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  #1 (permalink)  
Old 09-16-2007, 09:26 AM
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Default Weber popping question

According to my A/F gauge, I am really lean when I let of the gas at all rpm ranges. I believe this to be the cause of the popping. What is causing this? I seem to be running lean on the mains as well but that is an easier fix.

For future reference,

Edited for current settings

65 idle jet
120 idle jet holder
170 main
210 air corrector
f7 tubes
.00 exhaust/ float bowl valve
40mm chokes

380 ci 302 with 8.7 deck, Yates heads with huge CFM numbers.
cam - solid roller dur. I 244, E 247 lift 631, 666 110 LCA 106.5 LSA
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Last edited by dlampe; 11-04-2007 at 04:23 PM..
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Old 09-16-2007, 09:34 AM
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how far out are the idle mix screws?
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Old 09-16-2007, 09:44 AM
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They right at 3/4 turn.
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Old 09-16-2007, 09:52 AM
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I realize that is the ideal turnout, but I would try 1 full turn and see what happens, if it helps then going up in the idle jet may be in order. On the other hand if it works and the A/F is good , then leave it out. I have found that when you are dealing with more exotic motors you have to fudge on the conventional wisdom with webers.
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Old 09-16-2007, 10:04 AM
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This article at Inglese Inductions helps to understand the Weber carb and its problems and fixes, I found it was a good place to start:

http://inglese.com/tech.htm
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Old 09-16-2007, 10:05 AM
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that is a good article but all bets are off with Deans motor as well as a stroked 427
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Old 09-16-2007, 11:20 AM
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I am running really rich on the idle circuit. If I open the mixture screws, won't that make the engine run richer? If it fixes the popping would I want to go to a smaller idle jet? When the plates are closed, opening the screw should fix the popping to richen the mix. How do I lean out the off idle A/F ratio? Is that a larger idle jet holder? I am already at 120.
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Old 09-16-2007, 01:24 PM
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I thought I remember reading that these are so easy to set up?

Yeah, right.

Last edited by PatBuckley; 09-16-2007 at 03:50 PM..
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Old 09-16-2007, 01:30 PM
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It probably wouldn't be to bad if I had a jet kit. My thought is maybe go to a 55 or 60 idle jet. that would allow me to back out the mixture screw a full turn so maybe it wouldn't lean out when I am coasting down in RPM's with the throttle plate closed. Do you think that makes sense?
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Old 09-16-2007, 01:59 PM
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Deane,

Dont Coast, use the blasted brakes to slow down, thats what you put those big rotors on there for.
Now in regard to your jetting, if its a 'lean' pop then go to a slightly smaller air corrector in the main circuit rather than the idle-reason being that regardless of you having closed the throttle that big cube unit of yours will actually pull the throttle blades partially open under engine braking conditions. Think about it, you cannot get that much air thru those 3 or 4 idle and transition orifices of 1mm dia each.
As Pat has mentioned that combo is going to try your patience to get setup correctly-dont make it harder for yourself by trying to make the carburettor into a brake caliper.

Have just done a quick check on your jet sizes etc, its a mile away from where I would start , will post later, gotta go earn a buck- monday morning here.

Jac Mac

Last edited by Jac Mac; 09-16-2007 at 02:08 PM..
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Old 09-16-2007, 03:06 PM
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Thats how you are going to leave me? Come on, tell me where you would set the starting point.
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Old 09-16-2007, 04:13 PM
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I had the same problem, the cause was a broken spring in the MSD distrubutor. Just one item to look at.

Allan
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Old 09-16-2007, 04:21 PM
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Quote:
Originally Posted by dlampe
Thats how you are going to leave me? Come on, tell me where you would set the starting point.
Cant stand the thought of a grown man crying

380 cu in =778.38cc per cyl . Therefore;

Idle jet.............. 70 to 75
Idle jet holder..... your 120 should be Ok given that you will be increasing jet size.

Now you get to the main circuit- You have to get real honest about which category you would put yourself in here- Hi Perf Road or Competition. In your case I would suggest that you might have to consider having two jetting setups for your car especially if you intend comp use due to the extremely strong signal that motor is going to give the carb when you start to explore the upper RPM range.

