NASTY BACKFIRE ON START UP

mickjmag · Post by **mickjmag** » Fri Oct 13, 2006 9:30 am

Hi All,

Am wondering if anyone has a problem with their rex letting off a really nasty backfire when they press the starter after being stationary over night??????
I have 2 rex's, one with a k&n filter and megacycle round exhaust pipes its my road bike and does a huge backfire every morning much to my neighbours disgust at 6:00 am mon to fri, the other being my track bike,when it used to be my road bike it suffered the same problem, the set up was the same except for the cans, they were megacycle oval cans.
I've checked the carbs on both bikes and have found no wear to the emulsion tubes, even after a tune up they still do they backfire thing

Any clues would be greatly appreciated.

Cheers, Mick

cheap_trix · Post by **cheap_trix** » Sat Oct 14, 2006 12:04 pm

My bike does it occasionaly. Not enough to worry me though. Kinda put it down to a bit of fuel draining into a cylinder overnight, which kinda makes sense being downdraghts.

Red Rex · Post by **Red Rex** » Sun Oct 15, 2006 10:49 pm

I presume you mean a healthy BANG from the exhaust??? Mine occasionally does the opposite and spits back through the carbs, not anti-social at 6am but a bit more worrying. Could it be the same problem??

Waz · Post by **Waz** » Sun Oct 15, 2006 10:58 pm

cheap_trix wrote:My bike does it occasionaly. Not enough to worry me though. Kinda put it down to a bit of fuel draining into a cylinder overnight, which kinda makes sense being downdraghts.

Ditto

Post by **Jak** » Sun Oct 15, 2006 11:05 pm

Add mine to the list that does it occasionally - moreso since I've had the Scorpions on (and with them on it's stupidly loud!).

Dee · Post by **Dee** » Mon Oct 16, 2006 10:01 am

Mine too. I don't think it's anything to really worry about.

Killerwhale · Post by **Killerwhale** » Mon Oct 16, 2006 10:22 am

mickjmag wrote:Hi All,

Am wondering if anyone has a problem with their rex letting off a really nasty backfire when they press the starter after being stationary over night??????
I have 2 rex's, one with a k&n filter and megacycle round exhaust pipes its my road bike and does a huge backfire every morning much to my neighbours disgust at 6:00 am mon to fri, the other being my track bike,when it used to be my road bike it suffered the same problem, the set up was the same except for the cans, they were megacycle oval cans.
I've checked the carbs on both bikes and have found no wear to the emulsion tubes, even after a tune up they still do they backfire thing
Any clues would be greatly appreciated.

Cheers, Mick

I got it when i opened the airbox and put in K&N filter (and racecans)...
Never gets it now with the keihin FCRÃƒâ€šÃ‚Â´s....wich is nice because people was twisting their necks so it MUST have hurted!!

JL4049 · Post by **JL4049** » Tue Oct 17, 2006 9:31 am

I get it but not all the time, and only since I opened up the cans, either that or stock cans muffle so well you can't hear it. One loud bang as it fires, I like it

burty · Post by **burty** » Tue Oct 17, 2006 10:56 am

I have a theory that this is caused by unburned fuel remaining in the pipes when the engine is turned off. As it's heavier than air it sits there undisturbed then when you start the bike there is sufficient hot gas exiting the engine to cause the backfire.

Anyway that's probably wrong, but that's my theory.

My bike does it, very rarely. My ZRX (also on a race can) used to do it very occasionally too.

Post by **Max** » Tue Oct 17, 2006 11:49 am

I guess I classify a "backfire" as one through the carbs and anything out the other end a "pop"... Back through the carbs is not good (especially if you have oiled-foam filters that can ignite!), out the back is nothing to be worried about (unless you drive an old car with a rusty muffler that will eventually fall off if it happens often...). Muffler pops can be caused by a variety of things, excessive fuel build up which is fairly rare I think unless it was flooded. Open race type exhaust where deceleration allows a large volume of unspent gasses to grow untill the near the end a rush of O2 helps them ignite (my bike) or most commonly (on street bikes) a leak around the flanges in the front or mid section on joined pipes allowing oxygen to enter mid stream.

If you ever heard the first generation M1, they used an air induction system on the exhaust to help reduce engine braking... Sounded raucous for lack of a better word.

mickjmag · Post by **mickjmag** » Wed Oct 18, 2006 7:52 am

In my case its a"pop" a bloody loud one to be exact it sound like a .303 rifle going off, it scares the crap out off me every time! hate know what the neighbors think

Thanks for the clues folks will do some investigating and will let you know what I find

Mick

Olputt · Post by **Olputt** » Wed Oct 18, 2006 11:54 pm

Mine does this when i take the baffles out of the standard cans. I put it down to a lack of back pressure. I've also had issues starting it after washing - water in the cans. Had to put the baffles back in to get it going

cheap_trix · Post by **cheap_trix** » Thu Oct 19, 2006 1:24 am

Olputt wrote

I've also had issues starting it after washing - water in the cans.

There is no need to wash the inside of the cans.

Actually, a friend of mine has a 2005 KX250F. When he was washing it one day, his 5 year old daughter stuck the running hose in the muffler when he wasn't watching. When he jumped on the kick starter it threw a leg out of bed.
Not pretty.

Post by **Max** » Thu Oct 19, 2006 1:32 am

cheap_trix wrote: it threw a leg out of bed.

I do that every morning...

Sorry, I don't speakum Qld Oz. Did it break his leg, snap the connecting rod or what?

Does sound ugly (like me, every morning)!

cheap_trix · Post by **cheap_trix** » Thu Oct 19, 2006 1:39 am

Max wrote

Sorry, I don't speakum Qld Oz.

Obviously

Yes it broke the conrod. What you might call a compound fracture.

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