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/cgi-bin/YaBB.pl General Category >> Rubber Side Down! >> Keihin PWK Main Air Bleed Mod /cgi-bin/YaBB.pl?num=1676333024 Message started by DragBikeMike on 02/13/23 at 16:03:44 |
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Title: Keihin PWK Main Air Bleed Mod Post by DragBikeMike on 02/13/23 at 16:03:44 Spring will be here before you know it. The Savages will be coming out of hibernation. I thought the few of you running a Keihin PWK might find this information helpful. What I am about to describe worked good for me. If you decide to try this stuff on your own, you assume responsibility for the outcome. If you don’t have the skills, don’t do it. If you don’t understand something, STOP and get help. Get a manual. Read up. Comply with ALL the safety requirements outlined in the manual. Make sure you know what you are doing before attempting any of these modifications. |
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Title: Re: Keihin PWK Main Air Bleed Mod Post by DragBikeMike on 02/13/23 at 16:05:56 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). |
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Title: Re: Keihin PWK Main Air Bleed Mod Post by DragBikeMike on 02/13/23 at 16:06:41 If you remove the idle mixture screw, you see a .075” hole. It’s a fixed jet. Make that hole smaller and the main jet picks up earlier and feeds more fuel as the engine speed increases. Make the hole larger and the main jet picks up later and feeds less fuel proportional to engine speed. |
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Title: Re: Keihin PWK Main Air Bleed Mod Post by DragBikeMike on 02/13/23 at 16:07:19 The Mikuni VM has an adjustable main air bleed. That would be a nice feature on the PWK. |
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Title: Re: Keihin PWK Main Air Bleed Mod Post by DragBikeMike on 02/13/23 at 16:08:01 I was reluctant to drill & tap the carb body, so I simply made some inserts from old main jets. I used a #50, #60, #70, #80, #90 & #100. It’s way easier to turn down the OD and length of the main jets than it is to drill those teeny weeny holes. I ended up with a set of air bleeds that had holes ranging from .020” to 040”. The OD is .221”. The length is .150”. The original jet is shown on the left, finished product on the right. |
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Title: Re: Keihin PWK Main Air Bleed Mod Post by DragBikeMike on 02/13/23 at 16:08:34 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”. |
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Title: Re: Keihin PWK Main Air Bleed Mod Post by DragBikeMike on 02/13/23 at 16:09:11 The new air bleed slips into the hole behind the idle mixture screw. |
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Title: Re: Keihin PWK Main Air Bleed Mod Post by DragBikeMike on 02/13/23 at 16:09:53 When the mixture screw is reinstalled, the air jet is captured. |
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Title: Re: Keihin PWK Main Air Bleed Mod Post by DragBikeMike on 02/13/23 at 16:10:57 Swapping out the air jet is a breeze. Just pull off the air boot, remove the air correction screw, and snag the air jet with a large needle. I prefer a rusty needle; it grips the jet better. I made this tool from an old chop stick and a rusty sewing needle. |
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Title: Re: Keihin PWK Main Air Bleed Mod Post by DragBikeMike on 02/13/23 at 16:16:52 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: 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 |
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Title: Re: Keihin PWK Main Air Bleed Mod Post by TheSneeze on 02/13/23 at 21:27:03 Professor DBM chimes in with another amazing post. I still wish I had a lathe! Great info as always, Mike! |
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Title: Re: Keihin PWK Main Air Bleed Mod Post by ThumperPaul on 02/14/23 at 09:29:49 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.) |
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Title: Re: Keihin PWK Main Air Bleed Mod Post by LANCER on 02/14/23 at 10:04:22 Copy from Mikuni Inc. website: “ROLL-OFF: The Roll-Off technique is the quickest and is almost as accurate as the Roll-On method. First, one gets the engine warm on the way to a safe roadway. If there is room, use fourth gear as this allows more time to assess the result. Now, get the engine rpm high enough that it is on the cam and in its power band. This may need to be as high as 4000 rpm with some cam choices. Apply full throttle. Let the engine accelerate for a couple of seconds until it has settled in and is pulling hard. Quickly roll the throttle off to about the 7/8ths position. When you do this, the mixture richens slightly for a second or so. If the engine gains power as you roll the throttle off, then the main jet is too small and you need to fit a larger one. If the engine staggers slightly or has a hard hesitation, then the main jet is too large and you need to fit a smaller one.“ Basics: pilot jet…idle to 1/4 throttle; needle…1/4-3/4 throttle; main jet…3/4-WOT |
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Title: Re: Keihin PWK Main Air Bleed Mod Post by ThumperPaul on 02/14/23 at 11:07:09 Thanks Lancer! Exactly what I wanted to confirm! Looks like another bump up on the main jet for me. Let's try a #162. |
