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Slight backfire.
https://www.ausmini.com/forums/viewtopic.php?f=2&t=19701		 Page 1 of 2
Author:  Mick [ Tue May 30, 2006 6:01 pm ]
Post subject:  Slight backfire.
When I let the throttle go on my RE13'd 1275, I get a slight popping backfire It's very soft, not a bang. Also when I stop the engine I get a strage little pop in the exhaust pipe that changes note as unburnt gasses burn off slowly.
My timing is correct, and my mixture seems OK (set with a Gunson Colourtune), can anyone guess as to what might allow this to happen? Obviously something is out.

Mick
Author:  Stuartminidlx [ Tue May 30, 2006 6:13 pm ]
Post subject: 
it's all normal mate.
Author:  andbat [ Tue May 30, 2006 6:15 pm ]
Post subject: 
yeah i get that to, specially when i'm going downhill with the clutch out and slowing myself with the gears.
Author:  Mike_Byron [ Tue May 30, 2006 6:33 pm ]
Post subject: 
Also check the compression with a compression guage. A burnt exhaust valve will also a pop sound out the exhaust.

You usually have other symtoms as well like a miss but its worth a check
Author:  Mick [ Tue May 30, 2006 7:01 pm ]
Post subject: 
Cool, thanks guys. The compression is tops as I had the head rebuilt recently when the camshaft went in.

taa

Mick
Author:  Mike_Byron [ Tue May 30, 2006 7:09 pm ]
Post subject: 
No - make sure the compression is still tops now as well
It could also be an exhaust valve not closing properly due to either a sticking valve (valve guide grabbing) or tappet clearance too tight.

If nothing else it eliminates some possibles.
Author:  drmini in aust [ Tue May 30, 2006 7:47 pm ]
Post subject: 
Backfire in the exhaust is also symptomatic on an exhaust leak, usually up near the manifold.
It's burning unburned fuel.... :wink:
Author:  Mick [ Tue May 30, 2006 8:10 pm ]
Post subject: 
Okey Dokey, I'll check those too.

Mick
Author:  stuvz [ Tue May 30, 2006 10:24 pm ]
Post subject: 
maybe ur car is 'fully sick' 8)
Author:  Mick [ Tue May 30, 2006 11:18 pm ]
Post subject: 
Yeah like 'Fully hospital sik' bro :wink:
Author:  Mini Mad [ Wed May 31, 2006 1:54 am ]
Post subject: 
I wouldn't worry, ever since i put the RE-13 in mine does the 'pop' too..call it snap crackle and pop..minus the first two... pop..pop.pop 8)

It's very quiet though..
Author:  willy [ Wed May 31, 2006 7:41 pm ]
Post subject: 
Isn't this just valve overlap - ie both the exhaust and inlet valves are open for a very short duration?
If so, it did this in Stu's LS very "badly" (but i loved it), 286 Megadyne Scatter Pattern Kent cam... fantastic rally/fast road cam 8)
Author:  drmini in aust [ Wed May 31, 2006 8:06 pm ]
Post subject: 
Nah- my RE282 doesn't do it, nor did my RE13... both running 1.58:1 rockers. :lol:
Author:  Mick [ Wed May 31, 2006 8:13 pm ]
Post subject: 
I have also got 1.5 to one MS forged rockers. Hmmmm better look aty that manifold again...

I've got a question though, seeing that the exhaust is pressurised by the gasses exiting the engine, what compels the air to get into the exhaust via the gasket when there is a leak?
Author:  drmini in aust [ Wed May 31, 2006 8:54 pm ]
Post subject: 
At the manifold face there is a pulsating flow from pressure to vacuum in each exhaust port, not constant pressure. :wink:
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