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BigAl · P4 Joined: 06 May 2003 Posts: 2992 Location: U.K Surrey

Not that I'm complaining, i like it, but what causes it?
Occasionally when giving my car some, when i change from 2nd to 3rd gear there is a load muffled pop.
I heard its caused by a leak in the exhaust system, as air is sucked back in and ignites unburnt fuel, but mine has new gaskets all round and there are no leaks when you put your hand over the tail pipe. It started happening after i had my powerflow SS cat back system fitted.
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Ben_nz · Posted: Sat Jan 10, 2009 11:15 pm Post subject:

I'd like to know as well.
Until somebody knowledgeable comes along, have a look at this video I made of one of my 33s. It contains some exhaust poppings in the second half. Very Happy

http://picasaweb.google.com/oldschool.co.nz/Misc#5286573930582086034
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suds · Alfasud Joined: 25 Jun 2006 Posts: 70 Location: liverpool

Its to do with the valves they have a overlap were the exhaust valve doesn't quite close as the inlet opens its part of the design as the exhaust gases leave its creates a vacumn as the inlet valve opens it draw the new charge into the cylinder quicker and little bit of unburnt fuel enters the exhaust system then combust's that's the snap crackle and pop ciao daz Very Happy

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BigAl · P4 Joined: 06 May 2003 Posts: 2992 Location: U.K Surrey

sweet daz
it doesnt happen that often in my car, but when it does it makes me jump, then grin Laughing

thanks
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john 33_16v · Posted: Sun Jan 11, 2009 10:34 am Post subject:

Suds, You didn't teach day release car mechanics at watford college in the mid-late 80's did you? That all sounds very familiar, and has taken me right back Laughing

From memory you are spot on.

My sud and sprint (with dellorto/weber twins) use to crackle like crazy, but the 33 doesn't really do it Crying or Very sad

John
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BigAl · P4 Joined: 06 May 2003 Posts: 2992 Location: U.K Surrey

I don't know if my 8v ever did what Ben_nz's does and was never crazy enough to hang out the boot to check Laughing

However i remember once i was over taking a friend and pulled in front of them and he said a huge 3 ft flame shot out the tail pipe Laughing

I would like someone to drive my car and video it whilst they cane it past though Laughing

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suds · Alfasud Joined: 25 Jun 2006 Posts: 70 Location: liverpool

I did go to liverpool college around 1991-3 night school doing city and guilds repair and maintance of road vehicles and went back last year to do body side of it plus they have full work shop and spray booth and they encourage you to work on your own car booked the ramp for this week put the 33 on to change the exhaust ciao daz

I think on the ie models when you lift of the loud pedal the ecu cuts the fuel supply to stop exccess fuel entering the exhaust and helps the emissions at mot time Sad

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alfasud 1.5 ti
alfasud super 1.3
Alfa 155 2.0ltr 8v
Fiat croma 1.9jtd

BigAl · P4 Joined: 06 May 2003 Posts: 2992 Location: U.K Surrey

Will it damage the cat??
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33bits · Posted: Sun Jul 12, 2009 5:46 pm Post subject:

Suds you are dead right, the ie does cut off fuel supply via ecu when you lift off, when they 1st introduced the ie concept they had to prove in front of a body of "know it all's" that this fuel cut off system works. It did so the model name ie was allowed to be used. The "e" stands for economy, hense the fuel cut off system. Very Happy

God we are so sad!!! Very Happy

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BigAl · P4 Joined: 06 May 2003 Posts: 2992 Location: U.K Surrey

I'm blind, i see it now
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john 33_16v · Posted: Sun Jul 12, 2009 6:36 pm Post subject:

bobbber · Posted: Mon Jul 13, 2009 9:35 am Post subject:

Yeah John, you're right mate.
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33bits · Posted: Mon Jul 13, 2009 7:14 pm Post subject:

Sorry guys, yes John you are right, i've given myself a slap on the wrist for being wrong.
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33 16v Sportwagon
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156 JTS sportwagon
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Oggie · Posted: Mon Jul 13, 2009 7:50 pm Post subject:

Anyway , this over run pop thingy, its normally caused by a hole/leak in the exhaust system, suffered from this when my manifold cracked the day before Spa, all gone when it was tig welded up

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LDA · Alfa 33 Joined: 28 Nov 2004 Posts: 276 Location: France

turbo cars do it all the time but very high revs on a normal car and then lifting off hard can do the same thing. just getting unburnt fuel into the exhaust system tends to do the trick.

the huge amount of extra fuel the car can burn with a turbo means that when you lift off you really get a good bang from the exhaust. no leaks needed.

i find it tends to pop nearer the rear of the exhaust on NA cars and i have seen small flames from some of my friends cars while thrashing about. not from an alfa though.. but then i never followed the daft exhaust i used to have on one of mine.

this idea was later used to run antilag systems on turbo cars. 2 ways i know of doing that.. the proper way with extra injectors in the manifold to create an explosion that then spins the turbo up while off the throttle.. or just messing with the standard ecu timings so that excess fuel is injected while off throttle to create the same effect..
but the second way can cause fuel wash in the cylinders and isnt a great way of doing it.
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paulhide · Posted: Tue Jul 14, 2009 5:46 pm Post subject:

Dave Peters said your old P4 shot flames out the back once with your old exhaust when I was turning right in it once.
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LDA · Alfa 33 Joined: 28 Nov 2004 Posts: 276 Location: France

pimp

i've not had the luck i wanted getting it back but i've not given up yet. fingers crossed. mind you the old SS exhaust that was offered as part of the sale would be VERY dangerous here in the alps in the winter, due to it causing avalanches onto the road.

i'd still be tempted to put it back on just to hear the engine roar, even if it robbed the lump of some torque.
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Eddie_W · Posted: Tue Jul 21, 2009 11:20 am Post subject:

The pop on overun seems to be quite characteristic of the Alfa boxer motor but I've noticed that on miy s2 that it is much more obvious when my idle jets are partially blocked, suggesting that there is leaness involved. I use it as a diagnostic tool to tell me when they need cleaning. I don't know what is causing this progressive blockage as it has new lines and filters etc. however when it pops a lot on the overun it is also rougher on the progression phase from idle to main which on the carbed series 2 occurs a lot higher inthe rev range than you might think. A blowout of the idles makes a dramatic difference. Incidentally I now have JFZ 4 pots and bigger rotors on the front. I forget the size but they are not the 284mm ones as they seem to be hard to get out here. they are the next size down, from the 146. stops a lot better but I will look at 284s next time even if I have to import.
Regards Eddie
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Ben_nz · Posted: Wed Jul 22, 2009 6:53 am Post subject:

Eddie, I agree it seems possible to get more or less popping depending on carb tuning. I recall Pete from The Italian Job once telling me he'd set my car richer so it wouldn't backfire (but I like backfiring!)..
My 1.5 always blocks exactly the same idle jet (the one closest to the driver) completely and over and over again, and the other jets never block. When it's blocked, the car backfires softly just trying to idle. Rolling Eyes

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