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Message started by TheSneeze on 10/07/22 at 11:48:02

Title: A Tale of Two PWK40 Carbs
Post by TheSneeze on 10/07/22 at 11:48:02

The Keihin PWK40 carburetor that you find for sale at many locations on the internet may not be exactly what you think. There are companies out there, let's say over the pond, that are cranking out cheap knock offs. We have all read DragBikeMike's thread about rebuild kits and the questionable quality parts some of those contain. Well, here is some proof that even the carbs themselves can come with sketchy quality. So much so I purchased two of them (since I did not like the first one I got). The differences between the two is quite a long list, and I am still not finished documenting them. Here is what I have found so far. :o

The first carb I purchased on Ebay. The seller was (and still is) Blackhary7. The purchase price was $39.69, and it was delivered in a normal amount of time. This seller is currently listing the same product, but now at $85.63 (as of 10-7-22). The main thing I did not like with this carb was the plating on the slide. It was covered in pits and nodules. Coming from the metal fab industry as a career, I have dealt with a lot of different types of plating processes on many different base metals. This lack of plating quality really disappointed me. It was for that reason only I decided to buy another one from a different source. For around $40 how can you go wrong? The second carb I went with JFG Racing, since DBM had a good experience with the same product. It was $34.99 at the time, and still is today (I checked). This is what the plating looked like...

Title: Re: A Tale of Two PWK40 Carbs
Post by TheSneeze on 10/07/22 at 11:55:16

The purchase from JFG Racing arrived in a normal amount of time. But what came out of the box was quite different than the first one. The longer I look the more differences I find, and I have not finished disassembling them yet!

The first and most obvious thing was the latest one had more spaghetti than the first. It has the two top vents drilled with fittings, and hose attached to them. The next thing I noticed was the air intake sides - the first carb was bored to 40mm from a smaller carb, leaving a transition line from the radius taper to main ID. The JFG Racing carb had a smooth transition all the way into the main ID. Hmmm...

Title: Re: A Tale of Two PWK40 Carbs
Post by TheSneeze on 10/07/22 at 11:59:18

While looking closely at the air intake side of the carbs, I checked the slides. The JFG carb had good quality plating. I let out a little sigh of relief. But then I noticed the slide had a different angle cut into it. I haven't measured it yet, but they both came with a different slide. The JFG carb also had a notch cut into the needle housing, where the first one did not.

Title: Re: A Tale of Two PWK40 Carbs
Post by TheSneeze on 10/07/22 at 12:04:48

Some other observations about the exteriors of these two carbs. The JFG carb has a brass mixture screw. The other has one made of plastic. The choke housings are slightly different. The throttle cable boot on the top are different lengths. Upon removal, the fittings are not the same either.

Title: Re: A Tale of Two PWK40 Carbs
Post by TheSneeze on 10/07/22 at 12:07:31

After removing the float bowls, I found the inside of the JFG carb was wet. Not sure if it's old fuel or a light oil. The first carb was dry. The JFG carb has a two piece main jet set up, the other is a one piece.

Title: Re: A Tale of Two PWK40 Carbs
Post by TheSneeze on 10/07/22 at 12:13:36

The intake transition was smooth on the JFG carb, but I noticed the bore on the engine side was not a constant diameter. It reduces to about 37.5mm at the slide, and after the slide it tapers back out to 39.8mm where it enters the intake boot. The first carb was bored out to 40mm straight through. This find tells me these two carbs will not have the same performance with all the same jets and settings. I also noticed that the float bowl o-rings are not the same diameter. The JFG carb has a thicker o-ring than the first carb.

Title: Re: A Tale of Two PWK40 Carbs
Post by TheSneeze on 10/07/22 at 12:16:11

I will report more findings like jet size differences once I finish the disassembly and measuring. Both of these carbs were advertised and sold as a Keihin PWK40. What ends up going on my hot rod motor is yet to be determined.

