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Backfire on the overrun


Anglefire
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I had a nice run out in the cold this morning (See topic "Snow"!)  and I drove it reasonably hard - upto 70 on the dual carriage way and came down a couple of small hills (Bear in mind we are talking around Birmingham and not Wales!) and on the overrun was popping a little bit.

I've done a bit of a search - and the answer varies from its normal to a rich mixture to a lean mixture and to a leak in either the exhaust or manifold.

Interestingly enough the idle speed has increased as well and is a little too high (Again!) 

So my initial thought was timing - but I'm now inclined to think a leak?

I've not had another play as its too flippin cold!

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Dave as carbs are made up from all manner of bits and pieces , yes  variations and tolerances all play a part in making 

nothing standard or hard and fast ,  variables thats why all the bits are  adjustable,  

Everything from the fuel put in to the owner are all varyables   

 

Pete

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Hi,

is popping the sound you get with an overly lean mixture? Excessively lean mixture fails to ignite & fresh air with unburnt fuel exits the exhaust valve. Over several cycles, the AFR in the exhaust is good for ignition?

I know when I decrease the fuel pressure, my car pops too much on overrun. Gets attention for those that can't see the car.

Cheers,

Iain.

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it was normal to shut of ignition allow some overrun, switch  back on and    kaboom,

best done under bridge , or near guys working done a manhole,  

the commer pb with a side exhuast was perfect.

has a reputation for splitting open silencers 

then you get old and modern fueling wont work 

Is there no fun left 

 

Pete

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2 hours ago, Pete Lewis said:

it was normal to shut of ignition allow some overrun, switch  back on and    kaboom,

best done under bridge , or near guys working done a manhole,  

the commer pb with a side exhuast was perfect.

has a reputation for splitting open silencers 

then you get old and modern fueling wont work 

Is there no fun left 

 

Pete

Pete, if you did that on a modern car you would end up with a dead cat in the road!

Back then if you remember it was soot and bits of baffle that came out. One way of achieving a free flow exhaust system.

You will get thut thut (Did I spell that right) noises, but as we have said should not get any bangs on over-run.

An 88C thermostat helps the engine (and heater!) in the winter, but remember to fit the 82C in the spring. Been there done in, forgot that is.

Dave

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2 hours ago, Pete Lewis said:

it was normal to shut of ignition allow some overrun, switch  back on and    kaboom,

best done under bridge , or near guys working done a manhole,  

the commer pb with a side exhuast was perfect.

has a reputation for splitting open silencers 

then you get old and modern fueling wont work 

Is there no fun left 

 

Pete

I used to find the old petrol transits good for that, they often set off car alarms with the boom....... A jubilee clip around the silencer stopped them splitting.  The best fun was people pumping their tyres up at petrol stations :) 

Tony.

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I've bought another manifold gasket (They are only £6.80 ish from Ebay for a genuine Payen one) and if I get chance I'm going to whip off the manifolds, inspect the faces and refit with some high temperature silicone sealant (Yes I know you shouldn't need sealant) and see if that cures my backfiring and seemingly high idle again, I shall be satisfied.

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This afternoon I made a bit of time to whip off the carbs and replace the gasket - with little high temperature sealant both sides of the inlet holes.

Ran it up and it ran as rough as a rough thing - needed to apply some throttle - but choke didn't help after a few minutes. Idled at about 400rpm until it stopped!

So turned the idle screws 1/4 turn down and then idled nicely at about 7-800rpm. 

So does suggest there was a leak.

The only this I did notice was there was some oil lying in the inlet manifold - and I am using more oil than it should, so was wondering if that was normal - or whether it suggests the valve guides are worn?

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  • 6 months later...

Went out today for a 50mile round trip. 

I have backed off the needles (Su carbs) so it is running a little leaner at idle at least (exhaust is still black) 

and coming down a couple of hills on the overrun I was getting a slight pop from the exhaust. The idle speed that I did reduce after my last run out is running a little too high again (about 1000rpm) - I’ve not adjusted it again yet. 

When I got back home I sprayed the inlet manifold with some brake cleaner and no change to the idle speed (which is a relief!) but I sprayed it around the carb spindles and it did change slightly on both sides. Not much but a definite change.

I have looked at the inside of the carbs and the top of the one (front) needle is a bit damp from fuel. So I think I will check the fuel pressure (the pump is a new one (12months or so) and I wonder having read other things on here) that the new pumps tend to over pressure the lines. 

I already have a fuel pressure gauge that I can fit.

The float is obviously another possibility as well as the valve in the float chamber. But they have been replaced previously too   

Another other thoughts?

 

 

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I don't know about cars but on bikes, backfiring is usually a leaky exhaust, or most often not enough baffles on aftermarket exhausts. You only get a backfire in the presence of oxygen, it's getting in somehow.

Can someone explain to me why a lean mixture is often suggested? I'd have thought unburnt fuel was the issue and too rich would be the cause...unless being too lean means it doesn't combust properly.

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