weber tuning

[yeuporsch]

Ok, I followed all the threads on tuning webers on this board. The car is running very good, but not perfect. The problem is still some exhaust popping during idle. During the adjusting the carbs, I would get almost nonexistent popping. New cap and rotor, valve adjusted, accelerator pump adjusted ~.55 ml., timing and dwell adjusted to spec. However, after hard acceleration and letting off the throttle, the popping would come back. And coming to idle, it would pop again. There's hardly any misses or popping at >1500RPM and even to redline. So is this normal for weber carburators or do I still have more reading and adjusting to do? I was told that the wiring from the permatune to the coil over time can developed oxidation and build up resistance in the circuit;
therefore, it would pop during idle but not during acceleration. Any advice would be appreciated.

John

[TimT]

After you have followed all the directions and threads regarding tuning your webers, you now have a carb that is really closely set up. Now you begin to follow the zen path of weber tuning, seems like you have your webers really close, it will now take small adjustments of the air bleed, and mixture screw to make everything "right".

I tweek my carbs often, checking the sync, and playing with the adjustment screws (mostly the idle mixture). My engine barely burps or farts through the intakes now.

[jabb]

If you are running rich it can contribuite to exhaust popping also an exhaust leak. Check for exhaust leaks.

[yeuporsch]

Just how exactly does one check for exhaust leak? I am still a newbie.
John

[T Bird]

John
I presume that you mean that you followed the directions I posted months ago in the following:
Weber rebuild

So, you have the accelerator pumps issuing equal amounts, the float levels are set, and vacuum is balanced at idle and at 3000 RPM, but you are experiencing popping at idle. If you read my instructions carefully (steps 7 and 9), you will see that I advise to turn out the idle mixture screws 1/8th of a turn more if you experience popping, and test drive. Keep turning them out in small equal increments until the popping is eliminated. If you are out more than 3.5 full turns on the idle mixture screws, you probably need bigger idle jets. I presume that you are running .55 idle jets in your 2.4. If your jets are smaller than .55, you may need to bump them up, but only if you can't tame the popping at or under 3.5 full turns out on the idle mixture screws.

Tuning these carbs is no mystery. However, you must nail down each step in the process before moving on to the next. If you skip steps, or fail to sucessfully complete a step and then move on to the next operation in the process, you will surely end up with a mess. Once you have them in tune, they will stay in tune for quite some time and require no fiddling or constant adjustment, unless you have some other problem.

Also, carbs are adjusted LAST. That means that you tackle the adjustment process only if the valves are adjusted, and the points, cap, rotor, wires, plugs and air filter are new or nearly new.

Assuming the problem is in the carbs and not elsewhere, my guess is that you have not adjusted mixture evenly for all of the cylinders. This can be a daunting task and takes patience and a love for perfection. Keep doing it until you get it right.

Good luck.

[yelcab1]

I do not have a Porsche with webbers, but I do have a Ferrari with 4 webbers and tuning those is a chore.

In my experience, if you have all the carbs synched (check with a synchrometer to be sure0, and still have popping, then the problem is a LEAN mixture, hence turning the mixture screws 1/4 turn at a time will tend to leliminate the lean condition and the popping goes away.

You do have to compromise between running lean (low emission) and running richer (low popping). Life is not perfect.

Worked for me.

[Phoenix]

T-Bird:

I don't have Webers but rather PMOs which seem to be running fine (minor popping when cold and accelerating form idle) with 55 idle jets.

My only issue is that the fuel milage will not get any better than 13 mpg and is generally around 11 in mixed driving?

It has: new carbs, points, cap, rotor, wires, plugs and air filter and a nearly new 007 dist. Also the valves have been adjusted.

Overall the motor runs fine strong throughout the rpm range. It just gets VERY poor gas milage.

Any thoughts?

[911pcars]

yelcab1 is correct. An excessively lean mixture will cause popping. Inspect the throttle shafts/bores for excessive wear as this will allow air to bypass the throttle plates. The idle speed will also be inconsistent. If the bores are worn, they can be rebushed. Eurometrix and other companies can handle the rebuild.

Phoenix, in regards to your poor fuel mileage; let's assume a heavy foot isn't involved. The obvious place to look are the idle and main jet sizes. 55 idle jets sound okay. What size are the mains? What is the ignition timing set at? Have you checked the fuel level? Have you checked CO or air/fuel mixture? It sounds rich doesn't it?

Sherwood Lee
http://members.rennlist.org/911pcars
Early 911S Registry

[jabb]

A lean mixture will NOT cause popping out the Exhaust.
Exhaust popping is sign of a Rich mixture or exhaust leak.

You can check for an exhaust leak better when the car is cold. Jack it up and block it. get under it with a 3 foot piece of hose hold one end to your ear and run the other end by the exhaust flanges or other places that could produce exhaust leaks. Make sure the car is very secure and positioned before getting under it with the engine running.

