Backfiring SC on De- Acceleration

[geoff_k]

Hi,

I have noticed my ROW 83 SC backfires if I lift my foot off the accelerator (unfortunately there are times when you have to slow down on your Porsche) It is more noticeable when I change down through the gears.

At idle the engine sounds a bit lumpy for want of a better phrase

Where should I start?

thanks in advance

geoff

[Flat6pac]

I would check my timing first to spec but realizing backfiring is a generally lean condition. I would disconnect the ox sensor by #3 runner on the engine compartment and enrichen the mixture until the engine idle started seeking an idle and then back it off til the idle goes smooth. This completed, if you idle is too high turn it down at the throttle body.
Plug in the OX sensor. If youre changing the enrichment, know where you started from with the allen.
Bruce

[wolds]

I don't believe there is an OX sensor on an 83RoW. I know mine doesn't have one. geoff_k, you might want to post the particulars of your car. Any modifications? Did it start doing this after you or someone else worked on the car? Warren

[boyt911sc]

Geoff,

Check the following not necessarily in this order:

Ignition timing and valve setting
Fuel pressures (control and system, both cold and warm)
Unmetered air
deceleration valve opening pressure
exhaust leak (if any)

Check with Warren because he has one of the best running SC's with an incredible mileage better than my Subaru Outback Sport '06. We drove from Columbus, Ohio to Hershey without refilling his gas tank!!!!! Keep us posted.

Tony

[Flojo]

Dripping fuel valves max be the cause.

When sitting home and after some cooling off, does it smell of gas (best niticeable in a closed garage)?

[Stephen-911SC]

Is some degree of popping on decel acceptable or normal?

[Reiver]

Originally Posted by Stephen-911SC

Is some degree of popping on decel acceptable or normal?

With carbs yes with CIS not in my experience.

[snbush67]

Is the noise (backfire) from the exhaust or the intake?

Start simple,

A possibility is that one or more of your cylinders isnt getting spark check your plug wires to make sure they are connected correctly and in the correct order.

Check the distributor rotor to make sure it is not burnt or black.

As Tony mentioned above the unmetered air is a likely culprit, check for cracks in the air box or leaks. Check for any loose or cracked vacuum hoses.

[Stephen-911SC]

Quote de Stephen-911SC

Is some degree of popping on decel acceptable or normal?

With carbs yes with CIS not in my experience.

Well, it looks like something else just got added to my 911 to-do list...

[geoff_k]

thankyou for your replies. I will be trying to check it this weekend.
The car is standard as far as I know.

[stlrj]

Make sure this vacuum hose is connected. It's purpose is to limit the amount of manifold vacuum when decelerating to avoid backfiring and, at the same time, reduce emissions. Too much vacuum means very little oxygen to support combustion resulting in unburnt fuel leaving the combustion chamber igniting in your exhaust...Pow!

[Scooter]

Check for an exhaust leak at the muffler junction.

[Tidybuoy]

My '74 has the same problem and I'm hoping to cure it this weekend. I believe that I have an airbox leak because the idle is surging and I have previously replaced all vacuum lines, injector seals, manifold gaskets, as well as got my fuel pressures perfect. Even with my fuel pressures corrected, the car will not run properly with the mixture set to spec - this makes me suspect "unmetered air" is getting in.

My theory (and I'm no expert), is that I've had to richen the mixture to compensate for a vacuum leak but this only works for part of the driving range as the vacuum leak changes as the throttle is open or closed. The surging idle makes me think vacuum leak and it seams to be getting worse. The backfiring on decal is extremely annoying and makes the car sound like sh#$. I purchased a spare airbox and I'm going to use the good base on the spare and mate with the good top on my orig. I'm going to seal with silicone as well as replace the throttle body gasket and re-epoxy the pop-off. I'm also going to find a new o-ring for the pop off as I've been told that that can be a source of air leak.

If I am able to cure my air leak and backfiring, I'll let you know.

[Bob Kontak]

Originally Posted by Tidybuoy

My '74 has the same problem and I'm hoping to cure it this weekend. I believe that I have an airbox leak because the idle is surging

Ahhhhh, I remember fondly when lorenfb beat me about the head and shoulders when I said surging idle was the result of false air. Can you lean it out a touch and see if it does better? He was adamant about it being a rich situation.

You mention it's to spec, though.

[Tidybuoy]

Originally Posted by Bob Kontak

You mention it's to spec, though.

Actually, I can't really get it to spec. All of the pressures are correct but when I try and adjust the air/fuel ratio, the car runs bad as I get closer to the 14 mark. It will run at idle but then I get it on the road and it starts jerking. My only cure has been to richen the mixture. As a result, the car runs perfect at certain rpm's and not-so-good the rest of the time. And, I have an annoying backfire on decel.

Regardless, I'm doing some air box sealing work this weekend.