My Big Bore 4-Speed project was running good but it seemed to lack the low-end punch I’ve come to expect.  I’m running a 97mm pop-top with a short cylinder.  So, the compression ratio is bumped up a bit more than a standard Wiseco pop-top (from 9.5:1 to 10.5:1).   Even with the higher CR, it just didn’t have the part-throttle response that you get with a Wiseco flat-top.  It didn’t feel nearly as responsive.

Except for the main jet, the PWK was set up exactly the same as it was on the 97mm flat-top engine.  I was running a DGK slide needle and a 7.0 slide.  It had this killer hit about 3500 to 4000 rpm that made it seem really fast, but after riding it a bit I could tell that the hit was masking the lack of mid-range.  Even with a 145 main jet it seemed light on fuel.  So, I decided to switch from a DGK needle to an EGK.  The DGK has a 1°45’ taper; the EGK has a 2°00’ taper.  The tapers start at the same elevation and the basic needle diameters are the same, so low speed ops should not be affected.

With the EGK needle, part throttle ops were spectacular but now WOT was grossly rich.  That’s a valuable tidbit of info.  The only way that can occur is if the needle is metering fuel at WOT.  Reducing the main jet all the way down to a 132 brought the mixture at WOT to acceptable limits, but now it was light on fuel at part throttle again.

Time to look at the main air bleed.  On the PWK, the main air bleed is located behind the idle mixture screw (idle air screw).

The Mikuni VM has an adjustable main air bleed.  That would be a nice feature on the PWK.

I used an old needle jet as a holder so that I could turn the main jet in my lathe.  You could easily do the same thing with a hand drill and a file.  Once you reduce the OD to .221”, just cut off the threaded portion with a hacksaw and dress it down with a mill-smooth file until you achieve a finished length of .150”.

When the mixture screw is reinstalled, the air jet is captured.

After a little bit of fiddling, I got the Big Bore runnin like a top.  Perfect throttle response, loads of mid-range.  It’s a beast.  I know that not many of you are runnin the PWK.  For the few who are, this air bleed trick might be helpful.  You end up with a significantly smaller main jet.

Final jetting ended up as follows.

Pilot jet:            32     Ooooops!  My bad.  #35

Main Jet:            135

Main Air Bleed:            80 (.032”)

Slide:                  #7

Slide Needle:            EGK clip in center groove
     
Idle Mix Screw:            1.5 turns

I can’t explain why the same setup that I used on the flat-top engine didn’t work as well on the pop-top.  I do know that the pop-top uses a lot more fuel, and it runs best when the A/F is in the mid 11s while the flat-top seems to prefer mid 12s.  The pop-top is not nearly as efficient as the flat-top.  It’s always nice to have another tool to attack a problem.  Having the ability to adjust the main air bleed comes in handy.

Hope your weather warms up soon and all of you can get back in the saddle.

Best regards,

Mike

Very interesting DBM.  I'm running the PWK40 (ok, it's $45 knock off) with a Jardine pipe and stock air box/filter.  I'm at a 45 primary jet and 160 main jet with the e-clip in the middle slot.  It's about right given crazy Houston winter weather (warm and humid one day and dry and cool the next).  I'm wondering why such a big difference between your jetting and my jetting (I don't have any air leaks.)  

Also -- I always get this confused....  if I am WOT in say 3rd/4th, I accelerate well but RPMs will actually rise with better power if I gently roll off the WOT throttle.  Is that a sign of being too lean or rich on the main jet?  

(I'm about to move the e-clip down 1 notch richer on the needle in an attempt to alleviate a little afterfire I usually get when closing the throttle completely on deceleration.)

Thanks Lancer!  Exactly what I wanted to confirm!  Looks like another bump up on the main jet for me.  Let's try a #162.