Ask the Editors: Twin Cylinder Blues

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Two or nothing.

Dear ATVC: I have a Linhai 520efi with 2 cylinders. When I start it, it backfires and won’t run but when I remove the coil cable from the lower cylinder, it starts normally. Then when I reattach the plug cable, it backfires. I have checked timing and it’s okay. What could be the problem? Thanks in advance. Bels

You started your diagnostics in the right place. If the timing checks out, the next place to look would be to check the valve clearances in the lower cylinder. If those turn out okay, run a compression test of the cylinder. If it comes back significantly less than the upper, worn rings/ top end are your likely culprit.

If all of these diagnostics check out, we wouldn’t rule out the ignition itself. Begin by checking for spark on the lower cylinder; we’ve seen similar results caused by anything from a fouled spark plug all the way up to the coil or a faulty ECU.

If the the cylinder is receiving strong, consistent spark that really only leaves you two final possibilities. Is it getting fuel? A clogged fuel injector nozzle, torn fuel line or even a damaged fuel pump could be culprits.

If all of this checks out, we would then consider very unlikely possibilities like a bent or broken camshaft. But start by diagnosing the basics before the worst case scenarios. Begin by making sure the lower cylinder is getting adequate fuel, air and spark.

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