weber 500 problem

[toughy V8]

i have a weber 500 on my 3.5 and as far as im aware this should be calibrated to run fine on a 3.5 yet mine keeps cutting out its running rich and when i put my foot down nothing really happens makes loads of induction noise but just hesitates and goes nowhere. is there anyway i can check to see if its been tampered with

[mgbv8]

You need to have it apart and see what size jets, needles and springs you have fitted. I have the manual for the carb, assuming its a 1404 model. I'll have a look later to see what the stock settings are. You need torx drivers to get the top off.

[toughy V8]

it is the 1404 model

[Alley Kat]

Paul B recently got a set of jets & needles etc from RPi, specifically for a 3.5, his post is on here somewheres about that, pretty sure he said what sizes he was given.
The carb manual may be on Edelbrocks website, it used to be there, and also Volvord put a good version of it on his site http://www.telusplanet.net/public/giles ... tion_1.htm
and http://www.telusplanet.net/public/giles ... tion_2.htm

[ppyvabw]

Before, ripping you carb to pieces, Put magnecor leads on or other high spec leads.

You need a good ignition system with the edelbrock to avoid plugs sooting up.

Trust me, I had problems with mine, I chuffed about with it for weeks making no progress, then I tried a set of magnecor leads and it just fixed it. There expensive but it will work.

Also the cutting out could be carb icing, this is also a problem I have with it so next time look for ice forming around venturi

[sidecar]

The standard settings for the weber 500 over fuel a standard or even tuned RV8 3.5 quite badly.

The standard settings are 86 mains, 94 secondaries, needles 65x52 (1460) orange springs.

I spent quite along time messing about with my carb and ended up with 86 mains 94 secondaries and 67x55 needles (1463) and orange springs. The mixture screws are set 2+3/4 turns out. (I noticed that once the motor had big valves fitted they needed the extra 3/4 turn).

be careful when removing the top off the carb, the clips for the excelerator pump and choke mechanisum are microscopic. Also the gasket is very fragile across the middle of the ventruries.

A good way of comparing the varrious needles and jet combinations is too subtract the needle numbers (which are in thou of an inch) from the jet size. The bigger the number that is left the bigger the hole will be for the fuel to flow through will be, hence richer the mixture.
The two needle numbers are for cruise mode and power step. The thinner part of the needle is the power step, this is when the manifold vaccum is not low enough to overcome the needle spring. You do not need to remove the whole top off the carb to change the needles, just loosen the small top covers then slide the top to one side. (Don't totally remove the torx screw just slacken it off).
Oh IMHO the edlebrock manual is way too complicated and has at least one error in the mixture chart.

Also check the float heights, mine were miles out. (get the measurement from the edlebrock web site)

Here is a link to some more gumf on the carb!

http://www.cobraclub.com/forum/showthread.php?t=18898

Regards,

Pete

[Robrover]

They're really intended for a biger V8, more like a 289 Mustang, so out of box the jetting is a little on the big side for a 3.5.

You can get smaller main jets 080 or 083. Then just buy the kit and start experimenting with different combinations of step up sprinds, rods, etc until you find the one that suits your engine best. It's all a bit trial and errorish, although you can do it on a dyno too of course but this costs whereas testing it out on the road is for free.

[mgbv8]

I use a very small set of long nose pliers to take my clips off the accelerator pump linkage etc. I tried some tweezers first time, and lost one.

[Paul B]

Paul B recently got a set of jets & needles etc from RPi, specifically for a 3.5, his post is on here somewheres about that, pretty sure he said what sizes he was given.
The carb manual may be on Edelbrocks website, it used to be there, and also Volvord put a good version of it on his site http://www.telusplanet.net/public/giles ... tion_1.htm
and http://www.telusplanet.net/public/giles ... tion_2.htm

The new metering rods are # 6252, old ones were #6552, new primary jets
are 0.080", old ones were 0.086"

I've put the old primaries into the secondary position, bringing them down nicely.

The Weber jet kit set doesn't have jets as small as 0.080" so no point buying that. I got my stuff from RPI, just called them, asked what I needed, paid £18 by card over the phone, and it arived 24 hours later. I fitted it and it runs much better.

You also have to move the accelerator pump linkage out to the end hole, to give the smallest squirt. It effectively drowns the motor in the centre hole.

[KMCE]

Toughy,

You have only explained one part of the overall equation. As others have noted the 500cfm Weber as stock is too big for a 3.5, and will overfuel. Other things that affect performance are the manifold and cam. What type of manifold and cam are you using?
If the cam is high lift long duration, this can have a significant impact on vacuum signal to the carb, and affects fueling (ref comment regarding vacuum pressure on needle springs). The mainfold also has an impact, as a large bore inlet reduces velocity and charge inertia, again having an effect on vacuum signal, and charge quantities. Large bore is more suitable to continuous wide open throttle applications. Again a balancing act needed - free flowing vs. low velocity!!
As suggested, needle and jet changes will improve the situation, but it still may not give you the performance you are after.

