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General Category >> Rubber Side Down! >> PWK40 Knockoff with E-Tube
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Message started by DragBikeMike on 07/23/23 at 02:32:56

Title: PWK40 Knockoff with E-Tube
Post by DragBikeMike on 07/23/23 at 02:32:56

I have been dyin to get my hands on one of these for a long time. It may look like a regular Keihin PWK carb, but it’s a knockoff with an unusual brass contraption between the metering block and the main jet. The Duderino has one, ThumperPaul has one, and The Sneeze has one. I was anxious to learn more about it.

Title: Re: PWK40 Knockoff with E-Tube
Post by DragBikeMike on 07/23/23 at 02:34:37

Both the Dude and Sneezy got their carbs from JFG Racing. I’m not sure where ThumperPaul got his. When I searched the web, I always came up dry. Then I found this. The Amazon add said the brand is “JFG Racing” and the seller is “Jin Yao”. I decided to take a chance.

This is the link.

https://www.amazon.com/Carburetor-Universal-Compatible-Scooter-Motorcycle/dp/B094NJ466C

Title: Re: PWK40 Knockoff with E-Tube
Post by DragBikeMike on 07/23/23 at 02:35:38

It’s a knockoff, and like all knockoffs there’s risk. I might end up with a standard PWK like the ones I already have. Curiosity overwhelmed me. I really wanted to know exactly what’s inside one of these newer knockoffs with the unusual brass jet holder. The minimal entry fee was worth the risk. I ordered it, and in about four days the box was in front of my door. That was a big improvement over the one-month wait time for the last knockoff.

There isn’t much difference when looking at the outside of the carb. On the left side, you can see that the plastic idle stop screw has been replaced with a brass screw (nice upgrade), and you can see that the idle air screw looks different (it sticks out a bit and the head is larger). Other than those two items, it looked the same as the other knockoffs I had purchased.

Title: Re: PWK40 Knockoff with E-Tube
Post by DragBikeMike on 07/23/23 at 02:36:31

I screwed the top off and was pleasantly surprised. The slide looked good. The chrome plating was a lot better. This is the load bearing side. The chrome looks smooth, no pits or bubbles.

Title: Re: PWK40 Knockoff with E-Tube
Post by DragBikeMike on 07/23/23 at 02:37:03

The upstream side had a few funky spots, but overall, it was much better than the previous knockoffs.

Title: Re: PWK40 Knockoff with E-Tube
Post by DragBikeMike on 07/23/23 at 02:37:39

It’s a #7 slide. A letter “W” drill bit fits under the edge of the cutaway.

Title: Re: PWK40 Knockoff with E-Tube
Post by DragBikeMike on 07/23/23 at 02:38:21

The slide needle is different. The basic diameter is a lot smaller. None of the available Keihin needles are close to this small. If it needs a new needle, I am out of luck. Here it is next to a genuine Keihin DGK (DGK on top, new needle on bottom).

Title: Re: PWK40 Knockoff with E-Tube
Post by DragBikeMike on 07/23/23 at 02:39:11

Remove the float bowl and you will immediately see that the main jet configuration is different. There is an extra brass piece. Unlike the older knockoffs where the main jet was threaded directly into the aluminum metering block, these newer versions have some sort of brass piece between the metering block and the main jet.

Title: Re: PWK40 Knockoff with E-Tube
Post by DragBikeMike on 07/23/23 at 02:39:51

Turns out the brass piece is an emulsion tube. I’ll refer to it as the “E-Tube”. Those little holes in the side of the tube alter the fuel delivery curve and admit air bubbles into the liquid gasoline. The bubbles mix with the liquid and form an emulsion (i.e. foam). The foam is lighter, so it responds faster to changes in vacuum, and promotes atomization. That’s my limited understanding of the concept.

Title: Re: PWK40 Knockoff with E-Tube
Post by DragBikeMike on 07/23/23 at 02:40:46

For wide open throttle (WOT) operation, the holes at the top of the emulsion tube affect lower rpm, and the holes at the bottom affect higher rpm. Whack the throttle wide open at 2500 rpm, and the holes at the top of the tube come into play. As the fuel level in the emulsion cavity goes down, it begins to uncover more holes, so at maybe 4000 rpm the holes in the middle of the tube come into play. By the time the engine reaches 6000 rpm, the fuel level in the emulsion cavity has uncovered the holes at the bottom of the tube, so now all the holes are in play.

