popping back through the carbs?

[owbow]

hi all,

I'm a self employed mechanic based in Ibiza, Spain and have a 1970 911L here, the customer had overfilled with oil. I've sorted that problem, changed the plugs and she's now running nicely, idles good, drives ok.

when revved, stationary, to about 2 / 3k she pops and backfires thru the carbs (webers) occasionally flaming. I'm wondering if this is likely to be valve clearances set too tight? or condensor? points look good, not sure what the gap should be set at, so haven't touched anything much yet.

it seems to be happening on all cylinders

I'm trying to save the owner some money by saving myself time searching down avenues that i don't need to.... any ideas?

many thanks
Owain Morgans

[owbow]

oh yeah... the rear window folds away on this targa... is that rare? I see many with a glass rear screen but none like this on-line? or am I just being dense?

Owain

[JFairman]

It's possible the timing is retarded. That can cause backfiring on all cylinders, try advancing the distributor it to see if it stops.

The float levels in the carbs could be just a little too high.
That could cause fuel spilling over into the throats and make it cough or backfire through the carbs, but that is less likely on all 6 cylinders.

Look down inside the carb throats and make sure the carbs havn't caught fire in the past and melted the small auxilliary or primary venturi in the middle where the fuel comes out.
If someone ever sparayed a fire extinguisher into the carbs to kill a fire it can go down the throats and eat away or pitt the aluminum around the brass throttle butterflies.

[Flieger]

Originally Posted by owbow

oh yeah... the rear window folds away on this targa... is that rare? I see many with a glass rear screen but none like this on-line? or am I just being dense?

Owain

That is an early Targa- a "Soft Window Targa". They are generally more desireable and are rarer than the normal glass Targa. Porsche found that most people did not fold the window down so they could make a more weather proof and clearer window with glass and still get 90% of the open car feeling.

[BReyes]

Flames? Are you kidding?

Carbs may need rebuild. Can determine extent of plugged/corroded carb components, cleaning everything and using initial settings then reset mixtures.

Regards,

[BK911]

Mine popped and shot flames when the carbs weren't synchronized. I don't understand why, but they did. Once I balanced all the stacks the problem went away. Just something else to check. Have fun!

[FPB111]

It sounds like the carbs have been set as lean as possible at idle and are too lean at transition from idle circuit to main at around 2600 to 3000 rpm. This is a known "problem" with webers on 911 engines. If the fuel pump and pressure control are able to supply enough fuel at about 3-5psi then you may find that making the idle circuit a little richer will smooth out the transition.