K&B 3.5 NEED HELP WITH A FEW QUESTIONS.

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Rokieboater

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May 12, 2005
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I have a new guy setting up a .21 mono and he has a K&B 3.5. Was wondering if anyone remembers if that motor ran a short or long plug. Maybe a ggod starting point on head clearance. Without shims it is .032.
 

Kez

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Feb 27, 2003
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The K&B 3.5 runs a short reach plug.
I can't remember the head clearance. But I will try to measure mine
 

Tom Foley

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Oct 27, 2005
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I would run no more than .015 head clearance . Less is even better down to .006 but if he is new stay a little conservative . run a little rich for the rod life .
 

Al Hobbs

Legend of the NW
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Aug 25, 2009
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The early K&B 21 engines used a short plug. Later versions used a long plug.

I think that you would want 0.006' head clearance.

The weak point in that engine is the connecting rod.
 

Wasted wages

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Jan 2, 2006
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1,840
I've seen people run them on 25% to 65% with various degrees of success..
Higher percentage, and needles start to get finicky... 40-50% seems to work well..

Just check the rod and bearings often..
 

Mike Hughes

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Feb 23, 2003
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12mm cranks ran the short plug, 13mm changed to long. But both head buttons fit the same motor so there is a possibility that it could have change. If you see the side of the head and its about 1/8 showing then it is the short plug. More would be the long one.
Mike
 

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