Choke size.........42mm Road-45mm Comp
Main Jet............170 Road---180 Comp
Air Corrector.............On the Road application around 200 thru 220 air- for Comp air should be smaller than main jet by around 50/60 eg 110/120 Air.
Emulsion tube.......Keep the F16 for Road but try F7 for Comp
Pump exh .........50

Finally I must remind you to be real careful particularly in regard to full power runs in the higher gears particularly with the larger air correctors you currently have installed, a hi speed leanout is very likely under these conditions with all the tears ( real ones ) that will accompany it. Those 40mm chokes on a large cube motor only increase the risk factor. I am sure it feels nice & responsive now in short bursts of acceleration, but when you really start to get serious they will cause some problems. looking at your engine specs you have a race engine- not street engine, treat and tune it accordingly; ie dont spend any more time than you have to under 3000 rpm.

Jac Mac
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Old 09-16-2007, 06:44 PM
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Why would the air corrector need to be so small on a comp setup? Do you think that will help the popping? Could I solder the 210's and re-drill or is that a no no?
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Old 09-16-2007, 07:04 PM
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that does seem odd, when the air corrector comes into play at high rpm. However, some of this is definitly art.
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Old 09-16-2007, 08:06 PM
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The mains aren't triggered by RPM alone. Throttle position has a huge roll in it. If you are WOT at 2500 rpms coming out of a turn, your mains are flowing. I had a long conversation with a forum member and we talked about the 2 progression holes in the idle circuit. You can see them if you shine a light down the venturi and take out the inspection plug just below the mixture screw. You should only see 1. If you see 2 then your throttle plate is closed to far. The problem is if you set the idle screw to only show one hole, I am turning some serious RPM's, not just idling. I think I am going to try the smaller air correctors and see what happens. Tonight I raised my idle to 1100 and I backed the mixture screw out almost 1 turn. I will see what happens tomorrow.
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Old 09-16-2007, 08:39 PM
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Quote:
Originally Posted by dlampe
Why would the air corrector need to be so small on a comp setup? Do you think that will help the popping? Could I solder the 210's and re-drill or is that a no no?
Deane,
The easiest way to decide which category you slot into is look at it this way--Competition you are either accelerating or braking as in racing in all gears,
Road ,obviously you spend a lot of time at part throttle cruise mode and therefore less actual airflow thru the motor.

The smaller air corrector in the Comp setup is to increase the signal to pull fuel thru the main jet right thru the rev range since we are in a state of acceleration most of the time and require the enrichement. We dont require as much air thru the air corrector to emulsify the fuel/air mix at this point since the relatively high airspeed thru the secondary venturi will take care of most of that.

While you could solder the old air correctors its not really a permanent fix due to the erosion factor of the soft solder. A simpler way to check if this is reqd on your setup would be to make up eight small hairpins out of say 0.030" or0.040" stainless locwire and place one in each air corrector as a temporary measure- just make the 'legs' of the hairpin long enough so they cant jump out. Now if this cures your 'popping' at higher rpm/deceleration then you obviously have to richen up the main circuit by larger jet or smaller air. If it reappears lower in the rev range then the idle jet may need to be larger as well.

The 'popping' you are hearing is in the intake area isnt it ? as some other posters have mentioned the collector leak problem which usually occurs from an over rich unburnt fuel condition under deceleration and is a backfire in the muffler or tailpipe.

Jac Mac
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Old 09-16-2007, 08:40 PM
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Dean the best suggestion I can give you is to put the thing on a dyno and tune it. It will save you alot of heartache in the end.

Depending on what your doing with the car I wouldn't run the 40's they will just make tuning it that much harder. If your going to track the car only then set it up for that..if your going to drive it on the street then have either 2 configs or just run the street setup. If you remember David's Kirkham we only had 37's in it and didn't have any issues with fuel. THE DYNO will save your sanity.

The biggest problem is finding someone that has a slew of jets so you can set it up while tuning it.
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Old 09-17-2007, 05:46 AM
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Jac Mac, the popping is coming out of both pipes. It really doesn't sound like it is at the intake. My A/F gauge is reading lean not rich during the popping. I am going to try some smaller correctors.

Matt, you said harder to tune, not impossible, right! I am going to start changing a few things and see what happens. My timing is set at 32 deg. locked at the dizzy. Should i bump that or or does it matter as far as the weber tuning goes?
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Old 09-17-2007, 08:29 AM
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"Timing Locked @ 32 degrees" (??) Do you mean "limited", or is it running fixed 32 degrees all the time?
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