Going down this road myself at the moment, so hope this helps in some way.

Ken

[ppyvabw]

I disagree I think the weber is fine on the 3.5. Granted, the ideal size is the 390 holley. The weber has smaller primaries making it ok for bottom end, and also a secondary air valve so it only flows the air it needs on wide open throttle.

I really do suspect his problem is his ignition. I was advised to put magnecor leads on and I didnt believe it but it solved all the problems. The rolling road guy also said, it wasnt running rich.

In fact I had to go richer than the standard secondary jets on my 3.5 because it had a flat spot at wide open throttle leaner on the primary rod in high vac position though.

[sidecar]

I disagree I think the weber is fine on the 3.5. Granted, the ideal size is the 390 holley. The weber has smaller primaries making it ok for bottom end, and also a secondary air valve so it only flows the air it needs on wide open throttle.



I really do suspect his problem is his ignition. I was advised to put magnecor leads on and I didnt believe it but it solved all the problems. The rolling road guy also said, it wasnt running rich.

In fact I had to go richer than the standard secondary jets on my 3.5 because it had a flat spot at wide open throttle leaner on the primary rod in high vac position though.

I also disagree that the webber is too large for the 3.5. I believe that the primaries are infact smaller than the 390 holley so it runs OK on cruise and as the secondaries are vaccum controlled they open as much as the engine requires.

(Just read "ppyvabw" comments again....I've just repeated them, sorry about that!)

The problem could be down to either the ignition and/or the carb.
The carb did over fuel my motor and required another re-tune after I fitted stage III heads and dialled in the cam.

The standard ignition (35DML8) also caused problems as the bob weight springs allowed the advange to come in too quickly. (I found that using springs from the Real Steel kit, one silver and one gun metal grey allowed full advange at 4K). Also the mec advance caused too much total advance. I have made mods to my system so that I can run 10 deg BTDC at 1000 rpm (good tickover) and a total of 34 deg BTDC at 4k. I also run the vac advance.

Thinking about it the carb would have to be miles out to cause the described problem,....I agree its more likely an ignition issue.

Regards,

Pete

[ppyvabw]

The secondaries are mechanically controlled but they also have a rudimentary 'air valve' so in a sense they are both vac and mechanical

[toughy V8]

Toughy,

You have only explained one part of the overall equation. As others have noted the 500cfm Weber as stock is too big for a 3.5, and will overfuel. Other things that affect performance are the manifold and cam. What type of manifold and cam are you using?

i have an edelbrock performer and im using a 3.9 cam

[KMCE]

i have a weber 500 on my 3.5 and as far as im aware this should be calibrated to run fine on a 3.5 yet mine keeps cutting out its running rich and when i put my foot down nothing really happens makes loads of induction noise but just hesitates and goes nowhere. is there anyway i can check to see if its been tampered with

Toughy,

The performer manifold is dual plane, and a 3.9 Cam should give you a reasonable vacuum signal to the carb.
I cannot claim to be an expert on the Weber carbs, as I use a Carter four barrel (400cfm). Vacuum signal and air flow rate are what the carb is calibrated for. If the others are saying the primaries are sized to smaller than a 390 (surprising but accepted) then low speed & low throttle should be OK. Running rich on idle can be ammended by the idling controls, but this will affect tickover; it may be necessary to up the idle speed slightly.
Perhaps other can help here, but on the carter carb the primaries are controlled by needles in the jets, in turn controlled by pistons and springs. The pistons are actuated by vacuum signal, and controlled by the springs. Looking at the springs may help the richness at low throttle. Actuation of the secondaries, appears from the comments to be both mechanical and air. The timing of the opening of the secondaries will have an impact on the "going nowhere" issue. Opening slower will help the velocity through the secondaires and fuel pick up.
Another thing to check is the pumper circuit. Im not sure (others will correct me I hope) whether the weber uses a power valve or piston type pumper circuit. Its function is to make up the difference in fuel when you put your foot down quickly. When you open the throttles quickly the vacuum signal drops, and thus the quantity of fuel taken in by the engine drops and causes the engine to run lean, just when you need a richer mix. The pumper squirts fuel normally into the primary to allow the vacuum signal to catch up, and the primary and secondaies to catch up.

What I would recommend you do (if you havnt done so already) is to get a good book on the Weber carb, and take it off the car to your work bench. Disassemble according to the instructions, clean and re-assemble. Im not talking about a full re-build, just a clean. I had similar problems with my carb, and once cleaned and put back, runs beautifully. You would be surprised how much crap gets into them. This would be my first step, and if this does not cure the problem, then move onto other items as noted above.

Sorry for the long post, and forgive me if I have attempted to explain what you already know, but I have found that understanding goes a long way towards problem solving.

Best of Luck

Ken