Title: Re: A Tale of Two PWK40 Carbs
Post by zipidachimp on 10/08/22 at 00:10:53

Seems like everyone and his brother are selling these.
DC Motive have 2, at $85 each, Keihin or Koso.
Think I will stick to stock until this gets resolved.
Cheers! 8-)

Title: Re: A Tale of Two PWK40 Carbs
Post by DragBikeMike on 10/08/22 at 12:08:15

Thanks for the photos and comparison Sneezy. Two different animals for sure. I'm wondering how these things end up with that removeable needle jet. It may be some sort of modification they perform in conjunction with the enlargement of the carb throat.

I'm very curious about your needle and needle jet assembly. Can you compare your needle to the needle chart I sent you? Tell us how the JFG needle compares to that chart. Give us an idea of approximately what series needle you have in the new JFG carb. Would also be super-useful to know the size of your needle jet. As I recall, Thunderino's needle jet was up inside the metering block and was captured by the threaded main jet holder. This is a pic of Theduderino's.

Title: Re: A Tale of Two PWK40 Carbs
Post by DragBikeMike on 10/08/22 at 12:15:36

The slide cutaway info would also be quite valuable, and also the size of your main air bleed.

You are going to have to round up some short main jets. For refence purposes, a #140 standard Keihin PWK main jet has a .055" bore. I have attached the info I passed to the Duderino regarding a potential candidate for short main jets.

Sounds like ThumperPaul is having good success with his

Title: Re: A Tale of Two PWK40 Carbs
Post by TheSneeze on 10/11/22 at 11:12:20

This morning I disassembled the JFG Racing PWK40. Here is what I found:

#170 main jet (no washer underneath)
#50 idle jet
#7 slide
Needle clip on 2nd groove from the bottom
Needle tapers from .096" down to .055" at the tip

Title: Re: A Tale of Two PWK40 Carbs
Post by TheSneeze on 10/11/22 at 11:14:35

DBM - As filing is not my strong point, I cannot find the needle chart you sent me. Can you repost it here? Thanks...

Title: Re: A Tale of Two PWK40 Carbs
Post by justin_o_guy2 on 10/11/22 at 11:22:49

Everybody hates filing. That's why they make grinders.

Title: Re: A Tale of Two PWK40 Carbs
Post by TheSneeze on 10/11/22 at 11:44:29

The flea-bay carb had totally different components inside. The big surprise to me was the difference in the needles. This one was considerably larger in diameter, with a small step.

#150 main jet (with washer)
#40 pilot jet
#7 slide (crappy plating quality)
Needle clip on 3rd groove from bottom
Needle tapers from .105" to .071" at the tip

Title: Re: A Tale of Two PWK40 Carbs
Post by TheSneeze on 10/11/22 at 11:45:12

37282E2934330232023A28246F5D0 wrote:

Everybody hates filing. That's why they make grinders.

;D ;D ;D Good one, JoG!

Title: Re: A Tale of Two PWK40 Carbs
Post by TheSneeze on 10/11/22 at 11:50:44

Yet another difference - the main jet is further into the float bowl on the two piece main jet carb - by .100"!!

Title: Re: A Tale of Two PWK40 Carbs
Post by TheSneeze on 10/11/22 at 12:00:49

The slide springs are also completely different as well.

JFG carb
.043" gage x 3.115" long x .600" outside diameter

Fleabay carb
.035" gage x 3.900" long x .566" outside diameter

Title: Re: A Tale of Two PWK40 Carbs
Post by DragBikeMike on 10/12/22 at 11:43:18

Thanks for the pics Sneezy. Very informative. Your's is a very different beast.

Are you sure that there isn't an additional brass piece up inside your metering block? A piece like the item I show in reply #8. Theduderino said it fell out after he removed the long needle jet/emulsion tube.

I can see there are emulsion holes in the long brass tube. See the little holes in this snip.

