BK mod query

8 replies to this topic
  • TallJohn

Posted December 27, 2002 - 06:27 AM


Hi people in the know.

Not sure what I have done, but having done the mod, the plastic cam now doesn't travel to the edge of the carb case (or my BK mod screw either) when I open the throttle fully. As a result turning in the screw doesn't make any difference.

The only thing I can do is to screw in about 8mm, but that looks way off from the descriptions and comments I have seen elsewhere.

Anyone got any ideas?


  • tctrailrider

Posted December 27, 2002 - 10:09 AM


The black plastic cam will not turn until it hits the stop on the case, mine stopped well short of it. Without the BK mod it stops where it feels like stopping and this varies alot from bike to bike. With the BK mod you control where it stops. If the cam does not hit the screw the BK mod will not change anything. I would suggest that you pull the carb away fron the motor (so all the gas doesn't flood it) leaving throttle cables and fuel line on. Look in the back of the carb and twist the throttle and try to judge how long the squirt is. By turning in your new screw you will shorten the squirt duration. You want it 1/3 to 1/2 of a second. So when you twist the throttle the cam will turn. Turn in screw until it just touches the cam lobe at its stopping point. If you turn it in further it will shorten the squirt duration. Hope this helps

  • tctrailrider

Posted December 27, 2002 - 10:14 AM


Forgot to mention there is a very good description and photo's of the BK mod on Motomans site here on TT. Its in his Tech articles.

  • Hick

Posted December 27, 2002 - 05:21 PM


Without the BK mod it stops where it feels like stopping and this varies alot from bike to bike.

The spring coiled around the slide pulley forces the arm to operate the rod downwards until the tab on the pump diaphragm contacts the pump cover.

Stock the actuating arm should move downward almost 3 mm. I have mine set at 1 mm of total travel.

If, from closed throttle to open, your actuating rod is not moving dowward at least 2 or 3 mm it could be you have some crud causing the rod to bind in the carb body.

Hope this helps.

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  • TallJohn

Posted December 29, 2002 - 10:28 AM


Thanks for the advice guys. Will get to it.

  • TallJohn

Posted January 05, 2003 - 05:31 AM


Got my timing to about 0.5 seconds now, but still getting a 'bog' if I open the throttle quickly from idle. Its great with a few more revs, no problem then. Is this what I can expect, or have I still got something wrong somewhere.

I also timed the squirt to just miss the throttle slide opening up.

Anyone any ideas?


  • Chris_from_Oz

Posted January 05, 2003 - 05:37 AM


TallJohn, from what I have read on TT in the past, some of the guys have gone as low as 0.25-0.30 squirt duration to get rid of the bog. You may need to go below your current 0.50 setting?

  • John_Lorenz

Posted January 05, 2003 - 06:31 AM


This drives me nuts

some of the guys have gone as low as 0.25-0.30 squirt

I can not understand nor can I explain how someone can get a .3 or .5 secs of duration by eye.

It is humanly impossible to set a duration like that. Unless you are a Cyborg. I am not slamming anyone, I think it is funny that we see post after post after post, I have set it to .3 or .5 or .75 secs.

Simple the normal Human has a reaction time of .5 secs simple as that. So Picture this.

The person has to hold a stop watch in one hand, hold the Carb in the other right OK

Who turns throttle his buddie ok
so I say go push the button .5 sec reaction , buddy reacts reacts .5 sec later, I see it move Hits stop button on stop watch

Guaranteed you weill see at least 2 - 5 seconds on that watch.

Do this to test my theory

Simply take a stop watch and say start stop each time you say start press start on the watch every time you say stop press the stop and read the time it took for you to react. case closed..

Now setting the duration on the GBmod is very simple

I have a detailed report on setting up the GB/BKmod here
GB/BKmod Explained

Please read through this and tell me how well it works afterwords.


All bikes even with the mod will bog if you are twisting the grip at idle or on a stand. That is normal

  • Hick

Posted January 05, 2003 - 08:01 PM


I can not understand nor can I explain how someone can get a .3 or .5 secs of duration by eye.

I can't understand why more people don't just measure the travel of the rod. It is an easier and more accurate way to measure this.


I guess its sorta like the Betamax/VHS thing.


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