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Title: Re: Keihin PWK Main Air Bleed Mod Post by DragBikeMike on 02/14/23 at 20:55:17 Paul, you are definitely runnin some seriously BIG jets. The knock-off carbs come in a variety of configurations with all sorts of screwy parts. The ones I have run are faithful to the original Keihin design. When I look at an illustrated parts breakdown (IPB), all the parts in mine look like the parts shown in the illustration. This link will connect you with an IPB of a genuine PWK. https://www.jetsrus.com/carburetor/carb_keihin_PWK33-41_carb_exploded_view_parts.html |
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Title: Re: Keihin PWK Main Air Bleed Mod Post by DragBikeMike on 02/14/23 at 20:56:17 To help all of us learn more about the knock-off PWKs, it would be most generous of you to do a comparison of your PWK knock-off to the IPB. If you do so, please take pictures and post so we can all benefit from your efforts. The jetting you describe is so far removed from any of the jet combos folks have reported so far, it begs the question “what’s up with that?”. The PWKs came in two different configurations, Non Air Striker & Air Striker. The first PWK I got from Spammy was an Air Striker. It looked like this. |
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Title: Re: Keihin PWK Main Air Bleed Mod Post by DragBikeMike on 02/14/23 at 20:56:53 The next one I got from JFG Racing was a Non Air Striker and it looked like this. Which type do you have, Air Striker or Non Air Striker? |
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Title: Re: Keihin PWK Main Air Bleed Mod Post by DragBikeMike on 02/14/23 at 20:58:09 I screwed up when I reported my pilot jet size. It should be #35 which is the smallest OEM jet available. Your #45 is five-times larger. That seems pretty big to me, but the idle circuit can be affected by the slide cutaway. Sneezy got one of these knock-offs with a #5.5 cutaway. All three of the knock-offs I got had a #7 cutaway and they all needed the smallest pilot jet I could come up with. This shows the two cutaways, Sneezy’s 5.5 on the right and a 7.0 on the left. |
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Title: Re: Keihin PWK Main Air Bleed Mod Post by DragBikeMike on 02/14/23 at 20:59:01 When installed in the carburetor, you can see how the smaller cutaway tends to nest over the needle jet hood. This nesting causes the vacuum at the hood to rise which pulls fuel from the main circuit at idle (a similar condition occurs when you install one of Lancer’s UFOs on a VM). Is there a chance that you have a slide that is larger than a 7.0? A larger cutaway could necessitate a larger pilot jet to compensate. There’s always the possibility of a vacuum leak but you mentioned you have zero air leaks. See how the #5.5 cutaway on the right tends to nest the needle jet hood while the #7.0 cutaway on the left pretty much leaves the hood exposed. |
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Title: Re: Keihin PWK Main Air Bleed Mod Post by DragBikeMike on 02/14/23 at 20:59:40 Slides come in various sizes all the way up to #9.0. This sketch provides measurements for #5.5 through #8.0. |
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Title: Re: Keihin PWK Main Air Bleed Mod Post by DragBikeMike on 02/14/23 at 21:00:25 It’s easy to measure the cutaway. Just place the slide on a smooth, flat surface and use drill bits to measure the edge of the cutaway. Here you see a #7.0 slide being measured with a letter “W” drill bit. A letter “W” bit is .386”, close enough for me. Howzabout taking a measurement on your slide and let us know what you’ve got. |
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Title: Re: Keihin PWK Main Air Bleed Mod Post by DragBikeMike on 02/14/23 at 21:01:37 Then there’s the never-ending saga of needle jets. All three of my PWKs had fixed needle jets captured in the metering block. Note that on the IPB, no needle jet or metering block is shown. I figure that’s because the factory doesn’t consider those components as “replaceable” items. Got a problem, replace the whole carb. The metering blocks are held in place with tamper-proof screws (there’s always a way around those). The Duderino and Sneezy both got knock-offs with removable needle jets (a nice feature IMO). The carb on the right has a fixed needle jet, the one on the left has a removable needle jet. Note that the one on the left with the removeable needle jet must use a different main jet. It’s a lot shorter than a standard PWK main jet, like the one on the right. Which configuration do you have? |
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Title: Re: Keihin PWK Main Air Bleed Mod Post by DragBikeMike on 02/14/23 at 21:02:19 See how the removeable needle jet just unscrews from the metering block. Note that it also has emulsion holes, that might be beneficial, or possibly detrimental. Those holes and their placement can change the fuel curve. Holes at the top affect part-throttle performance, holes at the bottom affect WOT performance. Does your PWK have a removeable needle jet? |
