Help with Webers.. Spitting through the stacks

RetroSC · 09-09-2005, 08:03 PM

I've got three different sets of Webers and so far, all spit through the stacks. It's a 67S motor with 32mm venturies; 55idles; 125mains; F3 emulsions. I've gone through all the tuning on each set, set the floats, etc..

They idle great and run great at full throttle, or under any acceleration for that matter. It's just when I'm cruising along at say 2500-3000 rpms with little to no load. They'll spit and pop randomly. Sometimes it's pretty loud and causes the car to even buck a bit.

As an experiment I've tried to run the idle circuit's at as little as 1.5 turns out and all the way to 3.5 turns out. At 1.5 it is worse and spits up pretty good when just revving it up. At 3.5 I start getting subtle pop's through the exhaust (rich?) and it just doesn't run too well, but it still contines to spit while cruising. Anyway, when tuned properly, they're all somewher in the 2-2.75 range and it runs pretty good, except for the above problem. Thoughts??

RoninLB · 09-09-2005, 08:16 PM

only a guess and assuming the carbs are ok.

the cross linkage allows some play to orient one carb to another on my car. If the carbs are not in sync they will pop as one carb is feeding more than the other. An air meter on each is used to align the carbs to each other at a high idle, then at around 3.5k rpm etc. Two meters is much much better than one to do this imo.

Joeaksa · 09-09-2005, 08:46 PM

Spitting through the stacks usually means that its lean. Try richening the idle circuits a bit or check to make sure that the idle jets are totally clean.

It goes to a different circuit from idle to main as engine speed increases and the idle still works with the main circuit even up to the speeds you are talking about.

The reason it does not pop at full throttle is that the acc pumps are shooting raw fuel into the stacks and its mixture is ok.

JoeA

RetroSC · 09-10-2005, 11:16 AM

An update to this carb tuning saga.

I've been messing with these things for a month now.. tuning, syncing, you name it! It occured to me that throughout all this, I've never heard this car ping, even slightly under load or anything. Now I've gone through the timing several times and have settled on 5 at BTDC, and 32 ish at 6K. Anyway, I bumped it up about 2 degrees at idle and that solved it.

Strangely enough, about an hour later an old guy... well old to me (I'm only 29) cruises by on his bike and stops by to chat. I guess he had old alpha's and a 356 that he raced against Bob Hindson back in the day. I tell him about the carbs spitting, and before I get to what solved it, he goes, "yeah, my Healy 100-6 used to do that all the time.. distributor would work it's way loose and retard itself, always new it when those carbs would start spitting."

I said where were you about a month ago!!

73911guy · 09-10-2005, 01:04 PM

Have you had your distributor curved for the Webers? I sent mine to Barry at IEC in Detroit per Bruce A's book and I was amazed at how well it worked afterwards. I spent a lot of time and a lot of money before I did, and I regret I didn't do it sooner. Easily the best money I've spent improving performance.

Jim
'73 911

RoninLB · 09-10-2005, 06:12 PM

I use a 2.7 RS dizzy. Pelican has them brand new for around $300. It's not in the catalog, you have to ask for it with the pn. It comes with a non rev limiting rotor and cap. I don't use the vac canister and run 10deg initial. PMO used to recommend them yrs ago. There may be better curves specific to each engine but this curve is real nice for my act. Combustion is stable and no drop in EGT's at any rev range.

and Jim.. if you have specs on your curve I'm interested on what's happening. So far the only thing I could figure that's better than a good dizzy curve is an electronically adjustable curve. thx

Raynald · 09-10-2005, 06:53 PM

Hello Ronin !
is this possible the throttle shaft is too loose (used) ?
and can cause of popping in the stacks,

Ray

RoninLB · 09-10-2005, 07:26 PM

Quote:

Originally posted by Raynald

is this possible the throttle shaft is too loose (used) ?
and can cause of popping in the stacks,

Hi Ray.. nice to see you around here.

If the throttle shafts are too loose the air leaking in will disturb the venturi air from creating a lower than atmospheric environment. The normal venturi lowering of air psi allows the fuel droplets to expand similar to allowing a balloon to rise in the atmosphere. The greater the droplet expansion the easier it is to atomize/make smaller the droplets. The better your atomization the more energy that can be obtained from the fuel for combustion. That greater combustion means more power pushing the pistons. A very good but imperfect measurment of this combustion is EGT as related to carb design.

Loose throttle shafts disturb atomization. Greater atomization is what shought sought when carb tuning.

"can cause of popping in the stacks,"
------- it will/ may cause a lean fuel mixture because less air is available to suck a proper mixture thru the venturi, especially under engine load. It will disturb atomization of any extra fuel from the accelerator pump.

The cross shaft connection between my 2 carbs is adjustable on the left carb where it attaches. I use one air flow meters on each carb to equal adjust this at low, med and high rpms. If this is not correct my carbs will pop. This is detonation as one carb is working very hard and the other carb is working much less to maintain the acceleration load on the pistons.

2.7RACER · 09-10-2005, 09:48 PM

Try a set of 130 mains. PMO has em. You might even find some locally. Better than running lean.

RoninLB · 09-10-2005, 10:14 PM

Quote:

Originally posted by 2.7RACER

Better than running lean.

that's definitely the usual safest imo.

so, because this whole thread group is such a class act I'll keep hard info coming.

my 2.7 w/ 40, 34 vent, 60 idles, and 135 mains operates 25-50degF cooler EGT at 4-4.5k+ish than at 3.2-3.6k in 5th gear. That means that my 135 mains are relatively rich. I say rich because I know what EGT is best for my combustion and 135's is rich.. not much where it quickly becomes a spark plug problem at all but I wouldn't want it any cooler as far as power is concerned.. meaning richer. This is not dyno #'s, this is the real deal road #'s precisily measured and analyzed thru a computer.

hopefully this is a baseline

Raynald · 09-11-2005, 03:39 AM

Hello Ronin
Thanks for your information! my engine is 2.7 RS piston and Solex cams,
34 venturies 60 .idles, 135 main, msd... my mixture scews is open at 4 and 4.25 turns out(too much) and my carb popping some time at idles to acceleration,they have loose shaft... I try different jets size and the same problem ,
my distributor is not recurned it is to cis specs.
what do you thing about it ?

Ray

RoninLB · 09-11-2005, 10:45 AM

Quote:

Originally posted by Raynald

carb popping some time at idles to acceleration, they have loose shaft...

Hello Ray
Loose shaft causes a vacuum leak. A vacuum leak will cause popping. You must repair a vacuum leak.

The carbs can not be tuned with a vacuum leak. A vacuum leak will confuse all carb and ignition adjustments.

A vacuum leak causes misfire. A misfire during acceleration causes very high EGT.