As has been already stated, most performance engines will work well with around 10-12 degrees of vacuum advance. There are 2 different designs of the HEI coil. Another thing that is often overlooked, is if the cruise rpm is less than the rpm where the mechanical advance is all in by, the vacuum advance has to make up the difference to get the best mileage and drivability. Also the amount of advance supplied by the mechanical advance was set up for the specific application it was used on, and this is seldom what's needed for a performance application. For a race or a weekend street/strip vehicle this is probably no big deal, as long as fresh plugs are installed when needed. Click toolbar for various cam duration specs. Hold the distributor down against the intake (or block) with one hand and with the other hand lift up on the top plate of the distributor shaft (usually where the mechanical advance mechanism is located) to be sure there is some play and that the shaft isn't "solid"- which would indicate the distributor was bottomed out and there was no clearance. These later HEI distributors are not covered in this article, other than to say they're generally not used in performance applications unless used as just a trigger for an aftermarket ignition amplifier box or with a modified computer. Regardless of the brand of carb, opening the primary throttle blades too far in an effort to get a good idle will result in the idle being OK in neutral but will drop too far when the transmission is put into gear, along with the carb having poor off idle response. A word on the internal coil computer controlled HEI distributors: A computer controlled HEI distributor work basically the same as a non-computer controlled HEI except for the lack of a mechanical or vacuum advance mechanism (some early versions did use a vacuum advance); the ECM determines the advance curve electronically. For a daily driven street car, using a vacuum advance is always recommended. Using a manifold vacuum source will in many cases- depending on the cam and compression- allow you to close your throttle plates a little and still maintain the same idle speed. If you try to set your initial timing with the engine idling ABOVE the RPM that the mechanical advance has started to come in, getting a correct reading will be all but impossible. Often a stock distributor is set up to rely on the vacuum advance for a large proportion of ignition advance. This is located inside the distributor, and has the green and white wires from the pick up attached on one end, and a terminal block on the other. To run the engine efficiently without the ECM, sensors, and related equipment fully functional, the distributor and carb need to be replaced at the least. Ignition Coil Problems Ignition coils may be the cause of problems, or problems may be caused by failing spark plug wires. Baseline the throttle blades to give a transfer slot that looks like the image below, right (approximately as long as it is wide, or about 0.020", up to 0.040"): From this point the idle speed can be increased to the point where the transfer slot becomes over exposed. If looking for an HEI, choose one that has the 4-pin module seen at upper left, above. P0351: Ignition Coil A Primary circuit fault. In a stock application this wire is often pink and a relatively large gauge. On a Pontiac V8, the oil galley plug adjacent to the distributor gear can be carefully drilled with a small numbered bit to direct pressurized oil to the gears. If the connections are good, you next need to make sure the unit is getting a strong 12 volts. Remember, any change to the initial will also require the mechanical advance to be changed a like amount so as to keep the total advance where it needs to be. Don't be tempted to excessively widen the gap. There are times where the engine requires so much initial timing that the engine can be hard to start without kicking back against the starter. If an attempt to change the advance curve is made on a distributor that suffers from these problems, the mechanical advance may not work as smoothly as needed. Note: See link HEI vacuum advance specs, below. There is no more need to worry! This does a couple things: First, it will cure nozzle drip and a smelly, poor quality idle caused by the butterflies being opened too far at idle, which allows fuel to be pulled from the transfer slot. This can make up the difference between the mechanical advance you get at your cruise rpm and total amount of mechanical advance. For more distributor tech tips, check out Performance Distributors’ Professional Tips page on their website, it’s chock full of useful information. Holley has you covered with the new Terminator Stealth Throttle Body Fuel injection System. If you have been hesitant on switching to EFI because you like the traditional look of a carburetor? This can be accomplished in several ways. How many of you reading this remember a time when distributor diagnosis meant checking the points? The max mechanical at 3000 rpm is 18 degrees. A lightly loaded engine can tolerate more spark advance than a heavily loaded engine, all else being equal. And your plugs can develop carbon deposits within just a few thousand miles. It should read between 6000Ω and 30,000Ω. Total timing is not affected as much by the cam timing; that's more a function of the compression ratio, fuel octane, quench, engine temperature, air/fuel mixture, cylinder head design and material, etc. This page was last modified on 25 August 2016, at 10:53. In