Title: Re: A Tale of Two PWK40 Carbs
Post by DragBikeMike on 10/12/22 at 11:48:39

Those little holes are gonna break vacuum in the tube, so all bets are off on the size of your main jet, needle, needle taper, needle jet, etc.

Here is a tabulation of the various needles I have measured, but I'm not sure how useful it will be given how much difference there is between yours and a standard PWK. Would be nice if you could measure the two needles you have and post the measurements.

Title: Re: A Tale of Two PWK40 Carbs
Post by DragBikeMike on 10/12/22 at 11:56:36

This is a chart of main jet sizes measured with pin gages. It may be useful since you are shooting from the hip on your main jet. I'm thinkin you should just put it on the way it was delivered and see what it does, then move forward from there. The emulsion tube will probably make it behave differently from a standard Keihin PWK38. Would be great if you could measure your actual needle jet and post the size. I have a feeling it's still up inside your metering block.

Title: Re: A Tale of Two PWK40 Carbs
Post by DragBikeMike on 10/12/22 at 12:02:05

It would also be great if you could measure the size of your main air bleed. It's behind your idle mixture screw. You have to remove the mixture screw to see the air bleed. Mine is .075".

Title: Re: A Tale of Two PWK40 Carbs
Post by TheSneeze on 10/12/22 at 13:23:49

I really need to get a set of pin gages. The only bummer is they come in sets, and the good set (the smallest) is .011" to .060". This would force me to buy the second size set as well. One can never have too many tools, right?

Nothing else "fell out" when removing my needle. I will have to go back in there to see if is just stuck in there? I already put it back together.

Both carbs have the same long brass tubes, with the same pattern of holes.

Title: Re: A Tale of Two PWK40 Carbs
Post by TheSneeze on 10/12/22 at 13:28:33

080E017D7F787C4C0 wrote:

At this point, what is a "standard PWK"? We have clearly identified that there are differences with almost every one of these carbs people buy. How would you be certain that you are buying a "standard PWK", from any online seller? :o :o :o

Title: Re: A Tale of Two PWK40 Carbs
Post by DragBikeMike on 10/13/22 at 12:21:08

Yes, it's a crap shoot. But if you have a carb (knock-off or genuine) that complies with the Keihin illustrated parts breakdown, then I define that as "standard" (i.e. faithful to the OEM design). The three PWKs I got were all pretty much faithful to the OEM design.

This link will get you to a good illustrated parts breakdown. Note there are no removable needle jets shown. It even has a note that cautions you not to try and disturb any parts not shown on the illustration.

https://www.jetsrus.com/carburetor/carb_keihin_PWK33-41_carb_exploded_view_parts.html

Title: Re: A Tale of Two PWK40 Carbs
Post by DragBikeMike on 10/13/22 at 12:22:49

My PWKs all had a main jet that looked like this. It is identical to a genuine Keihin main jet intended for a PWK.

Title: Re: A Tale of Two PWK40 Carbs
Post by DragBikeMike on 10/13/22 at 12:25:16

Between the main jet and the aluminum metering block, there is a brass washer. It doesn't want to come out. You can see that the main jet threads into aluminum. There's no emulsion tube. The main jet goes directly into the aluminum metering block.

Title: Re: A Tale of Two PWK40 Carbs
Post by DragBikeMike on 10/13/22 at 12:26:48

Way up inside the metering block you can see what appears to be a brass needle jet.

Title: Re: A Tale of Two PWK40 Carbs
Post by DragBikeMike on 10/13/22 at 12:28:46

From the other side you can see the brass needle jet. It's fixed in the metering block. I suspect I could get it out, but I would most likely ruin the carb removing the needle jet.

Title: Re: A Tale of Two PWK40 Carbs
Post by DragBikeMike on 10/13/22 at 12:43:22

Of course, the relationship between the needle jet and the slide needle are critical to correct mixture control. There are no emulsion holes on what I define as a "standard" PWK. It's similar to a Mikuni VM. It's a simple port that connects the top of the needle jet to the air correction jet. My PWKs have a fixed air correction jet (main air bleed). That's faithful to the OEM design. Your PWKs have an emulsion tube and what sort of air correction? Those are drastic differences. It's gonna behave a lot differently than mine, and the main jet and needle selection will probably be very different from mine. Won't know until you try.

