Tuning Webers.

[Mycar]

Good morning, could someone help me decide what I'm to do with the idle air correction screws in my Weber IDS carbs.

My two reference pieces for tuning these carbs are, firstly Performance Oriented's excellent write up...

Weber Carbs Technical

And more recently I've found this information in Pelican resources...

Porsche 40IDA 3C Weber

My confusion arises from a discrepancy between the two sources in the initial settings of the Idle air correction screws.

Peformance Oriented states...

a. Throttle stop screws adjusted all the way out and turned back in until they just touch the throttle arm and then turn in 1/2 full turn. Alternately:- Remove the plugs covering the transition holes (Item #55 on the Weber Parts web page) and adjust the throttle valves until the bottom edges of the valves just begin to uncover the lowest progression circuit holes.
b. Idle air correction screws are turned in until they lightly bottom out and then turned 2 full turns out and locked in place with the 8mm lock nut
c. Idle mixture screws are turned in until they just bottom out and then turned out 2 full turns

The second method states...

Initial adjustments (PMO) are as follows:
Idle air correction: 1/8 turn out*
idle screws: ½ turn in after touching throttle arm on each carb
idle mixture screws: 2 turns out.

(*I suggest turning all the idle air correction screws in (gently!) until they hit their seats, and lock them down for the initial start up. You want a nice rich mixture for starters to avoid excessive popping which is really distracting, and then work from there.)

So, do I start with my Idle air correction screws turned out 1/8 of a turn... or 2 turns ?.... or locked down ??

Regards, Mike Ruddy.

[1QuickS]

The setting for the air adjustment screws isn't critical for initial adjustment. The point I'm trying to establish with initial air screw adjustment settings is this: when at idle the edge of the throttle plates should (by design) be completely covering the first (lowest) progression port. Unless you remove the throttle bodies from the manifolds and visually confirm how many turns of the idle speed adjustment screw it takes to reach this situation (and record that information for use later) you will be left with a "Best guess" as to their final physical setting.

The reason why you want the first progression hole covered is this: if it isn't covered by the edge of the throttle plate you will then be drawing fuel from it at idle and this situation will have you decreasing the fuel delivery from the mixture screw; again, at idle. Now; while driving, your fuel delivery from the progression circuit is the summation of fuel from the progression holes exposed by the throttle plates (below the edge of the throttle plates as they sweep past them) AND the fuel from the idle mixture screw. If the fuel from the idle mixture screw is set to be too little during idle mixture adjustment then total fuel delivery during progression is diminished; perhaps leading to a larger idle jet selection to overcome the deficiency.

Older carbs are less prone to this minutia since they leak air around edge worn throttle plates and through throttle shaft/bearing clearances so it is natural for the idle speed screw to set the throttle plates more closed than freshly reconditioned carbs would want.

The idea behind the two turns open on the air screws is to get enough air to the engine when idling without opening the throttles too much. Then, as air flows and mixtures are adjusted you would want to close the air screws down from the two turns open for balancing while keeping the idle speed screws 3/4 turn or less open; again in the effort to not prematurely expose the first progression hole.

Remember that the idle/progression circuit takes in air for fuel emulsification just as the main circuit does with its air correction jet mixing air via the emulsion tube with the fuel. The idle/progression circuit is a little different; air is injected into the fuel in the fuel gallery from both the air bleed (little brass "pill" pressed into the top of the main throttle body just inboard of the idle jet and the progression holes above the edge of the throttle plates which are at atmospheric air pressure. As the throttle plates sweep past the progression holes; those below the edge of the throttle plate add fuel and since the number of holes above the throttle plate has decreased then the amount of air available to emulsify the fuel has decreased which enriches the mixture.

[Mycar]

Thank you Paul, I appreciate the time you`ve taken to reply, sorry I didn`t get back to you sooner. You website has been a huge help in demystifying my carburetors.

My tuning has been hampered by a dead number two cylinder that has had me flummoxed for a long time. Turns out it was a crack in the mag manifold, so once I get that sorted out, I`ll get back to it.

Cheers , Mike.

[Peter Zimmermann]

Mike, I've got about 30 years experience tuning Webers. There is a very specific order to this, and you will need a Uni-syn and some simple hand tools. Feel free to pm me, if you need help, after you get your manifold issue handled.

[Mycar]

Thankyou Pete.

Regards, Mike.

[Mycar]

So gentlemen, I have a mystery on my hands.

I`ve had the manifold welded and put everything back together but as I tune the engine the idle mixture screw on number two is unresponsive. Using a ColorTune reveals a yellow flame on number 2 that becomes a blue flame as the revs climb to 1600 but as I say, at idle the mixture is too rich and the idle mixture screw has no effect. The plug is sooty when pulled after idling.

A leakdown test shows acceptable results, if anything number two is better than one and three.

A compression test shows uniform results at 165 psi

There are no manifold leaks.

