Backfire through intake
 

I am in need of some help with a backfiring issue my engine is having. Here are the details.

* 1975 2.7 long block with a 1977 CIS system and engine wiring harness on it. Engine is in a 1977 body.
* Long block has never been in car. The CIS system and engine wiring harness have, on a working engine.
* Long block was not built by me, so I do not know its build history.
* Cam timing has been checked and is OK.
* Compression on all cylinders ranges between 150 - 160.
* Ignition has been static timed at Z1.

The problem - The engine will start, run for about 3 -5 seconds, then backfire through the air box and quit.

I have tried - Checking cam timing, doing a compression test, new points, two different distributors, adjusting fuel mixture, banging my head against the wall, sitting quietly and meditating. I can almost roast a marshmallow over the flames coming out the toilet seat in the air box.

Any ideas?

Firing order?
Cold start valve original?

Too much fuel on cold start. Have you put a Bosch CIS fuel pressure tester on it and confirmed all control pressures are correct? System, Cold and Warm pressures (warm with the WUR heater, guess you can't get running...)

CIS Primer for the Porsche 911

Please don't tell me you adjusted the mixture without doing this first. The mixture is normally adjusted fully warm and with a CO meter when system pressures are in spec.

When I rebuilt mine, the timing needed to be advanced 5 deg BTDC to get it to run.
Kept doing the same as you describe until I did that.

Got it running today! After checking fuel control pressures were ok, we adjusted the 3 mm allen screw in the air box 2 1/4 enriching turns and the back firing went away. The CIS system came off a working but tired 2.7 with compression all around of less than 135. The long block it is on now has compression of between 150 and 160. Could this increase in compression have anything to do with the need to enrich?

Anyway, it is running and will be fine tuned tomorrow. Thanks for the suggestions.

I am not too familiar with this engine. Having said that I did have this very problem on my 993. 2 - 3 seconds after ignition. BOOM! and the engine died. The force was great enough to blow open the plastic intake manifold where it clamps on to the throtttle body. I had this going on for a period of weeks and it was driving me bananas.

In my case the culprit was the flywheel/RPM sensor being too far away from the flywheel resulting in the timing being so screwed up that fuel was in the chamber at the wrong time. I had the clutch off for repair and when putting things back together I did not pay enough attention to this sensor.

Not sure if this will help you - just posting as maybe it might help. Another common cause of intake backfire is a leaking fuel issue. When the car is turned off the fuel drips into the chamber and on start-up, KA-Boom!

Keep at it, you'll figure it out.