Are you sure the first knock-off you got has an emulsion tube. Your pics indicate a standard Keihin main jet in the first knock-off. Can't see how an emulsion tube would be held in. Got any pics of that first knock-off main jet and metering block, main air bleed?

This link might be useful. It's a selection of needles along with an explanation of how they measure the needles. It also shows the needle/needle jet configuration on a standard PWK.

https://www.jetsrus.com/a_jets_by_carburetor_type/needle_keihin_N427-48.html

Title: Re: A Tale of Two PWK40 Carbs
Post by TheSneeze on 10/14/22 at 15:43:19

Based on your definition of a "Standard" PWK, the first one I received off Fleabay is a standard. With some crappy quality parts. The newer one from JFG Racing is the one with the holes in the emulsion tube. Both of my PWK's have the small pressed in brass piece just below where the slide stops. Here is the JFG Racing carb with the filed notch. It sits proud of the throat diameter.

Title: Re: A Tale of Two PWK40 Carbs
Post by TheSneeze on 10/14/22 at 15:45:05

This is the same view of the "Standard" PWK off Fleabay. That same brass piece the needle goes through is about flush with the throat diameter. You can clearly see the notch has not been put in.

Title: Re: A Tale of Two PWK40 Carbs
Post by TheSneeze on 10/14/22 at 15:50:15

Based on all of this discussion, I am leaning toward mixing and matching the two carbs I received, using the the carb body, main jet with the washer (and no emulsion tube), and the needle that came with it. The higher quality slide will go into it as well. This first carb body is not drilled for the upper vertical vent tubes, but has the horizontal ones. As soon as I find a way (maybe using drill bits) I will try to measure the air bleeds on both.

Title: Re: A Tale of Two PWK40 Carbs
Post by DragBikeMike on 10/14/22 at 17:49:19

Thanks for the additional pics Sneezy. Those are two different animals. That mixin & matchin sounds OK to me, but I would keep careful records so you can always put stuff back where it originally came from. If that first carb, the one that is closest to standard, has a needle jet that is .1105", that seems like the best candidate. As I recall, you sent me the needle from that carb and it measured out pretty close to the needle that came with mine. So, if the needle jet is .1105" and the main air bleed is .075", it should run pretty good with a 145 main jet. At least good enough to get things rollin. That little notch in the hood is easy to apply with a tiny round file. If it needs the notch, you will have a rich condition at idle and just off idle.

That being said, if it were my problem to wrestle with, I'd be inclined to try each one as-is to see how they stack up. Then, if neither worked any good, I would do the mix & match.

I really appreciate the pics and info. Very interesting stuff.

Title: Re: A Tale of Two PWK40 Carbs
Post by ThumperPaul on 10/17/22 at 07:38:09

Good stuff Sneeze! Thanks for sharing! I mis-spoke on one of our earlier posts/comments - I checked again and I actually ended up buying from 'usamotoking' for $37.19. It compares more closely to the JFGRacing carb, but it has a straight 40mm bore on the spigot side (no tapering). I didn't do any measuring on other items so I can't comment on those specifics. Mine has the 2-piece main. Needle jet resembles the one with 'less taper'. The polish and finish on my carb is absolutely beautiful especially the slide and ports - you'd approve for sure - it's even better than the JFG in your pic. Mine also came with the #50 and #170 installed. With DragBikeMike's expertise, I am very happy with the re-jetting at #35 and #140 (stock bike at sea level).

Right now, my only 2 complaints about my PWK40 carb are:

1) it weeps a tiny amount of gas at the air/fuel mix screw when you hit about 3 turns out - not a huge deal for me as I aim to tune to 1.5-2.0 turns out, but I would expect 3 turns out to be okay (problem in the future? we'll see). My mix screw is soft brass and not plastic.

