Carb'd 308 Exhaust Flames Explained?

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unburnt gas is getting out of the engine and into the exhaust system and being ignited.

while cool, its not right imo.

so, too much fuel indicates a rich mixture.

however, on my 308, here is how i got the blue flame syndrome:

with the car at idle, i would run the mix screws in and out finding the optimal running condition at idle (ignoring the factory manual to set the screws at 3 out). this method resulted in a very lean condition, and as a result the were would be tremendous popping on overun, with the blue flames out the back. So how can it be so lean and getting too much fuel in the exhaust system? it was written somewhere that when its so lean, gas is pulled through the throttle plates from the progression holes above the throttle plate (due to fuel starvation...? dont know), and that is what causes the rich condition.

when i set my carbs to optimal idle (ie, the best running cylinder at idle), the screws are out about 1.5 turns (euro screws with 55 idles). This results in popping on overrun and blue flames. however when i set them to 3 out all around no popping and no flames.

geno

 

"I suppose a rich mixture that then uses all available oxygen by the time or around the time it exits the pipe would effectively "ignite" the excess fuel as it is introduced to oxygen."

There you go! The fuel is at a sufficient temp to burn but there is not enough oxygen available. When it gets far enough down the exhaust system where there is enough O2 and still enough heat, off it goes!

 

So how can it be so lean and getting too much fuel in the exhaust system?



this is from the fuel mix not igniting in the combustion and then heating in the exhaust system. Much of the explosion actually happen in the exhuast where it is not seen and the blue flame is just the end of it.

 

When you close the throttle abruptly, the engine, which is at high rpm (not idle), the pistons are still trying to pull the air/fuel mixture appropriate for whatever RPM into the combustion chamber. But since the throttle valve is closed, it can't pull air. The higher RPM/closed throttle valve generates a lot of vacuum which pulls a lot of fuel out of the bowls for a second or so while the engine RPM drops. So the combustion chambers are loaded up with fuel that can't burn due to insufficient O2.

 

that makes sense. but why does the problem go away when you richen up the idle circuit?

 

Is this a potentially damaging condition? I recently had a new exhaust installed on my 77 and when I wind it up and shift, I'm getting a flame out the back. I'm not hearing a pop or anything, but the person following me said it was pretty impressive. My concern is that I might be running a little too rich.

 

to be more precise, when i adjust the carbs at idle to run "best" by running the mix screws in and out until it idles perfect they are in about 1.5 turns. In this condition when I lift off the throttle there is fantastic popping. This also happens when stationary revving the engine. You can somewhat hear it in this video: [ame]http://www.youtube.com/watch?v=uTxm-j1UI-w[/ame]

i thought for a long tiem i had an airleak somewhere, but i didnt.

When i richen up the idle circuit, the problem goes away, and there is no popping on overrun, rather a really smooth wuuurgh

[ame]http://www.youtube.com/watch?v=J4Mqj1B1CfU[/ame]

 

what happens when you lift off the throttle after load at speed?

 

That's when I get the flames. I need to be winding it up a bit, say 5000+

 

no popping, snorting, or crackling? just flames?

 

I'm not hearing a pop or crackle. There may be a subtle sound, but my hearing isn't exactly the greatest. But there definitely isn't any loud backfiring or anything like that. I didn't know about the flames until someone following me told me about it.

 

Closing the throttle at high RPM does the same thing for a second or so that closing a choke does - it causes the engine to pull more fuel into the system related to the amount of air, thus richening the fuel/air. In order NOT to do that, there has to be a bleed/dump designed into the system that will vent the engine's vacuum "signal" upon closed throttle/high vacuum OR a device to prevent the throttle butterflies from abruptly snapping shut.

Back in the day, standard carbs used on most US cars had a dashpot that prevented the snap-shut. I have no idea what Webers have. Sounds like nothing.

 

so i guess backing out the idle screws (richening up the mixture) stops the fuel from being sucked out of the progression holes above the throttle plates?

 

I don't know a thing about Webers - have never even seen one up close enough to touch. So I have no idea how their circuitry is designed/works. But it sounds reasonable.

 

If you are getting flames out of the exhaust, I would check that your manifold has not developed any holes/leaks. This is where the oxygen could be getting in, creating the conditions for the un-burnt fuel to ignite. Especially, if you are running a tad rich.

 

my carb 308 GTB does it and I leave it, because sometimes it helps keeping away some dudes on the german Autobahn, which are following you too closely.
Shifting down, releasing throttle half way. In the beginning I wondered why some tailgaters then suddenly keep away.

Best Regards from Germany

Martin