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by HawgRyder, 1975 Shovelhead

This all began when somebody posted a question about the need to rejet when installing different exhaust pipes on a scooter….

Start with a couple of questions...does exhaust pressure/flow effect combustion? Yes! Are aftermarket exhaust systems less restrictive than stock? Maybe, but not always. Now, with this in mind I would recommend two things. First, contact the manufacturer of your chosen exhaust directly and ask their opinion...not just the parts person selling the exhaust over the counter, but the actual manufacturer. Secondly, after you install the new exhausts (with or without any other modifications to your bike), install a NEW set of spark plugs and then take the bike out for a full throttle run and "read" the plugs. Don't know how to "read" the plugs?

This is a set of very basic directions…it takes a while to really know what you’re looking at: ride your scooter to a remote area with a nice, straight stretch of road. Run it up through the gears until you are in top gear and run it full throttle for 1/8 mile or so. Now, this part is critical...pull in the clutch and hit the kill switch at the same time. DO NOT SHUT OFF THE THROTTLE !!! Brake and stop. Now let the motor cool down at least 15 minutes and pull out the new plugs you just put in before this run (wear gloves, they'll still be hotter than heck).

Look at the porcelain insulators around the electrode, especially way down inside the plug at the very base of the electrode…they should be a nice gray to tan color, dry and kind of 'dusty' looking. If they are absolutely clean white, your bike is running too lean...and if they're sooty looking, it's running too rich (not too likely with new pipes). Put the plugs back in and ride the bike for a few miles in high gear at freeway speeds (65 mph or so). Do the exact same stopping procedure…pull in the clutch, kill the motor, stop. Let it cool and read the plugs again…this time you are looking at the midrange circuit. And you’re still looking for dusty gray to tan.

Does all of this mean you absolutely have to rejet whenever you change your pipes? No, it means you may or may not have to rejet, but you ALWAYS have to check. Same exact rules apply whenever you change the intake, such as putting on a different style of air cleaner or different carbs.  Good-luck and have fun.

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