Swopping the idle mixture screws around makes no difference.

Plug leads are Newish Magnacors, swopping lead for lead has no beneficial effect. I have MSD fitted and the spark is excellent.

I`ve checked float level in carbs, the meniscus is between top two marks on gauge when the engine is running. I tickled the floats... and they are happy.

I`ve peered down the venturis with a mirror and flashlight and there is no untoward flow of fuel.

the idle jets are 55`s, new this week.

The carbs were professionally stripped, ultrasonically cleaned and serviced 2 years ago. I`ve had them on the bench, stripped and cleaned them with carb cleaner and compressed air this week... even the secret gallery. I`ve burnished out the divots and the floats are not compromised.

The Hardy pump delivers at a steady 3 1/2 lbs.....

.... not that that`s going to be the problem... just trying to be thorough.

Now I like a mystery as much as the next man but on this occasion I would appreciate a second opinion. And third and forth.

Any thoughts why my number two screw doesn`t work ??

Regards, Mike.

[1QuickS]

If you have early IDAs then the idle jet holder does not use an o-ring; later Webers have o-rings. If you have a later style idle jet holder and are using it in an earlier Weber then the idle jet won't seat properly and will allow ultra rich idling. Upper jet holder is the o-ring type.

[Mycar]

My carbs are `68 IDS`s and there are no O rings on the jet holders. I`ll check tomorrow and see if I have an errant later holder... excellent photo, thankyou very much.

Mike.

[Mycar]

Morning Paul, I felt sure we had the answer there for a moment... but alas, all the jet holders are all the same non O ring versions.



What else could it be.

Regards, Mike.

Edit to say, I should add that idle screws appear to be in good condition, identical and swopping them around changes nothing.

[1QuickS]

Drat, I also thought the issue was the jet holder.

Check to see the idle air bleed hole isn't blocked with the gasket or with a bit of debris. This is the brass "pill" that is just inboard from the idle jet holder and is pressed into the top of the main throttle body. If blocked it would enrich across the progression so I doubt this is it.

Are you sure all lead plugs in the fuel galleries are in place? When I service Webers I peen all lead plugs (including those in the tops of the accelerator pump jet bolts) after ultrasonic cleaning. They do loosen on occasion as a result of age and from the cleaning process.

There isn't much else I can suggest, a very interesting issue you have. Perhaps you could send it to me to have a look? Contact me via my email address if it comes to that.

By the way, without the Colortune you wouldn't have been able to provide the diagnosis as absolutely as you did; definitely rich at idle, leaning with increasing revs and a good spark. The tool is wonderful in this diagnostic role. I'm glad you have it to help focus upon the correct issue and (eventually) verify the issue is resolved.

[Mycar]

I suppose spraying each lead plug with carb cleaner, one by one, as the engine is running, cold and then hot, is as good a way of proving their integrity. All seems well on that front.

The idle air bleed pills are clear.

You`re right about the ColorTune, it gives me a certain peace of mind that number two cylinder is firing correctly once I get underway and for now that`s enough for me. The car is running well generally and whilst we have a bit of summer left, I`ll enjoy her while I can.

Thanks for all the help, I`ll try and get the carbs over to you over winter for a level 1.... maybe even a level 2.

Cheers, Mike.

[1QuickS]

Hi Mike,

I didn't catch you were in the UK, not so casual to ship to the US for me to have a quick look.

I had a project that the idle mixture on one cylinder was so rich that it would not even fire. What I thought was a "not enough fuel to fire" issue" was in fact a "too much fuel to fire" issue. When I investigated I found the mixture screw tip had broken off in the orifice and a previous repair actually shifted the hole to the side and enlarged it. I repaired the issue by tapping the wall and installing an aluminum, threaded plug; M5x0.8. I then drilled a new orifice and removed the stick-out from the inner bore. Once this was set right the issue went away.

Perhaps you could have a look for this issue? Also, I assume your mixture screw is the same as for the other cylinders and not buggered in some obvious way. Try removing the spring from the body of the screw and see if it allows it to seat deeper. If you are using the cup washers with o-ring seals then there may be the issue of grounding out on the spring/cup washer stack-up before the needle seats in the hole.

Grasping at straws here.