It works like a soda straw that has holes in the side of the straw.

Title: Re: PWK40 Knockoff with E-Tube
Post by DragBikeMike on 07/23/23 at 02:41:23

Put the straw in a glass of KoolAide (I like lime). When you first start drinking, it doesn’t take much suction to get the KoolAide up the straw.

Title: Re: PWK40 Knockoff with E-Tube
Post by DragBikeMike on 07/23/23 at 02:42:01

But as the level of KoolAide goes down, it starts to uncover holes in the side of the straw, and those holes permit air into the straw. The air breaks the vacuum. So, you must compensate by sucking a little harder. As the level of KoolAide continues to drop, you must suck harder and harder to get a mouthful.

Title: Re: PWK40 Knockoff with E-Tube
Post by DragBikeMike on 07/23/23 at 02:44:51

Venturi vacuum vs air flow is a quadratic function. It is not linear. As air flows through the venturi, the vacuum at the top of the needle jet increases exponentially. So, when the air flow doubles the vacuum increases like four times. That makes the fuel mixture go richer and richer as the rpm increases, but you want the air/fuel ratio to remain relatively constant (about 12.5:1). To compensate, carburetors use air correction to try and maintain a reasonable mixture over a wide range of rpm.

The standard PWK and the Mikuni VM use a simple air correction system that does OK, but it fails to achieve a perfect air/fuel ratio over the entire range. Both these carbs go rich at high rpm (when the throttle is wide open). That’s why the aftermarket has come up with third fuel circuits like the “Thunderjet”, or the “Dial-A-Jet”, or the “InteliJet”, or the “Power Jet”.

You can put in a larger main airbleed to try and correct the rich condition at high rpm, but then you end up with a lean condition at lower rpm. You can put in a smaller main jet to try and correct the rich condition at high rpm, but that will also result in going lean at lower rpm. You can fiddle around with the airbleed and main jet until you go nuts, but you will probably never be completely satisfied.

This problem only pertains to WOT conditions. At part throttle, the needle jet and slide needle are takin care of business. But snap the throttle all the way open and those two components are out of the picture. At WOT, you are relying on the main jet and air correction system to deliver fuel at the proper ratio. That’s a tall order for a fixed jet with a fixed airbleed.

The emulsion tube offers a little bit of relief. As the venturi vacuum increases exponentially, the fuel level in the emulsion well is dropping, and more emulsion holes are being uncovered. The additional air correction helps to compensate for the exponentially higher vacuum. It’s not perfect, but it should be an improvement over a simple airbleed system that only admits air at the very top of the needle jet. With an emulsion tube, the naturally occurring drop in fuel level adds more air correction as the rpm rises. It adds another dynamic component to the air correction system.

This YouTube vid does a nice job of explaining the quadratic behavior of a venturi.

https://www.youtube.com/watch?v=BvOqNwNuuAA

Title: Re: PWK40 Knockoff with E-Tube
Post by DragBikeMike on 07/23/23 at 02:46:11

Just above the emulsion tube is the needle jet. It comes right out when you remove the emulsion tube. Here’s a look at how things stack up in the metering block. Fuel enters through the main jet and fills the emulsion tube and surrounding cavity. At idle, the fuel level in the cavity is the same as the fuel level in the float bowl. As engine speed increases, the fuel level in the cavity begins to drop because the main jet restricts the flow of fuel into the emulsion tube. Pretty clever.

Title: Re: PWK40 Knockoff with E-Tube
Post by DragBikeMike on 07/23/23 at 02:47:30

With the E-Tube and needle jet removed, you can see how the main airbleed passage ties into the emulsion cavity. Just shine a flashlight up the hole in the metering block and look through the main airbleed.

Title: Re: PWK40 Knockoff with E-Tube
Post by DragBikeMike on 07/23/23 at 02:48:52

The hole through the center of the emulsion tube is .111”.

Title: Re: PWK40 Knockoff with E-Tube
Post by DragBikeMike on 07/23/23 at 02:49:40

But the hole in the needle jet is .101”. The needle jet is the controlling orifice, and it works in conjunction with the tapered needle to form the aperture that controls fuel flow during part throttle operation.

