Backfire?

[Jack Dalton]

Greetings, I have newly joined the ranks of RIB'ers and have just recently upgraded my old motor with a newer 2nd hand 15HP suzuki 2stroke.

During a boat test today I noticed a very loud bang / boom when accelerating up onto plane. It would only do this once each time - after i had switched the engine off then restarted it again, after periods of idle without turning the engine off it wouldnt happen. This scared the heck out of me as it was very loud and could be felt through the boat. Apart from the initial backfires the engine seemed to run fine.

Am I right in thinking this is a backfire? If not what could cause this to happen and what are my possible courses of action?

Thanks.

[m chappelow]

wonder if it could be unburned fuel getting into the leg then ignighting if its been over choked or over rich when trying to start but i wouldent think it would do it if its been running for more than a few seconds,once had an outboard jump about 10 feet into the air when trying to start it in a barrel of water ,the petrol had run into the exhaust in the leg with trying to start it ,when it did fire up there was a massive bang and the engine shot up into the air like a polaris missile with my ears ringing and my feet wet a great hole in the side of the plastic bin, just a thought.

[Jack Dalton]

That was my initial thought - because it makes a very loud and deep "BOOM" pretty scary when your not expecting it.... and a little less so when you are :P

A mechanic i called said that a loud noise like that could be caused by it jumping in and out of gear. But again he was puzzled by it only happening once each time after i'd turned the engine off. Thinking about it, those were also the only times i engaged reverse gear - start the engine reverse out - turn around - accelerate onto plane - *BOOM* so maybe the act of using reverse causes it to not engage into forward gear properly.... or something...

Either way ill have to test it more to know - ill buy some new plugs while im at it as faulty plugs could possible cause a backfire.... maybe..?

[Jack Dalton]

I have read elsewhere that faulty plugs could cause a situation similar to what i was seeing/hearing. It is hard to describe in writing exactly the type of noise happening.... So after buying some new plugs and testing the engine yesterday - I am very happy to say that the loud *BOOM* has not happened again. Yay. Must have been faulty plugs.
Cheers,
JD