How do I decrease the idle in Weber 40 IDA 3Cs?

bfinley · 11-12-2009, 06:05 PM

Um, I seem to be forgetting something. I've just moved so my manuals are stored, and I've just finished a bunch of work on my car (1971 911T) including installing oil-fed chain tensioners and set carbs and been trying to make the car keep running not matter how much it rains. But the other day I got it to the best it's ever ran for me (only had it a while and mostly up on blocks doing chain tensioners), despite constant rain on the dizzy! and now somehow I have an idle around 1200 to 1400 rpm...

It has Weber 40 IDA 3C carbs, and I've done the whole setup, but the stress of moving and packing away the books has a hole in my memory... How do I reduce the idle speed? The linkage is proper, the car has ran at 900 nice and steady the whole time I've had it, I set the float level, I adjusted the vacuum, I adjust for popping and backed out screws... and the idle lever screws are not pushing on the idle arms or plates or whatever they are called.

But what the heck? Now (after the rain is gone maybe?) the car idles at 1200 or so.

Like it was always running on 5 cylinders perhaps? Like a wet wire finally dried out?

Oh, and NO popping spitting or backfiring now, no hesitation or stumbling anymore (and I thought it was just my tuning), but after taking the best two round trips of it's life with me (just 50 miles each or so). Boom, I've got high idle. And there is nothing that I can remember to turn down.

The black knob/lever between the seats is not pulled on by mistake, the idle screws are not pushing the plates which would push the linkage, I can't think of a thing that is causing this.

And now I can't remember, okay, so if everything was fine and I wanted to reduce the idle speed anyway... like say to 600 for some reason... why can't I remember what would do that??????

JerryL · 11-12-2009, 06:33 PM

Briefly with your description(s) So adjusting vacuum sounds like you have all 6pulling identical vacuum, GREAT!!
You also say screws not touching the arm / stop, Hmm?
Main linkage cross bar with the 2 arms (1 to each carb) unsnap the short arms going to each carb lever arm, this should release the linkage to seat on the idle adjustment screw on each carb set, (10 : 1 your idle will drop off).
With the 2 arms disconncted from the carbs get the idle RPM right balancing btw the 2 carb sets, you will see one or the other adjustment screw touching, slowly get them both at about the same position a bit at a time keeping both drawing same air till you get the idle you like.
Then adjust the length of the 2 lever arms from main lnkage to each carb to where you can snap them on without changing the idle rpm and you are done.

You may well want to lube the linkage and the carb shaft areas as well to prevent any binding.

Regards

john walker's workshop · 06:39 PM

the levers on the butt end of each carb have an idle speed screw as you know. so you backed each one off and now they don't touch the stop block and the idle is still too high? they should still touch the stop. in fact they should be a turn or so in order to get some air flow so you can adjust the idle mixture screws. advancing the timing will up the idle. is it too far advanced? 1200 is not the end of the world though. some engines just like it up there a bit. they may not like 1000.

turbochad · 08:02 PM

Set your dwell (if you have points) and your ignition timing first. You set the floats so that is good. I assume your fuel pressure is set at 3.5psi. You next need to return all the air bypass screws (the ones next to the idle mixture screws) back in all the way and re-synchronize the cylinders (with an air meter) with the linkage disconnected from the carbs. This will eliminate the possibility that your problem isn't that you are bypassing too much air (around the butterflies) causing the high idle. If you find that you have to use a lot of air bypass on a single cylinder this is an indication that something could be off wrong with the shaft adjustment but that is another issue. If you still cannot get the idle adjustment screw to engage with the throttle arm then you have a problem with the carb and it may need to be rebuilt. Leaking shafts could cause an air leak or the butterflies could be not aligned allowing air into some cylinders and not others. This is what would cause you to need a lot of bypass air on some cylinders and not others.

Carbs don't generally go out of tune it is usually the other way round, the motor goes out of tune or the carbs were set up on a poorly tuned motor and then when you fix the motor problem the carbs need to be re-tuned to now match the properly tuned motor.

If I had to guess I would venture that you have too much bypass air causing the high idle.

bfinley · 11-12-2009, 08:15 PM

Hi, and thanks for the answers. Something is definitely slipping my mind here about how to lower the idle, but I thought I'd say I've tried all of those things.

They are pretty much part of the rebuild and set up process.

The idle has always been rock steady, and a good 900 rpm. It has been the best part of the whole carb setup since I got the car! Other parts (accell/decell were popping spitting backfire hesitation when I first got it).

