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Old 05-21-2004, 11:11 AM   #1
 
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Occasional backfire off-throttle

Any idea what would cause backfiring off-throttle? It's not too bad but seems to happen regularly. It also pop'ed real loud when I started it this morning. This is a 94 VFR750 w/20k miles.
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Old 05-21-2004, 11:54 AM   #2
 
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Re: Occasional backfire off-throttle

Could be the pair system. If the valve is stuck, it may be running out of sync causing air into the exhaust at the wrong time.
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Old 05-21-2004, 12:17 PM   #3
 
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Re: Occasional backfire off-throttle

An exhaust leak can cause it as well.
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Old 05-24-2004, 2:48 PM   #4
 
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Re: Occasional backfire off-throttle

Hmm, I'm gonna take it apart to check the air filter anyway, so I'll check out the pair valve.

The muffler was about to fall off it was so loose (I didn't install it?), but I thought I sealed it up real good. I'll have to check that too...

Thanks
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Old 05-24-2004, 10:29 PM   #5
 
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Re: Occasional backfire off-throttle

LOL.

Backfires? Trying to get rid of them?

Why?

Hell, I love the fact I can backfire my RC30. Nothing funnier than driving past a bus stop, rolling the kill switch off and back on and seeing people duck for cover. I've been told that the Kerker slip-on on the RC30 produces a very convincing BANG if you aren't expecting it.
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Old 05-24-2004, 11:46 PM   #6
 
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Re: Occasional backfire off-throttle

Originally Posted by brian48130
Hmm, I'm gonna take it apart to check the air filter anyway, so I'll check out the pair valve.

The muffler was about to fall off it was so loose (I didn't install it?), but I thought I sealed it up real good. I'll have to check that too...

Thanks
There's your answer. A loose muffler can suck in air on decel and make a combustible mix in the pipe. Also if your headers are slightly loose that'll do it too. What pipe are you running by the way. I had a Staintune on my 96 VFR and when I took the restrictor out it would pop a bit on decel. Sounded pretty highly tuned on the track

I don't think a 94-97 VFR has a pair valve. I never saw it mentioned in the shop manual.
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Old 05-25-2004, 4:39 PM   #7
 
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Re: Occasional backfire off-throttle

I bought my vfr 20k mi used with the Two Brothers slip-on already on. I haven't taken it apart yet, but I'll check the headers. The muffler seems tight now but it was pretty loose before I re-attached it. I may need to do it once more and use a ton of sealant.

Might be right about the pair valve, I never looked into it.

Doesn't backfiring damage the engine? If not then I certainly don't care. What about exhaust leaks, do they cause extra wear, less power, etc? I haven't been riding my blade because it seems to have a leak around the headers.
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Old 05-25-2004, 4:50 PM   #8
 
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Re: Occasional backfire off-throttle

Originally Posted by brian48130
Doesn't backfiring damage the engine?

It *can*. You should find the problem and get it fixed...end of story.
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Old 05-25-2004, 5:34 PM   #9
 
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Re: Occasional backfire off-throttle

Originally Posted by Pete
It *can*. You should find the problem and get it fixed...end of story.
That depends on "how" it's backfiring. Significant damage from backfires from the exhaust (caused by air being introduced into the exhaust stream igniting unspent fuel) is extremely unlikely and limited to higher exhaust header temps and possibly exhaust packing being dislodged.

Now "front-firing", ignition in the intake tract (detonation prior to the intake valves fully closing) is alot more serious and needs to be addressed as carbs/EFI/airbox sensors were not designed to handle little booms.
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Old 05-25-2004, 5:54 PM   #10
 
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Re: Occasional backfire off-throttle

Originally Posted by G-Man
That depends on "how" it's backfiring. Significant damage from backfires from the exhaust (caused by air being introduced into the exhaust stream igniting unspent fuel) is extremely unlikely and limited to higher exhaust header temps and possibly exhaust packing being dislodged.

Now "front-firing", ignition in the intake tract (detonation prior to the intake valves fully closing) is alot more serious and needs to be addressed as carbs/EFI/airbox sensors were not designed to handle little booms.
The amount of damage caused by an exhaust backfire is dependent on how far "downstream" the explosion occurs. I have seen exhaust valves bent by backfires caused by air leaking in at the exhaust ports. And since we don't know where his problem is, the safe thing to do is to find the culprit and fix it.
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