PWK Carb

[Billybream]

It me again investigating possible replacement for Mk1 930 Concentric,  started last year but decided to overhaul my original carb, results were still not acceptable trying to overcome body distortion and slide wear so now back on the hunt.
Was leaning towards Mikuni but then discovered PWK and liked it's features and the JRC version.
Just think they are all over priced and was keen to try and get a cheaper option or source and by chance find SRM offer PWK30 for £50.00 complete with mounting flange on Ebay.
Will I be brave enough to try, the story continues.

 

[harvey mushman]

I bought one from SRM at £56 inc of postage for my A65, there is same carb from another ebayer at £16 i took a chance and bought 2 at 16 for other A65.

Before everybody starts shouting i know the jetting might be different and the jets are hard to get etc

watch this space...

[muskrat]

G'day Billy/Harvey.
I have two on my Cafe (twin carb head). The std jetting may be OK for a single carb application but trying to tune two is a nightmare. I had to change everything about 1 million times. I ended up trying 3 slides, 3 pilots, 5 needles and a few mains in all combinations. Kehin jets/parts fit straight in but are expensive, slides are 60 quid each.
Cheers

[harvey mushman]

G'day Billy/Harvey.
I have two on my Cafe (twin carb head). The std jetting may be OK for a single carb application but trying to tune two is a nightmare. I had to change everything about 1 million times. I ended up trying 3 slides, 3 pilots, 5 needles and a few mains in all combinations. Kehin jets/parts fit straight in but are expensive, slides are 60 quid each.
Cheers

I think you know what my next question is!

Only if you really want to, could you give me an idea what way you went with jetting. 

Pretty please 

[muskrat]

G'day HM
I ended up not too far from the std jetting. Slow jet #40 instead of the #38, air screw 2,1/4 turns out, 3.5 slide, JJH needle with clip in middle, 135 main jet. Std float height is 19mm but on an alloy head with the downdraft angle it will flood (same as a concentric) so set at 21mm. Fine tuning the needle is a nightmare as there is about 30 to choose from to cover 3 throttle positions.
One thing I really like is the internal "jet block" is removable so those tiny (10-15 thou) orifices can be cleaned properly.
Lots of info out there re tuning.
Cheers

[harvey mushman]

Thanks for that muskrat, I will have a go and see how I get on. I think they will have left China now so another couple of weeks till delivery.

Cheers

Derek

[Billybream]

Just received JRC (PWK) carb from SRM, pleased so far with quality but will be a month before I get around to fitting it. Need to strip it down and check what jets are fitted and source inline fuel filter. Paid £50.00 plus carriage so a good bit cheaper than equivalent Amal.
Appreciate might need alternative jets but they seem readily available and good bit of advice regarding tuning now available. Bought some Gorilla glue to affix flange to carb. Fuel inlet is 8mm so will need to adapt from 1/4" supply from tank.
SRM do not offer any application details or advice as they do with Mikuni, but for cost saving worth a try.

[muskrat]

G'day Billy.
When I bought mine from JRC the flange was attached with glue and a grub screw. It was very tight to remove as I use spigot to rubber to manifold.
Cheers

[Sluggo]

I know Bill well at JRC who commissioned the PWKs and supplies them to dealers, including I suppose to SRM.
I Would caution people from assuming the cheap ones on ebay are the same carb.  I have been meaning to order one off ebay and compare side by side, So I cant comment, However what I CAN tell you is the ones JRC supplies (And I am still a dealer for them) need no fiddling rejetting or other issues.  They DO include a jet up and down from ones fitted, but you simply spec what the application is and it comes all dialed in.
I have sold a lot of these and no issues or significant problems.  Every one has been fit and forget.  Nor,,, have we had any problems fitting a pair and tuning them.  (Bonnies and A65 BSA twins) The ONLY issue you WILL find is on a Norton with twin carbs (Generally I prefer a single carb on Norton twins).  The adjusters are one sided so for a norton they supply an extended screw with flats and a small flat wrench to fit in between to fine tune.  But Triumphs and BSA no issues.
Unlike a Mikuni VM carb which IS actually a Amal design sold under license to Mikuni, Some people complain about the looks of the flat slide JRC/PKW carbs as they dont look english. Fair enough,, but they DO work, and work very well.

The whole purpose of them was to offer a simple carb with minimal tuning needs and fiddly adjustments like a Amal concentric but without the massive quality problems of the stock Amals.  The JRC/PKW delivers very nicely in that regard at a attractive price.

