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What is 8-stroking? What does it sound like?
#285458 11/23/09 7:18 pm
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Hello, I'm doing a bit of carb tuning and I am curious as to what is meant by the term "8-stroking." What does this sound like? Why is it called 8-stroking? I am assuming it is some pattern of irregular exhaust impulses, but I'm not really sure. Is there a way I can recreate this issue on purpose by changing jet sizes or other settings, in order to get a reference of what to listen for in the future? This is more of a general question than any specific problem I am having, but for what it's worth, I have a 1973 Daytona converted to a single Concentric carb with 3.5 slide, needle in middle groove and 180 main jet. Any information on any of the above is very much appreciated. Thanks.

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Re: What is 8-stroking? What does it sound like?
ridgeracer #285467 11/23/09 7:54 pm
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Turn the choke on or if you do not have a choke, remove the main jet.

Instead of the bike firing every 2nd revolution (as a 4 stroke would) it fires every 4th revolution (or eight strokes). It does this because the mixture is so rich that it needs the extra revolution to get enough air in to allow combustion.

So instead of broom, broom it goes Bah, Bah, Bah, Bah, Bah.

Hope this helps.


Re: What is 8-stroking? What does it sound like?
John Healy #285468 11/23/09 8:04 pm
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Since in this case the bike is a twin, it might be more clear to say "cylinder" in the above instead of "bike." I've often wondered why the mixture leans enough on the second intake stroke to allow combustion. I would expect it to more likely be even richer since it didn't fire the previous time and everything's probably wet with fuel, but nevertheless I've experienced it many times.

It's possibly because the non-firing stroke allows the cylinder to cool to the point where it requires a richer mixture to be optimum.


"Beer is proof that God loves us and wants us to be happy" - Benjamin Franklin
Re: What is 8-stroking? What does it sound like?
hh #285520 11/24/09 12:34 am
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If you miss a firing stroke (through excess richness),you also lose scavenging during overlap at the end of the exhaust stroke.No scavenge gives reduced intake velocity on the following intake stroke,and a slightly leaner mixture.
It can lean off enough to fire once,then repeat the whole procedure again.

Re: What is 8-stroking? What does it sound like?
John Healy #285573 11/24/09 10:11 am
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Originally Posted by John Healy
Turn the choke on or if you do not have a choke, remove the main jet.

Instead of the bike firing every 2nd revolution (as a 4 stroke would) it fires every 4th revolution (or eight strokes). It does this because the mixture is so rich that it needs the extra revolution to get enough air in to allow combustion.

So instead of broom, broom it goes Bah, Bah, Bah, Bah, Bah.

Hope this helps.

What an apt description JH!! Mind you, I guess you've had to describe this a few times in your life. RR


'72 Bonneville
"He who praises you for what you lack wishes to take from you what you have." - Don Juan Manuel
Re: What is 8-stroking? What does it sound like?
ridgeracer #285626 11/24/09 3:54 pm
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Thank you guys for all the information! I will take this all into account next time I get a chance to do a bit of experimenting. I really enjoy figuring out the how's and why's of it all, and this will be a big help.

Tim


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