AP Timing/ BK Mod: How to

Today I focused on the AP timing, opened the cover, read the service manual (SM) and attempted the operation. I found the SM to be very confusing. If the adjustment is made to my interpretation of the SM, the AP does not function at all. The SM says, "insert the 3.10mm rod", I did. "Fully turn the AP adjustment screw. Check the link lever 4 has free play by pushing on it slightly. Gradually turn out the adjusting screw while moving the link lever until it has no more free play".

Well, as I understood it, I backed off the adjusting screw until there was no more free play in the link lever or the push rod link lever assembly. This meant that the adjusting screw was just touching the link lever. This also meant that the AP was at the bottom of its stroke w/ nothing to pump. The SM illustration show a finger pushing the link lever to the front of the bike while making the adjustment.

As I twisted the throttle, there was no visible squirt of fuel. After reassembly & try out, the bogg was terrible. Taking a WAG, I turned in on the screw 1 1/4 turns and tried again. Much better, much less bogg. But still not right.

I have to understand this and I don't. Please offer some direction. I have spent hours searching these forums for some clearer discriptions of how to do this. Also spent hours looking for a good description of how to do the BK MOD. All I have found are testimonies of " Wow, the BK Mod is Bad @$$."

Go to the Performance Index and click the link to Motoman 393, it's all there...SC

I found the BK mod how to, and the description & fotos are excellent. I still need some clarification on setting the timing as described above. Thanks.

I am not sure what you are looking for. I will try and give you a simple approch to your AP timing issue. As stated in the BK mod remove the sub frame and air box. This will give you complete access to the carb throat. The Pump timing screw can and should be adjusted for when the squirt starts (initial timing) too soon and the squirt will hit the carb slide and cause a big drip (bad). too late and the carb slide will open too far and let in too much air whithout any coresponding fuel (lean). the squirt should be adjusted to start as soon as possible without hitting the slide for good atomization.

The BK mod screw which needs to be installed by you, controls the end of the squirt or the duration of the squirt which happens to be the amount of fuel introduced by the squirt. In stock form there is no way to limit the squirt so you end up with too much squirt.

By adjusting the bk screw you control when the squirt stops as well as the total amount of squirt. .5 seconds seems to be the favored amount of squirt for trail riding.

BTW you will then need to rejet inorder to correct the fuel ratio

if the SM is confusing then try this visual method outlined in the BK mod procedure:ride:

Thanks GCannon for the reply. I fully understand the BK mod, what it does and how to. Piece of cake. Clearly I will have to put the carb on the bench to figure out the timing issue. I am feeling seriously moronic about the proceedure as described in the service manual.

What direction is the freeplay when the screw is all the way in. The illustration shows the finger pushing to the front of the bike. Makes no sense. I think it should be a pull on the lever and run the screw in to take out the "free play".:worthy:

I will figure it out and post a better description of how to.

Thanks again

Easiest way to set AP timing is lay manual aside and do the following: (REMEMBER - setting timing also affects squirt so set timing first and leave it!)

1. Read this thread by Mark Cantrell to understand AP


ie. Screw AP timing screw in compressing spring. With fingers wiggle black cam back and forth and get a grasp of how the black cam depresses the rod that activates the diaphram. Now back timing screw out until the linkage has no slop (you can feel it when you understand the AP cam and linkage). Now turn timing screw in 1/2 to 1 turn and leave it.

I have an 06 wr450 and I did the BK mod and instead of removing subframe, I removed the carb and hung it from the right side. I reconnected the throttle cables and fed gas into the carb using a 4 ft long clear plastic tube from ACE hardware. Then move the carb up and down while working the throttle to clear air out of AP bowl. and carb bowl. When you get consistant good squirts with your BK mod screw backed out sufficiently, screw the BK mod screw in until it bottoms and you have no squirt from AP. Now back BK mod screw out till you have about a half second of squirt......your done. Since the BK mod screw is sooooooo easy to adjust you can try riding at using different squirts and see what you like best. I like tests that involve RIDING.

Nice thing about bk mod is it stops the diaphram from being depressed and you can shut down the duration of the squirt quite fast. You can also use different leak jets as so desired to regulate VOLUME of squirt. I have a #0 (shut off) leak jet. Most bikes with a typical AP setup have the initial squirt of gas needed when vacuum drops but then the squirt continues after it is needed and makes a rich condition. Try riding with no AP squirt and you'll see it runs better than ever when rolling the throttle on because it doesn't have a rich condition.....but when you whack the throttle, it'll bog. See Mark Cantrel, Motoman 393 articles.....thats where I learned.

Fantastic. :worthy: I understand the physics of how this all works perhaps better than most. I got hung up on the service manual description and illustration and the brief tinkering that I did on the bike w/ the timing adjustment. I am one of those guys that has to understand how it works and why it does what it does.

This has been very helpfull. Thanks again.

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