You might find the following useful: https://jrcengineering.com/technical-support/jrc-pwk-carb-instructions/
                                                  also
  https://jrcengineering.com/wp-content/uploads/2017/09/JRC-PWK-CARB-TUNING.pdf

If you have specific issues, let me know as I speak to Bill and his team frequently, But be advised they do not deal direct with the public. Dealer only.  That said, they are one of the nicest people I know in the Brit Bike industry and proud to know them.

[edboy]

hi sluggo
the carbs do seem impressive and as a fan of mikuni carbs i am sure they are a stage better. but the main drawback for me is the sizing. i use a 28mm concentric on my plunger a10 and the recommended 30mm would mean opening up the inlet port? why no 28mm option?

[muskrat]

G'day Sluggo.
It's been quite a while since I bought my two. May have been Lowbrow who I got them off. I didn't get any extra main jets. I need the extended pilot screw to get to the right carb. It's a pain having to loosen the carb and turn it 45 degrees to get to it. I may be the only one to have twin carbs on an A10. I'll also have to get the cable choke and either run it through a splitter (like my throttle) or have two levers.
Tuning wise I have straight through exhaust with reverse cones, no filters, 357 cam and 10.5:1 comp so the std jetting didn't work. There are so many needles to play with it can get very confusing.
In saying all that I am still very happy with them.
A friend is using the OKO ones on his CB450 and is happy also.
Cheers

[Billybream]

The carb supplied by SRM does not state JRC anywhere, on the carb body its stamped PWK and the packaging box PWK30J. It came with seperate metal mounting flange, with 2 × grub screw fixing and no spare jets, but during communication SRM did say the carb was supplied by JRC.
So far I,m pleased with the appearance, finish and quality, as for function that's still to try.

[Billybream]

Fitted the SRM supplied PWK (JRC) 30mm carb today, changed the main and pilot jets, made up new fuel lines and installed inline filter, filled with fuel and pulled up the enricher and first kick she fired up, adjusted idle speed and was ticking over well. Discovered leak from serviceable filter, so switched her off.
Well impressed so far with this carb, just had to shorten mounting stud on L H and existing throttle cable fitted great. The supplied alloy mounting flange was gorrila glued to the carb and secured with grub screws supplied and had to source flange O Ring.

[Sluggo]

Im glad it worked out (So far)  I have been off line a bit, and just returned from our vacation home,  However, in a twist of Irony right before we left, had a old shop customer out for a ride stop by, He said he was riding and decided to see if I was still at the same place.

The Mrs was upset figuring we would never leave the driveway on time, But it all worked out.  Mark discussed the JRC/PWKs we fitted to his 68 Bonnie many years ago.  Still working great and he is very pleased with them
(Duals) I still have his old Amals on the shelf and reminded him we should get together soon and Ill check over the carbs and see if they need adjustment, and return his old carbs to him.
The old original Amals had body distortion and warping. Slides kept sticking.  I took an exhaust pipe expander and gentily tweaked the bores and then honed them with a cyl. boring tool.  I expect they will warp again in use, But they are now functional albeit a bit sloppy with the slides.  The idea is if he ever wishes to sell the bike he does have the original parts to include, But he stated he has no intention of going back to the Amals while in his ownership.
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As to missing bits (O ring) I cant comment on why the dealer failed to include that, They ship from JRC with a flange O ring.When replacing O rings always use a Nitrile or fuel compatible Oring.  I cant say always, but I never liked the original slide springs. Way too soft and I like a firm response with the throttle.  Based on my feedback as well as many other dealers and users, there is 2 slide springs usually supplied. The original soft spring and the firmer revised springs.
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As to choke cables,  I never fitted or used them with the JRC/PWKs as the levers suffice, and cables become another issue on their own.  (Anyone familiar with modern cable offerings understands this).  However last year I have a friend with a collection of bikes and he got his dream bike.. (I told him he was better off with a BSA Goldstar, but some people, sigh...........)  Anyrate, He had been after a Vincent for many years and bought a Rapide with a side car.

