The 60/40 rule

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mikenixon
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The 60/40 rule

Post #1 by mikenixon » Sun Dec 04, 2016 9:48 pm

The 60/40 Rule
There is a troubleshooting technique known in the industry as the "less air/more air" test. Relied on by generations of home-grown mechanics and popularized in recent years by the customer service dept. at Dynojet, it is an easy way to divide the probable cause of an issue into the areas of fuel, electrical and mechanical. It's a classic test, and very effective if done correctly, which is what this article is all about.
To do the test simply duct tape up half of the air filter. Note the performance with the duct tape, and then for good measure reverse the procedure, that is, remove the tape and add some air by propping open the airbox cover or similar. Don't just take the air filter out -- in many bikes this is too much change. Again, note the performance. If the performance symptom did not improve in either case, you can be about 60 percent sure the cause of the symptom is not an intake issue.

But why only 60 percent? The reason is important and it is what I want to emphasize in this article. Even with the high probability of a fuel system issue when the less air test shows positive, 40 percent of the time it's electrical, i.e. ignition. The reason is simply that air/fuel mixture and spark are interdependant. Each needs the other. The air/fuel mixture works best when the spark is vigorous, and the spark is most effective when the air/fuel mixture is potently metered. If either is weak, the other has to compensate to maintain good combustion. If spark is weak, richer mixture is needed to make up for it. If mixture is weak, spark has to be stronger to compensate. They are inverse, or opposites.

Let's see the 60/40 rule in action. I was helping someone troubleshoot their bike. We used the less air test and found its result dramatically positive. The bike revved much better with the duct tape. So, a carburetion problem, right? No, not necessarily. Remember the 60/40 rule. After making sure the carbs were clean, there was good fuel flow to them, the air filters sealed well and were unobstructed, and there was a seal at the manifolds, we turned to the ignition system and found that one of the bike's spark plugs was firing, not at its electrode but down inside the plug, partway down the insulator, at a crack in the insulator. A misfire in other words. The partly drained voltage weakened the spark. The less air test we performed added mixture quantity, making up for the poor spark. This is a classic example of the interdependedness of fuel and ignition, and had we not applied the 60/40 rule we would have gone off on a carburetor tangent, when there was nothing at all wrong with the carburetors. Remember, just because more fuel gives a positive result does not mean the problem has to be fuel! More of one makes up for less of the other. Don't forget the 60/40 rule.
Mike Nixon

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gltriker
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Re: The 60/40 rule

Post #2 by gltriker » Sun Dec 04, 2016 10:04 pm

Very interesting, Mike!
What is meant by saying, " when the air/fuel mixture is potently metered." ?

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mikenixon
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Re: The 60/40 rule

Post #3 by mikenixon » Sun Dec 04, 2016 10:15 pm

gltriker, thanks for asking. Accurately, especially as to correctness for load and rpm. Specifically, it refers back to the principle that leanness needs more spark, richness needs less. The reverse is also true. Weak ignition requires more of carburetion (why modern, uber-precise fuel injected vehicles have wimpy cost-down ignition...they don't need more), and strong ignition makes less demands on carburetion (why the 70s car coil mods were so effective on RD350s and GS750s back in the day). :-)
Mike Nixon

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Angeltrumpettech
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Re: The 60/40 rule

Post #4 by Angeltrumpettech » Mon Dec 12, 2016 5:20 pm

Hello Mike, when performing the 60/40 test, is there a certain RPM we should get to , to get accurate test result?

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mikenixon
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Re: The 60/40 rule

Post #5 by mikenixon » Tue Dec 13, 2016 11:19 pm

Hi, Angel. Just the rpm or condition where you are experiencing the trouble. Or all rpm. The idea is, richer...what happens, leanner...what happens. By the way, some advocate using the choke intead of duct tape, and that is okay in certain cases, but it also can in some cases introduce more variables due to eddy air currents.
Mike Nixon


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