Spark plug help!!
 

ok i have a good one that has me stumped.

the car is a 72 911 with mfi.

i have had the car for about 2 yrs driven very seldom and decided to do a tuneup.

i adjusted the valves and added:
new coil, cap, rotor, wires, fuel filter, oil change, points
and everything was fine.

then decided to put new sparkplugs in it.
it had bosch w3dpo in it.

i first bought ngk bp8es, installed them and the car ran terrible. backfiring really bad.
so, i figured wrong plugs and put the old ones in it and it ran back to normal.

the old plugs look so awful i tried again and got bosch w5dpo.
installed them and it runs terrible again.

so again i put the old w3dpo plugs back in it and now its back to normal.

so before i spend another 80 dollars on plugs can someone tell me what is going on ?? !!

the other weird thing was when i had the w5dpo in it i pulled all the passenger side wires off and the car stopped backfiring and idled fine just had no power.
but i plugged the passenger side in and unplugged the driver side bank and the car wouldnt run at all.

i did a compression check and had good compression on all 6 cylinders.

i also have a syncmeter and measured each cylinder while idling and the passenger side is between 7 and 8. the driver side is at 10 to 11. i adjusted the air screws on the passenger side but they made no change in the measurement.

anyone have any ideas whats going on??

thanks

brant

Check to see that you didn't cross the spark plug wires. Sound like your firing order is off. You may have a vacuum leak and that would explain why you can't synch the two carbs.

okay double checked the wire order and they are good. searching for vaccum leaks now

Why did you change heat ranges?

You probably checked it but I'll add;

Spark Plug gap.

ok,
i havent found any vacuum leaks.

flieger, i changed heat ranges becuase it was my understanding that for heavy city driving the 5's would be better than the 3's. am i wrong?

snbush67, those platinum tipped bosch's i dont think i can gap. the ngk's bp8es i tried gapping several different sizes with no improvement.

NGK BP8 is way too cold for street use, try at least a 7 or 6. What size motor, compression ratio and octane rate fuel are you using? Also where are you located? Ambient air temp is also important for plug selection.

David

david,
the motor is a 2.4 with the mfi.
i do not know the compression ratio.
i use 91 octane fuel.
i live in las vegas.

this car has me so stumped... it does not make any sense.
can a car be that picky on plug heat ranges??

bump,

anyone else have any ideas?

It is very easy to "bonk" the electrode when inserting the plugs, and very easy to overlook a closed gap. Other than a crossed plug wire, I don't know.

You might try wiggling each plug wire while the engine is running poorly to see if it cleans up momentarily, indicating a bad wire. Watch them in the dark to detect arcing.

yes, you are correct. hotter for city driving.
i just went with BPR7's

use a plain copper plug. i lways have a hard time getting #1 wire to seat properly. infact i found 3 that were not seated all the way! still ran fine though

This is really a stumper. Is it possible that the new plugs have a different type connector nut on the top and (somehow) you are not getting the spark plug connector to seat solidly? It seems strange that only one side of plugs is misfiring. You said you had new wires but do you also have new end connectors? I assume you do and, if they're Beru, be sure you get that clear "click" when the connector seats properly--either by sound or feel. If you're certain all 6 plugs are solidly connected and the problem persists, then I'm out of guesses.
Good luck and keep us updated.

update:

just put in brand new bosch w3dpo plugs in the passenger side bank. ran fine for about 10 minutes then started backfiring and stumbling again.

so, i put the old w3dpo plugs back in and drove it around without any problems!

what in the world is going on with this thing?

Try this test. Old and new spark plug in hand, Plug both in a spark plug boot, compare side by side- depth and fitment of boot. Make sure the new ones are really seating well in the boot.

What do the plugs look like when you take them out, are they black as hell, or white? Does the three in that bosch number mean heat range, if so I would think that it is way to hot of a plug as mfi likes to run fat.
I could never get the ngk's to work in my car no matter what heat range I used, so I went to the tried and true 4 electrode bosch which work awesome for my car.

i was thinking about your problem today while enjoying a nice drive.
if the mixutre is lean, with the old dirty plugs it may run fine, but with the new ones, the lean condition could show itself.
i would still go with W5's. the W3 is more suited for very hi compression or race motor.
my brother's 930 uses W3 or W4's.

with W3's, the mixture would have to be very lean to prevent fouling

tobluforu,

what is the part number for the plugs you are using so i can give them a try?

thanks,

brant

when i take the plugs out, they are very black and you can barely see where they have been firing.

brant

hmmm,

t77911s, you got me thinking on that one.
you might be on to something.

brant

Yes, it can be that picky...you didn't say whether your car was a T, an E or an S...
In '72, all had relatively low compression ratios, designed to run on regular of that year. However, the factory suggested a different plug for each model. I no longer have my books here, since selling my car, but I do recall having better success with Bosch than NGK..