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Hagerty Employee

The tip of the problem with spark plugs and wires

A friend of mine messaged me recently asking if I could walk him through the process of adjusting the valves on a newly installed engine in his BMW 2002. Since, as I wrote in this story a few years ago, valve adjustment needs to be done on a dead-cold engine; the thing to do was for me to come to him. It wasn’t hardship for me to drive one of the cars the hour from Newton out to Worcester on a cool Saturday morning on Memorial Day weekend. The fact that my friend is a veteran made me all the happier to do him the favor.


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New Driver

I really enjoy these articles, however; I strongly suggest that this outstanding mechanic invests in a manicure 🙂


Or the long thumbnail's are an extra handy tool, always at the ready.

New Driver

Didn't some early BMW's use a weird resistor plug wire or something like that?

Intermediate Driver

With that high of compression, it sounds like he has an octane problem.


Good chance you are right.  High compression requires higher octane to correct detonation issues.  I experience the very same thing everytime I get cheap and put the cheaper premium lower octane alcohol free gas in my built car.  Its straightens out on the next fill with increased octane fuel.  Its possible he's tried this but the article didn't specify.  Can't hurt to try, and save on the shop labor.

Pit Crew

A small gap in the circuit at the plug to wire connection wouldn't make a difference. In some cases it could make the car run better. There is always a gap between the rotor and distributor cap terminals and some spark plugs were (are?) sold with a gap inside the porcelain insulator, called series gap type, which are used on engines that will oil foul normal plugs. Many years ago a device called the 'electronic spark intensifier' was marketed with live demonstrations at State Fairs and carnivals. An engine ran poorly because of one or more fouled plugs, then the device was installed and the engine ran normally. An air gap inside the device did the job.


A typical problem with Weber side draft carbs has to do with the air emulsion tubes that reside under a brass fine-mesh screen between the chokes. Also, was the distributor vacuum advance checked for the proper amount of movement? To get more out of a performance-enhanced engine, the installation of a standalone wideband O2 sensor would be a major benefit. Then, a more accurate diagnosis could be made regarding the fuel/air mixture.

Pit Crew

One thing I might check is the cap side of the plug wires.  As with inline 5 Audis of the day, not only did the plug end of some wire sets come with small and large connections, but the caps also could be "large" bore with "empty" centers requiring a wide plug connector, but they could come with small center towers in each cap opening requiring a similar type of clip needed on plugs without the terminal nut.

New Driver

Accelerator pumps: need to be tuned for volume, timing and duration, not easy, especially without a drawer full of pump parts.

Intermediate Driver

And I was so hoping that you had found the problem...


"...stumbles on light throttle..." indicates a fuel problem--not an electrical problem.


stumbling upon light acceleration can be two different things, both very common. The first is too lazy of a pump shot from the carbs. This means that the accelerator pump needs to be tuned to give an earlier shot of gas, and probably a larger and longer shot as well. The second could be in the spark advance curve. It could be coming on too quickly, or it could be too much initial timing, or a combination of both. 

I noticed that a hotter cam was listed. Once you deviate from factory valve timing events, in the even of a hotter cam like you listed, then factory spark timing events and specs are really out the window. Why? Because the valves are probably opening sooner, staying open longer, and also creating more valve overlap. I see this with a lot of cars, and have been guilty of it myself in my younger years. An engine is built, presumably with a comination of parts that are designed to work together in the same room range, etc., but the car does not perform due to the combination not being properly tuned. The result? Poor part throttle performance, decreased drive ability, and a frustrated owner. Try adjusting the accelerator pump first. Once done find a nice long stretch of road, get it in top gear, floor it (if it is an auto disconnect your kickdown so you csn do this in top gear). Take notice of any flat spots, hold it wide open until you reach the top of your power bsnd, shut it down, coast to a stop, and inspect your number 1 plug. It should be a brownish tan. If it isn't,  you have jetting issues. If you have flat spots, you can tune them out with your advance curve of your distributor. Bottom line: This combo needs tuned!