2) there is no float bowl drain drain/tube (not a huge deal).

My only slight performance problem/tuning question is that the bike idles very low when the engine is cold. It actually stalled once when I didn't keep my hand on the throttle long enough - I have hold it's hand for at least 30 seconds or it will stall. Once warned up to efficient operating temperature it's great. I know this is a normal dynamic of a carbureted internal combustion engine, but its very noticeable with this carb or my set-up. Related side note: If I use the choke, the bike dies almost instantly if I don't return the choke to normal position within 2-3 seconds and then I still have to hold the throttle for like 30 seconds. It's getting cooler here now, so I'll get a better sense of the choke behavior in the coming months. I've tried tuning this out by playing with the air/fuel mix, but really not accomplishing anything and I like my air/fuel mix tune when its warm and I really don't want to foul that up. Any expertise and advice here would be greatly appreciated. FYI - mine came with the e-clip in the 2nd notch from the bottom - I left it there (one notch to the rich side). I'm wondering if bumping the e-clip up to the middle notch (leaner) might address this issue - seems logical to me - it's like I'm too rich at cold idle, too rich with choke, but runs great when warned up... (thoughts?)

I have several things to rule out before I completely blame the carb or my tuning - 1) valve lash adjustment (it's too noisy up top), 2) new battery (I'm at my minimum acceptable fully charged level of 12.6 volts), 3) check my spark plug for evidence of lean/rich condition.

Also, I just swapped out the stock muffler for a Dyna muffler this weekend. It sounds soooo much better. I didn't re-jet and the 35 seems fine (I'm at about 2.5 turns out on the air/fuel mix). I haven't tested at WOT so I may need to bump up a size on the main jet to a #142 or #145 to optimize rpm horsepower. I will probably go ahead and bump up to the #38 primary jet and move the e-clip up to the center notch when I do the valve lash adjustment while I already have the gas tank off. I'm not doing any WOT testing until I get the valves adjusted and look at the spark plug.

I know I should probably do one tweak/modification at a time, but I'm an impatient 'chef' that prefers messy scrambled eggs over pretty omelets.

Title: Re: A Tale of Two PWK40 Carbs
Post by TheSneeze on 10/17/22 at 08:10:10

In all my years (too many to mention!) the one thing about carburetor tuning that I have heard over and over again is make sure you have a proper mechanical tune BEFORE you mess with jetting or air/fuel mixture settings. Timing, valve adjustment, etc. Once you straighten out your noisy top end (likely valve lash), you will be back at the drawing board on your carb settings. Every mechanic I have ever talked to said this same thing. So be prepared for this eventuality and don't be frustrated or disappointed that you have to go through all your settings once more after the valve adjustment. This may be more important on multi cylinder/multi carb motors that have distributors and what not, but I believe it to be good, sound advise on any four cycle engine. What about two strokes, you say? You're on your own with that one, pal!!

Title: Re: A Tale of Two PWK40 Carbs
Post by Yazman on 10/17/22 at 09:02:03

FWIW, I got the JFG PWK38 with #170 & #50 jets, and though I haven't done much testing (just a few short rides), it is 100% better then the stock carb and runs great on out of the box jetting :o Just to add more confusion! Only mod is Ryca kit, slip on, and pod intake.

Title: Re: A Tale of Two PWK40 Carbs
Post by ThumperPaul on 10/17/22 at 11:54:31

Yeah Sneeze. I hear ya. I've been jacking with bikes for over 45 years. I am bringing this bike back from the grave. I bought it without a carb. Bought this cheap PWK40 to try to see if it would fart. It farts, and it farts very well. Now I'll spend time doing the mechanic stuff - no point in doing all that harder mechanical work just to find out it has a blown motor. I bought it as a parts bike but it's actually worthy restoring now that I know it runs without any major issues.

