gums

I think it's more about valve overlap, there is an instant when both intake and exhaust valves are actually open in high revving engines, when you're off the gas there's no ignition, so raw fuel goes through to the pipes. Without mufflers to smother it, you get flame.

Geoffrey

With EFI (Motronic ECUs included), when you are off the throttle, the fuel is shut off but the ignition system is still operating.

gums

I had a feeling you'd know better, Geoffrey!
I'm still waiting for DeMan to straighten this out.

mark kibort

It could also be the incomplete burn of the engine, with the exhaust containing some raw fuel that is unburned. as it is in contact with the rear hot pipe, there is not enough oxygen to ignite it, so it just exits the pipe with the exhaust gases. BUT, when you lift, the volume of the exhaust is a lot less, but there is still a proportion of unburned gas present, which now has oxygen around the exit opening so, it ignites due to the tempurature of the pipe and the gases, in the presence of the air around it.

That's my theory. However, i think the inability of the system to stop the fuel, while stopping the air with the throttle plate, is proably more of the reason. There is a lot of "idle" fuel being dumped into the engine at 6000rpm, with little air. all this fuel is ignited out the pipe under decel. at idle, there would only be a fraction of the fuel present, to keep the engine running at 1000rpm, no load. Proof is in the vacuum the intake has under decel at 6000rpm vs idle. However, the fuel load is much greater at 6000rpm.

mk

mk

cornerhans

If the idle switch is out of spec or defective causing fuel to continue flowing after the throttle is closed, would that cause an erratic idle at idle? Or could it cause other symptoms?

Rick DeMan

Hi Frank...

Um, lets see....Because its not running right..!! Ya, thats it..!!

IcemanG17

The easy fix is longer pipes.....probably the best fix is to tune the engine so its not quite as rich....

gums

sometimes the simplest explanations are best....

J richard

Well, having been there....

With the full cup exhaust you do get some heat, and sometimes a bit of flame, It was worse if the car wasn't running quite right, (vacuum leak) , as well as no O2 sensor, but if you run a straight exhaust you will get a little puff of flame from time to time, it's just too shortly coupled. I added a heat sheild and reworked a dansk tailpipe by welding on a 2" section of stainless tube (it was just too short and was very close to the bumper lip, the factory tip had the same problem). No problem since, but occasionally it still pops a little flame.