Looking for suggestions. Uneven spark plug coloring. Some are super dark, some look good?

[Orig69]

Have you tried to eliminate the smell of fuel by lowering the fuel pressure to 3.5 pounds
I suspect it has a small fuel leak somewhere.

 

[RJRENTON]

Carb is not too big. It is actually smaller than the 850 TQs that Chry fitted to emission 440s & Inter trucks that had smaller engines....Plus the engine has a dual plane intake that divides the airflow. There would be little sense in adding higher flowing heads & intake, big cam, all of which move the power into higher rpm ranges where more carb airflow is needed.....& then choking it with a small carb.

Its not a function of the carbs total CFMs but the VELOCITY of the mixture in the manifold and its runners. Fuel is combusted as a vapor, NOT LIQUID FUEL....it must have time to change from an emulsified, semi liquid to a vapor before entering the combustion chamber. The T-Q carb as well as the GM Q--Jet were designed to provide maximum manifold velocity, promoting excellent throttle response and minimizing emissions. As stated, the engine has a dual plane intake manifold, while not ideal for performance, the mixture sees several turns and splits, plus unequal carb to runner lengths, promoting non linear or turbulent flow, which affects velocity (reference CRANE's Flow of Fluids laminar flow vs turbulent flow). The owner's larger carb reduces mixture velocity especially at lower RPMs, exacerbating even fuel distribution issues. My suggestion has nothing to do with changing the cam or heads or compression ratio ....or increasing rear gear ratio....but simply providing a possible solution to the origional stated problem of uneven mixture distribution. Did the OP state the intended use of the vehicle? I'm aware of results of over camming, over carburetorating......usually resulting in exactly what the OP states.....poor low RPM operation, fouled plugs, etc.......just my opinion of course....
BOB RENTON

 

[Some Car Guy]

IMO......as there are several carb "experts" contributing opinions, I'll just add my comments.
FIRST.....the carb is too big. As fuel mixture is a distribution function, large capacity carbs will result in low mixture velocity; low mixture velocities will result in uneven fuel distribution, which is also influenced ny manifold runner configuration (length and shape). The plenum, space under the carb, will cause a pressure drop and subsequent velocity drop of the fuel mixture.
True, timing and cam specs will influence mixture velocity as long overlap cams promote more exhaust gas left in the combustion chamber resulting in dilution of the incoming fuel charge. Once the idle mixture has been established, one possible solution to the mixture distribution issue MAY be to stagger jet fuel metering, either by a combination of jets, step up rods, depending on manifold runner length. There is NOTHING AS IN ANY HARD AND FAST RULE that prohibits stagger netering. Fuel pressure and volume applied to the carb as well as float levels will effect operation....stay within manufacturers guidelines......in this case, the "my buddy's rules" do not apply.
The problem is complex and likely that a complex solution will result.....nothing as simple as a few degrees of timing or an adjustment of delivered fuel pressure will fix. The spark plug coloration is a result of fixing mixture issues and MAY require a slight heat range revision of your "favorite" brand.
The first thing I would do, is to go to a 650 CFM carb....my preference is Holley, but it's your vehicle, and see if the mixture distribution issue (velocity) is resolved....then proceed .....and with DOCUMENTED changes of each change or revision........just my opinion of course....
BOB RENTON

The solution to a tuning issue is to put a smaller carb on a built up engine than the factory did on a lower horsepower one??? 800 carb on this is a total nonissue.

 

[Geoff 2]

Bob Renton,
Post #22.

You might have missed it, but this thread is about idle & idle quality. The idle cct is not dependant on the velocity through the main bores. The idle air comes from the idle air bleed, PCV, leakage around t/shafts & past the nearly closed t/blades, where there is high vacuum because the engine is throttled. The idle fuel is drawn from below the blades, once again it flows because of the high vacuum created by nearly closed t/blades.

 

[RJRENTON]

Bob Renton,
Post #22.

You might have missed it, but this thread is about idle & idle quality. The idle cct is not dependant on the velocity through the main bores. The idle air comes from the idle air bleed, PCV, leakage around t/shafts & past the nearly closed t/blades, where there is high vacuum because the engine is throttled. The idle fuel is drawn from below the blades, once again it flows because of the high vacuum created by nearly closed t/blades.

ABSOLUTELY AND TOTALLY INCORRECT ASSUMPTIONS ......velocity has everything to do with the carbs operation. I'm well aware how the carb functions, perhaps you only think you underdtand. The ancillary air sources are taken into account during the design calculations and are accounted for. To ENLIGHTEN you, a carb functions on the principles of DIFFERENTIAL PRESSURE. Which is DIFFERRNCE between ATMOSPHERIC PRESSURE (reference: sea level adjusted for temperature and barometric conditions) and MANIFOLD ABSOLUTE PRESSURE (which is apposite of vacuum, specified as millimeters of mercury). As suggested, after you review the principles outlined in Crane's Flow of Fluids and the fundamental principles of Boyle's Laws, you'll have a better grasp of carburetor operation as it pertains to the internal combustion engine and the fuel distribution problems at the various operating points.
BOB RENTON

 

[Geoff 2]

Bob,
Get a carb book such as the original Holley book & read about how the idle cct WORKS. What I said in the above post is 100000% correct.

All you have done in this thread is introduce red herrings & a lot of nonsense. Talking about carb CFM which was not relevant to the OPs problem, which as wrong anyway as shown by Chry. Then you try the bullshit baffles brains approach, introducing a lot of overly technical talk that is not helping the OP one iota & not relevant to solving his problem.

 

[Geoff 2]

FN,
I read the instruction sheet for your dist. Must have some dumb people at Holley, referring to the pressure drop for the VA unit to work as '15 lbs '.....
It should be 15" of Hg [ Mercury ].

This dist is NOT going to do the job for you if you want this engine to idle & run as best it can.
The VA unit is NOT adjustable; plus you were getting 7.5-12.5" of vacuum & the VA unit requires 15" to fully deploy. So it would be erratic in operation, with idle rpm/vac varying.

I tried to install a GM points Crane Adj vac unit, into a MSD dist & had no luck. The MSD had some type of bush, design was a bit different & the VA would bind up in operation.

You will need to get a new dist if you want to go further. Your idle vac increased by 5" just by turning the dist to advance the timing [ at idle ]; that is huge & man vac at idle is a measure of how efficiently the engine is burning the fuel & making power.

You need a dist with adj vac adv. You can buy them cheaply from Summit, they use the Chrys
mag trigger [ electronic ]. I have used them, no problem. You then need an ECU. Best bang for the buck is the HEI 4 pin module; some folk mount it under the dist base. Simple to hook up, wiring diagram on the web & probably pics of one mounted to a heat sink on the dist body. That's it, other than the coil, for your ign system!

Soooo, decisions to be made.....

 

[funknut]

Today I installed the 17 degree advance bushing and set initial timing to 20 degrees. With vac advance connected I am getting around 32 degrees at idle, which is a big increase over the 15 I was running previously. MSD doesn’t seem to have an adjustable vacuum advance option for this distributor.

Not yet ideal, but an improvement while I figure out next steps.

I’ll update this thread once I find my next distributor.

Thanks, everyone, for the advice and suggestions. I definitely learned a lot!