Backfire when you let off the gas!

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kjbtsi6686
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Joined: Sun Jul 06, 2014 8:16 am
Location: Jemez Mountains, NM

Backfire when you let off the gas!

Post by kjbtsi6686 »

I have a loud backfire when I let off the gas, am I looking at a exhaust leak? Or something more sinister, the exhaust isn't the best looking creation by a shade tree mech, I will post some pics of what I'm dealing with when I get a chance to not be at work.
1974 710M
undysworld
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Re: Backfire when you let off the gas!

Post by undysworld »

A backfire is a classic symptom of an exhaust leak. If you have leaks, fix that before you go looking further.
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pcolette
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Re: Backfire when you let off the gas!

Post by pcolette »

Here's a photo of the original exhaust slip joint between the header pipes and the heat exchanger. This is a common spot for exhaust leaks.
IMG_2181.jpg
IMG_2181.jpg (286.35 KiB) Viewed 3630 times
Many owners have replaced the slip joint with the flex kit from EI or SAV, like this:
IMG_2186.jpg
IMG_2186.jpg (301.46 KiB) Viewed 3630 times
A good exhaust shop could also probably weld in some SS flex pipe for you if that's where the leak is occurring.
Paul C.
_________
'73 Swiss 710M
'89 Puch 230GE
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kjbtsi6686
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Joined: Sun Jul 06, 2014 8:16 am
Location: Jemez Mountains, NM

Re: Backfire when you let off the gas!

Post by kjbtsi6686 »

Is this something I should be worried about as far as driving it?
1974 710M
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kjbtsi6686
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Location: Jemez Mountains, NM

Re: Backfire when you let off the gas!

Post by kjbtsi6686 »

Looking at this exhaust it was done very poorly, I'll have to post some pics, the welds are horrible. I can see spots where there is holes in the welds.
1974 710M
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rmel
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Re: Backfire when you let off the gas!

Post by rmel »

Back fire in the exhaust could also be caused by improper timing,
insufficient tappet gap, or running too high an octane versus the
compression you have.

You can also get exhaust back-fire when you shift up/down as
in that case the manifold pressure increases (Negative Bar's)
resulting in a fuel rich condition with some unburned gases going
out the exhaust. If you have an EFI you can dial that problem
out with appropriate fuel vs MAP.

Might want to look at your timing and a compression check on
occasion is a well worth test -- trust me on that one from my
own experience :wink:
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kjbtsi6686
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Joined: Sun Jul 06, 2014 8:16 am
Location: Jemez Mountains, NM

Re: Backfire when you let off the gas!

Post by kjbtsi6686 »

Adjusted the timing (it was quite off) and she seems to run much better now. Looking at probably doing the SS muffler from EI soon.
1974 710M
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