But, despite having those problems until the other day, the idle was not one of those problems. I had some room left on the idle stop screws (is this right?) that contact the plates at the very back end of the whole throttle linkage, but now there is free space and the plate won't touch the screw.

I have had the longest part of the linkage off (the part that goes forward to the firewall nearly) and have it properly lengthened/sized. I have popped off the two short linkage arms at the rear of the carb set up, and they are also not holding the idle up (when doing the carb setup you take these off to balance the carbs and get the car at 900).

But now the said screws are not touching, even with all THREE pieces of the linkage off an laying there (they pop off with an 8 mm wrench), the car runs at 1200 or so.

And the only way that I know to lower the idle rpm is to back out those rearmost screws that normally touch the plates. But they don't now touch the plates.

So I just think I'm forgetting something... like the throttle plates (or butterflies?) were never correct in step one? The accell pumps are pumping too much? They weren't?

I'm sure this is just the onset of memory loss. What am I totally overlooking?

turbochad · 11-12-2009, 08:22 PM

Are all the Air bypass screws turned in all the way on each cylinder? Air is getting into the cylinders some how when it shouldn't.

bfinley · 11-12-2009, 08:36 PM

But I think some things you have all said are right: I believe the air bypass screws are not happy, possibly the butterflies are not aligned for some reason! And/or there are teflon sleeves on the shaft that could have gone wrong.

Because yes, I think some of the air bypass screws took more turning than others to balance the 6 throats.

Thanks very much for the answers! Tomorrow or Saturday I'll go in there again and check all. It almost has to be air bypass problems/butterfly alignment problems, that sort of thing.

turbochad · 11-12-2009, 09:11 PM

I bought a 911 with a related issue. I had a butterfly alignment issue on a set of Zeniths that caused difficulty in getting a consistent idle. I took them off loosened the shaft connecting couplers and aligned the all three butterflies with the first idle circuit hole in the throttle and retightened the coupler. That solved the whole problem and the carbs no longer needed any air by pass at all. The synchronization was now only between the two banks. The carbs worked better throughout the idle circuit after that.

Webers only have one coupler per carb so only one butterfly can be out of sync with the other two. Still, this could be part of the problem.

Good luck.
Chad

T77911S · 11-13-2009, 03:45 AM

JERRYL and JW have given you good advice. start with the basics. when setting up carbs, disconnect the linkage to both carbs to make sure they are effecting the idle.
either you have the air bypass screws set too high or the linkage is effecting it. if you have a vacuum retard on the dizzy, check to see if that has come off or check for a lose dizzy.

i always start (like after a rebuild) with the air bypass screws all the way in. this is of course after timing has been set. dwell is not a big deal with these ignition systems, dont nit pick over it. use a feeler gage and your done.

once i have it so the car runs, you may want to leave the linkage connected until it is running, i check the vacuum on all 6. i note which ones have the highest on each side. then i adjust the 2 on one side up to the high on that side. then i adjust the 2 on the other side up to the high on that side. at this point, the 2 sides will be different. then i balance the idles. then i go back and repeat the process again. then i will go through them again and make minor adjustments until they are all the same.

after you are done with the carbs, connect the linkage. i had a way to adjust mine so it simulated my foot holding the RPM's up to around 12-1500. you will need to keep it low so the sync does not peg. now you need to adjust the linkage to the carbs so that both sides have the same vacuum. if it does not return to the idle you had before connecting the linkage, you have the linkage too tight. losen it up until it idles and repeat, you may have to adjust one side down to the other.

replace plugs, they are likey fouled after all the adjusting.

1QuickS · 06:31 PM

You might have too much air passing by your butterflies due to wear and can't adjust them closed. Check out my web site www.PerformanceOriented.com for info regarding wear items on the Webers on the Technical page. Butterflies edge wear due to uncontrolled axial throttle shaft movement (check out the picture showing this.)

snbush67 · snbush67's Garage

Quote:

Originally Posted by 1QuickS

You might have too much air passing by your butterflies due to wear and can't adjust them closed. Check out my web site (Performance Oriented: repair, restoration and modification of Weber carburetors) for info regarding wear items on the Webers on the Technical page. Butterflies edge wear due to uncontrolled axial throttle shaft movement (check out the picture showing this.)

Wow! That site is a keeper, thanks for posting this Great Resource.

T77911S · 12-01-2009, 04:48 AM

GREAT SITE!

do you have any more sites that go into detail of the operation of weber carbs?