Title: Re: PWK40 Knockoff with E-Tube
Post by DragBikeMike on 07/23/23 at 02:50:34

Here you can see the needle jet nested just behind the hood in the venturi. On the earlier knockoffs, you won’t see this brass piece sticking up. Looks like we have a quick & dirty way to see if we have the latest version.

Title: Re: PWK40 Knockoff with E-Tube
Post by DragBikeMike on 07/23/23 at 02:52:10

Since the needle jet is only .101”, a standard PWK slide needle won’t fit. The smallest basic diameter OEM needle is 2.605mm, that’s .1026”. Not gonna fit. Gotta trust that this thing runs good with the as-delivered needle. If it doesn’t, I will have to get creative.

Here is a DGK needle stuck in the .101” needle jet.

Title: Re: PWK40 Knockoff with E-Tube
Post by DragBikeMike on 07/23/23 at 02:53:11

I could see that the main airbleed on this new carburetor is significantly smaller than the original PWK knockoffs. My PWK36, PWK38 and PWK40 all had .075” main airbleeds. The new E-Tube carb has a .039” main airbleed.

Title: Re: PWK40 Knockoff with E-Tube
Post by DragBikeMike on 07/23/23 at 02:53:47

A .039” pin gage just fits.

Title: Re: PWK40 Knockoff with E-Tube
Post by DragBikeMike on 07/23/23 at 02:54:53

I find the smaller main airbleed an interesting tidbit. Some of you may recall that a few months back I did a post on modifying the older PWK knockoff by installing a replaceable main airbleed. In that post, I achieved a better fuel curve with a .032” main airbleed and a much smaller main jet (#135). What a coincidence.

http://suzukisavage.com/cgi-bin/YaBB.pl?num=1676333024

Title: Re: PWK40 Knockoff with E-Tube
Post by DragBikeMike on 07/23/23 at 02:55:35

I suspect that I could get at the needle jet in my older knockoff carb if I was willing to remove the metering block, but those tamper-proof screws are an indication that someone smarter than me doesn’t think I should be messin with that.

Title: Re: PWK40 Knockoff with E-Tube
Post by DragBikeMike on 07/23/23 at 02:56:28

I recall seein on some other sites that the metering block can be difficult to seal up. It has several air and fuel passages that interconnect, and it also has a joint between the float bowl and the venturi. Looks like a can of worms to me. Glad someone else figured out a way to get at the needle jet without disturbing the complicated joint.

This is the seal used between the metering block and carb body. No thanks. Emergency use only. Why struggle.

Title: Re: PWK40 Knockoff with E-Tube
Post by DragBikeMike on 07/23/23 at 02:57:12

This is the new idle stop screw. All brass, sweeeeeet! Nice rounded nose. Looks much better. I like the way the PWK carb has this screw captured. I snapped one of these off on a Mikuni VM years ago. It went straight into the engine and finished the motor off. RIP. This one can’t get out of the cavity.

Title: Re: PWK40 Knockoff with E-Tube
Post by DragBikeMike on 07/23/23 at 02:57:55

The idle air screw is a bit different. Might be easier to adjust. The screw head is a little larger.

Title: Re: PWK40 Knockoff with E-Tube
Post by DragBikeMike on 07/23/23 at 02:58:35

The float has a nice feature. They have added a brass bushing for the float pin. There’s a lot less slop in the mechanism. It swings free but doesn’t wiggle around as much. This should be an improvement, but I have never had any issues with the original float.

Title: Re: PWK40 Knockoff with E-Tube
Post by DragBikeMike on 07/23/23 at 02:59:19

I measured the bore of the carb. It’s billed as a 40mm.

Title: Re: PWK40 Knockoff with E-Tube
Post by DragBikeMike on 07/23/23 at 03:00:16

Even says so on the side of the carb (tee hee hee).

Title: Re: PWK40 Knockoff with E-Tube
Post by DragBikeMike on 07/23/23 at 03:01:00

But just like my current knockoff 40, it’s only 38mm. That’s OK, I’m already havin trouble keepin things glued together with a 38mm carb, I don’t need a 40.

