QuestionI have spark,fuel and new ignition units. Cylinder compression is 150psi in the front and 120 in the back. I squirted some oil in the back cylinder but there wasn't any improvement in compression. Sometimes it backfires through both carbs and the next time through the mufflers and sometimes it doesn't backfire at all. But, the motor doesn't start. Ideas? Thanks.
AnswerHi Gary,
Check the following:
Are the spark plug leads on the right cylinders?
Are the spark plugs fouled?
Flashover or current leaking from secondary leads (plug wires)?
Are plugs in good condition?
Are plugs properly gapped and correct plugs for the application?
Rear cylinder compression is more than 10% difference than front cylinder compression. The rear cylinder will need repaired, but engine should still start and run.
There's no history of repairs, modifications, or maintenance information with the question description. Was the MC stored for any periods? Has the fuel been flushed or carburetors repaired or cleaned?
I recommend performing electrical diagnosis on ignition system and charging system components.
I recommend inspecting carbs for evidence of varnishing.
Clean or replace the plugs and try restarting with the aid of starting fliud. Use one second bursts of staring fluid with choke and no throttle. Check plugs often and keep them clean.
Backfiring indicates carb, ignition, and cylinder head problems.
Perform a leak-down test on cylinders, especially the rear cylinder.
Respectfully,
Mark Shively
