K&B 3.5 NEED HELP WITH A FEW QUESTIONS.

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Rokieboater

Well-Known Member
Joined
May 12, 2005
Messages
1,591
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.
 
The K&B 3.5 runs a short reach plug.
I can't remember the head clearance. But I will try to measure mine
 
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 .
 
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.
 
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..
 
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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