[911pcars]

Jabb,
thanks for the correction. I forgot where his "Pop" was coming from.

Sherwood

[T Bird]

Jabb
I don't agree that a lean mix does not cause exhaust popping. Try turning an idle mixture screw in slowly. What you'll hear, after 3/4 of a turn or so, is a popping or banging sound in the exhaust, as the mixture leans out. You will also probably get some nasty popping or spitting up through the carb. It is true that exhaust popping may also be caused by other factors, as well.

Phoenix- I get about 17 mpg in my 2.4 T with S pistons. I drive pretty moderately. What is the displacement of your engine? Have any mods been done to it? Have you put it on an exhaust gas analyzer? What do the plugs look like? Are you running a hot enough plug? Do you drive like a maniac? Have you set the accelerator pump quantities correctly?

John- If everything else (valves, dwell, plugs, etc.) is properly setup, go back and redo each step in the Weber set up process. Make sure that you have nailed everything correctly before you touch those mixture screws. Redo the mixture as many times as it takes to get it right. By the way- make sure that you are running a hot plug-- not the spark plug recommended for an MFI car.

Good luck.

[Chuck Moreland]

I've had my webers for a few months now and I'm developing some comfort level. Tbirds thread was a great start for me.

I think part of the contradictory info above is ambiguity of terms like "popping".

I observe at least 5 types of bad behavior, my own made up terms:

- Spitting, puff comes back through the carb. Not a full backfire, just a little pfffft.

- Missing, cylinder fails to fire.

- Popping, small popping sound comes from the exhaust.

- Booming, a deep sounding explosion inside the muffler.

- Backfire, explosive reversion through the carb. I have never seen this on my webers, but have on other carbs.


Those with more familiarity please correct me, but here is my take:

Spitting - Rich? I still can't figure this one out.

Missing - Likely ignition problem, could be any number of things.

Popping - Imbalance between throats. May be due to wear.

Booming - Lean idle adjustment.

Backfire - Likely ignition timing problem.

[john70t]

I am getting a lot a lot of loud back fires through the carburators now. I think it is the result of the mixture burning for way too long, even after the intake valve starts opening. There could be several reasons for this.
I found bad plug wire ends after moving them took away the tapping(arcing) felt. Tweeking them took some of the backfire on accelleration away.

[DavidPorter]

Back in my Norton days we always thought spitting back through the carbs was a sign of lean misfire, the evidence being that they did it almost exclusively when cold. Not an explosive backfire, more of a sharp quick "pfft" with a little puff of smoke, usually making the slides rattle in the carb bores (since the slides and bores wore quickly).

The Amals I had didn't have any enrichening devices other than the dreaded Tickler, which simply overrode the float valve and dumped raw gas into the engine.

And then there were the SUs . . . .

[clevy70911T]

I've never understood the exhaust leak popping theory and am very skeptical. I'm no mechanic or even close, mind you. These opinions are just the result of hours of thought and experimentation.

As I understand the theory, the unburnt gases would mix with oxygen entering the exhaust through leaks and pop or backfire in the exhaust.
I would think that if there was an exhaust leak and the exhaust is trying to get out of the exhaust system through the tailpipe, it would have some positive pressure in the system. As with all gases, they follow the path of least ressistance and that would be OUT the leak rather than IN as some would suggest. To have air enter the tiny exhaust leak there would have to be some negative pressure/suction and I'm not sure there is ever a situation like that in the exhaust. Possibly in a fancy muffler? If there is, correct me.

I think lean causes carb popping and rich causes exhaust backfires/ popping as the unburnt fuel from a rich mixture enters the hot exhaust system regardless of leaks or not.

My car occasionally backfired, popped on decelerations and gurgled during steady driving, but smooth on accelerations. I leaned the mixtures slowly until backfiring and gurgles are gone.
When I leaned it too much, it popped up through the carbs. But as always, what worked for me may not work for you.

My final point being, I've had many exhaust leaks through the years on other cars and they never backfired as a symptom.

Hope this helps someone. Goodluck!

[Wayne 962]

See the project in "101 Projects" for Weber Tuning...

-Wayne

[john70t]

Clevy, air is sucked into, rather than out of, the exhaust (at least near the engine) because of the venturi effect: there is a series of fast moving pulses that are all going in the same direction.

Unless there is some kind of backpressure, the column keeps moving with the negative pressures between the pulses pulling the chain along.

An exhaust leak (or a non-firing plug) before the O2 sensor will make the whole system run rich because the computer"sees" high oxygen content and tries to compensate.

[Chuck Moreland]

Clev, to expand on John's comments have a look at this:

http://www.elephantracing.com/techtopic/headers.htm

[RoninLB]

not that I know what I'm writing about..but nobody mentioned worn throttle shafts..which would cause an "overly lean" adjustment on the idle circut.. thus carb popping.......Ron