weber settings and backfire

[Gaz64]

A few things we need here Paul.

All of the Camshaft specs, preferably manufacturer amd grind number.

Compression ratio.

Ignition timing at idle and maximum.

All of the carb specs, idle, main, air, emulsion tube, venturi etc.

Gary

[Tom Wells]

Paul,

A few observations to add to the discussion. Be forewarned I run EFI so have no direct experience (except #3 LOL).

1) Lobe separation is a factor in designing a cam for Webers to minimize fuel "standoff" above their venturis. The internet says 108-110 is a good range. Less seems to give problems, more may reduce power.

2) Backfiring on deceleration is not usually associated with intake vacuum leakage. It is much more often a result of an exhaust header flange leak, either at the head, or at the collector. Don't think it is usually a carb problem.

3) A friend used platinum plugs believing the more expensive plug would be better. He had a backfire usually on startup that could be heard for a mile. Changing to normal copper electrode plugs eliminated that backfire.

Just sayin...

Tom

[PaulProe]

Paul
I ran Webers on my big block prior to going to an EightStack system. It's really difficult to get rid of the backfire on decel. Hard to cut the fuel supply with the throttles closed, they still leak slightly around the blades and you get oxygen via reversion, coming backup the pipes. I could never dial it out completely. Some days were worse than others, I am guessing due to air density but don't know for sure.

I learned to keep the blades open slightly when throttling down, that seemed to help a little bit. Also, make sure you're not running too much timing, that is also a factor.

Look on the bright side - it really looks cool at night seeing the flames pop out of the side pipes.

Paul

[moar_carbz]

Originally Posted by PaulProe

Paul
I ran Webers on my big block prior to going to an EightStack system. It's really difficult to get rid of the backfire on decel. Hard to cut the fuel supply with the throttles closed, they still leak slightly around the blades and you get oxygen via reversion, coming backup the pipes. I could never dial it out completely. Some days were worse than others, I am guessing due to air density but don't know for sure.

I learned to keep the blades open slightly when throttling down, that seemed to help a little bit. Also, make sure you're not running too much timing, that is also a factor.

Look on the bright side - it really looks cool at night seeing the flames pop out of the side pipes.

Paul

Hey I know I'm hijacking this thread, but I had a thought: has anybody tried putting a throttle damper anywhere on these carbs to close the throttle more gently and maybe this would go away?

[paul cass]

Hey guys
I know that it's been a couple of weeks but this is the first time I've been able to sit down in front of the computer. I talked with several of my Cobra buddies at our annual event last week and they offered a couple of ideas. I tried playing around today with settings and seem to be getting close. In spite of what I was told by Red Line I turned the mixture screws in so that now they are set at 1 turn out. During our cruise last week I found that the FIA would sputter on occasion while cruising at a set speed. By going in 1/2 turn I found that today I could cruise with no sputter. Still a little sputter, but not anything like before, before shifting as the engine reaches 2500 rpm. Much happier.
Paul

[Gaz64]

Hi Paul,

I don't know where Redline get their adjustments from since many Webers I've played with (DCOE, IDA, IDF), have their mixture screws at an average of 3/4 turn out IF the idle fuel AND idle air are correct for the engine.

The rest of the carb can be all over the shop, and won't affect the idle/transition area of engine operation.

DCD, DGV etc tend to be in the 1- 1.5 out zone.

So by your info, Redline has had you running the engine a little fat (rich) at idle with the mixture screws at 2 turns out, and probably a little more throttle blade at idle to achieve your target idle speed.

Gary

[paul cass]

I think you are tight Gary. One of my car buddies said that "you will always be dealing with adjustments on those things". He may be right.