Damon Kuhn 3/10/2010
Many times I have been asked about tuning an unresponsive carb only to find out the ignition was the real culprit- so here's a little info on that. One thing that many people fail to consider is that ignition and fuel are not only both required but also interact with each other in such a manner that if one is wrong, the other will not function properly. Also, if one is wrong , it may seem the other is the problem. Many bogging, acceleration deprived or other malady stricken engines have been mis-diagnosed as carb problems. Another common misconception is that you should set your engine according to what the shop manual says. While this may work, it surely will not be optimum. While much of the reasoning for the advance comes from chamber shape, etc., each engine is different, and if you have managed to improve on what you started with, it will surely take different tuning than a stocker. This paper will address mainly performance oriented information/ modifications, however many of these can be used to get better mileage/ drivability out of everyday drivers.
The reason ignition advance matters, in basic terms, is because of mixture dilution due to low velocity and overlap at low speeds and, probably more importantly, the faster the engine turns, the less time there is for mixture burn initiation to start. By advancing the spark at idle, the less volatile mixture is given more time to burn, and at high rpm, the spark can initiate the burn far enough ahead of time to achieve maximum pressure. I realize that is a very elementary explanation and leaves a bit out, however it will suffice for purposes of this discussion.
First off, there are two main systems and a couple of concerns with each that need to be understood before undertaking modifications. The first is the centrifugal advance mechanism, the second being vacuum advance. Centrifugal advance is controlled by a weight/ spring system and is wholly dependent on rpm to function. Vacuum advance is a "load” sensor and is controlled by engine vacuum which varies directly with the amount of load on the engine. There are two main terms you will hear repeatedly- base and max or full timing. Base timing is the number of degrees advanced (or retarded) the engine is from zero at idle, generally assumed to be without the vacuum connected. Full or max timing is the maximum number of degrees advance reached before no more occurs (again this is generally discussed without regard to the vacuum system). One other item of note is the advance "curve" which doesn't seem to enter in to many conversations. This is the "curve" if you will, representing how the base to max timing is reached, usually plotted as a curved line (graph). All of these things must be taken into consideration if you want to properly tune your engine.
Before getting into specifics on each, there are a couple of things to check. You can skip this, but the results you obtain will be directly proportional to how precise the operation of all systems is. The first requires removing the distributor from the engine, which is described in many places so I will forgo the description of properly aligning the distributor for removal / replacement. What we want to check is the endplay on the distributor shaft. This should be between .008 and .012 inches. The reason this is important is that the drive gear is helical and any excess up and down travel will modify timing- I've seen some vary by over 4 degrees because of this. Excess play can be corrected by adding shims between the shaft collar (or gear for non- Mopars). Simply push the roll pin out and add shims until the proper endplay is achieved. ( measuring can be done with a feeler gauge between the collar and the distributor body). While you are at it try to move the shaft side to side in the body- any perceptible movement means you've got a worn out distributor which will not respond well to any real tuning. DO NOT assume that because you have a high dollar distributor that all is well-I've seen quite a few brand new and / or high dollar units with problems.
On to centrifugal advance. The first thing to check on is play in the springs. If the distributor is in the engine, simply wiggle the rotor back and forth- if not, lock the shaft in a vise and do the same. What we are checking here is the springs / pins themselves- there should be no unrestricted movement at all. In order to work properly, the springs must restrict the advance. On many distributors there is a light spring on one side and a heavier one on the other side- the heavy one may be slightly loose, but the light one should be tight. By leaving the heavy one loose, it does not affect the curve until part way thru. As the engine spins the weights attempt to move outwards, restricted by the springs- as they move outwards, they cause the rotor to advance on its axis a specified amount. The amount they can move is controlled by a slot on many, and built in stops on the weights on the others. The rate at which they advance is controlled by the rate, or tension of the springs. The shape of the weights on some affect this, but modifying the shape of these is outside the scope of this paper.
