Author Topic: Tuning Help  (Read 1055 times)

Offline matt63

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Tuning Help
« on: September 17, 2007 - 12:52:08 PM »
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? 
Matt in Edmonton

'68 Valiant
'73 Cuda 340 4 speed (408) SOLD




Offline tactransman

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Re: Tuning Help
« Reply #1 on: September 17, 2007 - 12:55:17 PM »
What color is the smoke?
Terry-tactransman 
Torqueflite/Automatic Transmission Specialist
Union, Mo.
Give a man a fish and he eats for a day,teach him to fish and he eats for a lifetime.

Offline matt63

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Re: Tuning Help
« Reply #2 on: September 17, 2007 - 01:46:46 PM »
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).
Matt in Edmonton

'68 Valiant
'73 Cuda 340 4 speed (408) SOLD

Offline MEK-Dangerfield

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Re: Tuning Help
« Reply #3 on: September 17, 2007 - 05:08:30 PM »
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

Mike

1970 Challenger - SOLD
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Offline matt63

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Re: Tuning Help
« Reply #4 on: September 17, 2007 - 06:25:51 PM »
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.
Matt in Edmonton

'68 Valiant
'73 Cuda 340 4 speed (408) SOLD

Offline matt63

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Re: Tuning Help
« Reply #5 on: September 17, 2007 - 10:09:15 PM »
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). 
Matt in Edmonton

'68 Valiant
'73 Cuda 340 4 speed (408) SOLD

moparniac

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Re: Tuning Help
« Reply #6 on: September 20, 2007 - 05:39:02 AM »
figure out the issue......

Offline matt63

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Re: Tuning Help
« Reply #7 on: September 20, 2007 - 10:05:09 AM »
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.
Matt in Edmonton

'68 Valiant
'73 Cuda 340 4 speed (408) SOLD

Offline Chryco Psycho

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Re: Tuning Help
« Reply #8 on: September 21, 2007 - 12:04:27 PM »
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 

Challenger - You`ll wish You Hadn`t

Offline matt63

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Re: Tuning Help
« Reply #9 on: September 21, 2007 - 01:10:27 PM »
Thanks CP. Is that normal? (I do lift)
Matt in Edmonton

'68 Valiant
'73 Cuda 340 4 speed (408) SOLD

Offline Chryco Psycho

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Re: Tuning Help
« Reply #10 on: September 22, 2007 - 02:47:18 AM »
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

Challenger - You`ll wish You Hadn`t

Offline matt63

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Re: Tuning Help
« Reply #11 on: September 22, 2007 - 10:02:00 AM »
I  do have the 1/16 rings.
Matt in Edmonton

'68 Valiant
'73 Cuda 340 4 speed (408) SOLD