many cases the vacuum advance should use a full manifold vacuum source on the carb- but this is not written in stone. The resistance should be somewhere around 600Ω to 1000Ω. But once the RPM goes up and the cylinder pressure increases, the chance of a misfire increases dramatically with a wider plug gap. A bad ground can also keep you scratching your head. Cheap/offshore buttons can have excessive resistance; this causes heat and can in extreme cases melt the surrounding plastic of the cap and the engine will quit. The nylon distributor shim kits are sold through Summit and Jegs, etc. Once the engine is turning over, the switch is released. Newer HEI distributors (1981-86 for passenger cars exc. Depending on the cam specs, adding initial timing in addition to or instead of tipping the secondary throttle blades open more may give the best results. Chevy Hardcore NEWSLETTER - SIGN UP FREE! This is done by measuring the amount of play between the distributor gear and the thrust washer. Yes, you can remove a computer controlled HEI and drop an old-style mechanical advance distributor in its place; it will physically bolt right in. To use the interrupter, the switch is depressed (or toggled, depending on the design) and the engine is cranked via the key/starter switch. To check the secondary resistance, remove the screws that hold the coil in the cap, remove the coil and touch the negative meter lead to the ring terminal on the black wire (between the red and yellow) and touch the positive lead to the bottom of the coil where the rotor bushing makes contact. The kits typically contain 0.010", 0.020", and 0.050" (or 0.053", depending on brand) shims. Another adjustable vacuum advance can for the GM HEI is the Accel p/n 31035 that is said to allow infinite adjustment to both the amount and rate of advance. If you have a computer controlled distributor in your car right now you can’t do much to increase performance other than to make sure it is correctly communicating with your car’s ECM and upgrade the coil to a better unit. You can run without a vacuum advance but expect your highway mileage to suffer, possibly more. Talking Hydraulic Clutches With Silver Sport Transmissions, New ’69 Camaro Billet Taillight Bezels From Eddie Motorsports. Any time during the adjustment procedure that the curb idle becomes too high or low, readjust the curb idle for proper idle speed. If that's not your #1 plug wire, or your vacuum can is pointed in another direction, the distributor may be installed one or more teeth off. Failure to do this can lead to an early failure of the module. Example of a "typical" performance ignition advance curve, Wells catalog to be used for looking up representative part numbers to find specs for testing, GM points-type vacuum advance can specs and info (Lars), How to find the number one cylinder in an engine, https://www.crankshaftcoalition.com/wiki/index.php?title=Hot_rodding_the_HEI_distributor&oldid=1064098, You will want to begin by knowing the timing tab and line on the damper are accurately indicating. Use ported vacuum in this case; you don't want or need any more advance at idle. You will need to remove it and take it to an auto parts store that has a module tester. A relay that delays the ignition from being energized- giving the starter time to get the engine turning over- can be wired into the ignition feed wire. But it will cause the ECM to throw a code (the check engine light will be illuminated) because the ECM senses if the computer-controlled portion of the distributor is functioning every time you start the engine. The primary resistance value should be between 0.6 – 1.5 ohms. The drawing on the left shows an overexposed transition slot. Note: Other carbs have similar transition circuits, and they need to have the throttle blades in the 'sweet spot' same as a Holley. On the SBC there were three different combinations of damper lines and timing tabs that go together. Crane vacuum advance limiter plate pn 99619-1. The distributor drops right in the place of the factory distributor and uses GM large-cap HEI technology with an integrated coil and module. You want the mechanical advance in as soon as the combination will allow, without causing pinging. This is something an adjustable vacuum advance can help; adjusting it to give advance at a vacuum level just below the vacuum seen at cruise RPM will let the engine run smoother and get better mileage. So fix it first or get another HEI to start improvements on; just make sure you are getting the right one for your engine- they were used on ALL makes of GM inline and V6/V8 engines and all look similar. The "missing" 6 degrees can come from the vacuum advance. If this is the case, the engine will idle high when out of gear and then the idle speed will drop down once it's put in gear, and the off-idle response will be poor. from Moroso, Mr. Gasket, Jegs brand, and others as well. This mod will have you grinning ear-to-ear with the nice seat-of-your-pants improvement! This isn't a good plan for the street, but in some cases it'll be about the only way to get a cam to work on the street. The button has to be assembled into the cap first, then the rubber insulator. The mechanical advance should be "all in" by about 2800-3200 RPM for a typical street performance motor (additional advance above this RPM point is neither needed or wanted; increased turbulence in the combustion chamber offsets the need for further ignition advance).
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