Many stock type late model distributors have too much centrifugal advance built in the plate as they come which, without modification, prevents proper tuning of a performance engine. Many stock engines use -4 to + 12 degrees of initial advance and another 24- 40 degrees of centrifugal advance. Most performance engines want somewhere between 34-38 degrees of total advance and something close to the below for initial:
Cam Duration @ .050 Initial Advance
220 or less 10-12
Looking at this, if you have an engine with a cam featuring say 238 @ .050, and you use 16 degrees of initial timing and your stock distributor has 28 “on the plate” (centrifugal), we would end up with 44 total-WAY TOO MUCH (with a few rare exceptions)- or if we set it at 36 total, we have only 8 initial and the engine will not want to idle, or at least will be very sluggish at low rpm. Many peoples solution to this is to lock the distributor at maximum “cause my buddy did his racecar like that”. Race cars typically operate at comparably high rpm’s where mid range advance does not matter. However, on a street car and even some automatic race cars that launch below 2800 rpm , some form of advance is not only beneficial, but in many cases necessary. Otherwise, the low to midrange will be over- advanced causing loss of power, knocking and possibly engine damage. (That rattling sound you hear is the fuel which was ignited too early trying to push the pistons backwards). The proper solution to this is to shorten or lessen the amount of advance on the plate. In this manner you can have enough initial without too much total. This is accomplished on aftermarket units with bushings or adjustment screws. On stock style units the advance slots must be shortened. How much to take out is best figured out on a distributor machine, but can be done (with some patience) without any more than a welder, grinder and some math. First, find out how much you have to start with by subtracting the initial advance from the max(without vacuum line connected). Then remove the distributor and take it apart far enough to get to the advance weight portion. Measure the amount of available TRAVEL- not the length of the slot / weight to pin distance. Divide the travel by the number of degrees obtained in step 1. For example, if you had 38 degrees total minus 8 degrees initial equaling 30 degrees on the plate and measure and find you have .300 travel. Dividing .300 by 30 gives you .010 inch per degree. Now multiply that figure by the number of degrees you need- say you want 36 total and 20 initial or 16 degrees “on the plate”. This means you need .160 inches of travel. Simply weld up the slot / move the pin (a little further than it needs) and grind it back / round it off to get the desired figure. In real life, the figure will probably not be that round, but usually you can get within a degree or so- again, it may take a couple of tries, but precision is reflected directly in the results.
Once this is done, the other area for experimentation revolves around what springs to use- while a dyno is extremely helpful for this, typically the modern wedge style head will perform well with a fairly steady rise with total in by somewhere between 2500-3500 rpm. The lighter the spring, the quicker the rise. While on that note, it is generally best to have a spring on both sides- unrestricted movement of one side can cause instability and binding. By the way, if you’re not a Rockefeller or just enjoy bragging about how you made something of nothing, old cassette decks, vcr’s, and a variety of other cast off devices have springs you can experiment with. While it’s a bit of a rough way to test things, a point just before knocking is at any rpm at full load is very close to optimum timing. Keep in mind also, some light pinging at very low rpm is quite common –LIGHT –if it sounds like bowling balls in a washing machine, you’re over advanced.
Vacuum advance is used as a form of load control and is designed to disappear under heavy load and come back as load diminishes. Some extra advance at idle and part throttle cruise tends to help emissions and to some degree power. The vacuum canisters with a hex shape have a 3/32 inch Allen adjuster inside the hose bung which can be used to adjust them. On most, less advance is obtained by turning the adjuster clockwise and vice-versa. A good base adjustment is one which causes the advance to just start working between 10 -11 inches. You do NOT want vacuum advance at less than 10. This is also a matter of experimentation- keep increasing a little at a time until light acceleration gives a hint of knock and then go back enough to get clean acceleration and you’ll be in the ballpark. I mentioned earlier that many lower rpm race cars/ street machines could benefit from some vacuum advance- for you non believers- hook up a vacuum gauge and power brake the car like you do waiting for the tree to come down and watch the vac signal slowly increase- adding just a little advance for the first few feet generally will help. On a street car it will help with a cleaner overall burn and provide snappier acceleration.
For now, before attempting any of this make sure the components you are working with (plugs, cap, rotor, etc) are in good shape first. Also if you are way off from the suggested figures, something is wrong such as too high/ low an octane fuel, excessively rich or lean carb, etc. While on that note, many times I’ve heard people announce they put 148 octane race fuel in their 8:1 family fan and it went faster- NOPE- higher octane burns slower, in effect by using too high an octane you are somewhat chasing the piston down the bore. Tune for the best lean mixture and a reasonable amount of advance on the lowest octane your engine will run on. There are more things you can do, such as indexing the reluctor tangs on electronic distributors, experimenting with plug and reluctor gaps but my fingers can’t think anymore and my brain is sore-