Spark - Gap That Matters

The question - For those of us with stock CDI units, how much can be gained (or lost) by changing the timing from narrowing the plug gap??? On your Website there's a story from a guy named Grabowski (sp?) that states that a difference in plug gap of .002 can make a difference in how a 2-stroke motor runs. Comments?

The response - I don't know what I can add to that article except my personal experience. George Grabowski seems to pretty much know what he's talking about. He may not be a Jennings or a Cameron (as far as technical writing goes) but he covers his bases well and his procedures seem to be first rate.

My 1986 TRX250R runs static compression of about 210 PSI here in town. If I check it at Glamis it will be about 10% higher - Glamis is below sea level. This makes for great low end and midrange power, though it does give up some rev-a-bility on top. With this much static compression there is a lot of pipe effect at high RPM. That is to say that when the pipe is working and the motor is "on the pipe" the compression will be high enough to stifle a wide gap (or even normal gap plug). The compression will blow out the flame - or at least upset it a lot. This makes for an engine that sounds rich and blubbery, though in fact it is running poorly because the spark cannot jump the gap and ignite the incoming charge under pressure.

Closing the gap a couple of thousandths can make a (big) difference and in my case is indicated. I have to run the gap closed up a bit. The stock ignition can not supply the amount of voltage required to make the spark jump the gap at high RPM. My alternatives are to lower the static compression - or make a pipe with less pipe effect at high RPM. By an after market ignition in hopes that it can produce the power needed. I know of the MSD unit though I have been hesitant to buy it because it requires a permanent modification to the flywheel. Though I may have to get it in order to be done with this problem. I would rather find an ignition unit from another Honda (CR something) that would work.

Generally speaking, gap the plug at it's widest setting. This will expose the most amount of charge to the most amount of spark. If up against the compression wall - reduce the gap a bit, though not a perfect solution - it will help the motor run better. Whenever possible torque the spark plug. It does matter.

Rick

I should look into all aspects of this problem/solution before I make any changes. Whether that be a higher output coil or other means of increasing the voltage to the plug I'll search out whatever I can find. It is such a small problem that it is (almost) not worth dealing with... though... it may be worth a couple HP peak and a little before and after the peak as well. Now that I think about it I should get on it right away.

Rick

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