Hello:

My 1973 914 2.0 backfires/pops on deceleration. My mechanic replaced the aux air regulator and the car now idles properly after re-starts when warm. The head temp sensor was replaced, the valves and timing were adjusted and the vacuum lines were re-routed (with the factory workshop manual schematic). The car has been checked for exhaust leaks and I don't think that is the problem. My mechanic also tested and adjusted the the throttle position switch. The car is stock accept for a Pertronix Ignitor (installed after the poping/backfiring was an issue). The pressure sensor has been evacuated and checked. The pressure sensor on the car is a 1.7 litre Bosch part # 0280100049. The car was built in 11/72 as a 1973 model. The four fuel injectors have been replaced because one was leaking. The poping/backfiring began after this repair. Should the injectors be cleaned and/or replaced? Should I go back to points and condenser? What is causing the car to run rich on decel making the car pop/backfire?

Thank you, Bob

http://members.rennlist.com/pbanders/
http://members.rennlist.com/pbanders/DJetParts.htm

These two links might help identify your parts. You have the correct sensor. If the wires from the distributor are hooked up backwards the car will not run proper and backfire. A faulty diaphram in the MPS can make it backfire. Did you do a test to see if it holds vaccume? Timing could be off. Hope that helps.

http://members.rennlist.com/pbanders/
http://members.rennlist.com/pbanders/DJetParts.htm

These two links might help identify your parts. You have the correct sensor. If the wires from the distributor are hooked up backwards the car will not run proper and backfire. A faulty diaphram in the MPS can make it backfire. Did you do a test to see if it holds vaccume? Timing could be off. Hope that helps.

What is your fuel pressure?

Where is your MPS mounted?

How about your decel valve?

1973 FI use the same brain (ECU), but different MPS's and the 1.7 MPS and 2.0 MPS for 1973 are 1973 1.7 and 2.0 ONLY. You cannot have a 1974 2.0 MPS on a 1973 ECU or a 73' 1.7MPS on a 73' 2.0, doesn't work.

It goes on and on. There's good resources (have been pointed out already) how to match up your FI parts.

Find someone that has a 73' 2.0 that runs good, plug in the MPS, go test drive. I've done that in the past.

I would agree with you that you have the "wrong" sensor. But when you take your foot off the gas, vacuum increases, the cells expand, and the diaphram seats onto the part throttle stop. When MPS's are operating in this region, they are pretty much the same. Take a look at Brad's calibration chart. Four MPS, and yet at 15" Hg, their inductance is .72, .72, .71, and .71 H. The significant differences come in when the diaphram starts to rise off the part throttle stop- when it starts, transition slope, and how far it goes before it hits the full throttle stop all tune it to the needs of part and full throttle operation.

So with your sensor, I would probably expect lean miss at part or full throttle, and/or hot running at full throttle. Of course, if this is mitigated by something, like higher fuel pressure, then you may seem to run fine at part/full throttle, but then too rich on idle and overrun. And if you're too rich AND your timing is retarded, then the popping could result.

So I'd probably check fuel pressure first. Followed by timing. Is your distributor retard hooked up and operating? Out of curiosity, what's your idle vacuum?

Still figuring these things out myself...

Jim

Good catch, I seen 1.7. Not 2.0 intially

Ok, let me ask again. What is the condition of the decel valve. If you did all the other tune up bits, and new injectors i'd say you need to look at that. They will pop on decel without it.

...............HOLD IT.........he said he had a pertronics ignitor....IF wired in backwards they more or less instantly burn out....FRY...Right!....so the distributor wiring is not the issue.(unless of course it just ain't burned out yet due to lack of use.)....probably need to check distributor wiring like yesterday.

I had a shop wire the pertronix wrong. The brain fried before the pertronix did. Took a while too.

Your car is backfiring on overrun because the mixture is lean. If this happens only during warm-up, you can fix it by adding a spacer to the CHT, which will also probably improve your start-up drivability. If backfiring continues once the car is warmed up your basic mixture adjustment is lean. Normally, I'd say this was due to your 1.7L MPS, but as you said it started happening when you had the injectors changed, perhaps the wrong injectors were fitted. As others have mentioned, fuel pressure is also a possibility. Check my pages for the correct MPS, CHT, ECU, and injectors for your car. Lastly, I don't think that the decel valve is going to cause backfiring, or the lack of a decel valve, either.