nib1980
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| posted on 3/5/09 at 09:15 AM |
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Massive Backfire when turning engine off
Hi All,
It's only a minor irritation at first. When I turn my kit off, about 2 secs later there is a massive backfire, sounds like a gun going off and
enough to set off my tintop car alarm.
At first i thought not too bad. But going to a petrol station or coming home late is rther embarassing.
Whats the cause and is there anyway I can stop it?
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jpindy3
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| posted on 3/5/09 at 09:21 AM |
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what engine is it mate?
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nib1980
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| posted on 3/5/09 at 09:26 AM |
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quote:
Originally posted by jpindy3
what engine is it mate?
1600 xflow on twin webbers, with 4-1 exhaust
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Mark G
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| posted on 3/5/09 at 09:34 AM |
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any chance the exhuast could be blocked?
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nib1980
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| posted on 3/5/09 at 09:37 AM |
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quote:
Originally posted by Mark G
any chance the exhuast could be blocked?
nope runs fine, spits and pops a bit at idle but no blocked exhaust.
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jpindy3
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| posted on 3/5/09 at 09:39 AM |
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check all air ways,in and out,take cam cover off and have a look at the rods ect.im no expert on xflows
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SeaBass
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| posted on 3/5/09 at 09:48 AM |
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Backfire from exhaust I presume - I did have one from a carb at one stage...
Wikipedia is your friend...
http://en.wikipedia.org/wiki/Back-fire
I'd say basically has to be excess fuel / poor timing...
How well set up are the webbers? Does the exhaust pop and crackle on overrun?
JC
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blakep82
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| posted on 3/5/09 at 10:39 AM |
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i'm not expert, but i'd doubt it being the timing being out if it does it 2 secs after the engines switched off.
air leak in the exhaust maybe? allowing extra oxygen in to get amongst the unburnt fuel in the hot exhaust (from the few turns of the engine it makes
after it turned off, as it would still suck fuel from the carbs)
________________________
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tomgregory2000
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| posted on 3/5/09 at 11:00 AM |
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quote:
Originally posted by blakep82
i'm not expert, but i'd doubt it being the timing being out if it does it 2 secs after the engines switched off.
air leak in the exhaust maybe? allowing extra oxygen in to get amongst the unburnt fuel in the hot exhaust (from the few turns of the engine it makes
after it turned off, as it would still suck fuel from the carbs)
That gets my vote as well
I have twin 45s on my vaux and a small air leak on the exhaust and get rather loud bangs when slowing down under engine braking
[Edited on 3/5/09 by tomgregory2000]
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Dave Ashurst
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 posted on 3/5/09 at 11:21 AM |
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I've had the same set up as you for the last 7 or 8 years, similar issues too, based on your recent posts.
For a popping and backfiring exhaust I would always look for an air leak first.(especially if it was OK previously and nothing else has changed).
integrity of exhaust manifold gaskets seal - you might see soot or feel gas escaping.
tightness of exhaust manifold nuts, go round and tweak them up
tightness of any joints in pipes or silencer - how are they joined? Slotted and clamped? very difficult to seal.
look for any cracks especially at welds and rigid connection points
Look for incomplete welds too, the 4-1 collector might have been tricky to weld (mine was)
D
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nib1980
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| posted on 3/5/09 at 12:01 PM |
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excellant suggestions, I found a leak on the box, little bit of paste and problem solved.
many thanks
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bitsilly
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| posted on 3/5/09 at 02:49 PM |
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I'm off to do the same then!
Started to tell people it was because it was soooo highly tuned!!
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JoelP
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| posted on 3/5/09 at 04:24 PM |
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mine used to do that too, i loved it! 
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dhutch
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| posted on 3/5/09 at 09:02 PM |
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quote:
Originally posted by JoelP
mine used to do that too, i loved it!
Yeah, mine does it a lot.
- Not stupidly, but it certainly pops and crackles on the over run. With a load band on turn off.
But to link with the above, it does have a holed manifold.
- It just seams a shame to fix it and loose the overrun noise!
In the short term, if you bottom the accelerator pedal just after turning the igition off it stops it on mine.
Daniel
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