[T77911S]

Originally Posted by 1QuickS

Remember that the idle/progression circuit takes in air for fuel emulsification just as the main circuit does with its air correction jet mixing air via the emulsion tube with the fuel. The idle/progression circuit is a little different; air is injected into the fuel in the fuel gallery from both the air bleed (little brass "pill" pressed into the top of the main throttle body just inboard of the idle jet and the progression holes above the edge of the throttle plates which are at atmospheric air pressure. As the throttle plates sweep past the progression holes; those below the edge of the throttle plate add fuel and since the number of holes above the throttle plate has decreased then the amount of air available to emulsify the fuel has decreased which enriches the mixture.

paul, i love your web site and all the info on it. i post links to it when people need help with carbs. where can i get more like the the explanation you have above?

there were 2 things in the other guys procedure that i had questions about (did not read all of it yet).
when i had carbs, back in the 90's, i could only raise my RPM's to around 1500 or so to keep my unisyn from pegging when balancing the 2 sides. (more info than a question).

i did not know the idle air correction effected the mixture. he said to turn it all the way in to make it richer.

if you remove the carbs and set the idle screws so the plates just cover the forst holes, should you not touch them after that? it sounds like that is more of a mechanical setting or visual setting than and idle setting. that would then require the actual idle setting to be done with the air correction screws.


i remember having the problems that you state will happen if the idle screws are set wrong.
i use to start with the air correction screws all the way in, then set the idle with the idle screws and then adjust each air screw for the highest barrel.
i do remember having the air screws out too much and making a lot of noise. is that a problem if they have to be set that high?

the car always drove Ok, i just use to foul plugs a lot when starting. (i was learning as i was going back then. unfortunatley idid not have the resources there are today.

i had converted a 914 to a 6 with a 2.2E.

[1QuickS]

The whole thing with the idle air screws is to be sure your throttle plates are set so that the edges of the plates are aligned with the first progression holes when at idle. IDA carbs are designed to allow for this where the IDTP series do not; these typically expose 1/2 of the first progression hole at idle.

When carbs are in great shape (as when they have been reworked with fresh throttle shaft bearings and throttle plates) they will leak less air than ones with more miles on them. I have come to believe that 90k miles is a typical wear-out mileage for our Webers. When older Webers are being tuned there is adequate air leakage to initially adjust the idle air screws to be closed as much as possible and then opened up to achieve air flow balance.

The interesting thing comes when the 40mm carbs are used on larger displacement engines; larger displacement calls for more air at idle which is all too-easy to achieve by opening up the throttle plates...and exposing the first progression hole. The exposure of the first hole would have you closing the idle mixture screw to achieve a "Lean Best" idle mixture but since fuel delivery during progression includes the fuel contribution from the mixture screws you would then be inclined to install a larger idle jet to make up for that fuel deficiency.

I believe it is always better to get the throttle plates adjusted correctly at idle and then make jetting decisions based upon driving issues such as lean progression due to insufficient idle jet size or to deficiency of IDTP progression circuit operation.

The idle air screws don't really affect mixture setting; they just pass air from ones side of the throttle plates to the other. The idle air bleeds are jets (pressed into the top of the main throttle body just inboard of where the idle jets are located) function just like the air correction jets that are used in the main circuit; they add air that is mixed with the fuel passing through the idle jets and emulsify it for better atomization during introduction into the intake manifold.

[Mycar]

Hi, it`s been a while but I`m able to report back about why adjusting my number 2 idle mixture screw wasn`t having any effect. As it turned out , the number six screw wasn`t responding too well either.

I stripped the carbs and removed the lead plugs just to make sure but didn`t find anything untoward. I Changed the plugs, measured the leads, checked the valve clearances, put everything back together again and.... Same as before.

As a last ditch attempt to try something I decided to replace the points with a Petronix I have for this distributor and it was then something turned up.

The rotor arm was loose on the distributor shaft. Not much, but enough.

I bought this distributor off US ebay after searching for the correct one for my car for a few years. It was NOS but I had the vendor send it directly to a specialist in the States to have it serviced and the curve checked before sending it on to me here in the UK.

It arrived here a few weeks later as a plug and play distributor... so I did.

It seems unlikely that a loose rotor arm would have such an effect on number 2 and number 6 mixture screws but wrapping a bit of black tape around the distributor shaft so the rotor arm is a good interference fit has cured my engine problems completely !!

Now I can tune the car accurately and there is no missing as there was before.

It's a miracle.

So after spending a summer tearing my hair out, thinking I should buy a new MSD or some new PMO carbs or at least have the engine rebuilt, it all came down to 2 inches of black plastic tape.

Now I get to enjoy fine tuning my engine and see just how good I can get it. The thing that has made the most difference over the last couple of days has been adjusting the air flow balance @ 3000 revs by adjusting the cross bar brackets. Never done this before, thanks Paul.

Oh and if you do see this Paul, I'd be interested to know what spark plug gap you use.

Cheers, Mike.

[yelcab1]

Wow,
The set of webers I was working on had the same issue and I may need to look inside the dizzy for that. Plus, it is slow to come back to idle as well. Dizzy inards, it is.

[1QuickS]

Hi Mike,

Congratulations! Cylinders 2 & 6 fire in sequence so it seems you have some eccentric rotor motion which would affect 1/3 of the rotation of the distributor shaft. I'll put this info into my notebook.

As for plug gap...ask others for this. I believe stock ignition uses anywhere from 0.014" through 0.024" but MSD and the like can use up to 0.045".