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Author Topic:   engine misfire
posted September 05, 2000 12:35 PM
Engine runs fine at shop.When the green flag drops and I get on the gas there is a misfire.The sound reminds me of an engine that has hit the rev limiter.Fresh engine.What should I look at?I use an MSD system.Jetted up,no difference.

Earl Parker II
posted September 05, 2000 06:08 PM
Assuming the engine itself is OK it sounds like a case of insufficient electrical potential at the plug gap.

If you ever suspect an ignition problem and want to perform a quick-and-dirty check, gap a new set of plugs at .025" (assuming that's smaller than the gap you normally use) and run them a few laps. If the problem gets better or goes away, you can reasonably deduce that you have an ignition problem.

Assuming the problem is lack of ignition energy there are a number of things you can do to optimize your ignition system and eliminate the problem:

1) The one thing that will help you the most here is to install an alternator and have it charging the battery at all times. I realize that conventional wisdom says that an alternator robs horsepower and slows you down. Nothing could be further from the truth. If the battery doesn't need charging, the horsepower absorbed by the alternator is negligable; if it does need charging, the alternator will return far more performance than it will ever take away. You will also hear that the MSD ignition will perform at it's optimum with as little as 9 volts of input voltage. If MSD has figured out a way to get the same output, in terms of both voltage and amperage, from 9 volts of input voltage as they have from 14 volts of input voltage then they have managed to violate one or two laws of physics. You can believe that a properly setup alternator is essential for maximum performance.

They following will help with alternator setup: 1) You want it to produce about 14.0 to 14.5 volts. 2) Add up the amperage requirements of everything electrical on the car, increase the total by 50%, and that's the amperage you want it to produce. 3) Use deep groove pulleys to keep the belt in place. 4) Size the pulleys to turn it just fast enough to get maximum output and no faster. 5) Ground the body of the alternator to the cylinder head with at least a 10 gauge ground wire.

2) Proper ignition system grounding is also essential. You'll need several feet of 2 gauge ground cable and four eyes. Find a spot on the roll cage that is central to everything that has a ground wire. Weld a stud onto this spot- this will be your central grounding point. Ground everything that needs to be grounded to this stud. Next, cut a length of the cable that will comfortably reach the stud from the cylinder head. Crimp and solder an eye onto each end of the cable. Find an unused, threaded hole in the cylinder head (be sure to scrape any paint away) and bolt one end of the cable to the head. Bolt the other end to the central grounding point. Prepare another length of cable in the same manner and run it from the central grounding point directly to the negative post of the battery- do not use the roll cage or the body of the car as a ground path. If you ground you ignition system in this manner then you're grounded as well as possible.

3) If you have a master on/off switch, check for a voltage drop across it. Very often these switches will experience a voltage drop due to a very light oxidation on the contact points. If your switch is more than a year old, replace it anyway.

4) In general, every connection in your ignition system should be crimped and soldered- crimping alone isn't good enough.

There are several more things that you can do to help your ignition system but if you start with the above you should eliminate a big chunk of your problem.

[This message has been edited by Earl Parker II (edited September 05, 2000).]

posted September 05, 2000 09:53 PM
I know alot of guys have trouble trying to determine if the engine miss is electrical or fuel related. Remember, electrical misses are sharp and fuel misses tend to roll or stumble.

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