My son has a 70 mav with a 250. He finally did a tune up on it for the first time since he bought about 6 months ago. Found out that instead of points someone replaced them with electronic but, keeping the same distributor. We set the gap on the plugs for 44 and now its running real rough. What is the correct gap with this setup?
I pulled the plugs off today and reset the gap to .049. It started right up and it runs smooth. Thanks for the advise.
Mike, maybe i'm waisting my time trying to explain this but if your regap fixed the running issue, you have a deficiency in ignition voltage to begin with. Coil may have low output or there is some other fault limiting coil output. The gap at idle should never be that sensitive and suggest you may still have a missing condition under power. Here is how the ignition basicly works: The spark voltage needed is only that amount needed to break down the plug gap with the air/fuel supended in it. The rest of the voltage the coil may be able to provide is waisted and is correctly called the reserve voltage to handle all conditions normally found in the cylinders due to normal rich and lean conditions that occurr during operation. When reserve voltage is exceeded a miss results. This is the basis of saying you still may have an ignition issue if the plug gap made that difference at idle, otherwise a much larger gap would still work, providing there are no faults. As a real world example, much of the time it only takes somewhere between 8,000 and 10,000 volts to run most low compression motors. The coil normally has the ability to output in the range of 18,000 to 25,000 volts, of which only the lower values are normally needed with the rest in reserve. Again since you needed to reduce the gap, this suggest the reserve voltage is not present. Better take another look at the whole system.
the design of most any ignition system doesn't have anything called "reserve spark". A coil is nothing more than a glorified transformer. You put 12v in and get 12,000 out, for example. Simple. At a given RPM, the plug sees the same voltage at very little load as it would at high(er) load...meaning that if it was running 2000 RPM, it could have 15KV whether it was wide open or just decelerating down a hill. I just went through this with a CD ignition system on an ATV the other day. As RPM increases, the pickup charges more voltage INTO the module, which supplies more voltage TO the coil's primary, which is still amplified a certain number of times (depending on the primary to secondary winding ratio). In a nutshell, as RPM increases so does voltage at the plug. As load increases, voltage will stay the same, or nearly the same. You will always have some loss due to resistance. Now that said, most electronic ignition systems function best between .035 and .045 gap. If you have an MSD system, or something else with more than about 50KV secondary output, you can get away with more but more gap increases resistance, which is VERY hard on plug wires, caps, rotors, and the coil & module(s). And most factory style ignitions have about a 15KV to 30KV output. Most on the lower side. Boosted engines can and do have a little more toward the high end of the scale. I tested my Merkur the other day at work just for giggles...and it only had about 28Kv. I would have thought it would have more being that it's turbocharged.
The car was running good before tune up. After tune up it with new plugs set at .044 it was running bad, no power, hard to start. After regaping to .049 car runs smooth and more power easy to start. If there is an ignition problem it would be hard to detect. It could be the coil.
Mavman, I'm not going to argue with you about what I am well qualified to explain. You sir have a lot to learn. Go find out and learn instead of guessing what you think you know! Your missing a lot of knowledge about the subject.
I would say a change of .005 gap would not make such a big differance. More likely one of the ignition wires was making poor contact at the plug electrode, and when you pulled the plug to re-gap, the faulted ignition wire was installed propperly, so now it runs on all cylinders, leading you to believe the gap was the issue. Quote: Originally Posted by Bluegrass Again since you needed to reduce the gap, this suggest the reserve voltage is not present. Better take another look at the whole system. I believe he increased his gap from 44 to 49 thousndths
Well, teach me then. Explain to us how and where the "reserve" is stored. Also explain to us how the reserve is released, and when. I'm really curious. I know CD and TC ignitions quite well (I work in the ATV and motorcycle industry) but I'm not that familiar with points.
Here is where you went wrong with your reply and caused a bite back. This thread is about the basic 'inductive' ignitions on all these cars, not CD, digital or any other types you so quickly brought into the reply. If you will reference other sources on inductive ignition, you will find the following being explained; The coil is of a design that needs to magneticly SATURATE it's core with a magnetic field (called dwell time) such that when it collapses from the points opening or the trigger device cuts the current to the coil, the collapsing magnetic field 'self induces into the secondary and provide the high voltage output the coil was designed to give. Note that the voltage does indeed step-up but it is not only the 'simple' tranformer action you refer to. Here is the second part for the reserve concept; In the plug gap is the air/fuel mix that is in the 'electric field' between the center electrode and the ground strap. This A/F mix takes a certain amount of voltage to allow it to be broken down (electrons pulled out of molicular orbit) and becomes conductive as the begining of the spark kernal for combustion. Normally it will take between 7000 and 10,000 volts to break over the plug gap. If the coil will output any higher voltage, only that amount it takes to spark the gap and start the kernal will be used with the rest waisted by the low resistance of the breakdown to ground appearing as short circuit to the coil after breakdown has begun, so the full volage cannot be develped each time. This is true on all 'inductive' saturating ignition systems. The rest of the 'reserve' story is this; If the A/F ratio goes lean, the plug gap widens, the plug wires get leaky, the wire carbon center resistance goes up, the distributor cap or rotor has faults; this all RAISES the voltage needed to fire the mix, HENCE some or all the reserve from the coil is used. Note that the rotor to cap gap is in series with the plug gap and is often as much as .100" in addition to the plug gap. Once the coil max voltage (reserve) is exceeded by any circuit conditions, a cylinder missfires because the combustion kernal can't be reliably developed. As an example: on a late FORD 4.6 or 5.4L mod motor with coil over plug configuration, there is a condition not many recognized until I made it known on some other boards and have the experience with. Here is how that goes; on those motors with inductive coils, yes they are still inductive saturating; at cruise light loads and speeds between 40 and about 60 mph, the fuel is cut back, ignition timing advanced and EGR introduced back into the intake resulting in air/fuel ratios as high as 20 to 1. These a/f ratios are very hard to fire such that coming from a nearly closed throttle, a coil with shorted turns has run out of reserve capacity and can't fire the extremely lean a/f ratio so causes the motor stumble UNTIL the a/f richens up again and takes less break-down voltage, again. THIS IS THE RESERVE DISCRIPTION AT WORK. Even with the sofistication of Ford's modern PCM driven FI system, it can't ID the offending cylinder because the fault does not last long enough before it clears, to build any history in the PCM memory for code setting and clears as soon at the stumble condition is 'driven' out of by heavier throttle application calling for richer mixtures. What is the fix? The condition in now known and the repair is to either do a coil output 'stress test' or change all 8 coils and hope none of the new coils have the same fault or it still will not known which cylinder is offending. Am I forgiven? Please!
Additionaly, other systems such as CD does operate differently and does often use the more simple stepup coil types as a high voltage pulse tranformer by having the ignition 'amplifier' generate a very high (coil driving) voltage independently of the coil stepup such that a saturating type coil is not always used but could be. Example a mfger will spec either the stock coil or his or some other speciality coil as compatable but they arn't always the same designs. There are other types of driving systems but that's enough for the purposes here. Lastly, many are under the impression and from advertising hype that high voltage coils make more power. Not true because the plug and mix in the cylinder still only take the same voltage to break down the gap. The rest is just higher reserve and make the life of the wires , cap and rotor shorter. If one sees an improvement, there were faults present with the original hardware. Going one step more is the multispark ignition coil driver. This indeed often will be felt as an improvment but don't fit the usual ignition operation as far as referenced to a stock system and shows there is not the same combustion conditions everytime a cylinder comes up to compression at varying engine speeds and the fact that conditions are not the same everytime, so multspark tends to help fix this.