The Vincent issue with him is a long story in itself and not germaine.. However the Concentrics fitted were knackered badly, along with some other questionable maint by the DPO.  I ordered him the JRC/PWK carbs and he wanted the cables for the chokes.  We got them and a splitter and I also supplied a handle bar choke lever.  The plan was to photograph it and document the install. (And alienate most of the Vinnie community) But to date it remains incomplete due to other time constraints and other issues.  I do expect by summer it will be completed and the other repairs as well.
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One note on design,installs & sizing.  It depends on the application. Since these carbs are often fitted to a wide variety of machines,,
(I have a Kawasaki KZ750B, which is a 750cc vertical twin and similar to a unit BSA/Triumph but it was fitted with CV carbs and problematic so fitting these JRC/PWKs to it as well)

But there is such a wide variety of mounts and designs.  Some unit models used a phenolic insulating block which is quite thick. Hence long studs.  Many concentrics use a thicker O ring and a odd setup of washers, cupped rubber bits and locking nut to secure the carbs.  Many mouth breathing-knuckle dragging owners over tighten these and fail to understand you are only tightening to seal the Oring, NOT mash it down so tight it squeezes out the poor suffering O rings.  This accelerates as well the warpage problems many Amals are prone too. But often, studs might be too long, or, if you need a heat and vibration insulator and wish to use a spacer/insulator, the studs may be of insufficient length.
So,, some fettling required..!! 

As to the flange and spigots, The original bodies as supplied are spigot design.  Because most applications are flange mount, JRC manufactured suitable flanges.  Originally they were assured that a epoxy resin with the close machined fit would be indestructible and last forever, However early versions experienced failures, so I believe they changed Epoxys, and now fit a grub screw to help secure them.   When I ordered the carbs for Jeff and the Vinnie, Because of the manifolds previously on the bike, I wanted to mount them racer style without flanges and requested them without flanges.**
Jeff Changed his mind and while the bike had some really questionable manifolds fitted (2 piece with lots of RTV )  He elected to order new manifolds from some supplier in the Vincent community, so we are back to flange mount again.

** Racer style.  If you look at period race bikes and even some of todays classic racers, You will not see many Amals being raced with the exception of some of the Mk11s which is similar to the Mikuni VM style (Again, both the Mikuni VM and the Mk11 are Amal design).  But you will see often times is tuned length intakes for the application.  These are usually spigot mounts.  Some run a alloy tube, some run flex pipe. 
I have some Triumph race bikes and some of these were modded many years back with a variety of intakes and using the tuned length intakes. (Funny story here**)

I cant speak for everyone, But what we often sourced was from workplaces, I have used Commercial Sandblast hose, Naval and Maritime fuel and fluid hose, And food grade industrial hose that is chem resistant.  So, in a pinch at the time, I couldnt find a good source for Jeffs project so I went to an industrial supplier and bought a radiator hose for a Caterpillar Diesel engine.  Very attractive black thick wall material and chem resistant.  Also some heavy duty stainless clamps, not cheesy hose clamps like you see on some cars, but really nice clamps with bolts for tightening.  Since no longer needed, I will repurpose it for a cafe racer build but cost a small fortune.

**So funny story,    I often would grill old guys who raced, And there was a local named George who was somewhat famous in local racing series, Sonny Burres, Glen Adams and several other nationally known racers rode for George.  Most of these guys went to great lengths to throw off their competitors as well as closely guard their secrets.   George fed me many absurd bits of misinformation over the years as a matter of habit, but eventually stopped telling me tall tales and gave me the facts (Which was appreciated).

So, they often fitted crazy or mismatched parts their race bikes knowing full well that other racers would copy it and detune or slow them down.  At times they went to great lengths to do so with hidden modifications to make these work.  One of which, Sonny went out and qualified with a Triumph with absurdly long tuned intakes using the industrial sandblast hose.  They hid a sleeve inside to mask the true ID of the tube and did some creative tuning to overcome the shortcomings.  Sonny was fast because he was a outstanding rider.  So when the races start, they went to the line with the normal setup they had kept hidden.  Mind games!         There is rules for being able to race with what you qualified with, But the game was simply, keep the other racers guessing.
Here is a Unit A65 Tracker that illustrates a extended intake .

[Sluggo]

Eh, went on a roll there and forgot to address one issue. 

This could be helpful as well See: https://jrcengineering.com/technical-support/jrc-pwk-carb-instructions/

Also: Additional information on your JRC carburetor.
Accessories and kits available;
1. Cable operated choke assembly for single , twin , and 3 cylinder applications
2. Fuel line and fuel fittings to make custom fuel lines
3. Air filters , custom and original type
4. Throttle cables for single , twin or 3 cylinder applications
5. Carburetor kits for Norton Commando single 30 or 32mm
6. Carburetor kit for TR6/7 to fit standard air filter box, 30mm
7. Carburetor kit for T140E 1979-82, replaces Mk2 Amal or Bing
8. Carburetor kit for new Royal Enfield 500, 30 or 32mm
9. Carburetor kit for Ural twins
10. Trident and Rocket 3 gantry conversions to keep stock air box and gantry

&

Check out our carb fitted to various applications. Click the links below:

