I'm having a ittle confusion reading plugs on my newly carb'ed 
(DCNF) DP car.  We all know that the plugs should be a light tan 
(closer to white on unleaded).  However, looking at my plugs 
after a weekend's autoxing, I find I can reach any conclusion I 
want, from slightly rich to too lean, depending on which part of 
the insulator I look at. One side of the insulator is lightly 
gray sooted, possibly making one think "rich".  This is only on 
30% of the insulator, tho, and the rest looks stone white (too 
lean).  I supose that this has to do with the way the plug ends 
up in the chamber, ie the sooty part faces the intake valve, the 
white faces the exhaust??

Anybody resolved this to their satisfaction? 

What kind of plugs?

MikeR
'77 Fiat 124 Spider
'76 Fiat 128 4 door
Beaverton, Oregon

steve soar wrote:
: I'm having a ittle confusion reading plugs on my newly carb'ed 
: (DCNF) DP car.  

You should be most concerned about how the plugs look at WOT; 
that's where the engine normally operates while racing.  To do 
this, shut the engine off when it's up to temp and is at or near 
WOT, don't let it idle.  You should see a fairly even 
distribution of color.  My guess is you may be a little lean, 
light grey is ok.   To really nail your jetting down, you'll 
need a set of EGTs and a dyno.

steve soar wrote:
: I'm having a ittle confusion reading plugs on my newly carb'ed 
: (DCNF) DP car.  

Hey Steve how do you like the DCNF compared to the DMTR carb?  
Not that I have had time or the money to get started on my car, 
but I had a really interesting phone con with the guy at pierce 
manifolds and came away doubting the DCNF for this application.  
So what do you think?

-clay-
Bent Rabbit
74 X-1/9 waiting for a massage.

Fritz Costley wrote:
: You should be most concerned about how the plugs look at WOT; 
: that's where the engine normally operates while racing.  To do 
: this, shut the engine off when it's up to temp and is at or 
: near WOT, don't let it idle.  

I concur - the British call this a "plug cut".  Not sure what its 
called here, but run the car up to operating temp, and then cut 
the ignition under load (not always WOT, but if that's 
appropriate...) in a gear which makes sense.  When I was 
rejetting my Spider after autocross use it was idling way too 
rich (actually fouling plugs), but WOT throttle cuts in 4th 
produced some of the most beautiful plugs I had ever seen.

Doug

Clayton Horste wrote:
: steve soar wrote:
: : I'm having a ittle confusion reading plugs on my newly 
carb'ed 
: : (DCNF) DP car.  
: Hey Steve how do you like the DCNF compared to the DMTR carb?  
: Not that I have had time or the money to get started on my 
car, 
: but I had a really interesting phone con with the guy at 
pierce 
: manifolds and came away doubting the DCNF for this 
application.  
: So what do you think?
: -clay-
: Bent Rabbit
: 74 X-1/9 waiting for a massage.

Clay:

I made a couple of changes at the same time.  I changed the cam 
to an IAP performance cam (35-75), used an intake manifold I had 
cut apart on a bandsaw, ground all the passages open, then JB 
welded back together, and changed from the 34DMTR to a 40DCNF.  
I know the 40 is technically illegal but I wanted to see if the 
change was worth it before I sprang for a 36 (I had the 40 lying 
around).  Besides, air flow ought to be limited by the chokes, 
not the throttle plates, so a 40 with 32mm chokes ought to be 
the same as a 36 with 32mm chokes, and wrote Denver requesting 
that the 40 be allowed.

Here's a warning.  the idle jets for the DCNF look just like the 
jets for the DMTR/DATRA, which I had a ton of lying around.  
However, the DMTR idle jets are maybe .050" shorter than the 
DCNF jets and don't seat in the carb. (the mains, aircorrs and 
emuls _are_ the same...stupid Italians..) So if you use DMTR 
idles in a DCNF, they won't control gas flow properly, and 
you'll spit out huge clouds of black smoke, cause yourself to 
curse and swear, because no matter what jets you put in the @
#$$%%& thing, it still runs too rich.  You will also score the 
cylinder wall in cylinder #3, causing about 20% leakdown  :-(.

Even after wrecking the engine, the new combo makes a good 20-
30% more power than it did the day you drove it.  It really woke 
the engine up.  I wish I knew what it would feel like if I 
hadn't wrecked cylinder #3.

So here's a jetting for a single DCNF sideways on a modified 
stock manifold.

choke   31mm
main jet    160
air corr    195
emul tube   F24
accel pump   170
accel cam   11
idle         50

This runs and makes good power.  It's not optimized yet (too 
rich in the transition region, and stumbles if the throttle is 
oponed real quick).

Caveats:  The idles are too rich.  45's will make it idle 
sweetly, but agravate an off-idle stumble.  The transition ports 
are wrong, so you put in idles that are too rich, and run the 
mixture screws most of the way in. The 70 pump is big, but 
necessary to prevent the stumble.  The 160 mains are too rich as 
the main ckt begins to come in, maybe a change in emulsion tube 
could help, as they seem OK at WOT.

For all my comments about stumbles, the throttle response is 
much better with the DCNF.  The top end power is way better, and 
I didn't see any drop in midrange.  As for chokes, I started 
with 29mm (what I had) and opened them up to 30mm on my lathe, 
then 31mm when I saw no drop in low end torque.  I'd go to 32mm 
ultimately.  Oh yeah, I suspect the rules are hosed, I don't 
think you can put 34mm chokes in a 36.

The DCNF also is better about not cutting out in hi G corners.


So I'd go to the DCNF.  Don't even mess with the 34DMTR.

anybody got a 36 to trade for a 40?? :-)


-steve