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  #1 (permalink)  
Old 03-27-2018, 01:14 PM
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Default weber settings and backfire

Hey guys
I know that it probably has been answered before but with all of my research I've found conflicting answers. 3 basic questions. I have a 289 that is not exactly stock. .532 lift solid cam, head work with larger valves, and bored .060. With a Holly 750 it put out around 350 HP. I've replaced the Holly with 44 IDF Webers. Can't say I'm that happy with the help I've gotten from Top End-who I bought the set up from, or Redline who built the Webers. Anyway here are my 3 questions. How much do you turn out the idle mixture screw to start. Told both 1 1/2 and 2 turns. Big difference. What is the order of the 4 carbs when adjusting the mixture screw. Heard that when you adjust one of the four first it sets the vacuum so that it makes it easier as you adjust each of the next 3. And finally when you get an occasional backfire or popping thru the exhaust pipe on deceleration what is the adjustment. There are no leaks. Someone said to turn out the mixture screw? Any help here would be greatly appreciated. Still waiting on the catch can. Any hints on how to post new pictures?
Thanks, Paul
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Old 03-27-2018, 03:26 PM
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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
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Old 03-27-2018, 03:51 PM
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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...

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Old 03-27-2018, 06:04 PM
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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
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Old 03-27-2018, 08:23 PM
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Hey guys
Thanks for the responses. I don't readily have the specs on the cam as I installed it almost 20 years ago. Compression is 10.8 to 1. I run a mixture of 110 leaded and super gas. I have an AOD with a 2900 stall convertor and 4:10 gears. FIA weighs around 2000 pounds. Accelerating is not a problem. It goes like stink! At first when I was adjusting the mixture screws I was able to get everything to work pretty good. No backfires. Then I had to remove the distributor to fix an intake leak and when reinstalled I developed this backfire or popping problem. Not sure why removing and installing the distributor would cause all this. I originally ran a vacuum advance. It is plugged off. I've experimented with the timing ranging from 12 to 24 degrees initial advance. I've installed soft springs for the mechanical part of my distributor and have found that regardless of my initial timing the total advance changes by no more than 15 degrees. I've read that Webers like a lot of advance. Top End and Red Line gave me NO specs on these Webers. Told me I could take them apart if I wanted to know the jets, etc. And I assume these do not have a choke. At least in the diagram one is not shown. So, back to two of my original questions. How far out do I turn the mixture screws and which carb do I start with and then what is the order for the next three?
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Old 03-27-2018, 11:55 PM
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Hi Paul,

My Webers ran best at 3/4 turn out.

If you need to go to 2 turns, your idle jet is lean.

When adjusting mixture strength, doesn't matter which cylinder you start with, or what order you follow from there.

First up, they need to be balanced accurately with a synchrometer.

Then playing with each mixture screw should make that cylinder go rich or lean, the cylinder "goes away", almost like pulling a plug wire.

IDFs can have a starting circuit, cable operated, or come with a blanking plate.

A little disappointing your supplier didn't give you info for your settings.
But that is easy enough for most of it.
Can you look down the venturi and note the number cast into it?
Factory settings for 44 IDF are 36mm venturi, 4.5 aux, 135 main, F11 emulsion tube, 180 air, 50 idle, 50 pump, 50 exhaust (pump inlet valve).

Gary
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Old 03-28-2018, 12:34 AM
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Hi Paul
I cannot reply on your original question about the mixture screw.

But as you mentioned that it worked perfect before you removed the distributor, to fix an intake leak, I would search on this side again.
It worked before!

