Tuning Help

[matt63]

The 408 is running flawlessly except for some smoking at WOT as seen in the rear view mirror.   I've probably got 500 miles on the engine.  I have a Holley 750 Street HP vacuum secondary unit which has been jetted down front and back to 72's.  The spring is the std silver one and the power valve is the std 4.5.  The bowl levels are at the bottom of the sightglass and fuel pressure is 6-7 psi (regulated) I'm hoping that this is a carburetion problem and would like to try to find another carb to swap if I can't resolve the problem with this one.  I might try a heavier vacuum spring again but my recollection was that this just hurt performance.  Any suggestions? 

[tactransman]

What color is the smoke?

[matt63]

It's hard to tell in the rear view mirror.  I might try a WOT run with a new plug installed to see what it looks like.  As I said in the initial post, the car runs well and makes good power in my judgment.  I really don't like the smoke though.  I would say it is moderate (not light and not very heavy).

[MEK-Dangerfield]

How do you spark plugs look? The smoke could be gas or oil. Does the engine have any bogs or missing with this carb?


  Mike

[matt63]

I'll do some more street testing tonight if I can and pay attention to the power curve.  I don't believe that I have major bogging or missing.  I usually nail it at 2500 rpm or above when "testing". 

The plugs were fouling before I dropped the float levels.  I checked them last week and they were dry black from fuel on the first part of the ground strap and the ring.  There was also some white "powder" on the tip of the ground strap - fuel additive deposits??.  The center electrode was all white on the top half.   I can't recall whether I changed the plugs just before or just after this adjustment.  I have a couple of new plugs to test with.  The car doesn't smoke at start-up, idle, cruise or moderate throttle. Thanks.

[matt63]

I took the Cuda for a cruise with a new plug in #2.  The pulling power and sounds from this 408 always impresses me.  The smoke seems to come mainly between shifts so it is a little less concerning.  I can't feel any bogging.  It runs pretty good.  I pulled the plug after getting home after a 20-30 minute drive so I don't know of how much use this will be.  It was still a little busy to be pulling plugs on the side of the road.  The center insulator is pure white.  The ground strap is white for the first half from the end of the electrode.  It can easily be wiped off as can be seen in the last picture. The base ring is dark (it looks oily but it is not). 

[moparniac]

figure out the issue......

[matt63]

I'm not sure if there really is one now unless anyone suggests that I be concerned about the plugs.  I've got to get the car out more but the driving season is pretty much over here.

[Chryco Psycho]

I would be installing 2-4 size larger jets , the smoke could be pulling past the rings on shifts assuming you lift on the throttle 

[matt63]

Thanks CP. Is that normal? (I do lift)

[Chryco Psycho]

Normal ? no
 what you have high RPM & you close the throttle the vacuum goes very high & will pull from anywhere , if you have low tension or thinner rings than a normal street engine it can pull oil past them though , aslo an intake gasket leak under the intake into the lifter valley can do this as well

[matt63]

I  do have the 1/16 rings.