1537.

It can be ordered from the following sources:

Direct from Generac 1 - 8 8 8 - 4 3 6 - 3 7 2 2 I do not know the cost from them.

For $204.29 from http://www.partsfortechs.com/asapcart/generac-c1537-control-board-p-261.html?zenid=9d7002edcdc42d4755682969cf8d3c3a

Or, what I would recommend, is sending your board in to have it rebuilt for $150.

Go to http://www.flightsystems.com/Generac_generator_controls/rebuilding_services.htm for more information. Flight systems makes a great product, and warrants them for 2 years - far better than any OEM out there.