Did you replace any gasket or could there be some small dirt in between?
If I would be you I would try to go back with the settings on prior the intake work and check the work you did.
Not saying that you did something wrong.
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Old 03-28-2018, 10:30 AM
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Hi Gary and Alf
Have the morning off tomorrow and plan to start over with all of my settings. Gonna recheck to make sure I don't have any leaks. One of the guys from Redline is supposed to be available for help. But, I'm not fond of the idea of taking apart the carb to get tech info which is what he first suggested. I may pass on him until I've exhausted all other options. I have all of the carbs in sync using the synchrometer. At idle they read 5.5. For what its worth when I first built my Cobra I had the engine built by one of the drag racing engine shops and know that the engine is sound. It always ran best with the 750 although I tried a 650. So even though I gave all of the tech info I had on my engine to Top End I've wondered if the 44's might be a little lean for my engine. One of the "Weber experts" told me that I could adjust the mixture screw to richen the carbs instead of changing jets. I've seen several posts about if you have to go out more than two turns you should change the idle jet. Anyway, starting over I hope I can get back to where the engine will run satisfactorily. At age 71 I'm past the age of being the fastest car.
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Old 03-28-2018, 10:54 PM
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A note here. The intake leak was a small oil leak on the front seal behind the distributor.
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Old 03-29-2018, 02:15 AM
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Hi Paul,
If where your mixture screws are now make the engine run correctly, not rich or lean, at an A/F ratio of around mid 13s, (technically 14.7 would be nice), then turning them out further will only richen the engine at idle.

2 turns out, says the idle jets are too lean.

The idle jet is really the low speed jet, carries the engine up from idle as the transition circuit, to overlap with the start of the main circuit.

The mixture screw can be used to analyse the transition circuit.

For example, we wind the speed screw up to 1400 rpm or so, we note the engine behaviour, (gas analyser preferably), then nudge the mixture screws to add or remove so fuel accordingly.

If we need to open them, then the idle jet is too small, eg: step up from 50 to 55.

And then vice versa, if you need to close them.

Gary

Last edited by Gaz64; 03-30-2018 at 01:29 AM..
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Old 03-29-2018, 08:45 PM
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A quick update Gary, Alf, and Paul. Didn't have a lot of time today but I did accomplish something. Went back to square one. Re torqued everything. Found a few of the header bolts a little loose, a couple of the carb bolts lose, started at 0 and turned out the mixture screws 1 1/2 turns, reset the idle, and set the timing at 18 degrees. Started up fine. Went for a quick test run and ran great! No pops and only a slight stutter if I accelerated slowly in hi 2000's RPM. May not be a perfect tune but it seems to be working well enough to drive now. Needs to be as next week is our annual Texas Cobra Club Spring meet. Won't have time to fool with it for a while as I'm off to visit my grandson for his 5th birthday. Happy Easter everyone!
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Old 03-30-2018, 01:36 AM
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Sounds like you are close enough for now, if you are happy Paul.

Happy Easter to you and family.

Gary
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Old 04-07-2018, 11:01 PM
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Quote:
Originally Posted by PaulProe View Post
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?

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Old 04-13-2018, 07:30 PM
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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
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Old 04-14-2018, 04:09 AM
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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
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Old 04-14-2018, 07:10 AM
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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.
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Old 04-14-2018, 02:34 PM
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Quote:
Originally Posted by paul cass View Post
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.
Yeah, I am pretty tight, but I hope you mean I'm right Paul,

Gary
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Old 04-14-2018, 06:04 PM
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Sorry Gary, you are "right".
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Old 04-15-2018, 01:20 AM
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Once you have your Webers adjusted, they will stay set for ages, especially with a zero slop rose jointed linkage,

Tell your friend from me, that I have, and many will agree, that multi webers are easy to maintain.

Gary
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Old 04-18-2018, 04:41 AM
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Hi Gary
I'm going to try your suggestion of running the engine at 1400 rpm and then play with the idle mixture screws. I was sent one of the synchrometer gauges but no info on how to use it. So my plan was to use it and see what the air flow is. I know that when I used it at idle all of the stacks read 5.5. Say that it now reads 7.5. I'm assuming that as I turn the mixture screw the reading will go up or down, and that I'm trying to adjust to a steady reading and where the engine noise sounds the best. I will have to do this one stack at a time. A motorcycle guy told me that he was able to adjust his mixture by adjusting the float level on his carb which was similar to a weber. Maybe I'm missing something here but I don't see how the float level could change the mixture. Unless maybe it was set too high and fuel would bleed over. Thanks for the help.
Paul
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