When 911 is backfiring?

[andreas panayio]

I am sure this issue has been discussed before, but I would really like someone to give me the possible reasons why my would back fire.
With my SC it happens usually shen its cold and I try to step on the pedal a bit fast.

[livi]

Good to see another European!

Among other things, backfire may be caused by a too lean fuel mixture or exhaust leaks.

Better informed will hopefully respond. Do a search in the mean time. Plenty of previous threads on the subject.

[andreas panayio]

thanx livi
i was in sweden in december.
malmo, lindvalen and goteborg.
didn't see a lot of 911s there ....

[livi]

No you wouldn´t. Particularly not in December. Most owners have another daily driver for the winter months. The salt is killing vintage cars.

[JBO]

I think Markus is right and you need to richen the mixture. This is common with CIS in the winter, and you can lean it back slightly when the weather gets warmer.

[81 911 SC]

If you have not already done so I recommend installing a pop-off valve in the air box. This will prevent it from blowing up when the engine backfires.

[Superman]

Originally posted by livi


Among other things, backfire may be caused by a too lean fuel mixture or exhaust leaks.

This is true. Backfiring up through the intake suggests and lean mixxture. So does subtle surging while cruising. If the mixture cannot be tamed, you may have already cracked your airbox. That would create an intake leak. This should be addressed.

Gurgling and popping when you let off the gas while cruising is usually caused by an exhaust leak. Not a big problem.

[Gunter]

Andreas:
Check the airbox for a pop-off valve, install one if needed.
When the engine is cold, the mixture is all over the place (Rich) and backfire is possible.
For a tune-up, the valves are set first, then the ignition timing, then the mixture.
Assuming that all components like WUR, AAR and Decel. Valve are good, mixture is set when the engine is hot.
Your mixture may be too rich and the Decel. Valve may not be set right.

[holtjv]

Andreas--see if your car will cold start with no throttle whatsoever. If it dies, crank it again and it will most likely hold the idle from that point.

I know that each time I even lightly added throttle during cold start it would backfire and sometimes pop the popoff valve.

Enrichen your mixture if you can't get going with this tip. Jack

[snbush67]

Air, fuel vacuum leak, timing, wur etc. I think you should do a search for trouble shoot a CIS. The most severe problem would probably be a cracked air-box as mentioned. If you want to search for leaks there is a mist aerosol spay that is used to determine air leaks. It could even be the rubber boots that are leaking.

You say it happens when you step on the gas really fast. So the throttle linkage may be possibly be pulling the airbox slightly enough to split or expose a crack that is not normally exposed.

Just one thing you should check for.

Good luck

[jtkkz]

What causes carbs to backfire , mainly when idling at a stop light ?

[artisticent]

I have a similar problem. I took my 76 2.7 to the porsche guy to have the fuel mixture adjusted and he said there are too many vacuum leaks that need to be fixed before it can be adjusted. I have a pop off and no cracks so I guess I need to find where else I have leaks. Oh JOY
Nathan

[Gunter]

Vacuum leaks can be a real problem with CIS because you won't be able to set the right mixture.
Fortunately, there is an easy way to find the leak.
Best is an acetylene torch with a small nozzle.
At idle, point the small nozzle at the usual suspects:
Intake runner gaskets and boots. The runners are mounted on the heads and tightening the 2 screws will help. Or use new gaskets.
Same for the rubber boots.
Check around the airbox as well and all vacuum hoses for the brake and breather etc.
You found the leak when the idle suddenly sounds smooth; Eureka!
When you remove the oil cap and the RPM doesn't drop by ~50 RPM, you have a leak in the hoses going to the oil filler stud.
Put clamps on each end of the 2 hoses.
Propane will work but the spray is too general; acetylene torch with a small nozzle is best.
I don't recommend carb cleaner because of the fire hazard.
Go for it, find that leak.

[artisticent]

Wow that was awsome Gunter, Thankyou. I will print that and start right away. Where is the best place to look for a vacuum diagram? I know for sure there are a couple hoses disconected and just blocked off with a plug?
Thanks
Nathan

[Gunter]

I believe PP has the vacuum diagrams available here for various years, do a search.