Can't get rid of that Rich Stinky Exhaust at idle?
Without enough Initial timing it is impossible to set up the carburetor correctly as it won't respond to adjustments without enough timing to burn the fuel.
Incorrect timing can also be responsible for not only a rich idle but overheating in traffic, stumble off idle, lazy throttle response, lost horse power, burned out header gaskets, discoloration of ceramic coatings and poor fuel economy. 
The FBO Limiter Bushing kit will solve many of these issues if the distributor is correct for the application.
The Black bushing supplied by MSD with your distributor will allow 18* mechanical advance. Most American V-8 engines require a total of 34-36* under load. With the largest bushing they supply you can only get 16-18* of Initial timing.  In most performance engines that is not enough to achieve a lean, clean, and crisp idle and get rid of the eye watering raw fuel fumes caused by inadequate initial timing. 
As you have already discovered you can't get a clean idle with carburetor adjustments, because your performance engine needs more initial timing. Of course, when you add more initial timing the total goes up the same amount and causes detonation and other tuning issues that could seriously damage your engine. MSD will tell you to lock it out at 36 and buy one of their spark retard units so you can get it to crank over at 36*.   We disagree with this solution because it can cause even more tuning and performance issues.  Your Race motor may like 36* all the time, but in most applications and street/cruise use  it will cause hot hard starting, overheating and detonation issues at low RPM and high load.


The MSD distributors come with 3 bushings with the largest one being 18* giving you 18* of mechanical advance.  If you're trying to achieve 36* total engine timing you would set the initial timing at 18* and the MSD black bushing will limit mechanical advance 18* to bring your total to 36*. By swapping out any of the stock bushings provided by MSD and using the FBO 10* or 14* bushing, it will allow you to increase your initial timing and not exceed your desired total timing. (34* for Mopar, 35* for Ford, and 36* for Chevrolet)
Big cams bleed off cylinder pressure allowing you to run big initial timing numbers. This will help you get clean quick starts. We need the motor to have as much initial timing as possible without kicking back or dragging down the starter and at the same time still have plenty of initial timing to get clean and quick starts.
Keep in mind that the MSD distributors out of the box do not allow the timing to come all in until 5000+ RPM.  You will need to run a heavier spring combination so that your timing mark is not jumping around when you are trying to set your initial timing.  Too light of a spring will allow the weights to bounce off the stop at idle and cause the timing mark to go crazy when you are trying to set your initial timing or even be allowing the distributor to be advancing at idle causing too much timing at the top end. If you’re running a 3000-4000 or bigger stall convertor there is no need to bring the timing all in until just below the stall, you won't load the motor until the convertor locks, so there's no performance gain in running your distributor all in at 1500-2000 RPM.
We recommend all engines that are producing enough vacuum, to use a distributor with an adjustable vacuum advance connected to a constant manifold vacuum source.  The constant source gives you the additional vacuum timing when the vacuum is present and drops when there is not enough vacuum to the source.  This helps give additional timing at idle and part cruise throttle to continue running clean. Our rule of thumb is "If the engine will make 7-8" of Manifold Vacuum at idle with the idle timing set at 30* then it should have a properly curved vacuum type distributor and connected to a CONSTANT Manifold vacuum port.


These bushings will fix most rich idle conditions.  The 10* bushing is now milled on 2 sides to allow easier identification.  The 14* bushing is smooth on all sides.  
 
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