backfiring at 3000 RPM 1971 911E

ed911 · 09-20-2001, 04:43 PM

I need some help here!
I have a 1971 911E Targa that I just bought a month ago...it has 74,000 miles on it. As you accelerate up between 3000 and 3200 RPMs, I have a backfire.

So far, I have changed the air filter, the spark plugs, the rotor and cap, checked the fuel lines, and checked the fuel pump - and everything seems fine .. but still I have this backfire at 3000 to 3200 RPMs ....

Does anybody have any suggestions as to what I should tackle next, or what could be causing this?
Thanks!
Ed

Bill Douglas · 09-20-2001, 05:16 PM

Ed, is the performance ok in general? Just the backfire and is it running on all five cylinders all the time.

ed911 · 09-20-2001, 05:27 PM

Bill,

Performance is very good all the way except for at 3000 to 3200 ...

It runs on all six cylinders (?) very well.
Any ideas?

marc weintraub · marc weintraub's Garage

Ed,

Do you still have the mechanical fuel injection on the car? If you do, it sounds like it might be the MFI. Many of the earlier cars have flat spots in that RPM range that are associated with the fuel injection. This can be tuned out of the system by someone familiar with the specific injection system on your car. If it has carbs, I will pass this off to guys much better versed on carborated engines.

ed911 · 09-20-2001, 05:46 PM

Quote:

Originally posted by marc weintraub:
Ed,

Do you still have the mechanical fuel injection on the car? If you do, it sounds like it might be the MFI. Many of the earlier cars have flat spots in that RPM range that are associated with the fuel injection. This can be tuned out of the system by someone familiar with the specific injection system on your car. If it has carbs, I will pass this off to guys much better versed on carborated engines.

Yup, it has MFI .. and the belt was changed just before I got the car...
Ed

Bill Douglas · 09-20-2001, 08:31 PM

Sorry Ed, I wsan't tying to be funny saying all five cylinders. I got a bit confused - don't 944's have 5 cylinders that they inhertited from Audi. Anyway, could it be something like a condenser or coil? The backfiring is unburnt fuel making it through to the exhaust so makes me think it's ignition related.

ciaranchallis · 09-21-2001, 06:01 AM

I had a backfire on my 1971 2.2T just after I had bought it. It happened when the revs were falling past the same point you mention, so on over run not acceleration. I checked my exhaust and found a small hole in one of the heat exchangers that I didn't know about. I have only patched it up for now but the backfire disappeared instantly.

I hope this helps.

Ted Stringer · 09-21-2001, 10:25 AM

Audi is the only company to do a five cylinder engine in a car as far as I know. But then again, I could be wrong. (wouldn't be the first time)

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Ted Stringer
nuke3@juno.com
'84 911 Targa aka pocketrocket

Early_S_Man · 09-21-2001, 12:02 PM

Ed,

If you want to find out where you need to go with the MFI, get a Gunston gas analyzer, and take some measurements while driving. The following threads discuss the Gunston tester:

http://www.pelicanparts.com/ultimate/Forum3/HTML/007782.html
http://www.pelicanparts.com/ultimate/Forum3/HTML/012328.html

http://www.pelicanparts.com/ultimate/Forum3/HTML/005190.html

http://www.pelicanparts.com/ultimate/Forum3/HTML/012730.html

Ted ... Mercedes did the five-cylinder 3.0 Diesel (300D and 300TD) before Audi built any 5's!

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Warren Hall
1973 911S Targa
1992 Dodge Dakota 5.2 4X4 parts hauler

ed911 · 09-24-2001, 10:42 AM

Thanks for all the responses.
Ed

psychotron · psychotron's Garage

thanks for everyones feed back to Ed's post. i have had problems in the 3000-4000 RPM range on my MFI 911, so i just ordered a Gunson analyzer. My problem is strange, good torque from 1000-3000 then a lot of struggling and it comes on smooth as ever at 4000 and screams all the way to redline in every gear. i'll let you all know what happens this weekend, Priority mail was FREE!