Author Topic: Back fire from exhaust  (Read 545 times)

Offline Drew Back

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Back fire from exhaust
« on: 12.07. 2015 11:28 »
I know this is a long shot but what does it mean when a bike backfires out of the exhaust when you shut it down..I will be honest this is happening on a 1959XLCH with mag and linkert carb but the same principle should apply none of the harley guys could answer this for me so im giving it a shot with all the knowledge on this board please dont hate me for asking!!

Online Klaus

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Re: Back fire from exhaust
« Reply #1 on: 12.07. 2015 11:35 »
Hi,

control your exhaustsytem, l'll be sure you found a leak.
blow by a gasket, or worn push in.

cheers


If you think, everything is under control, you are not fast enought.

Offline Greybeard

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Re: Back fire from exhaust
« Reply #2 on: 12.07. 2015 11:48 »
I suggest it might be a leaky exhaust.

Online bsa-bill

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Re: Back fire from exhaust
« Reply #3 on: 12.07. 2015 12:16 »
surely all things being correct there should be nothing in your exhaust to ignite, so are we looking at fuel/air mixture getting past exhaust valve (when it should be closed) combined with a spark from somewhere (does this engine used a wasted spark system) or timing issue.
Good question tho, we may all learn something from answers
All the best - Bill
1961 Flash - stock, reliable, steady, fantastic for shopping
1959 Rocket Gold Flash - blinged and tarted up  would have seizure if taken to  Tesco

Offline Greybeard

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Re: Back fire from exhaust
« Reply #4 on: 12.07. 2015 13:18 »
There will always be some explosive gas in the exhaust; just add some oxygen from a badly sealed exhaust system and it can ignite causing backfires.

When I was young I used to make my old Ford Consul, like this one https://goo.gl/j59Scr backfire by turning the ignition off, putting my foot on the gas pedal then turning the ignition on again. Great fun making pedestrians jump out of their skins! I stopped doing this when I destroyed the silencer, (muffler)  *conf*

Offline Drew Back

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Re: Back fire from exhaust
« Reply #5 on: 12.07. 2015 16:10 »
Thank you all for the great responses it uses a Morris mag which is set fully advanced I just got this put together a few weeks ago since it fried an exhaust valve a couple years back..It ran like a dream for the first 60 miles but the last 60 it was sluggish and very noisy and the backfire thing started to happen I have yet to check plugs still have the honey do list first..

Online bsa-bill

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Re: Back fire from exhaust
« Reply #6 on: 12.07. 2015 17:34 »
Quote
backfire by turning the ignition off

well then there would be an expolisive gas in the exhaust cause you stopped it getting burnt, but I don't see there being much in there if valves are good and ignition is correct, if a plug failed to spark for some reason then I can understand it or if valve timing allowed some overlap and incoming charge got mixed up pushing out burnt gasses.
Of course I do know it happens  *eek*
All the best - Bill
1961 Flash - stock, reliable, steady, fantastic for shopping
1959 Rocket Gold Flash - blinged and tarted up  would have seizure if taken to  Tesco

Offline Greybeard

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Re: Back fire from exhaust
« Reply #7 on: 12.07. 2015 17:38 »
...fried an exhaust valve a couple years back..

I hope that hasn't happened again. It could explain a backfire.

Offline Greybeard

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Re: Back fire from exhaust
« Reply #8 on: 12.07. 2015 17:41 »
Quote
backfire by turning the ignition off
well then there would be an expolisive gas in the exhaust cause you stopped it getting burnt
Of course, that is understandable but I know from personal experience that a leaky exhaust system can be the cause of backfiring.

Offline Drew Back

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Re: Back fire from exhaust
« Reply #9 on: 12.07. 2015 17:57 »
I pulled the plugs and rear is burnt bad front looks great now I will do compression check and readjust valve..Thank you all so much for your knowledge..

Offline muskrat

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Re: Back fire from exhaust
« Reply #10 on: 12.07. 2015 21:39 »
Air leak at the inlet manifold on the rear cylinder giving a lean burn and taking out the ex valve.
There is always unburnt fuel mix in the exhaust no matter how well it's tuned. An air leak in the pipe will give popping on back off.
Cheers
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Offline duTch

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Re: Back fire from exhaust
« Reply #11 on: 12.07. 2015 22:53 »

 How about slightly over advanced ignition, and/or idle slightly too high....? (I wind up the idle just a bit of my Hilux when cold, and if I forget to wind it down, when turning off it pre-ignites bad- a bit different, but maybe related..?  )
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Online bsa-bill

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Re: Back fire from exhaust
« Reply #12 on: 13.07. 2015 08:48 »
Quote
There is always unburnt fuel mix in the exhaust no matter how well it's tuned

I'm beginning to see that *smile* *smile*

OK suck it back through with a blower
All the best - Bill
1961 Flash - stock, reliable, steady, fantastic for shopping
1959 Rocket Gold Flash - blinged and tarted up  would have seizure if taken to  Tesco