Triumph Bonneville 650 – pair 30mm

Triumph TR6 – 30mm

Triumph TR7 – 30mm

Triumph Daytona 500cc – pair 26mm

Triumph Bobber 500cc – pair 26mm

Triumph Trophy 500cc – 26mm

Triumph and BSA 3 Cylinder Machines – three 26mm

Triumph and BSA Unit Singles 250-500cc – 26mm

BSA A7 500cc – 26mm or A10 30mm

BSA A50 -26mm BSA A65 -30mm

Norton Commando 750 and 850 – pair 30mm

Norton Commando 750 or 850 Single carb -32mm

Norton 500 Single – 26mm

Royal Enfield 350 Bullet – 26mm or 500 Bullet 30mm

Royal Enfield 500 Bullet – AVL motor – 32mm

Royal Enfield 750cc twin – pair 30mm

Ural Dneper 750 – pair 30mm

Douglas Dragonfly – pair 26mm

Moto Guzzi V50 500 V twin – pair 26mm

Ariel Square 4 1000cc – 30mm

Montgomery Wards Lawn Tractor -26mm



As to sizing of whats available, I Did talk to Bill and his guys at JRC about this.  Its always a guess what to offer and what the marketplace will respond too, further, then it comes down to What can you source or what can you get manufactured?  ** (See below for more fun with manuf)

So IIRC and I will resubmit this today for any update,  But my understanding was to try and offer the basics for what  most will order.  Triples, singles and twins.  While a 26mm-28mm-30mm &32mm all were fitted to BritIron at one time or another by Amal, They were limited on carb bodies sourceable.   Originally Bill tried a lot of different companies to supply replacement carbs, And there is a lot of issues in investment, engineering and then having a marketable product, many of which the consumer never sees. So that is why what they have is whats on the market.

Keep in mind also, Different carbs respond in their own way. A round slide carb does not function the same as a flat slide design.  A VM round slide style Mikuni does not function at all like a CV style Mikuni and the sizings are different for the same size motor.  Secondly, RPM and usage, as well as temp and altitude impact as well.  I built a 71 Bonnie for a customer and we fitted 28mm twin carbs as he rode it primarily in city traffic and urban areas, Rarely ever saw RPMs over 3500 and no freeway-interstate riding.  With 30s or 32s, it was just a dog.
I serviced for many years a Triumph with a big bore kit (810cc) with dual 34mm carbs and it was a dog off the line but once you got it up to 5000+ RPM it just kept accelerating.  Was not much fun to ride on the street except the odd track day when they allowed street bikes on.

In hot rodding days with cars, people chronically overcarbed.  Common to see a SBC 350CI engine with Holley 750 and 850 double pumpers with a 600cfm with vac secondaries was faster, more power, more responsive and gas mileage was around 15-20 mpg vs the 850 double pumper which was a dog off the line and managed 8 mpg.

And heres a link to an interview with Sonny where he talks about psych tricks and racing..
See: https://www.soloshawn.com/sonny-burres.html

Heres some period race clips, Sonny winning Peoria TT in 1975, along with interviews of others.
See: https://www.superbikeplanet.com/1975-dirt-track-footage-sonny-burres-gary-scott/

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I was in a discussion about manufacturing, JRC is coming out with some very cool flat track racing and desert racing reproductions.  Bates type seats, Belly pans, and air boxs.   One discussion is the old flat blade type fenders many 50s and early 60s racers used.  While its not complicated to make these, Many people want a ready to install kit along with fender stays. 

Materials wise, here in the US, its a problem.  My wife is logistics lead for a machine shop as well as Master scheduler.  Getting tooling, materials and machine time is a complicated juggling act.  While there is a lot of news medic covering trade issues, tariffs and import/export in global news, Here is the real problem.

Many large companies are now hedging their bets and stockpiling certain types of materials, which can complicate sourcing certain specs and dimensions.  US made stainless, steel, aluminum, copper & Brass is a very volatile market.  Imported materials rarely meet spec, and JRC told me that there is a worldwide shortage of Stainless steel fenders for certain British bikes because the Eastern Europe and Asian made stainless sheet has issues.  When stamping in a die press the Stainless work hardens and cracks too easily.
Industry wide, there is a ongoing issue with this, I know personally I get about a email or phone call every week asking If I have any stainless fenders laying about.  Even used ones.

My wife had ordered some specialized mill run copper tubing for a customer job and it was damaged in transit.  Insurer and transport company suggested "Why not just bend it back???" 
So, its scrap.  The mill said, it will be 6 months before they make any more. The customer is NOT happy.