If I remember correctly our books say the spark plug gap should be .023 to .028. I have allways shot for the mid point of .025 but I don't know anything about how these settings change how the bike will run. Can someone please explain to me how combustion is effected by say a tight gap as opposed to a loose gap? And also the same thing pertaining to valve clearances. I use NGK plugs but don't want to start a fight between the diff types, I just want to learn why we gap these things the way we do. Thanks. Glenn.
Spark plug gap & valve clearances & what they do?
- Thread starter gtsun
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Plug gap is a compromise between being small enough for the spark to jump reliably under compression and being big enough to reliably light the fire under the leanest conditions. You will probably not be able to discern a difference in running from min to max spec.
Valve clearance allows the valves, head, cylinder all to expand at running temp and still have clearance to live.. that is, allow oil between the tappet and cam, pushrod and rocker, tappet, valve stem before the next opening event and insure the valve head seats reliably. Too little, valves will not seat properly and hot combustion gases will begin the "burned valve" scenario rather quickly. Too loose, the actuation mechanisms will hammer themselves looser still, and degraded performance from late/low opening. Again, from min to max alowable specification, you won't be able to notice a difference in running.
Valve clearance allows the valves, head, cylinder all to expand at running temp and still have clearance to live.. that is, allow oil between the tappet and cam, pushrod and rocker, tappet, valve stem before the next opening event and insure the valve head seats reliably. Too little, valves will not seat properly and hot combustion gases will begin the "burned valve" scenario rather quickly. Too loose, the actuation mechanisms will hammer themselves looser still, and degraded performance from late/low opening. Again, from min to max alowable specification, you won't be able to notice a difference in running.
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The range of gap in the manual will not cause detectable differences, that why its given that way. If the ignition can jump a bigger gap that above then some benefit in better combustion as more plasma kerna exposed to mixture. One small effect of opening gap is it takes longer for the coil flux collapse to build up enough voltage to jump, so slightly delays ignition timing. Is that detectable, yes but only on HIGHLY refined tuned race engine. If ya care to experiment for us start opening gap up till hard to start or running mistfires then close up till just don't and let us know what ign system and the gap and brand/type of plug and fuel run and compression ratio.
Oh yeah plug indexing may be more detectable than plug gap that ain't misfiring. Mixture swirl is said to swing past the exht valve towards plug so maybe face the open hook at the exht. valve. Plug indexing washers used for this. Just me guessimating.
I lett a down dirty lash adj. post to be spot on valve lash, which ain't fixed in stone but at the extremes does as written above. If we could set lash like my old chevy ya warm up with all of em on loose side then diddle lash down till quiets best and not click-clanking in lock up. Type of push rods matters as does barrel material but most us Cdo'rs got what factory issued.
Oh yeah plug indexing may be more detectable than plug gap that ain't misfiring. Mixture swirl is said to swing past the exht valve towards plug so maybe face the open hook at the exht. valve. Plug indexing washers used for this. Just me guessimating.
I lett a down dirty lash adj. post to be spot on valve lash, which ain't fixed in stone but at the extremes does as written above. If we could set lash like my old chevy ya warm up with all of em on loose side then diddle lash down till quiets best and not click-clanking in lock up. Type of push rods matters as does barrel material but most us Cdo'rs got what factory issued.
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