BK Go-Go ! Like a Pro Stock Dragster . . .

36 replies to this topic
  • yzernie

Posted November 15, 2001 - 07:47 PM


Originally posted by Boit:
On the advice of Rich Rohrich, I've been experimenting with VP MR2 fuel. While this fuel was aimed at the 2-strokers, it is also an excellent choice for the 426 due to its light-ends.[ November 15, 2001: Message edited by: Boit ]

VP MR-2 is a great fuel. I simply cannot justify $9.00-$10.00 a gallon for it. I use Unocal 110 (The official fuel of Nascar!!) and have found that it works better than VP Red. It is usually a bit cheaper that VP Red fuel and performs well all the way up to the rev limiter.

As Boit mentions, race fuel will enhance performance over pump gas. Jetting the bike correctly will make it run better regardless of what fuel is used.


  • yzernie

Posted November 15, 2001 - 08:13 PM


Originally posted by Frank B:
I didn't change the pilot jet, and I'm not sure what stock is.

Frank B,

I don't know what year your bike is but if it is a 01 the stock pilot is a 42. Since you are at sea level, I would try a 48 pilot jet, lean the needle one clip position and a 168 main. The fuel screw can be used to fine tune. That should get you pretty darn close.


  • Psycho426

Posted November 15, 2001 - 09:00 PM


Frank - That definitely a jetting issue, mine did the same before I changed my jetting, follow Ernie's advice

Fastest1 - Sounds like you acc. pump is disabled or might be malfunctioning, remove the plastic throttle cover on the carb and check your acc. pump delay screw, your delay might be set so high that the pump never kicks in. Record you current settings get your pump working and try the mod if you don't like the results you can always go back, you'll never know if you don't try!

  • MX_Tuner

Posted November 16, 2001 - 02:16 AM


VP Red is not a very good choice for a big bore four stroke.

  • Fastest1

Posted November 16, 2001 - 05:46 AM


Thanks for the quick reply Psycho426. Today I think I will investigate the settings etc. This bike runs so well I was sure somebody had done the mod, BK or other. Have you heard of anyone disconnecting their AP? This bike absolutely has no bog it revs all the way from idle in neutral or with a load. No stalling , sputtering nothing but a hard pull! I have never ridden another 426, But I have ridden my buddy's Husky 610 and it pulls that good at least. He says it is damn fast(my 426). I also have an R6 and a VFR 750 both road race bikes and this bike still pulls hard. Thanks again.

  • vaughn

Posted November 16, 2001 - 05:57 AM


Feel like I need to give my $0.02 worth.

I had done the BK a couple of months ago to my 01 426. Since I had not done any other jetting changes from stock, I also leaned everything up one position due to 6000' altitude. The bike stock had about a 1.5 sec squirt and I adjusted it to about .4 sec and went to the track. The bike ran really good but still could be bogged with a quick WOT (Wack-of-throttle).

After 2 months of riding I decided to undo the BK and leave the jetting because I wasn't sure if my jetting changes or the BK caused the better running bike. With the BK mod removed the bike ran just as good thus my jetting adjustments were the primary reason for the better running bike over stock. I also adjusted the fuel delay screw 1/2 turn in as suggested by others here and that seemed to really improve the WOT bog issue.

My final jetting is 158 main, 1 position leaner on the needle, 45 pilot and 1.75 turns fuel screw.

I'm inclined to believe that the bog can be reduced/eliminated by the fuel delay screw as much or more than the BK mod. It makes sense that by reducing the fuel squirt to a measilly .3 seconds is effectivly muting the effect of too early of a squirt. By delaying the squirt allows the engine to start picking up RPM before the fuel dump and allows it to process the fuel without beeing overwhelmed with fuel.

Just my thoughts.

  • Fastest1

Posted November 16, 2001 - 04:07 PM


OK I was asleep last night during the initial carb removal. I thought my AP was not spraying but I think my bowl level was low. My carb which performed excellent was set up as follows MJ#170, PJ#48, Pilot screw @ 1&1/2 out, Needle is OBEKR 4th clip down. I did the BK mod lessened the spray to .3-.4 seconds and it does seem to start easily. It also wheelies like all hell but it did before! Why are my jet so much richer than others in my area? The bike is otherwise stock except a Uni filter. I am going to buy the next 2 jet sizes(down) and pilots to check if I cant make it run better. It seems a hair to rich by looking at the plug and exhaust. Thanks

  • yzernie

Posted November 16, 2001 - 05:18 PM


Originally posted by Fastest1:
[QBWhy are my jet so much richer than others in my area?[/QB]


All bikes are just a tad bit different. My buddy and I got our 426's at the same time, from the same dealer and the VIN numbers are very close. I went one up on my pilot and he had to go two up on his. We ride in virtually the same locations so there is no difference of elevation, have the same exhaust systems and run the same air filters/filter oil. We even run the same race fuel out of the same drum.