Title: Re: PWK40 Knockoff with E-Tube
Post by DragBikeMike on 07/23/23 at 03:03:03

The carb came with a #170 main jet and a #50 pilot jet. Don’t know what these jets would be used on. I suspect a very large two-stroke engine. In Savage Land, those jets are like a flash-flood. I will probably start with a #140 main and a #35 pilot, needle clip in the center groove.

I am cautiously optimistic about this new knockoff. I like the idea of having the emulsion tube to flatten out fuel delivery, and the other new features look like they will improve the carburetor as well. Overall quality looks good.

I can’t wait to try this carb. My old school PWK is running well. It purrs like a kitten and roars like a lion. It will be a hard act to follow, but there is always room for improvement. Maybe I can squeeze out a few more miles per gallon, or get crisper response. As a minimum, I will learn a few things.

If any of you have a PWK knockoff with the E-Tube, please share your experience. How does it run? Was it hard to tune? How’s your fuel mileage? Starting, idle, throttle response, power???

I am currently doing a long-term test on my lubrication system. I don’t want to change anything while I’m in the middle of that test. I want to go 10,000 miles and then do a thorough inspection to verify that the gears and bearings are holding up. If all goes well, I should be installing this new E-Tube carb in a couple of months.

Stay tuned.

Title: Re: PWK40 Knockoff with E-Tube
Post by ThumperPaul on 07/23/23 at 16:49:30

Hey Mike! Amazing analysis!! I really liked the Kool Aid illustration. LOL.

I bought mine from USAMOTOKING. https://www.ebay.com/itm/155136268286

I actually sold my S40 project, but still checking-in here for good stuff like this. As you recall, I kinda called it good enough at a #40 pilot and #145 main with the Jardine exhaust. If I would have kept the bike, I could have dropped down to a #38 primary. I ended up landing at about 50mpg which was my goal after correcting those big jets and loose manifold connection (again, thanks for all your guidance on that!!).

I honestly don't fully comprehend a lot of your analysis, but it doesn't sound like you found anything significant enough to alter your mind about starting with a #35 and #140, huh?

Let us know how it goes! I'm interested! FYI - I bought a 2006 Honda Shadow Spirit VT750DC in mint condition and only 2600 miles. It was owned by a guy that treated it like a trophy bike in his garage and I got a great deal. I couldn't pass it up. I'm really enjoying it!

Title: Re: PWK40 Knockoff with E-Tube
Post by DragBikeMike on 07/29/23 at 10:48:09

Paul, sorry to hear you switched rides. :'(

What can you tell us about the Honda in comparison to the Savage? Maybe you could do a comparison and post it. Power, comfort, handling, brakes, suspension, etc. How my Savage 650 stacked up against my Honda 750.

Thanks for the info on your source for the carb. Looks like they're right around the corner from Dave and they suggest it for a honkin BIG 2-stroke.

Title: Re: PWK40 Knockoff with E-Tube
Post by ThumperPaul on 07/29/23 at 14:37:20

I am looking for my next Savage project. I love the smoothness of the Honda and its stability over 65+mph (night and day comparison). I wanted something that I would feel a little more comfortable, stable, and confident on the highway for the occasional times I highway ride. I do miss being able to flick around the Savage. The Honda is about 520lbs compared to 380 for the Savage (wet weight comparison).

Tinkering with the Savage is way easier! The VT750 has dual carbs and I just pray they stay in sync. I don't have the equipment or know-how to sync carbs. Dialed in the air/fuel mix hoping I'm properly synced. I think it is. The prior owner had the carbs cleaned/rebuilt by a buddy. The air box duct work wasn't put back together well and I had to get all that straightened out and dial-in the air/fuel mix. Runs great! Looks awesome!

I think you're right about it somewhat intended for a big 2-stroke. I think some ads referenced a big Honda 500 2-stroke.

Title: Re: PWK40 Knockoff with E-Tube
Post by DragBikeMike on 07/30/23 at 16:09:54

Paul, you weren't kiddin when you said that Honda is in mint condition. That's a beauty. Thanks for sharing the pic.

Title: Re: PWK40 Knockoff with E-Tube
Post by ThumperPaul on 07/30/23 at 16:16:52

Thanks Mike! I couldn't pass it up! It's like new.