Webers popping at part throttle.
 

Hi All,

I'm trying to cure popping at the Weber inlets and hesistation to accelerate at part throttle, on my 2.0 litre 911S.

The carbs don't pop at tick-over but pop badly as I bring the revs up. I've had to turn the mixture screws out to prevent this. When driving, the car struggles to accelerate until about 3000rpm and they goes off like a scalded cat!

Both Weber banks are popping equally and the mixture screws are currently about 3.5 turns out on each one. The carbs are balanced throat to throat and bank to bank.

Is it normal that when adjusted to not to pop at tickover, the carbs pop badly at mid revs? I thought you set the mixture at 900rpm and the jets should look after the rest.

Also, should I have seals / soft O-rings on the idle jets? Mine don't at present.

(I have 40IDS3C Webers with 130 Main Jets, 180 Air Correction Jets, 55 Idle Jets).

Any suggestions welcome.

Thanks,
Chris

Specs for a 2.0 with IDS is: 110 Main jets, 50 Idle Jets, 185 Air Correction Jets, F2 Emulsion Tube. per factory manual. The popping may be caused by the dirty jets, especially if the butterflies are not closed all the way or the bushings are worn. Also, you set mixture at 1200 RPM, not idle. Check the linkage as well and make sure that the carbs are balanced at 3000 RPM (adjust linkage to balance).

Thanks for the response.

Are you sure those jet sizes are correct for a 2.0S? I thought mine were correct to the original workshop manual.


Cheers
Chris

Woops... sorry, flipping between windows on computer I somehow got the page shifted down. Just rechecked and should have been 130 main jet, 55 idle, 180 air correction, and F3 emulsion tube (I checked the page each time I flipped windows this time) so that is good on yours.

Give the carbs a good once-over and make sure everything is inordnung. Then go thru the carb tuning procedure in full. Plan on spending a lot of time sitting behind the engine, and make sure that you have the exhaust blowing away from you.

Here is a link to the tuning procedure:
http://forums.pelicanparts.com/porsche-911-technical-forum/67957-weber-rebuild.html

http://forums.pelicanparts.com/porsche-911-technical-forum/409376-new-webers-adj-help-needed.html

Yup, you are way too rich. 130s are pretty big depending on your altitude. I am running 135s in a 2.7 and my wideband shows 12.5 A/F in the midrange.

tuning Webers
 

Before you start tuning you should really check the float level with a float sight. It is impossible to tune the popping out of a set of Webers when the floats are not perfect. I am at high altitude so we set ours to the low side of normal. The float level dramatically affects the timing of the main circuit. Once you have the floats set then proceed to tuning and like others have mentioned the idle should be set at 1100-1200 when setting the idle mixture.

The idle jet holders should have o rings on them. While they are out blow the jets all out with compressed air for good measure.

One more thing, set the timing and dwell before you start trying to tune carbs.

I have tuned Webers in so many different ways at different times in my life and I have found the method described in the thread by T Bird (John) to be consistent with the best methods. If you follow that method your carbs will work as well as possible, given their condition. If you leave out a step or do it in some different order then it is basically a crap shoot that you will have wasted you time on. It is a ground up procedure if you will and when I tune my car I do the whole thing over from the start because you can't know what part of the setup is causing the problem. I rarely have a problem with the carbs going out of tune following this method (they are static metering devices and have limited ability to change properties through use). Once the carbs are set correctly you can know that future performance changes are caused by other systems. These can include popping caused by ignition issues(timing, dwell, fried points, bad wires, bad plugs), valve train issues (tight valves, bent valves, walking rocker shafts, cam timing) or vacuum or exhaust leaks.

I love carbs because they are so predictable once you really get the hang of working on them.

I would look more into the condition of the teflon bushes ESPECIALLY the one closest to the drop arms of the crossbar. 130 mains are fine. I have a 2 liter running 145 mains so do not change a thing. Excessive throttle shaft clearance pulls air thru the shaft leaning out that particular cylinder.


Also, block off the high rpm enrichment circuit. This was a bad idea from Weber and causes all sorts of problems. Poor fuel control is the main issue. The IDS did not use o-rings at the Idle jets so do not install any.

Chris68,
Welcome to the Pelican forum.
I don't see a venturi size in your specs.
From what I can find, you can go as large as 32mm with a "S".
I would be inclined to go with 30mm if you are near sea level.
Your 130 mains are fine based upon your over 3000 performance description.
O-rings or gaskets around the idle port caps must be in good shape.
It wouldn't hurt to use O-rings at the idle jet holders, even if they weren't supplied 40 years ago. Air leaks at any of these points just as at the butterfly shafts is not good.
My best analogy is like sucking through a soda straw with holes in the side.

There is no provision for an O-ring on the IDS. You would have to machine the carbs in order to accept the o-ring. Otherwise the o-ring squeezes around the head of the screw. Also if you run an o-ring on such a carb, it can prevent the idle jet from seating.

Aaron,
Are the IDS carbs and jet holders different from the IDA's? I am thinking from your post that they must be. Here is an IDA jet holder that will accept o rings. Thanks for the info.
Chad
http://forums.pelicanparts.com/uploa...1256492461.jpg

The jets and holders are identical, the casting is different and the machining is as well
Note the IDA has been machined to accept the o-ring. The IDA casting has more material around the jet boss to compensate for the design change. I would assume that early IDA's have no O-ring as well.
IDS on left.... IDA on right