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Discussion Starter · #1 ·
I've been hearing this weird "pfft" sound coming from the front left side of my engine after it's been running in the city about 20minutes. It starts up just fine and seems to be running ok, except for when the "pfft" sound happens it feels like it loses a bit of power. Doesn't happen consistent enough to really get an idea of a cause.

I was thinking it might be a loose spark plug, but it feels snug, and like I said, it starts without issue. Spark plugs were changed at the end of last season so I'm guessing they should be good still.

Anyone heard a similar sound and maybe know what, if any the issue could be?
Thanks
 

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That's going to be a tough one to diagnose. It sounds like you are losing spark in the forward jug when the engine gets hot. Usually a bad ignition coil. You can swap them to see if the pfft moves to the rear jug. ;) Since the plugs were changed recently and the wires were disturbed, it could be as simple as a bad spark plug wire connection. Could also be a bad plug...I've seen it happen even with brand new ones.
 

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+1 ^^^^^^^^^^^^^

Just for giggles replace the spark plug. Its rare but I have seen the insulator come loose from the metal jacket.

The "pfft" sound is it constant or just every now and then or just once then stops.
 

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1983 Honda vt750 Shadow
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A quick test of the condition of the spark plug and wire and coil insulation is to spray water while it is running and see if there is any arcing anywhere. It may be the intermittent sound and loss of power you hear. Especially at night when it would be more visible.
 

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1983 Honda vt750 Shadow
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Thank you XXXXXX.
I am fairly new to bike mechanics but worked on Japanese cars for 46 years so I am trying to help other people out with the "nuts and bolts" of diagnostic and repair.
Sometimes another view of a problem can find a quick fix. I know I asked a lot of questions for help over the years.
 

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I have heard you say that before. The water suggestion tickled me because it reminded me of a uncle of mine. He put his first finger and his small finger in his mouth to wet them. Then he would place one finger on a spark plug end that did not have a rubber insulator then the other finger on an engine and kill the engine. did not bother him what so ever.
 

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1983 Honda vt750 Shadow
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I am not that brave!

My old father-in-law said during the WW2 years an Army friend of his would bet guys that he could put his finger in a light bulb socket and not get shocked. What he did was slide his finger first against the ground side of the socket and then up to the center connector so if any voltage would get through it was just the finger tip. Or maybe he didn't even touch the center connection.
Still I am not that brave. LOL
 

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Discussion Starter · #12 ·
Thanks for all your help guys.

I just took my seat in to get re-covered and so I have a week until I can get out to ride it long enough to test if a new plug helps. I'll keep in mind a new exhaust gasket if the plug doesn't fix. Luckily it doesn't sound like it's a problem that might leave me stranded on the side of the road. I haven't gotten out of the city yet this season, so maybe it won't happen at all at highway speeds.
 
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