Oh and FYI - I've owned and worked on over 70 2-stroke dirt bikes (that's a blast and barrel full of monkeys)! Just not very familiar with this Savage/S40, and especially it's stock CV carb that has more technology and overkill than this bike needs.

So while I’m a newbie to this forum and bike, this ain’t my first rodeo. I greatly appreciate your help and other senior members help! I plan on keeping at least one of my bikes for a good while and hopefully one day I can be a valuable contributor. I am very grateful for this site, it’s commitment, and all the senior member, old-timers, and Savage gurus here!

Anyway - any advice on my questions is appreciated.

Title: Re: A Tale of Two PWK40 Carbs
Post by TheSneeze on 10/17/22 at 13:42:56

Back in the day my best friend had a Kawasaki H1 triple with tuned chambers. OMG!!! Faster than believable, with a rubber frame between the wheels. Went crazy fast in a straight line and scared the crap out of you in the corners.

Online you never know the level of someone's experience unless they bring it up first. And like all the people on here, just trying to help. I would have verified the same thing, btw. Make sure it's not a dead motor.

Title: Re: A Tale of Two PWK40 Carbs
Post by ThumperPaul on 10/19/22 at 07:12:09

Just FYI, but here's the item description of the cheap 40 piece jet set that I recently bought. With Yazman having success with a #50 and #170 straight out of the box, I found the differentiation in the listing interesting and relevant. Note how the description says the 100-140 Main jets are for a PWK 21mm - 34mm and the 155-178 are for a PWK 28-42mm. Now this obviously has to be taken with a grain of salt - how did they make this distinction?, is it based on 2-stroke applications (common application for the PWK40)?, what else??

With this in mind, I'll be re-jetting over the next few days to a #38 and #155 (currently at 35 and 140). This is also in response to upgrading from stock muffler to the Harley Dyna muffler. Other items on my to-do list for the next few days include: valve lash, check de-comp cable play, new battery, change from NKG spark plug to E3.36. Anxious to see what happens...

Item Description:
Size of slow pilot jets: 32, 35, 38, 40, 42, 45, 48, 50, 52, 55; Thread pitch: 6 x 0.75 mm; Nut diameter: 5 mm; Overall length: 28 mm

Size of main jets: 100, 105, 108, 110, 115, 120, 125, 130, 135, 140(PWK 21mm-34mm); Thread pitch: 4.7 x 2.5 mm; Nut diameter: 5.7 mm; Thread length: 5 mm; Overall length of the hole: 7.9 mm

Size of main jets: Carburetor Main Jet #155 #158 #160 #162 #165 #168 #170 #172 #175 #178 (PWK 28mm-42mm),PLEASE Attention:Not fit for CV, VE and CVK above 3cm carburetor.

Title: Re: A Tale of Two PWK40 Carbs
Post by ThumperPaul on 10/23/22 at 12:10:58

Update on my PWK40. Rejetted to #38 primary and #155 Main. Performed valve lash adjustment (need to try again to hit perfection, ticky ticky...), adjusted decomp solenoid cable free play to 4mm (it was way off, like 7+mm, and it was sticking too). My weird random top end noise is gone after adjusting. I'm 99% sure my noise was due to the sticky decomp and cable free play out of spec..

I am extremely impressed with performance! This bike rips now! Clean idle, smooth power, throttle response is sweet, no bogs, no lags, no after fire kaboomers just a little crackle... I'm at 1.5 turns out on the air/fuel mix screw with the side cover removed on the air box. Time to call this a sweet success experiment!!

Now back to the valve lash... When you are at TDC on the compression stroke, how much wiggle (free play) should I expect on the rocker arms? I had very little and not easy to wiggle. Are the timing marks sometimes not 100% accurate (am I cockeyed maybe?)? I had the bike on the stand and leaning and I'm laying on the floor wearing my bi-focals. I'm 99% sure I was spot on the marks... Thoughts/Experience anybody? And what a pain getting feeler gauges in there (no room to work...)