There must be very minor tolerance differences in the bikes that cause these issues. Anything that is mass produced will have very slight differences. As long as it runs good, fine tune your jetting and haul ass!!!


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  • Psycho426

Posted November 17, 2001 - 10:42 PM


Vaughn - That might be a good test! I rode her at the track this weekend; she pulls like a bat out of hell right from the bottom (with BK mod .3s duration).
Before next weekend I’ll record my current settings undo the mod, reset my delay and then see how she runs. Before I did the mod my jetting was stock standard it will be interesting to see what effect the jetting changes had with out the mod in place (stock duration).

Fastest1 - How did she do at the track? Did she perform better with or with out the mod?

  • yzernie

Posted November 19, 2001 - 02:58 PM



Did you return it to stock this weekend?? If so, how does it compare with the BK mod??


  • Fastest1

Posted November 19, 2001 - 09:13 PM


I havent gotten a chance to run it on the track yet. Mostly due to my refusal to stand up on the doubles and came up a little short!(Ouch)I feel shorter all of a sudden? So I am taking it easy for a couple of weeks. I am 40 so this doesnt heal like it used to. I have just had a chance to ride it in the driveway/street it seems to still pull your arms off. It ran as well as could ever be expected before. But I will get it on the track soon and evaluate it. Cya

  • Psycho426

Posted November 19, 2001 - 10:37 PM


Ernie - I will only return it to stock this weekend (24-25 November) I'll let you know how she ran.

Fastest1 - I agree take it easy, the last thing you want to do is injure yourself again :) only to sit out for another couple of weeks!

  • Psycho426

Posted November 24, 2001 - 06:28 AM


I undid the mod today but left the jetting changes...

I must say, she did run better than stock but there was still a bit of a bog, I tried to remedy that by adjusting the pilot screw but was unsuccessful.

With the mod the speed pickup is instantaneous, if you wack the throttle she just rockets :) I am going to re-install the BK duration screw tonight and reset my duration, I'll never go back :D

  • yzernie

Posted November 24, 2001 - 07:42 AM



I knew you would like it!!!!


  • vaughn

Posted November 26, 2001 - 10:52 PM



Did you try to turning in the fuel pump DELAY screw 1/2 turn (see my previous post in this thread above)? I know you said you played with the fuel screw but I found that delaying the fuel prime a little would reduce the bog at WOT.

Seems to me that the throttle wack bog would occur with either the BK mod or no mod if the fuel prime delay is not sufficient. Most people adjust the delay to just miss the slide. I think to just miss the slide plus a 1/2 turn in on the delay screw (for good measure) works well.

Remember that the "just missing the slide" does not take into account air flow under power. The suction from the running engine will pull the spray past the slide faster than a bench test will (thus the reason for 1/2 turn in extra).

  • MN_Kevin

Posted November 26, 2001 - 12:34 PM


For you guys that are hesitant to do the BK mod, just check your APJ (Accelerator Pump Jet)actuation. Pull off the carb w/ a FULL float bowl, or keep the carb attached to the petcock via a longer fuel line. Wack the throttle open observing the fuel stream shoot out ~ 2 feet. If it hits your slide, YOU ARE LOSING PERFORMANCE. If nothing else, as mentioned already, at least delay the timing to set your squirt to where it shoots down the throat of your carb, instead of puddling in the carb and allowing engine vacuum to suck it all in.

If you have jetted your carb w/ out the BK mod (and your APJ setting is AFU), you have leaned it out to compenstae for an overly rich condition. It is kind of like putting lipstick on a pig. Don't live with it if you don't have to. Set your APJ to work as hoped, play with your jetting, as tedious as it might be, and get your bike to run as efficient as it can. You are losing that instantaneous bottom end performance the APJ is designed to give you.

Just like practice riding: It is fun to blast around the track, but is it as good practice as working on one difficult corner until you have it nailed(?)...


My 2 cents (sence)

  • Psycho426

Posted November 26, 2001 - 09:48 PM


Vaughn - I did not adjust the delay screw after I undid the mod last weekend(I can't believe I forgot that).

You are running:

<ul type="square">[*] ACC pump with stock duration. [*] Reset the delay screw so it just misses the slide + 1/2 a turn in.[*] 158, 45, clip 3, 1.75 on the fuel screw.
And your bog is gone.

I'm at 5000'-5500' temperature ranges from 77 - 85.

I'm running:

<ul type="square">[*] ACC pump with .3s duration.[*] Delay set so the spray just misses the slide[*] 165, 45, clip 5, 1.5 on the fuel screw(reset it from 2 turns last night, seemed a bit too rich down low)
I also have no bog whatsoever.

If I run into problems with the mod (hope that never happens 'cause she's running like a dream now) I'll give your settings a shot.

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