Backfiring thru the exhaust

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Well-known member
Staff member
Supporting Member
Dec 3, 2009
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Duncanville, Texas
My Bike Models
1981 GL1100 Innerstate("The Turd")SOLD!!, 1996 GL1500 Innerstate
Dadgummit! Recently, as in the past few months, my 1500 has been a lil cantankerous when starting after sitting over the weekend, or longer. Cranks fine, but just don't wanna fire off for a while. During the cranking process, it sometimes will backfire(LOUDLY) thru the left side exhaust once, and it always happens when I release the start button. It will only do it once, then it will fire off and run usually the next time I crank it. I have to assume this has the same "wasted spark" setup my 1100 had?(haven't taken the time to actually look in the service manual.) Bike has just over 100,000 miles on it, and as far as I know, still has the factory sparkle plugs in it.
Could the carbs be drying/leaking out overnight and causing the long crank times, and then the surge of fuel entering the chambers be force out the exhaust and firing with the valve open? I have no raw fuel smell, and no visual leaks on the floor. Other than this issue the dang thing runs great, idles smooth, and always fires right up after the initial morning sickness. Not that I care, but I'm sure the neighbors are prolly gettin a lil tired of me waking them up before the crack of dawn. I'm tired of it myself. Any real ideas on what should I be looking at/for as far as a rudimentary diagnosis? Ignition? Fuel? :unsure:
(I keep a battery maintainer on it at all times when not riding, even on weekends. Battery is always hot, and it spins over just fine.)
Well let me start by saying good luck with fixing the bike. I found some info from another site that might be of use...

The summation of what I read was..... Verify battery at 10.6v + while cranking, evidently they are very voltage sensitive. Coils tend to fail with this result, test per FSM. Vacuum lines breaking from being brittle, cause this issue.
Thanks! I know very well the issue of it not wanting to start with a weak batt/low voltage, hence the maintainer at all times while parked(at home). It's possible, I s'pose, the batt is getting's 3 years old, I think. Bike starts fine anytime after the initial day or more scenario. Even after sitting at the shop all day, it will start immediately when I'm ready to go home in the afternoon.....and I do not have a maintainer on it here at the shop. Got some investigating to do! 🥸
Knowing next to nothing about the 1500 I would say change sparkles and load test the battery at a minimum.
When I was messing around with a single carb, new plugs made a world of difference.
Update: Went out and checked cranking voltage. It's been sitting since Monday without starting, maintainer plugged in. Cranking voltage dropped down to just a smidge over 9 volts...but wouldn't fire.🙄😒 Hooked up jump box, crank voltage stayed above 10 volts, and fired right up. Battery was installed 4-20, so it's 4 years old. Guess I'll put a new battery in it and see how that goes. Funny how the stoopid thing will fire up instantly the rest of the day after the initial morning start. 🤔
Is that an AGM battery? When my AGM batteries were ready to fail, there was no warning. Start up fine in the morning for the commute to work and at lunch time, nothing but lights. Battery was usually 3 - 4 years old when they would just stop working.
It is AGM, same as I have been using for years. We'll see how it starts up in the morning. This morning, I went ahead and hooked a jump box up first, and it started right up immediately, no fuss no muss.
I have a new AGM setting here now to put in it, same brand/size that it has already....just not 4 years old. I do remember before having a hard time getting it started, and a new batt solved it....had same issue on The Turd too.
Installed new battery last nite. Fired up this merning within a couple of seconds, no problem. I'll not be totally convinced until it sits over the weekend, but I believe the battery was most likely the problem. I did load test the "old" battery....and it passed that test. Strange....anyway, we shall see! :rolleyes:
Agree with the battery issue. We tend to want to get every ounce of power out of a battery without realizing the consequences. Battery 3-4 years old, get new. It's like a car, a power pig. If a newer car has a weak/failng battery all sorts of wondrous things start to happen. Even more critical with FI.