Backfire??

Here's the deal, 2005 Feej, 16,0000 on the bike. Generally runs flawlessly. Last week, for the first time, I got a little backfire sound. Details: bike was cold, just started, when I was moving in first or maybe second gear, rolled off the throttle as I was coming to a stop sign or something, heard a short but distinct backfire sound. I could reproduce it once by rolling on the throttle, then off again. I only did it as I was coasting and the rpm was higher, and a lot of engine breaking going on. Only does it once or twice, then when the bike is warmed up, it doesn't do it again. I rode 600 miles on the bike over the weekend and it didn't happen at all.

So, is this something that will get progressively worse? Like maybe a bad ring in the cylinder or something? What say you?

Thanks.

Chris

I can't remember, are your pipes and header stock? Probably no big deal, especially if it goes away.

The engine is cold and the popping is unburned fuel. thats why it doesn't do it when warmed up.

Feejer/Weejer man said:

The engine is cold and the popping is unburned fuel. thats why it doesn't do it when warmed up.

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And yes, it is normal.

Yeah, everything is stock. And it only does once or twice when I first start off (cold engine.) But it's a new sound, so I was just wondering if it's a problem. Relieved to hear you guys think it's normal. Does anyone else have this happen?

BTW, thanks again to this board for being an instant answer provider. I only posted this about 15 minutes ago.

Keith, thanks for the cool route last week. I used most of it, but cut some off as I was running a little late and needed to get home. I went through Pineville, KY then down into eastern TN to stop for the night. Got up and ran through NC via Asheville to Charleston on Sat. I'll be back up there with my bike after July 4th and will be staying about three weeks before returning to get my household goods. Oh yeah, my wife too. Looking forward to riding again later this summer.

Cheers,

Chris

You might want to check this post: ai on the fjr. Surprising that it just started doing this.

As someone else pointed out, its unburned fuel that ignites in your exhaust pipe. With 16,000 miles, you might check/clean your air filter. That can lead to rich conditions. I don't know if the Gen I FJR had a closed loop engine control module, but if it didn't, a dirty air filter could reduce air flow and cause enrichment which could cause backfire (really, these are after fires... true back fires come up through the intake side). You might also do your valve adjustment and tension cam chain if it hasn't been done. These are generic suggestions. I have not (yet) taken apart a FJR and my service manual won't ship until tomorrow.

Another generic observation is that engine control modules include an enrichment strategy for cold start up. Gasoline has a relatively low percentage of components that vaporize at cold start temps. Because of this, enrichment can be a very large multiplier on the normal "on-time." So there's a lot of gas shooting at the back side of the intake valves when the bike is started when it hasn't yet been warmed up, and its only reasonable that some of it could be finding its way unburned into your exhaust.

The mystery here is "why did it just start doing this?" and that's why I'd be taking a look at the air filter.

Another generic observation is that lean conditions can also lead to after fires because of the air to fuel ratio is too high, you can't sustain uniform combustion, and some of the unburned slugs of air and fuel sneak out into the exhaust, where they may ignite if they hit something hot enough. Usually this is more "decel popping," rather than what folks worry about as "backfire."

So.....is backfire.....kinda like.....um....frontfire, then? Or do I have it forwards?

odot said:

So.....is backfire.....kinda like.....um....frontfire, then? Or do I have it forwards?

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Just like lightin' farts....

El Toro said:

As someone else pointed out, its unburned fuel that ignites in your exhaust pipe. With 16,000 miles, you might check/clean your air filter. That can lead to rich conditions. I don't know if the Gen I FJR had a closed loop engine control module, but if it didn't, a dirty air filter could reduce air flow and cause enrichment which could cause backfire (really, these are after fires... true back fires come up through the intake side). You might also do your valve adjustment and tension cam chain if it hasn't been done. These are generic suggestions. I have not (yet) taken apart a FJR and my service manual won't ship until tomorrow.
Another generic observation is that engine control modules include an enrichment strategy for cold start up. Gasoline has a relatively low percentage of components that vaporize at cold start temps. Because of this, enrichment can be a very large multiplier on the normal "on-time." So there's a lot of gas shooting at the back side of the intake valves when the bike is started when it hasn't yet been warmed up, and its only reasonable that some of it could be finding its way unburned into your exhaust.

The mystery here is "why did it just start doing this?" and that's why I'd be taking a look at the air filter.

Another generic observation is that lean conditions can also lead to after fires because of the air to fuel ratio is too high, you can't sustain uniform combustion, and some of the unburned slugs of air and fuel sneak out into the exhaust, where they may ignite if they hit something hot enough. Usually this is more "decel popping," rather than what folks worry about as "backfire."

Click to expand...

Good idea about checking the air box. I've pulled it out and blown it out, but I just took a pretty long trip and it very well may be dirty, or filled with cicadas (holy crap there were a lot of them smashing into me all last week). I'll pull it tomorrow to check the filter, but I know my Yamaha dealer won't carry one and I'll have to order it. And the noise is more like a decel popping, not a true backfire.

Thanks for the good words.

CS