Deaccelarate backfire

[calais24/7]

Is there anything you can do to stop that sh**y backfire to stop when you back off the go pedal and slow down ?

[Nackerz]

sounds like its in need of a good tune my friend

[VL_5SPEED]

Is there anything you can do to stop that sh**y backfire to stop when you back off the go pedal and slow down ?

How big is your exhaust and does it have a rear muffler or just a pipe. When i got my fagwag it had a 2.5" exhaust cut before the diff and used to backfire and pop on deacelleration. Then i had a 2.5" pipe and rear muffler welded up. Pretty quiet exhaust now with a nice tone and still quietly pops on deaccell... Think its to do with the size of the pipe....

[I drive a VL Commodore]

Get your injectors leak- and flow- tested. If it's properly backfiring it means fuel is getting into the cylinders when it shouldn't be.

If I recall correctly RB30's shut down fuel completely (or almost) on 0% throttle over a certain RPM. In my 5 speed Skyline it was about 3,000rpm ish. So it shouldn't be backfiring.

[calais24/7]

Get your injectors leak- and flow- tested. If it's properly backfiring it means fuel is getting into the cylinders when it shouldn't be.

If I recall correctly RB30's shut down fuel completely (or almost) on 0% throttle over a certain RPM. In my 5 speed Skyline it was about 3,000rpm ish. So it shouldn't be backfiring.

How much roughly is a leak and flow test? Also i dont have a stock exhaust its 2.5"

[moose man]

An exhaust backfire on deceleration is usually due to an air leak in the pipe somewhere up near the engine - possibly the manifold gasket at the head or flange with the rest of the system. It sucks oxygen into the pipe when there's a slight vacuum on deceleration and allows any incompletely burnt gasses in the pipe to burn.

[greenfoam]

More timing during deceleration fixes it. So perhaps your timing isn't quite right

[snowybill]

If I recall correctly RB30's shut down fuel completely (or almost) on 0% throttle over a certain RPM. In my 5 speed Skyline it was about 3,000rpm ish. So it shouldn't be backfiring.

Just out of interest, coz im new to the whole scene, what exactly do you mean by "shutting down fuel completely"

Cheers

[tombomb]

Any good ideas on checking vacuum leaks fellas?

[detox_nath83]

put a gauge on and you should get a reading of 20-23psi vacuum at idle
thats what i get

[tombomb]

and what should one do if it's outside specs? isnt it possible to spray something on the hoses and see bubbles? or am i mistaken? cheers

[snowybill]

If you put soapy water on your lines, if theres any leaks, you'll see bubbles start to form.

[HARVZ-6]

Or, check/ listen for exhaust leaks in the front section of the exhaust.. this will cause it

[calais24/7]

Alright cheers for all that, also could it be something simple like a foul plug? I just changed the fuel filter and still not it. I'm going to change the plugs and see if that helps

[calais24/7]

Not spark plugs, not leads, not fuel filter, also it is a bit sluggish at the moment and its shitting me. I know it should have more poke then what it does, any more ideas. It has to be something with mixtures or fuel, thats my guess anyways

[leigh 7005]

sounds like it might be running lean or the timing

[greenfoam]

More timing during deceleration fixes it. So perhaps your timing isn't quite right

I think this guy was right

[Calaber]

I'll go with Mooseman. Leaks in the exhaust towards the front of the system will cause backfiring for exactly the reasons he says. When you back off, the amount of unburnt fuel fed into the pipe only needs a bit of oxygen to set it off and, when you back off, there is a vacuum created within the pipe which draws air in through any leaks that exist. The leak needs to be towards the front of the system because of the heat present there which is sufficient to assist combustion.

[calais24/7]

Alrighty ill look into the timing and see if i have issues, could that be the source of the problem and causing it to be sluggish aswell? Also calaber how could i check for a leak like that?
Cheers guys

[calais24/7]

Found out my AFM is cactus so this should be the the cuase to the problems, now to track down a working afm asap

[detox_nath83]

if u get one from wreckers check it there man as it may be damaged already... keep reciept :P

[calais24/7]

Will do man espicially if the have those adjustable screws but do you reckon it would be making it sluggish and backfire?

[Calaber]

Alrighty ill look into the timing and see if i have issues, could that be the source of the problem and causing it to be sluggish aswell? Also calaber how could i check for a leak like that?
Cheers guys

The leak can be very slight. Simplest way is to put the front of the car on ramps and start the engine, leaving it idling. Crawl under with a small piece of light paper like tissue and run it carefully along the exhaust, without actually touching the pipes. If you can't hear the leak, the gases may move the tissue paper as it passes over them, but usually, up close, you will hear the leak. Most likely place is any pipe joints, starting with the manifold and working back. Also, look for point along the pipe where it has scraped when passing over speed humps or rocks. Sometimes, this happens so often the pipe can actually wear through. Obvious points to look for would be the lowest point in the system.

[moose man]

Get someone to put their hand over the exhaust pipe at the back (with a rag if they have lady hands) and listen for the hiss while you're underneath. You'll hear it pretty quickly.

Don't let too much pressure build up though, or you could blow out a rust hole in your muffler or something.

[Calaber]

Get someone to put their hand over the exhaust pipe at the back (with a rag if they have lady hands) and listen for the hiss while you're underneath. You'll hear it pretty quickly.

Don't let too much pressure build up though, or you could blow out a rust hole in your muffler or something.

Agreed. Easier and more effective than my method.