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Title: Re: Keihin PWK Main Air Bleed Mod Post by DragBikeMike on 02/14/23 at 21:03:02 The relationship between your slide needle and needle jet is critical for the majority of your operating time. You are almost always running on the needle jet/slide needle. Those two components control your air/fuel ratio during the majority of the time you are riding. Generally, at WOT the needle and needle jet should be out of the picture and the main jet should be in complete control of the fuel flow. On this particular project, it became apparent that my DGK needle was affecting WOT fuel mixture. If the aperture between the needle and needle jet is too small, it will flow less fuel than the main jet and become the limiting factor. Under those conditions, you can keep throwing in larger main jets with little or no effect on the WOT mixture. To me, that sounds like a reasonable explanation for your BIG main jet. You might wanna check that. A quick & dirty check involves removing the main jet and riding the bike. Without the main jet in place, it should run normal until you go WOT, then at WOT it should stumble, hiccup, buck, and in general run absolutely terrible. If it doesn’t run lousy without the main jet, the needle & needle jet are controlling flow and need to be adjusted. A needle with a larger taper is in order. |
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Title: Re: Keihin PWK Main Air Bleed Mod Post by DragBikeMike on 02/14/23 at 21:04:30 My needle jets are about .111”. They are fixed and I’ve never tried to remove one. Generally, needle jets are provided in sizes taken out to four decimal places. I don’t have the tooling necessary to measure a needle jet to the degree of accuracy demanded, but I do have pin gages (only three decimal places). My needle jets are about .111”. You can try and check yours with the shanks of drill bits. Be gentle, this component is critical. A #35 is .110”. a #34 is .111”, and a #33 is .113”. If you do check the needle jet, please let us know the results. I do have the ability to accurately measure slide needles. This table provides measurements taken off the actual needles. The needles labeled with three letters (DGK, EGK, etc.) are genuine Keihin needles. The other needles came from knock-off repair kits or knock-off carbs. You may find the table useful. As noted in this report, I had good results with a DGK combined with a .075” air bleed and a #140 to #145 main jet, but the needle was controlling WOT mixture. I had superb results with an EGK combined with a .032” air bleed and a #135 main jet. I use a wide band O2 sensor and A/F meter to confirm my jetting. It’s far from perfect but it gets me very close. I also should note that I favor rich mixtures, generally trying to hold WOT mixture in 11.0 to 12.5 range. |
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Title: Re: Keihin PWK Main Air Bleed Mod Post by DragBikeMike on 02/14/23 at 21:05:32 Your main jet (#160) is through-the-roof BIG. If you look at this chart, you will see that a VM38 (carb with a similar design) would need a #330 or #340 main jet to come close to the #160 you are running. If my memory serves me right, on an LS650, VMs usually run best with around a #190 to #210 main jet (Can you confirm that Lancer?) Those Mikuni jets are in the range of .048” to .052”. Assuming you are running standard Keihin jets, your PWK main jet should have an orifice about .063”. All’s I can say is Wow! What kind of main jet is in your carb? Is it a standard PWK main jet? |
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Title: Re: Keihin PWK Main Air Bleed Mod Post by DragBikeMike on 02/14/23 at 21:06:56 This report is about the main air bleed. As you can see from my results, that air bleed has a significant influence on your main jet size. As delivered, my PWKs had a .075” air bleed. If your air bleed is larger than .075”, it will require a larger main jet. Can you check your main air bleed and let us know what you find. Just remove your mixture screw and use drill bits to check this hole (use the shank, not the cutting tip). It’s easy to check. You can use drill bits. If it’s .075”, a #49 drill bit will slip in but a 3/64” bit will not fit. |
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Title: Re: Keihin PWK Main Air Bleed Mod Post by DragBikeMike on 02/14/23 at 21:09:53 These knock-off carbs are certainly a bargain. You can learn a lot and have fun setting them up. Set em up right and they make good power and run well. They can also be a real puzzle. That’s why it’s important to share what you learn. If you get into your PWK, please take lots of pics and post what you learn. These links provide some good tuning guidance. Keihin North America https://www.keihin-na.com/aftermarket/tuning/ Duncan Racing (note 2-stroke section has 4-stroke info too) https://www.duncanracing.com/carbjetting |
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Title: Re: Keihin PWK Main Air Bleed Mod Post by DragBikeMike on 02/14/23 at 21:14:05 Thanks Sneezy. Hey, you don't need a lathe for this project. You can trim that jet down with a drill and a file. |
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Title: Re: Keihin PWK Main Air Bleed Mod Post by ThumperPaul on 02/15/23 at 03:58:19 Thanks DBM! Can’t say I’ve completely torn mine down, but what I can see and remember is the same as the illustration. I need to go to Harbor Freight and get one of those needle measuring gizmos. I’m beginning to think my jets aren’t true to size (cheap knock off jets too, lol). Maybe I’m getting lucky with these knock offs, but it sure works well for 1/10 the cost of the stock Mikuni (stupid expensive). Love the snappy responsiveness of the PWK especially in the mid-range (no issues, much better than the Mikuni CV carb). |
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Title: Re: Keihin PWK Main Air Bleed Mod Post by TheSneeze on 02/15/23 at 07:33:21 Yeah, but I still need a lathe... 8-) |
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