"Popping" Webbers

[Alan.UK]

Hi all,

I had my 69 911engine tuned a few months ago on a rolling road. The engine was set up perfectly (it's a standard 69 110bhp engine) and run very well. Recently the webbers have started "popping" back through the carbs. Since the tuning nothing has changed. The only thing which has changed is the weather which has recently become a lot colder here in the UK. It seems to do this at no specific time or cylinder, i.e. does it at idle and also when revving, does it when cold and also when hot. And does it both carb banks.

I removed the air filter and (using a mirror) I can see what seems to be the fuel igniting in the barrel of the carb. I thought I’d get a bit of advice before tampering with the set up..

Does it sound a bit lean.....Any ideas !!

Thanks in advance
Alan
67 – with 69 911 engine

[Joe Bob]

I have the same motor in my 914/6....it does the same thing. You need to richen up the mixture or live with it...my plugs look good, so I just see it as a Porsche Quirk....

[Keith S]

As winter draws on in the UK, the air becomes more dense and damp = more oxygen. This means mixture gets slightly leaned out, so a readjustment of idle mixture screws is in order to stop popping at idle. It is similar to the reason why racers have to rejet their cars according to whether they are racing at sea level or in the mountains.

If the jetting was sorted out on a rolling road in summer, the chances are that the idle jets may be a tad too small - most rolling road operators set the jetting up as lean as they can get away with as most customers want optimum performance with minimum fuel wastage.

When you rev the engien at rest, the chances are you are only running on the idle jets. Webers (and Dell'Ortos also) draw on the idle jets until about a third opening. Out on the road, with the car under load, you use wider throttle openings and the chances are that you won't experience the popping you describe except when you throttle back and cruise on part throttle.

If you can't dial out the popping with the idle screws, try going up half a size on the idle jets.

Hope this helps!

[Superman]

Yes, this is evidence (or proof) of a lean mixture. DO NOT locate your face above the carburetor venturis while the engine is running. It is very dangerous. Guess how I know this.

[Doug Zielke]

That's a tidy lookin' engine room ya got there Alan.UK

[rstoll]

Hey Super, how long does it take for eyebrows to grow back??

[Joeaksa]

Alan,

The above is correct about its being a lean mixture but try looking at the idle jets. My 72 had the same problem and I found it to be rust in the tank/fuel lines. Clogged up every few months until I put a couple of extra filters inline. Never did find the problem as the car was stolen.

Blow the jets out with air (there are six of them) and then try the car. The 911 motor runs on the idle jets up to 2500 rpm or so when the mains take over, but the acc pump will get it off line OK. Then after its still running on the idle jets (but off of the acc pump) uthey tend to pop a bit when lean.

JA

[Early_S_Man]

Alan,

By all means, check for any rust or particulate matter in your carb bowls, as you never know what the last fuel truck brought to your local gas station!

But, you also may want to consider bumping up your idle jets from 55 to 60, or 60 to 65 ... because your engine runs on the idle jets ALL the way to the 3500 to 4000 rpm range, and that makes them Quite Important!!! So, if your engine is sensitive to minor changes in humidity, then perhaps it is on the lean side.

[Alan.UK]

Hi all
Thanks for all your help and advice (Keith, JA, Warren and others)
I'm going to richen up the mixture a little on each carb later tonight. Like Keith says it's probably due to the cold. If anyone is interested in the results, send a private mail and I'll keep you updated.

I will also look into larger idle jets, but when the car was first tuned, boy did it run well. Sounded great and pulled fantastically especially considering it was the lowest BHP of the 911’s. I've also added a few extra pics when it was at the rolling road.

Thanks again
Alan

[joehahn]

I like that fuel set up. Is that a Facet fuel pump? What is it mounted to? Are those standard fuel lines available anywhere (pretty colors!)

[Alan.UK]

Hi Joe,

I was going to mail you later. The pump is a Facet (spelling) electric pump, which pushes out 3.5 psi. I have attached to the rear of the engine bay using two rubber fittings. Being rubber it stops all the noise and vibration. They are really cheap and for sale in most local auto stores. They were designed to attach the exhaust to the car. The fuel pipe is standard 8mm unleaded stuff I bought from the same local auto store.

HTH
Alan

[Joeaksa]

Alan,

Boy you bring back memories with the last picture! Jeez... the hours I spent on my knees massaging the webers in my car! I wish that there would have been an internet and Pelican site to help at this time.

Remember that the car may have run fine six months before but that was not winter. If you could bump up your idle jets a notch or so it might be a lot more driveable in winter. Also, the webers have no choke so starting is more difficult and any "help with the idle jets" will make the motor start easier.

When you are checking your idle jets for dirt, they are so small that it takes very little to clog them. I used to take them off and blow them out with compressed air even when they looked OK, and it usually helped with the idle and running up to when the main jets take over.

When you take the tops of the carbs off, if you find any sediment in the bowls then I would start to put extra filters inline.

Let us know how it works!

JA

[not_sure]

Alan, I see you have your pump in the rear of the car, do you also have one by the tank to get the fuel to the rear? I also see a small black line coming off the T fitting, is that for a guage?

Thanks
Tom
71T 2.4

[Alan.UK]

Hi Tom,

I just have the one pump which works fine. The T piece you see is connected to a tempery gauge which I used to measure the fuel pressure whist on the rolling road.

HTH

Alan

[not_sure]

My car had a return line to the tank, I guess you do not need that. I have it blocked off now but was not sure what to do with it, now I know. Thanks.

[john70t]

Get some kneepads.