MFI or valves?!!

sessa · 10-30-2001, 06:27 AM

Well I'm new to this BB I purchased a sweet 1970 911S about four months ago after two years of searching. The car ran great all summer ( BTW checked out fine at the porsche dealer when I purchased it). About one week before I wanted to put the car away for the winter the car started to misfire. I pulled the plugs an they were wet with gas and black. I know this means the fuel system is running rich. I timed the valves as part of my pre winter storage, this is the first time I've done it but I checked the clerances four times and I left the plugs in. The distributor cap looked worn so I decided to order a new cap rotor and points and in the mean time cleaned up the old ones which cured the misfire( I live in central Canada and it takes at least two weeks to get any parts). Now the car is idoling rough and has a backfire I haven't attempted to take it out of the garage yet as I'm concerned about the damage the backfire could cause. Is my MFI system at the issue, I have been working though the "check measure adjust" manual and plan to check the timing of the ignition and fuel pump next. Do I have a problem with my valve adjustment? or as the CMA manual sugests is it with the micro switch, rpm tranducer or stop valve(I've read the other strings but don't fully understand the checking of these items. Sorry for the overload of info. but I want to do this myself I bought the car as a hobby and have a real hard time having to admit defeat and have it towed to the shop.

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1970 911S

[This message has been edited by sessa (edited 10-30-2001).]

[This message has been edited by sessa (edited 10-30-2001).]

bfranklin · 10-30-2001, 07:53 AM

". The distributor cap looked worn so I decided to order a new cap rotor and points and in the mean time cleaned up the old ones which cured the misfire( I live in central Canada and it takes at least two weeks to get any parts). Now the car is idoling rough and has a backfire I haven't attempted to take it out of the garage yet as I'm concerned about the damage the backfire could cause."

I don't understand, you cleaned the cap and points and it cured the misfire, but your car idles rough and backfires? Seems like its still misfiring, or misfiring worse than ever. What did you use to clean your points and cap. Did you remove the points from the distributor (if so, did you set the dwell correctly?) If your plugs are wet and black, its probably because they're not firing right, not necessarily because the MFI is messed up. Before digging into the MFI, I'd put a new cap and rotor on, new points, and set your dwell correctly. If that doesn't solve it, check to see if your plug wires are worn, and check the transmission ground strap.

Brad

Rot 911 · 10-30-2001, 08:08 AM

I agree with Brad. I seriously doubt that it is your MFI. I also doubt that it is your valve adjustment. I would recheck everything you did on the tune up. Also check your timing. MFI's just don't go out of adjustment. The backfiring won't hurt anything on an MFI car, just annoying and embarassing. You might also want to do a search on this board on "MFI" lots of good postings.

Kurt V
Was 72 911E, now '86 Carerra

Early_S_Man · 10-30-2001, 08:14 AM

sessa,

Check the following thread and the links it contains near the bottom for an explanation of the speed switch and microswitch operation as it relates to backfiring -- only on overrun over 1500 rpm:

http://www.pelicanparts.com/ultimate/Forum3/HTML/012702.html

You probably should disassemble your distributor, clean, check for bushing wear and proper centrifugal advance operation ... an exploded diagram abd instructions are found in the fiollowing thread:

http://www.pelicanparts.com/ultimate/Forum3/HTML/009677.html
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Warren Hall
1973 911S Targa
1992 Dodge Dakota 5.2 4X4 parts hauler

[This message has been edited by Early_S_Man (edited 10-30-2001).]

sessa · 10-30-2001, 08:16 AM

I don't understand, you cleaned the cap and points and it cured the misfire, but your car idles rough and backfires? Seems like its still misfiring, or misfiring worse than ever. What did you use to clean your points and cap.
Brad[/B][/QUOTE]

I cleaned the black carbon deposits from the spots where the rotor contacts the cap with emery cloth. I checked the dwell but I'm not sure I did that correctly, and cleaned out the debris from around the bottom of the rotor area(dirt and dust). All of the wires carry the same amount of current and the distibutor cap distibutor and also carried a much stonger signal once the carbon was removed. Im not hearing the same pop(or slight hesitation) of a misfire as before that is why I had assumed it was fixed and now the air/fuel mixture was causing the car to run rich. please point out the flaws in my logic as I really don't want to touch the mfi.

bfranklin · 10-30-2001, 08:35 AM

No real flaws in your logic persay

Its just that personally I've learned when things go wacky, many times the ignition is the culprit. I find the points on these cars to be REALLY finicky, i.e. any tiny film of grease or oil or dirt between the contacts can cause your car to run MISERABLY. As you alluded to, the MFI is a much more complicated system, requiring special tools to work on it, and ALOT of reading. I find it always cheaper to check/replace ignition parts first, and as Kurt said, typically the MFI is a fairly stable system when its running right. Warren mentioned the speed switch item, which can make your car run a bit funny and backfire if it fails. You can check to see if its working by moving the MFI rack to bring the revs up above 2000 rpm, and then closing the microswitch on the left side of the rack. The engine should drop in rpm, and then oscillate between 1300 and 1500 rpm. That being said, a wierd problem like MFI drive belt slippage might cause a sudden change in MFI operation, but I'd recommend installing the new points and cap you ordered before anything else.

Brad